18 ms·

Why can’t I go faster than the speed of light?

271 points by lanna 5y ago

programmer_dude 5y ago
Wow, this is the first time I have understood why nothing can go faster than the speed of light and why time dilation is a thing. A very easy to read article, give it a try. Also if you are curious why a moving charge creates a magnetic field watch this Veritasium video: https://www.youtube.com/watch?v=1TKSfAkWWN0 https://www.youtube.com/watch?v=1TKSfAkWWN0
- deleted 5y ago
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- thriftwy 5y ago
  Frankly though, I not sure why they couldn't just use a simpler correction: postulate that moving charge only creates magnetic field with regard to objects moving relative to it. Which is also basically true? This way, the stationary observer will sure notice that there's F_B, but will also know that the charge moving alongside the (also moving) rod does not experience it.
  - DangitBobby 5y ago
    That's the thing though, in the article the charged object does not move relative to the rod. The rod, object, and person B are stationary relative to each other. I was actually really surprised to read that two charged objects moving with the same velocity relative to each other generates a magnetic field from A's perspective.
    - whatshisface 5y ago
      Two long charged rods moving with the same velocity relative to each other are two parallel wires carrying current, the second of which is a typical and easy-to-calculate example of electromagnetic attraction. In fact, that situation is so prototypical that it is used to define the Ampere in terms of what current is required to produce a certain force between parallel wires. [0] [0] http://www.physics.louisville.edu/cldavis/phys299/notes/mag_force_2wires.html http://www.physics.louisville.edu/cldavis/phys299/notes/mag_...
      - thriftwy 5y ago
        In this scenario, rods are stationary but they have current running in them - a different situation. Can it be translated to the relativistic one? Maybe what they really wanted to say is "there is a speed c at which the charge of a moving rod is indistinguishable of current running through stationary rod"? Now that would make a lot of sense.
        whatshisface 5y ago
        It's not different as far as electrodynamics is concerned. The motion of the charge is what matters; whether or not the neutral part is stationary or moving is of no concern.
soneil 5y ago
The simplest way I could ever wrap my head around this, is to understand that everything travels through spacetime at the same speed. There is no travelling faster or slower, just crossing the graph at a different angle.
- juped 5y ago
  Yes, the magnitude of something's 4-velocity vector is always c^2 (set it to 1 in your unit system) by definition. It's not as simple as a R^n vector because you have a time coordinate of opposite sign and you switch coordinate systems by Lorentz transformation, but it's the right intuition. (It's complicated further in the presence of gravity, where you need an enriched differential geometry notion of a velocity vector since the spacetime is curved.)
- minitoar 5y ago
  Right, this is like the 2d projection of it which makes it easier for our 3-space brains to comprehend.
- kevinventullo 5y ago
  Hm, but if I can make my angle orthogonal to the time axis, isn’t that like instantaneous movement?
  - bollu 5y ago
    Yes, which is why a photon experiences zero proper time.
    - RHSman2 5y ago
      They should make a movie called ‘Photon’ and track its life. A parody based on science.
    - raattgift 5y ago
      That proper time is zero everywhere along a photon's geodesic does not mean that the photon cannot evolve from point to point along it, and we can show this by taking advantage of total coordinate freedom. We can parametrize (as in make parametric) arbitrary curves through spacetime however we like. Parametric representations of unique curves are generally nonunique. Some of the infinite possible parametrizations of a chosen curve have useful properties, such as uniquely labelling every point on the curve with some monotonically ordering value and keeping the form of some set of equations reasonably simple. For timelike geodesics, particularly in the Minkowski space of Special Relativity, proper time (being a Lorentz scalar) is a good option. However that is not true for all geodesics in Minkowski space (as you note, the proper time is everywhere zero on a null geodesic, and so a bad option), much less all curves through general curved spacetimes. For null geodesics, following the logic of GP's question, we may wish to preserve the tangent vector under parallel transport; this requires the parametrization to be affine. Some gory details at https://en.wikipedia.org/wiki/Geodesic#Affine_geodesics https://en.wikipedia.org/wiki/Geodesic#Affine_geodesics and a brief useful summary at https://www.reddit.com/r/AskPhysics/comments/9aenid/what_actually_are_affine_parameters/e4uwfda&context=3 https://www.reddit.com/r/AskPhysics/comments/9aenid/what_act... As is noted below the comment directly pointed to by the second link, labelling a timelike geodesic with proper time is choosing one specific affine parametrization on that geodesic, and that this choice is driven by convenience. One of the neat outcomes of affine parametrization is that we can take a point on an affinely-parameterized null geodesic and look at the derivative with respect to the affine parameter there, and define a momentum k^{\mu} = \dot X^{\mu}. In a Lorentzian spacetime, with curvature, we can compare the momentum at two different points on the null geodesic, giving us the gravitational redshift between those two points of the photon's wavelength equiv. frequency.
sylr 5y ago
With gifs: https://imgur.com/gallery/BT2mG https://imgur.com/gallery/BT2mG
- zfxfr 5y ago
  Wow thank you ! I have read the whole article but I couldn't wrap my head around it. I found the "rod story" confusing then I followed your link (and also watched the related Youtube video).. This really makes things clearer. I am really not into maths but this version seems much easier to understand and very logical.
leonardp 5y ago
I always feel like this questions is really asking "What is time?". This book gives a new perspective on time which I think is crucial for understanding the nature of it: https://www.harvard.com/book/the_janus_point/ https://www.harvard.com/book/the_janus_point/
otterley 5y ago
My favorite way to think of it is that the speed of anything is measured in distance per unit time. You can’t change the constant that is the speed of light, but what if you could change what “time” means? The constant holds, but time doesn’t; it varies among observers. A second from the perspective of observer A is not identical to a second from the perspective of observer B. Freaky, but true.
- stareatgoats 5y ago
  I still don't get this. I simply can not accept that the speed of light is that same for all observers, irrespective of their respective speeds vs the light source. It is obviously a cornerstone of Einsteins theory (I forget which; either the special or general relativity), but to me this is simply not logical. To state that the speed of light is the same only because the relative time is variable for respective observers is unconvincing, which requires that things like blueshift/redshift to be explained by logical gymnastics, and not the straightforward reason that the speed of light for all observers is a direct function of the relative speed vs the light source. But what do I know.
  - DangitBobby 5y ago
    It's a consequence of the time dilation. You are not stationary towards the photon to an outside observer, so to them it looks like the photon moves away from them faster than it moves away from you. And from the obserer's perspective, your clock is ticking slower. That's the key. The amount that your clock is ticking slower is such that you, the one moving, would calculate the speed of the photon the same as the stationary observer, because you would do so from your own slower clock.
    - stareatgoats 5y ago
      Thanks, that sounds like the explanations I've seen too, and that I hope to understand some day. Atm I'm at the point where I gladly accept that time may seem to dilate, but not that it actually does. Maybe I just need to dive deeper into it than I've been willing to do so far.
deleted 5y ago
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redwood 5y ago
I think a lot of people assume that this means you couldn't go more than a few (~100) light-years in your lifetime... But this is not actually correct. Counter-intuitively you can theorically go any number of light-years (essentially) in your lifetime, as long as you are able to approach the speed of light because when you do so the distance is dilated and hence you're covering far more ground within your reference frame (of course you'd be in the deep future from the perspective of anyone in our normal reference frame).
- rkagerer 5y ago
  It just means you can't within the lifetimes of the people you left behind. For that you need more exotic contraptions (wormholes, warp drive, etc).
  - ghayes 5y ago
    If they were to wait a year and then come find you far away, would it net out and you’d be the same age?
    - Guillaume86 5y ago
      Yes
- xyzzyz 5y ago
  To put it another way, time travel machines are very much theoretically possible, and in fact are “only” an engineering problem (an extremely hard one, however). But, only forward travel is allowed: time travel machine could take you as far forward as its engineering would allow it, but there is no going back.
  - nimish 5y ago
    They are only theoretically possible if you allow for negative mass and energy--not an engineering problem so much as a "need to find exotic matter" Basically people ran the EFE "backwards" to see what matter distribution makes the wanted curvature. You get either negative mass-energy or the bubble doesn't travel ftl iirc.
aero-glide2 5y ago
I've been trying to understand this since high school. No Eureka moment but I think every year I understand just a little bit more.
StephenAmar 5y ago
I highly recommend watching https://youtube.com/playlist?list=PLoaVOjvkzQtyjhV55wZcdicAz5KexgKvm https://youtube.com/playlist?list=PLoaVOjvkzQtyjhV55wZcdicAz... They do such a good job at using video to explain Special Relativity.
webwielder2 5y ago
I prefer the question, “why can’t light go faster?”
franciscop 5y ago
A bit all over the place, the author assumes "It is even less well-known that the rule “nothing moves faster than the speed of light” is a consequence of the laws of electricity and magnetism" but at the same time "This is also a well-known property of electricity and magnetism" and then drops some equations. Most people for whom this property of electricity and magnetism is well known, also know that the speed limit from general relativity comes from Maxwell's equations (which are on themselves a compilation of other previous rules). I also dug deep into this problem a while back, but without proper physics background couldn't get too far. The speed of light comes directly from the Vacuum permeability and Vacuum permittivity, since light is an electromagnetic wave. If these were different, then the speed of light would be different. Both of these seem to be values of the behavior of vacuum in our universe (and possibly electron's [1]). It is Maxwell's equations gives us that the speed of electromagnetic radiation needs to be c ^ 2 = 1 / (e * u) [2]. [1] https://en.wikipedia.org/wiki/Fine-structure_constant https://en.wikipedia.org/wiki/Fine-structure_constant [2] https://en.wikipedia.org/wiki/Electromagnetic_wave_equation#The_origin_of_the_electromagnetic_wave_equation https://en.wikipedia.org/wiki/Electromagnetic_wave_equation#...
- DangitBobby 5y ago
  The author's assumptions were spot on for me. I am familiar enough with E&M to know the equations and roughly understand relatively but did not know where the intuition for a universal speed limit comes from.
- shusaku 5y ago
  People can disagree with your assessment, but it’s pretty sad that you write the only comment in this thread that actually engaged with the content of the linked article, and it’s downvoted.
crdrost 5y ago
That you can’t go faster than the speed of light is a direct, simple consequence of everybody agreeing on the speed of light. It's a real-life Zeno paradox. Alice and Bob are in some spaceships by a long racetrack in space. Alice fires a very brief pulse of light down the racetrack, maybe we see the rays that don't go straight through successively illuminate some rings around the track. Alice challenges Bob to race another light pulse, Bob revs his engine. As the countdown hits zero, Bob accelerates to speed c/2 relative to Alice, then checks the reflected light from these rings only to find out that the light pulse is still traveling at speed c away from him. So he drops a beacon at his current speed then accelerates to speed c/2 relative to that, but no dude: the light is still moving at c away from him. Bob realizes that he can never win, so tries to instead measure the speed of Alice, who he expects to be moving at speed c away from him, only to find that she is instead moving at speed 0.8 c away from him. Now the question is, how can this be? Consider a much slower spaceship. When Alice fires the light pulse and Bob starts moving forward at a slow speed, Alice sees this bubble of light expanding in kind of a uniform sphere centered on her. (Say it reflects off of space dust instead of a track.) Since they were at the same position when the light Bob also sees an expanding bubble of light, with himself at the center. The weird stuff about time dilation and length contraction does not apply at low speeds, if Bob goes at c/1,000 say, then these are only one part per million. They both basically agree on how far this bubble of light is from Bob, in the directions perpendicular to the motion. The motion is parallel to the bubble in those directions, and to first order those parallel lines will not get any closer or further away. (This is why I want Bob to move at a slow speed!) They only disagree along the motion. Alice thinks the light is receding from Bob at speed c–v ahead of him, at speed c+v behind him: Bob sees the light recede at speed c in both directions. So they come back together to repeat the experiment and Alice decides to force the contradiction. Alice puts a clock ticking out every millisecond out at distance 1 light-second, but it will start at the moment she fires the pulse, stopping when the pulse hits it, showing 1000. She puts one of these in the direction Bob will travel, and in the opposite direction. Surely he must agree that the light started from here and that it intersected those two clocks when they both said 1,000. Bob agrees that the clocks look synced up and films all of this with a high-speed camera to make sure that there is no funny business, and they repeat the experiment. Right when the light hits, Bob accelerates at his usual 1,000,000 gees for 0.03 s, to get his final speed of c/1,000. (I need to rewrite this to make the numbers more reasonable LOL.) Here's where something weird happens, and it is entirely contained to those first 30 ticks of both cameras, as Bob looks at them in his high-speed camera footage. Bob corrects for Doppler shift like you do, and agrees that these clocks appear to be ticking during the other 970 ms at one tick per ms, his camera has maybe microsecond resolution and not the nanosecond resolution you need to see time dilation. But during those first 30 milliseconds when Bob was accelerating, even after correcting for the Doppler shift, his best guess is that Alice artificially slowed down the clock behind him and artificially sped up the clock ahead of him. Because the clock ahead of him definitely ticked 31 times in those 30 ms, while the one behind definitely only ticked 29 times. So Bob says the light did hit these clocks when they said 1000 ms, but the clock ahead of him should have said 999ms at that time, and the light should have gone past it a bit by 1000ms, while the clock behind should have said 1001ms, and the light was actually not yet there at 1000 ms. This anomalous Doppler shift is proportional to both the distance of the clock you're looking at, and your acceleration. It is also called the relativity of simultaneity, and it is the only new prediction of relativity, in that length contraction and time dilation are second-order consequences of it.
ergocoder 5y ago
"Nothing can go faster than light" is just a convention and is not experimentally confirmed. IIRC there are a few things that go faster than speed of light (e.g. universe expanding). "Spooky action at a distance" is also not known very well. That could also break the speed of causality law. While Bell's theorem hints that this is not the case, there are some exceptions to Bell's theorem. Right now a lot in physics are just convention like energy conservation and symmetry. We need another paradigm upgrade to understand these things.
- Choc13 5y ago
  Hmm not sure if troll, but energy conservation is definitely not just some convention. It’s a fundamental thermodynamic law. Spooky action at a distance does not violate speed of light information propagation either. The particles have to be entangled before they’re set off in opposite directions. Only once one is observed does the other also collapse, but it doesn’t mean you can communicate faster than the speed of light because you had to prepare the information when the particles were together IIRC, it’s been a decade since I studied quantum information theory.
  - ergocoder 5y ago
    Why do you feel the need to say somebody is a troll? Here: https://phys.org/news/2017-01-violations-energy-early-universe-dark.html https://phys.org/news/2017-01-violations-energy-early-univer... Here: https://phys.org/news/2015-02-space-faster.html https://phys.org/news/2015-02-space-faster.html - we are trying hard to reconcile this and categorize universe expanding as something else (e.g. not a movement that has speed because time itself is a dimension or something). But this is still up for debate, tbh. Another example was one where we said CP symmetry was true (it was a law like a lot of things in physics) until it was violated by a weak nuclear force experiment. And now we are holding the fort at CPT symmetry as the law. In Physics, the evidence of anything is kinda light. A lot of reasonable extrapolations has been made. Still they are extrapolations (e.g. intelligent guess). Even the big bang itself is just an extrapolation from the "theory" that the universe expanding. To be fair, it is difficult to find good evidence because we can't dial back time, can't go observe things on Neptune, can't measure gravity at the subatomic scale, and etc. So, we have to work with what we can experimentally observe. Our tools are getting better, and this is where the physics paradigm shift will come from. You say like these are 100%. It is just a theory that we currently hold according to the little evidence that we have. Failing to recognize that is straight up unscientific.
cletus 5y ago
The best analogy for this I've read is that your motion through spacetime is constant. Think of a quarter circle dial with an arrow that goes all the way from 0 degrees (no relative movement in space) to 90 degrees (relative speed of light). The faster you go through space, the slower you go through time. The effects of this are barely noticeable until you get near the speed of light. This effect is time dilation. This, I believe, is the general relativity view of spacetime. What I like most about this is that it highlights the importance of understanding the domain of a function. The above shows how any object with rest mass has a space velocity domain of [0, c). People really don't want this to be true so latch on to any wild theory that would seem to bypass this cosmic speed limit, be it wormholes, FTL drives or whatever. Pretty much all of these theories rely on taking an equation with a domain over real numbers and plugging in negative values for things like mass. Garbage in, garbage out. I mean if mass can be negative, why limit yourself to real numbers? Why not a complex number for mass? As to why this is the case and how to reconcile it with a quantized view of spacetime... is beyond my pay grade.
ghego1 5y ago
On traveling at the speed of light I also highly recommend to watch this Veritasium video https://youtu.be/vVKFBaaL4uM https://youtu.be/vVKFBaaL4uM
dejongh 5y ago
Thanks. Fun to know that the speed of light prevents the universe from splitting up. Cool. Also I was not aware that movement create magnetism.
PaulDavisThe1st 5y ago
In the 70s, I remember reading "The Quincunx of Time", which made great use of a key distinction (for the purposes of some classic 70s scifi): It's not that nothing can move faster than the speed of light, it's that nothing can accelerate to a speed faster than the speed of light. If you could bring particles into being that already moved faster than light, they would not violate our understanding of relativity or the rest of physics. Hence ... the tachyon. https://en.wikipedia.org/wiki/Tachyon https://en.wikipedia.org/wiki/Tachyon
unyttigfjelltol 5y ago
Looks to me like the short answer to the question posed in the headline is that matter is composed at an atomic or subatomic level of energies similar to light and so it would be quite inconvenient to try to propel the mass as a whole faster that its constituent parts can go. A similar principle may well be true for straight-line inertia, which might be caused by the same as the force that makes gyros stay on course. In general the scientific discussions seem to touch these ideas but approach theories in reverse of intuition so it's hard to know if they align.
dvh 5y ago
This example has a flaw. Skateboard man sees object stationary, but the cable is moving. There should be magnetic force too.
deleted 5y ago
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ySteeK 5y ago
wouldn't it go backward in time if it goes faster than light? Light travels in null-time from point a to point b (light-perspective of course) - anything slower, needs time to go from a to b. if i travel faster than light, i can - but i travel to b before i start at a. Maybe because this is not possible.
- drran 5y ago
  How much you will travel back in time by using a teleport (instant travel, infinite speed)?
holoduke 5y ago
Interesting to know that if we would have a 1g acceleration spaceshil with almost unlimited energy onboard we can cross a large part of our galaxy in less than 70 years.
subroutine 5y ago
Q1. Does the future already exist? I realize this might be purely philosophical, but if I leave Earth, and fly through space near c kph, and return home a few hours later to find everyone aged 50 years... are they the "same" people, or are they a future-instance of the people left? To clarify, theoretically I could leave earth, and return home in exactly 1 hour (from my ref frame), and basically make people whatever age I want by varying my speed. Since the same 1 hour passes for me no matter the age for them I chose, it seems like I'm selecting an already existent future from a stack, not fast-forwarding the present I left. Q2. How do black holes exist if time stops inside a black hole? That is, they continue to move through spacetime, even though spacetime is not moving within the hole? I accept that a black hole can form, and can stop time, I'm just curious how something that stops time continues to persist in the present. The way I'm visualizing this, is like a lava lamp about to bleb off some goo from the top [1]. As the object gets more and more dense, it curves and drags spacetime more and more, until eventually its density passes the Schwarzschild radius and blebs off. [1] https://m.media-amazon.com/images/I/615deDvfDkL._AC_SS450_.jpg https://m.media-amazon.com/images/I/615deDvfDkL._AC_SS450_.j...
- the_gipsy 5y ago
  Q1b. Are you the same person you were yesterday?
  - subroutine 5y ago
    I think yes, and the answer is more concrete for "oneself", since the molecules that make-up oneself are all accelerating and decelerating with you/as you in your own reference frame.
    - mewse 5y ago
      I mean, apart from cells and etc dying and being replaced as we age. ;) Truly, the Ship of Theseus was inside us all along. (Which is super convenient, considering how often we step into the same river twice!)
- ww520 5y ago
bambam24 5y ago
Do you have zero mass? Otherwise just to a 150lbs person will have mass of a galaxy at 99% of the speed of light. And to go remaining %1 you will need to have a power to push entire galaxy 0.1% of speed of light. At 99.999998% of speed of light the man will need to burn all the elements in the universe. Basically a credit card or bank loan cannot buy you such energy.
thom 5y ago
You can’t go faster than light for the simple reason that it would defeat the very many performance optimisations necessary to keep the universe running.
masa331 5y ago
Of course you can go faster than light, way faster. When it comes to physics there are no limits how fast, far or deep you can go. People constraint themselves with articifial limits and it's not necessary a bad thing. But real limits doesn't exist. I'v heard someone saying space is a big ball and outside of it is nothing. I imagine nothing as a black space but that actually is something. So there is something after all. And however weird or normal these other places are, it goes like this infinitely.
- chrisoverzero 5y ago
  > Of course you can go faster than light, way faster. OK. How?
  - masa331 5y ago
    I don't know
holoduke 5y ago
What would happen if you had a very large rotor of let's say 50.000km in diameter. And let it rotate at 1r/s. The inner part would only move at very low speed and the outer part at near lightspeed. What kind of time dilation effects would be seen? If you would sit on the tips for a few days and then stop the rotor, travel back to the center, you would see a very old one right?
- hinkley 5y ago
  I think veritasium discusses that as well. Essentially at relativistic speeds there are no chemical bonds that can hold and the device shreds itself.
midrus 5y ago
I have a probably very dumb question. If speed is relative, when we say something travels at... I don't know... 50% the speed of light, that speed is relative to what? how do you know it is 50% and not 53%? How do we know we're not already moving at 99% the speed of light (like our observable universe as a whole having that speed )? I love this stuff, but it is so counter intuitive for the average human.
- drran 5y ago
  Higgs field is present everywhere and mediated by Higgs boson, so just measure speed relative to Higgs field. Or you can measure speed relative to CMB.
- hinkley 5y ago
  No matter of fast you are moving, you will agree with a “stationary” observer about the speed of light. What you won’t agree on is the color of the light. Apparent frequency shifts to maintain the speed of light. The color shift is how we know how fast stars are moving relative to us.
- RogerL 5y ago
  "that speed is relative to what" To you. There is no such thing as absolute speed. You say cosmic rays are crashing into the earth at 99% the speed of light. The cosmic ray says it is sitting still and you (and the Earth) are approaching it at 99% the speed of light. Some other particle moving at 70% relative to you will give a different speed for the cosmic ray. All of you are right.
- caf 5y ago
  When we say something travels at 50% the speed of light, it's always relative to some inertial, approximately inertial, reference frame, often implied. So eg. if you hear about the particles at the LHC travelling at near the speed of light, it's implied that's relative to the Earth. The speed limit comes in here: no-matter what nearby object you look at, its speed relative to you always be less than the speed of light. (Here, nearby means something like "within the local group of galaxies" - objects a long way away can be receding faster than the speed of light due to the expansion of space in between you and the object). You can accelerate as much as you like for as long as you like and when you stop accelerating and check, that'll still be the case.
dhosek 5y ago
My favorite general relativity paradox: A runner is running with a 20m pole at a sufficient velocity so that a stationary observer sees the pole having length 10m. The runner runs through a barn which is 10m long. And two people outside shut the doors simultaneously so that the runner is completely enclosed in the barn for a moment. But from the runner's perspective, the barn has length 5m long. What does she see happen when the doors are closed on her?
- gridspy 5y ago
  How do the two people at opposite ends of the barn agree on when to close the doors as a runner approaches at relativistic speeds? Also, relevant xkcd (what-if) https://what-if.xkcd.com/1/ https://what-if.xkcd.com/1/ Most likely from the runner's perspective the world turns white as they dissolve to plasma prior to reaching the barn.
- n3k5 5y ago
  > And two people outside shut the doors simultaneously No, they don't. I'd phrase it differently, e.g. “each door is shut for an instant when the end of the pole is just inside the door”. (Also, make it clear that the barn is long enough to fit a 10m pole between the two shut doors.) When you put it as “shut the doors simultaneously”, it's more troll physics than a paradox. Meta-puzzle: figure out how to still imply that the runner is completely enclosed for an instant, without incorrectly insinuating that it is so from her perspective.
- deleted 5y ago
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- contravariant 5y ago
  From the frame of reference of the runner the pole will first hit the door at the end of the barn (unless it opens in time), and the door at the front of the barn will only close just after the back of the pole has passed the door. If the doors actually don't open in time then the resolution for the fact that the pole doesn't physically fit is that nothing can be rigid at relativistic speeds. So either the door or the pole or both will deform (violently) to allow the pole to fit inside the barn.
wizardforhire 5y ago
You can go faster than the speed of light. Its slowing thats the hard part!
OneEyedRobot 5y ago
Can a thing with no mass ever go less than the speed of light? Does it have a concept of time?
- jandrese 5y ago
  Sure, light is slowed down when it travels through different substances. That's why we have refraction.
  - micw 5y ago
    Is it really slowed down? I always assumed that it just need to take a longer way through matter than through vacuum.
deleted 5y ago
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noobermin 5y ago
The beginning is the best line, that no one really knows "why" and what I have said to even other physicists who don't really understand it. A lot of the counterintuitive consequences of the paradox of the moving charges detailed here are a fact of nature but we don't really know why it must be that way. For example, in a modern particle physics starting point for qed, we force the equations to be lorentz invariant from the start but that is a starting assumption. But that's it, there's no real deeper answer to "why" beyond it being a experimentally observable fact, and the only resolution of these observations is that lorentz transformation and thus things like time dilation and length contraction must happen. Small relativity related tidbit: I hate when people say the phrase "the faster you go, time slows down for you." This is a problem because it implies that the moving observer notices their own time dilation which is reverse of the case. Of course, every non-accelerating observer is in their own rest frame, so it doesn't make sense to say "time slows down for you," because you are your own reference and there is no other frame to base your measurements on (I mean that was the whole point of relativity, there is no universal rest frame). Instead, when you measure the rate of change for other reference frames moving relative to you, their clocks move slower when measured by your clock. So the actual phrase should be something like "the faster others go, the slower their time appears to you."
sytelus 5y ago
If you just extend time dilation beyond zero (I.e. negative time) then speed faster than light is possible. The constraint described in the article will still be maintained. Also, notice that negative time would be the perception of external observer.
wruza 5y ago
If anyone wants to play with a speed of light, there [still] is a fun web game called “velocity raptor”.
lifeplusplus 5y ago
Velocity is a relative number. So let's say your are going 0.9 c but I'm going at 0.8 c now according to me you can go 0.7 more at least. But you're already going 0.9 and adding 0.7 will put you above speed of light. You can say you're already travelling faster than light relative to something out there... So what the hell are we talking about here?
France_is_bacon 5y ago
As far as I know, we can, and do, go faster than light all the time. Right now, our bodies are going faster than light, relative to other astronomical bodies. Space can expand faster than the speed of light. Therefore, since WE are on one galaxy, we are traveling faster than the speed of light away from other galaxies. Oh...did you mean us PERSONALLY traveling faster than the speed of light to other parts of the galaxy? This is probably the best analogy I've seen: https://www.youtube.com/watch?v=A2JCoIGyGxc https://www.youtube.com/watch?v=A2JCoIGyGxc
eyelidlessness 5y ago
Light isn’t limited in the speed it can travel. Mass is. Light can’t exist without matter. The fundamental limit is mass versus all of the forces which act as drag.
- pfalafel 5y ago
  Why are gravitational waves limited by the same speed limit? There is no mass traveling in that case, I suppose.
  - eyelidlessness 5y ago
    I wouldn’t dare to hazard a guess here, but I do think it’s worth remembering that matter is always in motion.
  - jandrese 5y ago
    What we think of as the speed of light could also be thought of as the rate at which change propagates throughout the universe. The rate at which cause and effect travel.
    - pfalafel 5y ago
      Two runners overtake the field at incredible speed, only to cross the finish line at exactly the same time. Would also raise eyebrows.
  - raattgift 5y ago
    This is an excellent question, which predates the theory of General Relativity by a few years, and has only been astrophysically verified after 1974 (Hulse-Taylor) or even later. As far as I am aware, a complete theoretical answer in General Relativity itself is still elusive. I'll largely stick with the theory, which I guess is what you are interested in.[1] The topic is in Part VIII (Chapter 35) of Misner, Thorne & Wheeler's Gravitation ("MTW"), which is the gold standard reference/textbook for General Relativity. I will later try to briefly summarize the section. Instead, first, at the root of my answer is the non-linear nature of the Einstein Field Equations, which imply that gravity self-gravitates. This is usually side-stepped by textbooks, which instead proceed to linearize the Einstein Field Equations, that is, they consider the weak limit of gravitation. This is usually to split a metric with a gravitational wave into some minimally-or-even-non-dynamical background and the wave-part, using the former to define the speed of the latter. For strong gravitational waves, or a very dynamical background, we cannot do this. I think this regime is best studied in a vacuum solution, i.e., where there is only gravitation that self-interacts, although most of the work in this regime appears to have the aim of resolving questions about the distribution of matter in the very early universe (e.g. Misner's mixmaster). An interesting exact solution of the Einstein Field Equations is the https://en.wikipedia.org/wiki/Kasner_metric https://en.wikipedia.org/wiki/Kasner_metric which can generate singularities and other features formed by gravitational-wave interactionsn. That is, there is manifestly non-linear gravitational self-interaction in the highly-dynamical Kasner chaos. (This is the worst case for the linearized treatments in textbooks). The Kasner metric can be applied to a Lorentzian manifold (3 space, 1 time dimension), and so is consistent with our universe's causal structure. An interesting feature of this solution is that it engages only two constants in the Einstein Field Equations: c and G. The only velocity scale that we can construct from any combination of these two constants is c itself. This is highly suggestive that in a universe like ours gravitational radiation must propagate at c. Again, this is just an argument that there may be an answer within the theory of General Relativity itself, without treating the speed of gravitational waves as a postulate. This argument is made without regard to the obvious craziness in a Kasner chaos universe compared to our own. General Relativity admits complete gravitational solutions for all sorts of craziness, including universes with any number of space and time dimensions as long as there are at least two total, stress-energy which is distributed very differently from ours (including negative energy, or energy that pops in or out of existence without a cause), and so forth. It is very General. (Special Relativity is very Special: it's defined on -- and only on -- a gravity-free (flat, no gravitational waves) spacetime of exactly 3 spatial and 1 timelike dimension.) Returning to MTW, they conclude that the propagation speed instead can be no greater than c. The authors develop an exact vacuum plane-wave solution in §35.9, where the only thing in the spacetime is single large pulse of gravitational radiation in otherwise totally empty flat spacetime, and a set of "test particles", which are well defined probes in General Relativity defined so as to not perturb the solution. They proceed to compare this solution to that of an electromagnetic plane wave in Special Relativity, and arrive at a more physical viewpoint where the gravitational plane wave, if it has anything like a physical source (e.g. a pair of masses in mutual orbit; they return to this in Ch. 36) should be more like a set of "ripples in the spacetime curvature ... propagating on a very slightly curved background spacetime ... The most striking difference between the background and the ripples is not in the magnitude of their spacetime curvatures, but in their characteristic lengths". There is a characteristic length of this background spacetime, determined by its (much much larger) radius of curvature. They then grind out effective stress-energy tensors, which would couple with any matter in a non-vacuum environment. The argument is that anything in the stress-energy tensor must in certain causal structures (like the one in our universe) must propagate at no more than c. Their treatment is the basis for a particular type of graviton (Ex 35.16), the scattering and redshift of gravitational waves (.17, .18), and various ways to express them without resort to an effective stress-energy tensor. In these approaches, c is the limiting speed of gravitational radiation, but gravitational radiation may move slower than c in non-vacuum. In vacuum, the pure pulse may in some circumstances develop a trailing edge that propagates at less than c, but when this can even happen the effect is weak when the wavelength is short compared to the background length scale, or when the amplitude of the pulse is small. Perhaps gravitational astronomy can hope to find someday a high-enough amplitude wave to put this to the test. For clarity, though, the linearization-is-good-theory claim is on solid footing. LIGO uses the linearized equations and have found agreement between the speed of gravitational waves they've detected to their theory to something around nineteen decimal places. A good multimessenger signal, which seems inevitable, will almost certainly improve that. There is no good reason to expect the speed of gravity in a full solution to the Einstein Field Equations to differ. One can make the same argument about the march of results from post-Newtonian expansions (as below) too. The only "wiggle" room is that cosmic inflation is probably much more dynamical, and the gravitational waves are of much greater amplitude. - -- [1] The non-theory answer is that the behaviour of orbits in known astrophysical systems which are very post-Newtonian (think black holes, or extremely fast-moving galaxies containing predictable spectra from hydrogen or light curves from supernovae) are consistent with a speed limit on any gravitational interaction, and that the speed limit is very close to c. It turns out to be hard to measure the speed limit exactly. See e.g. Will @ https://arxiv.org/abs/astro-ph/0301145 https://arxiv.org/abs/astro-ph/0301145 which discusses light from quasars being gravitationally lensed by Jupiter and how it would look different under theories that admit a propagation speed c_{gravity} different from c_{electromagnetism}. One could also compare bimetric theories of gravitation in which in the early universe gravitational self-interaction propagates differently from electromagnetism, with a view to resolving some questions in the distribution of galaxies in our sky. These generally have to decay the additional metric (meaning gravitational radiation propagates like electromagnetism) in the very early part of the universe or we get something very different from the cosmic web of galaxies that we observe.
gnarcoregrizz 5y ago
there's a vertical asymptote in e=mc2, with near infinite energy required to accelerate something to near c. however, does it prevent something from existing thats already (or always has been) traveling faster than c?
Khaine 5y ago
PBS Spacetime has done some fantastic episodes on this including this one https://www.youtube.com/watch?v=msVuCEs8Ydo https://www.youtube.com/watch?v=msVuCEs8Ydo
timonoko 5y ago
Lightspeed is infinite, it is the simultaneity which is slow.
mzimbres 5y ago
Nobody postulated that the speed of light is constant because it was obvious: light just like every other wave has a constant speed in the medium though which it propagates. The fact that nothing can go faster is a consequence of the Lorentz transformation, so nothing had to be postulated. The important thing that Einstein recognized is that the Lorentz transformations apply for all physical phenomena and not only the Maxwell equations and that started a revolution on physics.
- smusamashah 5y ago
  This is very interesting. Does it mean we can observe the constant speed phenomenon for other waves e.g waves in water or sound waves or waves in a spring or a string? That will be very comprehensible

271 points by lanna 5y ago

Wow, this is the first time I have understood why nothing can go faster than the speed of light and why time dilation is a thing. A very easy to read article, give it a try. Also if you are curious why a moving charge creates a magnetic field watch this Veritasium video: https://www.youtube.com/watch?v=1TKSfAkWWN0 https://www.youtube.com/watch?v=1TKSfAkWWN0

deleted 5y ago
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thriftwy 5y ago
Frankly though, I not sure why they couldn't just use a simpler correction: postulate that moving charge only creates magnetic field with regard to objects moving relative to it. Which is also basically true? This way, the stationary observer will sure notice that there's F_B, but will also know that the charge moving alongside the (also moving) rod does not experience it.
- DangitBobby 5y ago
  That's the thing though, in the article the charged object does not move relative to the rod. The rod, object, and person B are stationary relative to each other. I was actually really surprised to read that two charged objects moving with the same velocity relative to each other generates a magnetic field from A's perspective.
  - whatshisface 5y ago
    Two long charged rods moving with the same velocity relative to each other are two parallel wires carrying current, the second of which is a typical and easy-to-calculate example of electromagnetic attraction. In fact, that situation is so prototypical that it is used to define the Ampere in terms of what current is required to produce a certain force between parallel wires. [0] [0] http://www.physics.louisville.edu/cldavis/phys299/notes/mag_force_2wires.html http://www.physics.louisville.edu/cldavis/phys299/notes/mag_...
    - thriftwy 5y ago
      In this scenario, rods are stationary but they have current running in them - a different situation. Can it be translated to the relativistic one? Maybe what they really wanted to say is "there is a speed c at which the charge of a moving rod is indistinguishable of current running through stationary rod"? Now that would make a lot of sense.
      - whatshisface 5y ago
        It's not different as far as electrodynamics is concerned. The motion of the charge is what matters; whether or not the neutral part is stationary or moving is of no concern.

soneil 5y ago

The simplest way I could ever wrap my head around this, is to understand that everything travels through spacetime at the same speed. There is no travelling faster or slower, just crossing the graph at a different angle.

juped 5y ago
Yes, the magnitude of something's 4-velocity vector is always c^2 (set it to 1 in your unit system) by definition. It's not as simple as a R^n vector because you have a time coordinate of opposite sign and you switch coordinate systems by Lorentz transformation, but it's the right intuition. (It's complicated further in the presence of gravity, where you need an enriched differential geometry notion of a velocity vector since the spacetime is curved.)
minitoar 5y ago
Right, this is like the 2d projection of it which makes it easier for our 3-space brains to comprehend.
kevinventullo 5y ago
Hm, but if I can make my angle orthogonal to the time axis, isn’t that like instantaneous movement?
- bollu 5y ago
  Yes, which is why a photon experiences zero proper time.
  - RHSman2 5y ago
    They should make a movie called ‘Photon’ and track its life. A parody based on science.
  - raattgift 5y ago
    That proper time is zero everywhere along a photon's geodesic does not mean that the photon cannot evolve from point to point along it, and we can show this by taking advantage of total coordinate freedom. We can parametrize (as in make parametric) arbitrary curves through spacetime however we like. Parametric representations of unique curves are generally nonunique. Some of the infinite possible parametrizations of a chosen curve have useful properties, such as uniquely labelling every point on the curve with some monotonically ordering value and keeping the form of some set of equations reasonably simple. For timelike geodesics, particularly in the Minkowski space of Special Relativity, proper time (being a Lorentz scalar) is a good option. However that is not true for all geodesics in Minkowski space (as you note, the proper time is everywhere zero on a null geodesic, and so a bad option), much less all curves through general curved spacetimes. For null geodesics, following the logic of GP's question, we may wish to preserve the tangent vector under parallel transport; this requires the parametrization to be affine. Some gory details at https://en.wikipedia.org/wiki/Geodesic#Affine_geodesics https://en.wikipedia.org/wiki/Geodesic#Affine_geodesics and a brief useful summary at https://www.reddit.com/r/AskPhysics/comments/9aenid/what_actually_are_affine_parameters/e4uwfda&context=3 https://www.reddit.com/r/AskPhysics/comments/9aenid/what_act... As is noted below the comment directly pointed to by the second link, labelling a timelike geodesic with proper time is choosing one specific affine parametrization on that geodesic, and that this choice is driven by convenience. One of the neat outcomes of affine parametrization is that we can take a point on an affinely-parameterized null geodesic and look at the derivative with respect to the affine parameter there, and define a momentum k^{\mu} = \dot X^{\mu}. In a Lorentzian spacetime, with curvature, we can compare the momentum at two different points on the null geodesic, giving us the gravitational redshift between those two points of the photon's wavelength equiv. frequency.

sylr 5y ago

With gifs: https://imgur.com/gallery/BT2mG https://imgur.com/gallery/BT2mG

zfxfr 5y ago
Wow thank you ! I have read the whole article but I couldn't wrap my head around it. I found the "rod story" confusing then I followed your link (and also watched the related Youtube video).. This really makes things clearer. I am really not into maths but this version seems much easier to understand and very logical.

leonardp 5y ago

I always feel like this questions is really asking "What is time?". This book gives a new perspective on time which I think is crucial for understanding the nature of it: https://www.harvard.com/book/the_janus_point/ https://www.harvard.com/book/the_janus_point/

otterley 5y ago

My favorite way to think of it is that the speed of anything is measured in distance per unit time. You can’t change the constant that is the speed of light, but what if you could change what “time” means? The constant holds, but time doesn’t; it varies among observers. A second from the perspective of observer A is not identical to a second from the perspective of observer B. Freaky, but true.

stareatgoats 5y ago
I still don't get this. I simply can not accept that the speed of light is that same for all observers, irrespective of their respective speeds vs the light source. It is obviously a cornerstone of Einsteins theory (I forget which; either the special or general relativity), but to me this is simply not logical. To state that the speed of light is the same only because the relative time is variable for respective observers is unconvincing, which requires that things like blueshift/redshift to be explained by logical gymnastics, and not the straightforward reason that the speed of light for all observers is a direct function of the relative speed vs the light source. But what do I know.
- DangitBobby 5y ago
  It's a consequence of the time dilation. You are not stationary towards the photon to an outside observer, so to them it looks like the photon moves away from them faster than it moves away from you. And from the obserer's perspective, your clock is ticking slower. That's the key. The amount that your clock is ticking slower is such that you, the one moving, would calculate the speed of the photon the same as the stationary observer, because you would do so from your own slower clock.
  - stareatgoats 5y ago
    Thanks, that sounds like the explanations I've seen too, and that I hope to understand some day. Atm I'm at the point where I gladly accept that time may seem to dilate, but not that it actually does. Maybe I just need to dive deeper into it than I've been willing to do so far.

deleted 5y ago

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redwood 5y ago

I think a lot of people assume that this means you couldn't go more than a few (~100) light-years in your lifetime... But this is not actually correct. Counter-intuitively you can theorically go any number of light-years (essentially) in your lifetime, as long as you are able to approach the speed of light because when you do so the distance is dilated and hence you're covering far more ground within your reference frame (of course you'd be in the deep future from the perspective of anyone in our normal reference frame).

rkagerer 5y ago
It just means you can't within the lifetimes of the people you left behind. For that you need more exotic contraptions (wormholes, warp drive, etc).
- ghayes 5y ago
  If they were to wait a year and then come find you far away, would it net out and you’d be the same age?
  - Guillaume86 5y ago
    Yes
xyzzyz 5y ago
To put it another way, time travel machines are very much theoretically possible, and in fact are “only” an engineering problem (an extremely hard one, however). But, only forward travel is allowed: time travel machine could take you as far forward as its engineering would allow it, but there is no going back.
- nimish 5y ago
  They are only theoretically possible if you allow for negative mass and energy--not an engineering problem so much as a "need to find exotic matter" Basically people ran the EFE "backwards" to see what matter distribution makes the wanted curvature. You get either negative mass-energy or the bubble doesn't travel ftl iirc.

aero-glide2 5y ago

I've been trying to understand this since high school. No Eureka moment but I think every year I understand just a little bit more.

StephenAmar 5y ago

I highly recommend watching https://youtube.com/playlist?list=PLoaVOjvkzQtyjhV55wZcdicAz5KexgKvm https://youtube.com/playlist?list=PLoaVOjvkzQtyjhV55wZcdicAz... They do such a good job at using video to explain Special Relativity.

webwielder2 5y ago

I prefer the question, “why can’t light go faster?”

franciscop 5y ago

A bit all over the place, the author assumes "It is even less well-known that the rule “nothing moves faster than the speed of light” is a consequence of the laws of electricity and magnetism" but at the same time "This is also a well-known property of electricity and magnetism" and then drops some equations. Most people for whom this property of electricity and magnetism is well known, also know that the speed limit from general relativity comes from Maxwell's equations (which are on themselves a compilation of other previous rules). I also dug deep into this problem a while back, but without proper physics background couldn't get too far. The speed of light comes directly from the Vacuum permeability and Vacuum permittivity, since light is an electromagnetic wave. If these were different, then the speed of light would be different. Both of these seem to be values of the behavior of vacuum in our universe (and possibly electron's [1]). It is Maxwell's equations gives us that the speed of electromagnetic radiation needs to be c ^ 2 = 1 / (e * u) [2]. [1] https://en.wikipedia.org/wiki/Fine-structure_constant https://en.wikipedia.org/wiki/Fine-structure_constant [2] https://en.wikipedia.org/wiki/Electromagnetic_wave_equation#The_origin_of_the_electromagnetic_wave_equation https://en.wikipedia.org/wiki/Electromagnetic_wave_equation#...

DangitBobby 5y ago
The author's assumptions were spot on for me. I am familiar enough with E&M to know the equations and roughly understand relatively but did not know where the intuition for a universal speed limit comes from.
shusaku 5y ago
People can disagree with your assessment, but it’s pretty sad that you write the only comment in this thread that actually engaged with the content of the linked article, and it’s downvoted.

crdrost 5y ago

That you can’t go faster than the speed of light is a direct, simple consequence of everybody agreeing on the speed of light. It's a real-life Zeno paradox. Alice and Bob are in some spaceships by a long racetrack in space. Alice fires a very brief pulse of light down the racetrack, maybe we see the rays that don't go straight through successively illuminate some rings around the track. Alice challenges Bob to race another light pulse, Bob revs his engine. As the countdown hits zero, Bob accelerates to speed c/2 relative to Alice, then checks the reflected light from these rings only to find out that the light pulse is still traveling at speed c away from him. So he drops a beacon at his current speed then accelerates to speed c/2 relative to that, but no dude: the light is still moving at c away from him. Bob realizes that he can never win, so tries to instead measure the speed of Alice, who he expects to be moving at speed c away from him, only to find that she is instead moving at speed 0.8 c away from him. Now the question is, how can this be? Consider a much slower spaceship. When Alice fires the light pulse and Bob starts moving forward at a slow speed, Alice sees this bubble of light expanding in kind of a uniform sphere centered on her. (Say it reflects off of space dust instead of a track.) Since they were at the same position when the light Bob also sees an expanding bubble of light, with himself at the center. The weird stuff about time dilation and length contraction does not apply at low speeds, if Bob goes at c/1,000 say, then these are only one part per million. They both basically agree on how far this bubble of light is from Bob, in the directions perpendicular to the motion. The motion is parallel to the bubble in those directions, and to first order those parallel lines will not get any closer or further away. (This is why I want Bob to move at a slow speed!) They only disagree along the motion. Alice thinks the light is receding from Bob at speed c–v ahead of him, at speed c+v behind him: Bob sees the light recede at speed c in both directions. So they come back together to repeat the experiment and Alice decides to force the contradiction. Alice puts a clock ticking out every millisecond out at distance 1 light-second, but it will start at the moment she fires the pulse, stopping when the pulse hits it, showing 1000. She puts one of these in the direction Bob will travel, and in the opposite direction. Surely he must agree that the light started from here and that it intersected those two clocks when they both said 1,000. Bob agrees that the clocks look synced up and films all of this with a high-speed camera to make sure that there is no funny business, and they repeat the experiment. Right when the light hits, Bob accelerates at his usual 1,000,000 gees for 0.03 s, to get his final speed of c/1,000. (I need to rewrite this to make the numbers more reasonable LOL.) Here's where something weird happens, and it is entirely contained to those first 30 ticks of both cameras, as Bob looks at them in his high-speed camera footage. Bob corrects for Doppler shift like you do, and agrees that these clocks appear to be ticking during the other 970 ms at one tick per ms, his camera has maybe microsecond resolution and not the nanosecond resolution you need to see time dilation. But during those first 30 milliseconds when Bob was accelerating, even after correcting for the Doppler shift, his best guess is that Alice artificially slowed down the clock behind him and artificially sped up the clock ahead of him. Because the clock ahead of him definitely ticked 31 times in those 30 ms, while the one behind definitely only ticked 29 times. So Bob says the light did hit these clocks when they said 1000 ms, but the clock ahead of him should have said 999ms at that time, and the light should have gone past it a bit by 1000ms, while the clock behind should have said 1001ms, and the light was actually not yet there at 1000 ms. This anomalous Doppler shift is proportional to both the distance of the clock you're looking at, and your acceleration. It is also called the relativity of simultaneity, and it is the only new prediction of relativity, in that length contraction and time dilation are second-order consequences of it.

ergocoder 5y ago

"Nothing can go faster than light" is just a convention and is not experimentally confirmed. IIRC there are a few things that go faster than speed of light (e.g. universe expanding). "Spooky action at a distance" is also not known very well. That could also break the speed of causality law. While Bell's theorem hints that this is not the case, there are some exceptions to Bell's theorem. Right now a lot in physics are just convention like energy conservation and symmetry. We need another paradigm upgrade to understand these things.

Choc13 5y ago
Hmm not sure if troll, but energy conservation is definitely not just some convention. It’s a fundamental thermodynamic law. Spooky action at a distance does not violate speed of light information propagation either. The particles have to be entangled before they’re set off in opposite directions. Only once one is observed does the other also collapse, but it doesn’t mean you can communicate faster than the speed of light because you had to prepare the information when the particles were together IIRC, it’s been a decade since I studied quantum information theory.
- ergocoder 5y ago
  Why do you feel the need to say somebody is a troll? Here: https://phys.org/news/2017-01-violations-energy-early-universe-dark.html https://phys.org/news/2017-01-violations-energy-early-univer... Here: https://phys.org/news/2015-02-space-faster.html https://phys.org/news/2015-02-space-faster.html - we are trying hard to reconcile this and categorize universe expanding as something else (e.g. not a movement that has speed because time itself is a dimension or something). But this is still up for debate, tbh. Another example was one where we said CP symmetry was true (it was a law like a lot of things in physics) until it was violated by a weak nuclear force experiment. And now we are holding the fort at CPT symmetry as the law. In Physics, the evidence of anything is kinda light. A lot of reasonable extrapolations has been made. Still they are extrapolations (e.g. intelligent guess). Even the big bang itself is just an extrapolation from the "theory" that the universe expanding. To be fair, it is difficult to find good evidence because we can't dial back time, can't go observe things on Neptune, can't measure gravity at the subatomic scale, and etc. So, we have to work with what we can experimentally observe. Our tools are getting better, and this is where the physics paradigm shift will come from. You say like these are 100%. It is just a theory that we currently hold according to the little evidence that we have. Failing to recognize that is straight up unscientific.

cletus 5y ago

The best analogy for this I've read is that your motion through spacetime is constant. Think of a quarter circle dial with an arrow that goes all the way from 0 degrees (no relative movement in space) to 90 degrees (relative speed of light). The faster you go through space, the slower you go through time. The effects of this are barely noticeable until you get near the speed of light. This effect is time dilation. This, I believe, is the general relativity view of spacetime. What I like most about this is that it highlights the importance of understanding the domain of a function. The above shows how any object with rest mass has a space velocity domain of [0, c). People really don't want this to be true so latch on to any wild theory that would seem to bypass this cosmic speed limit, be it wormholes, FTL drives or whatever. Pretty much all of these theories rely on taking an equation with a domain over real numbers and plugging in negative values for things like mass. Garbage in, garbage out. I mean if mass can be negative, why limit yourself to real numbers? Why not a complex number for mass? As to why this is the case and how to reconcile it with a quantized view of spacetime... is beyond my pay grade.

ghego1 5y ago

On traveling at the speed of light I also highly recommend to watch this Veritasium video https://youtu.be/vVKFBaaL4uM https://youtu.be/vVKFBaaL4uM

dejongh 5y ago

Thanks. Fun to know that the speed of light prevents the universe from splitting up. Cool. Also I was not aware that movement create magnetism.

PaulDavisThe1st 5y ago

In the 70s, I remember reading "The Quincunx of Time", which made great use of a key distinction (for the purposes of some classic 70s scifi): It's not that nothing can move faster than the speed of light, it's that nothing can accelerate to a speed faster than the speed of light. If you could bring particles into being that already moved faster than light, they would not violate our understanding of relativity or the rest of physics. Hence ... the tachyon. https://en.wikipedia.org/wiki/Tachyon https://en.wikipedia.org/wiki/Tachyon

unyttigfjelltol 5y ago

Looks to me like the short answer to the question posed in the headline is that matter is composed at an atomic or subatomic level of energies similar to light and so it would be quite inconvenient to try to propel the mass as a whole faster that its constituent parts can go. A similar principle may well be true for straight-line inertia, which might be caused by the same as the force that makes gyros stay on course. In general the scientific discussions seem to touch these ideas but approach theories in reverse of intuition so it's hard to know if they align.

dvh 5y ago

This example has a flaw. Skateboard man sees object stationary, but the cable is moving. There should be magnetic force too.

deleted 5y ago

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ySteeK 5y ago

wouldn't it go backward in time if it goes faster than light? Light travels in null-time from point a to point b (light-perspective of course) - anything slower, needs time to go from a to b. if i travel faster than light, i can - but i travel to b before i start at a. Maybe because this is not possible.

drran 5y ago
How much you will travel back in time by using a teleport (instant travel, infinite speed)?

holoduke 5y ago

Interesting to know that if we would have a 1g acceleration spaceshil with almost unlimited energy onboard we can cross a large part of our galaxy in less than 70 years.

subroutine 5y ago

Q1. Does the future already exist? I realize this might be purely philosophical, but if I leave Earth, and fly through space near c kph, and return home a few hours later to find everyone aged 50 years... are they the "same" people, or are they a future-instance of the people left? To clarify, theoretically I could leave earth, and return home in exactly 1 hour (from my ref frame), and basically make people whatever age I want by varying my speed. Since the same 1 hour passes for me no matter the age for them I chose, it seems like I'm selecting an already existent future from a stack, not fast-forwarding the present I left. Q2. How do black holes exist if time stops inside a black hole? That is, they continue to move through spacetime, even though spacetime is not moving within the hole? I accept that a black hole can form, and can stop time, I'm just curious how something that stops time continues to persist in the present. The way I'm visualizing this, is like a lava lamp about to bleb off some goo from the top [1]. As the object gets more and more dense, it curves and drags spacetime more and more, until eventually its density passes the Schwarzschild radius and blebs off. [1] https://m.media-amazon.com/images/I/615deDvfDkL._AC_SS450_.jpg https://m.media-amazon.com/images/I/615deDvfDkL._AC_SS450_.j...

the_gipsy 5y ago
Q1b. Are you the same person you were yesterday?
- subroutine 5y ago
  I think yes, and the answer is more concrete for "oneself", since the molecules that make-up oneself are all accelerating and decelerating with you/as you in your own reference frame.
  - mewse 5y ago
    I mean, apart from cells and etc dying and being replaced as we age. ;) Truly, the Ship of Theseus was inside us all along. (Which is super convenient, considering how often we step into the same river twice!)
ww520 5y ago

bambam24 5y ago

Do you have zero mass? Otherwise just to a 150lbs person will have mass of a galaxy at 99% of the speed of light. And to go remaining %1 you will need to have a power to push entire galaxy 0.1% of speed of light. At 99.999998% of speed of light the man will need to burn all the elements in the universe. Basically a credit card or bank loan cannot buy you such energy.

thom 5y ago

You can’t go faster than light for the simple reason that it would defeat the very many performance optimisations necessary to keep the universe running.

masa331 5y ago

Of course you can go faster than light, way faster. When it comes to physics there are no limits how fast, far or deep you can go. People constraint themselves with articifial limits and it's not necessary a bad thing. But real limits doesn't exist. I'v heard someone saying space is a big ball and outside of it is nothing. I imagine nothing as a black space but that actually is something. So there is something after all. And however weird or normal these other places are, it goes like this infinitely.

chrisoverzero 5y ago
> Of course you can go faster than light, way faster. OK. How?
- masa331 5y ago
  I don't know

holoduke 5y ago

What would happen if you had a very large rotor of let's say 50.000km in diameter. And let it rotate at 1r/s. The inner part would only move at very low speed and the outer part at near lightspeed. What kind of time dilation effects would be seen? If you would sit on the tips for a few days and then stop the rotor, travel back to the center, you would see a very old one right?

hinkley 5y ago
I think veritasium discusses that as well. Essentially at relativistic speeds there are no chemical bonds that can hold and the device shreds itself.

midrus 5y ago

I have a probably very dumb question. If speed is relative, when we say something travels at... I don't know... 50% the speed of light, that speed is relative to what? how do you know it is 50% and not 53%? How do we know we're not already moving at 99% the speed of light (like our observable universe as a whole having that speed )? I love this stuff, but it is so counter intuitive for the average human.

drran 5y ago
Higgs field is present everywhere and mediated by Higgs boson, so just measure speed relative to Higgs field. Or you can measure speed relative to CMB.
hinkley 5y ago
No matter of fast you are moving, you will agree with a “stationary” observer about the speed of light. What you won’t agree on is the color of the light. Apparent frequency shifts to maintain the speed of light. The color shift is how we know how fast stars are moving relative to us.
RogerL 5y ago
"that speed is relative to what" To you. There is no such thing as absolute speed. You say cosmic rays are crashing into the earth at 99% the speed of light. The cosmic ray says it is sitting still and you (and the Earth) are approaching it at 99% the speed of light. Some other particle moving at 70% relative to you will give a different speed for the cosmic ray. All of you are right.
caf 5y ago
When we say something travels at 50% the speed of light, it's always relative to some inertial, approximately inertial, reference frame, often implied. So eg. if you hear about the particles at the LHC travelling at near the speed of light, it's implied that's relative to the Earth. The speed limit comes in here: no-matter what nearby object you look at, its speed relative to you always be less than the speed of light. (Here, nearby means something like "within the local group of galaxies" - objects a long way away can be receding faster than the speed of light due to the expansion of space in between you and the object). You can accelerate as much as you like for as long as you like and when you stop accelerating and check, that'll still be the case.

dhosek 5y ago

My favorite general relativity paradox: A runner is running with a 20m pole at a sufficient velocity so that a stationary observer sees the pole having length 10m. The runner runs through a barn which is 10m long. And two people outside shut the doors simultaneously so that the runner is completely enclosed in the barn for a moment. But from the runner's perspective, the barn has length 5m long. What does she see happen when the doors are closed on her?

gridspy 5y ago
How do the two people at opposite ends of the barn agree on when to close the doors as a runner approaches at relativistic speeds? Also, relevant xkcd (what-if) https://what-if.xkcd.com/1/ https://what-if.xkcd.com/1/ Most likely from the runner's perspective the world turns white as they dissolve to plasma prior to reaching the barn.
n3k5 5y ago
> And two people outside shut the doors simultaneously No, they don't. I'd phrase it differently, e.g. “each door is shut for an instant when the end of the pole is just inside the door”. (Also, make it clear that the barn is long enough to fit a 10m pole between the two shut doors.) When you put it as “shut the doors simultaneously”, it's more troll physics than a paradox. Meta-puzzle: figure out how to still imply that the runner is completely enclosed for an instant, without incorrectly insinuating that it is so from her perspective.
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contravariant 5y ago
From the frame of reference of the runner the pole will first hit the door at the end of the barn (unless it opens in time), and the door at the front of the barn will only close just after the back of the pole has passed the door. If the doors actually don't open in time then the resolution for the fact that the pole doesn't physically fit is that nothing can be rigid at relativistic speeds. So either the door or the pole or both will deform (violently) to allow the pole to fit inside the barn.

wizardforhire 5y ago

You can go faster than the speed of light. Its slowing thats the hard part!

OneEyedRobot 5y ago

Can a thing with no mass ever go less than the speed of light? Does it have a concept of time?

jandrese 5y ago
Sure, light is slowed down when it travels through different substances. That's why we have refraction.
- micw 5y ago
  Is it really slowed down? I always assumed that it just need to take a longer way through matter than through vacuum.

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noobermin 5y ago

The beginning is the best line, that no one really knows "why" and what I have said to even other physicists who don't really understand it. A lot of the counterintuitive consequences of the paradox of the moving charges detailed here are a fact of nature but we don't really know why it must be that way. For example, in a modern particle physics starting point for qed, we force the equations to be lorentz invariant from the start but that is a starting assumption. But that's it, there's no real deeper answer to "why" beyond it being a experimentally observable fact, and the only resolution of these observations is that lorentz transformation and thus things like time dilation and length contraction must happen. Small relativity related tidbit: I hate when people say the phrase "the faster you go, time slows down for you." This is a problem because it implies that the moving observer notices their own time dilation which is reverse of the case. Of course, every non-accelerating observer is in their own rest frame, so it doesn't make sense to say "time slows down for you," because you are your own reference and there is no other frame to base your measurements on (I mean that was the whole point of relativity, there is no universal rest frame). Instead, when you measure the rate of change for other reference frames moving relative to you, their clocks move slower when measured by your clock. So the actual phrase should be something like "the faster others go, the slower their time appears to you."

sytelus 5y ago

If you just extend time dilation beyond zero (I.e. negative time) then speed faster than light is possible. The constraint described in the article will still be maintained. Also, notice that negative time would be the perception of external observer.

wruza 5y ago

If anyone wants to play with a speed of light, there [still] is a fun web game called “velocity raptor”.

lifeplusplus 5y ago

Velocity is a relative number. So let's say your are going 0.9 c but I'm going at 0.8 c now according to me you can go 0.7 more at least. But you're already going 0.9 and adding 0.7 will put you above speed of light. You can say you're already travelling faster than light relative to something out there... So what the hell are we talking about here?

France_is_bacon 5y ago

As far as I know, we can, and do, go faster than light all the time. Right now, our bodies are going faster than light, relative to other astronomical bodies. Space can expand faster than the speed of light. Therefore, since WE are on one galaxy, we are traveling faster than the speed of light away from other galaxies. Oh...did you mean us PERSONALLY traveling faster than the speed of light to other parts of the galaxy? This is probably the best analogy I've seen: https://www.youtube.com/watch?v=A2JCoIGyGxc https://www.youtube.com/watch?v=A2JCoIGyGxc

eyelidlessness 5y ago

Light isn’t limited in the speed it can travel. Mass is. Light can’t exist without matter. The fundamental limit is mass versus all of the forces which act as drag.

pfalafel 5y ago
Why are gravitational waves limited by the same speed limit? There is no mass traveling in that case, I suppose.
- eyelidlessness 5y ago
  I wouldn’t dare to hazard a guess here, but I do think it’s worth remembering that matter is always in motion.
- jandrese 5y ago
  What we think of as the speed of light could also be thought of as the rate at which change propagates throughout the universe. The rate at which cause and effect travel.
  - pfalafel 5y ago
    Two runners overtake the field at incredible speed, only to cross the finish line at exactly the same time. Would also raise eyebrows.
- raattgift 5y ago
  This is an excellent question, which predates the theory of General Relativity by a few years, and has only been astrophysically verified after 1974 (Hulse-Taylor) or even later. As far as I am aware, a complete theoretical answer in General Relativity itself is still elusive. I'll largely stick with the theory, which I guess is what you are interested in.[1] The topic is in Part VIII (Chapter 35) of Misner, Thorne & Wheeler's Gravitation ("MTW"), which is the gold standard reference/textbook for General Relativity. I will later try to briefly summarize the section. Instead, first, at the root of my answer is the non-linear nature of the Einstein Field Equations, which imply that gravity self-gravitates. This is usually side-stepped by textbooks, which instead proceed to linearize the Einstein Field Equations, that is, they consider the weak limit of gravitation. This is usually to split a metric with a gravitational wave into some minimally-or-even-non-dynamical background and the wave-part, using the former to define the speed of the latter. For strong gravitational waves, or a very dynamical background, we cannot do this. I think this regime is best studied in a vacuum solution, i.e., where there is only gravitation that self-interacts, although most of the work in this regime appears to have the aim of resolving questions about the distribution of matter in the very early universe (e.g. Misner's mixmaster). An interesting exact solution of the Einstein Field Equations is the https://en.wikipedia.org/wiki/Kasner_metric https://en.wikipedia.org/wiki/Kasner_metric which can generate singularities and other features formed by gravitational-wave interactionsn. That is, there is manifestly non-linear gravitational self-interaction in the highly-dynamical Kasner chaos. (This is the worst case for the linearized treatments in textbooks). The Kasner metric can be applied to a Lorentzian manifold (3 space, 1 time dimension), and so is consistent with our universe's causal structure. An interesting feature of this solution is that it engages only two constants in the Einstein Field Equations: c and G. The only velocity scale that we can construct from any combination of these two constants is c itself. This is highly suggestive that in a universe like ours gravitational radiation must propagate at c. Again, this is just an argument that there may be an answer within the theory of General Relativity itself, without treating the speed of gravitational waves as a postulate. This argument is made without regard to the obvious craziness in a Kasner chaos universe compared to our own. General Relativity admits complete gravitational solutions for all sorts of craziness, including universes with any number of space and time dimensions as long as there are at least two total, stress-energy which is distributed very differently from ours (including negative energy, or energy that pops in or out of existence without a cause), and so forth. It is very General. (Special Relativity is very Special: it's defined on -- and only on -- a gravity-free (flat, no gravitational waves) spacetime of exactly 3 spatial and 1 timelike dimension.) Returning to MTW, they conclude that the propagation speed instead can be no greater than c. The authors develop an exact vacuum plane-wave solution in §35.9, where the only thing in the spacetime is single large pulse of gravitational radiation in otherwise totally empty flat spacetime, and a set of "test particles", which are well defined probes in General Relativity defined so as to not perturb the solution. They proceed to compare this solution to that of an electromagnetic plane wave in Special Relativity, and arrive at a more physical viewpoint where the gravitational plane wave, if it has anything like a physical source (e.g. a pair of masses in mutual orbit; they return to this in Ch. 36) should be more like a set of "ripples in the spacetime curvature ... propagating on a very slightly curved background spacetime ... The most striking difference between the background and the ripples is not in the magnitude of their spacetime curvatures, but in their characteristic lengths". There is a characteristic length of this background spacetime, determined by its (much much larger) radius of curvature. They then grind out effective stress-energy tensors, which would couple with any matter in a non-vacuum environment. The argument is that anything in the stress-energy tensor must in certain causal structures (like the one in our universe) must propagate at no more than c. Their treatment is the basis for a particular type of graviton (Ex 35.16), the scattering and redshift of gravitational waves (.17, .18), and various ways to express them without resort to an effective stress-energy tensor. In these approaches, c is the limiting speed of gravitational radiation, but gravitational radiation may move slower than c in non-vacuum. In vacuum, the pure pulse may in some circumstances develop a trailing edge that propagates at less than c, but when this can even happen the effect is weak when the wavelength is short compared to the background length scale, or when the amplitude of the pulse is small. Perhaps gravitational astronomy can hope to find someday a high-enough amplitude wave to put this to the test. For clarity, though, the linearization-is-good-theory claim is on solid footing. LIGO uses the linearized equations and have found agreement between the speed of gravitational waves they've detected to their theory to something around nineteen decimal places. A good multimessenger signal, which seems inevitable, will almost certainly improve that. There is no good reason to expect the speed of gravity in a full solution to the Einstein Field Equations to differ. One can make the same argument about the march of results from post-Newtonian expansions (as below) too. The only "wiggle" room is that cosmic inflation is probably much more dynamical, and the gravitational waves are of much greater amplitude. - -- [1] The non-theory answer is that the behaviour of orbits in known astrophysical systems which are very post-Newtonian (think black holes, or extremely fast-moving galaxies containing predictable spectra from hydrogen or light curves from supernovae) are consistent with a speed limit on any gravitational interaction, and that the speed limit is very close to c. It turns out to be hard to measure the speed limit exactly. See e.g. Will @ https://arxiv.org/abs/astro-ph/0301145 https://arxiv.org/abs/astro-ph/0301145 which discusses light from quasars being gravitationally lensed by Jupiter and how it would look different under theories that admit a propagation speed c_{gravity} different from c_{electromagnetism}. One could also compare bimetric theories of gravitation in which in the early universe gravitational self-interaction propagates differently from electromagnetism, with a view to resolving some questions in the distribution of galaxies in our sky. These generally have to decay the additional metric (meaning gravitational radiation propagates like electromagnetism) in the very early part of the universe or we get something very different from the cosmic web of galaxies that we observe.

gnarcoregrizz 5y ago

there's a vertical asymptote in e=mc2, with near infinite energy required to accelerate something to near c. however, does it prevent something from existing thats already (or always has been) traveling faster than c?

Khaine 5y ago

PBS Spacetime has done some fantastic episodes on this including this one https://www.youtube.com/watch?v=msVuCEs8Ydo https://www.youtube.com/watch?v=msVuCEs8Ydo

timonoko 5y ago

Lightspeed is infinite, it is the simultaneity which is slow.

mzimbres 5y ago

Nobody postulated that the speed of light is constant because it was obvious: light just like every other wave has a constant speed in the medium though which it propagates. The fact that nothing can go faster is a consequence of the Lorentz transformation, so nothing had to be postulated. The important thing that Einstein recognized is that the Lorentz transformations apply for all physical phenomena and not only the Maxwell equations and that started a revolution on physics.

smusamashah 5y ago
This is very interesting. Does it mean we can observe the constant speed phenomenon for other waves e.g waves in water or sound waves or waves in a spring or a string? That will be very comprehensible