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Do we really travel through time with the speed of light?

354 points by laurex 6y ago

ta1234567890 6y ago
> Now, of course there is a difference between time and space, so that can’t be all there is to space-time. You can move around in space either which way, but you cannot move around in time as you please. Not really. The instant you move in space, you also move in time, and since everything is moving and there is no fixed frame of reference, technically you can never move back to the same spot where you started. So even though it might appear to us, at our perception scale, that we can move back and forth in space, in reality whenever we move, space changes and so it's just like moving in time, there is no going back.
CGamesPlay 6y ago
What really cemented this for me was actually looking at spacetime diagrams. Light moves at 45 degree angles on a spacetime diagram, but EVERYTHING moves at the same speed on these diagrams. I have seen several good YouTube animations demonstrating the Lorentz transformation, here's one that might help clarify what I mean: https://www.youtube.com/watch?v=k73psdcmzEY https://www.youtube.com/watch?v=k73psdcmzEY
- Zamicol 6y ago
  Nice find on that YouTube link. Thanks.
- quickthrower2 6y ago
  What I’m still confused about is if you travel through time at a certain speed and time is like any other dimension doesn’t there have to be a underlying concept of “this the real damn time” underneath it?
  - pas 6y ago
    There is, the damn spacetime, under all it's completely tangled together, you can't separate them, because relativity. It's a very useful "illusion" for everyday life, to think of time as something completely distinct and always present... but it seems, our best experiments and thus models suggest otherwise. Similarly with quantum stuff, there doesn't seem to be hidden variables, it's truly a probability distribution.
    - quickthrower2 6y ago
      Thanks in this case I am trying to understand what is meant by “moving through spacetime”
      - pas 6y ago
        Sitting at home, orbiting a star, falling into a blackhole, or being a photon, an electron, or a GPS sat. All are moving through spacetime, of course on different trajectories. Especially when looking at them with the lens of General Relativity.
      - nybble41 6y ago
        > what is meant by “moving through spacetime” AIUI the equations describe a path through spacetime. There is no "speed" or "movement" along this path, or at least none with an actual physical interpretation. The path simply has a certain shape. The equations define the path as a parametric function, and you could in principle calculate the change in 4D spacetime coordinates with respect to the change in that parameter, but it would have no physical meaning. To keep things simple, in a canonical representation of the path this ratio (the "speed" through spacetime with respect to the arbitrary scalar parameter—though "speed" is not really the right term since the parameter does not represent time) is held constant and defined as the speed of light. To put things in more familiar (or at least less spacetime-y) terms, these very different parametric equations both describe a 2D unit circle with respect to an arbitrary parameter r with range (-∞, ∞): p(r) = [cos(r), sin(r)] q(r) = [abs(2 - t) - 1, ((2 - t) / abs(2 - t)) * sqrt(1 - (abs(2 - t) - 1)**2)] The "speed" of p(r) with respect to r is the magnitude of the derivative [-sin(r), cos(r)], a unit vector, and thus equal to one for any value of r. The magnitude of the derivative of q(r) is something rather more complicated, and not even well-defined for most values of r. However, they both describe the same circle. The parameter r is not part of the path; only the set of [x, y] coordinates counts, and p(r) and q(r) describe the same sets of 2D points.
  - CGamesPlay 6y ago
    There is, it's called the "spacetime interval". It's relative to the observer, because everything in spacetime is, but it's the "true" measure of separation in spacetime.
nitrogen 6y ago
Today I want to answer a question that was sent to me by Ed Catmull who writes: "Twice, I have read books on relativity by PhDs who said that we travel through time at the speed of light, but I can’t find those books, and I haven’t seen it written anywhere else. Can you let me know if this is right or if this is utter nonsense." I wonder, was this question asked by the Ed Catmull (https://en.wikipedia.org/wiki/Edwin_Catmull https://en.wikipedia.org/wiki/Edwin_Catmull)? ---- Edit: to make this comment less useless, I'd like to point any laypeople like myself who are interested in the nature of the universe to the PBS Space Time show/YouTube channel: https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g I'm pretty sure they have videos related to this question.
- acidbaseextract 6y ago
  The moment I saw that name I wondered the same thing. It would be cool to be running in the same internet circles as Ed Catmull. I'm in awe of his contributions to both computer graphics and Pixar, and I really liked his management book, even if he definitely did some sketchy stuff with wage fixing.
antognini 6y ago
Here is another explanation. In Newtonian physics lots of common quantities are represented by vectors: position, momentum, force, the magnetic potential, etc. Other important quantities are represented by scalars: time, energy, power, the electric potential, etc. One of the deep insights of special relativity is that these scalar and vector quantities are actually unified into a single, new entity called a "four-vector". Each four-vector has three spatial components and one temporal component. So in the two lists I wrote above, each scalar quantity gets paired with the corresponding vector quantity: time & position; energy & momentum; power & force; the electric & magnetic potentials. In Newtonian physics a rotation in three-dimensional space will transform one component of your position vector into another (say, some of your x-component becomes a bit of y-component). In special relativity observers moving at different velocities are related by a similar kind of transformation of their four-vectors --- a bit of the time component mixes with the spatial component and vice versa. But what happens if you do this with velocity? Velocity is a vector. It's corresponding temporal quantity is a bit weird: change in time per change in time. But it's a little more subtle than that, because it's actually the change in coordinate time per change in proper time. Basically this is the ratio between how fast you observe a clock tick in an observer's reference frame relative to how fast they see it tick within that reference frame. But the really weird thing about this particular four-vector is that it always has a magnitude of exactly the speed of light. No matter how fast you go, the magnitude of your four-velocity does not actually get larger. All you do by going faster is just mix some of the time component of your four-velocity into the spatial components. (This is why the clocks of moving observers tick more slowly.) And when you slow down, you just mix some of the spatial components into the temporal component. And when you're at rest the spatial components are all at zero, so the only component of your four-velocity is in the temporal direction. So at rest your four-velocity points directly in the future with a magnitude equal to the speed of light. This is what is meant by the statement that we travel through time at the speed of light.
- vnorilo 6y ago
  This is great, thanks! This is the first time I hear it explained like this and it just somehow clicked immediately. I now have this (probably wildly incorrect, total layman here) vision of the time dimension of velocity being "a normal" simultaneously to all the spatial dimensions and acceleration as rotation of the four-vector away from the time axis... sorry for any terminology errors.
ta1234567890 6y ago
> If you want to calculate a distance in space, you use Euclid’s formula. A distance, in three dimension, is the square-root of the of the sum of the squared distances in each direction of space. Here the Δx is a difference between two points in direction x, and Δy and Δz are likewise differences between two points in directions y and z. I really dislike it when explanations about reality become explanations about a model of reality. Can the original question be answered without assuming some model is the perfect reflection of reality, or just without using the model?
- lokedhs 6y ago
  What would that even mean? The model is a description of reality. When you talk about the model, you are referencing a mathematical description of reality. In a similar way, the ratio π is the mathematical description (or model, if you like) of a circle's circumference relative to its diameter. I'm not sure how to describe this relationship without invoking the mathematical description of said relationship.
  - guerrilla 6y ago
    > The model is a description of reality No, not neccessarily. It's purpose is to make predictions. It need not be an accurate description of reality. We know this from our own internal model of reality and from classical mechanics, neither of which seem to be accurate descriptions of reality but both of which are very good models for predicting outcomes.
    - saagarjha 6y ago
      There really is no reality, just models that describe our observations more and more accurately.
      - i-am-curious 6y ago
        Of course not. We can prove things that are absolutely true. For example, we can show 2+2, with suitable definitions of 2, + and 4, always equals 4 anywhere in the universe.
Xlythe 6y ago
If you like hard sci-fi, Greg Egan has a written a few books that cover what life would be like if there were 2 dimensions of time (http://www.gregegan.net/DICHRONAUTS/DICHRONAUTS.html http://www.gregegan.net/DICHRONAUTS/DICHRONAUTS.html) or if the dimension of time was sumed instead of subtracted when measuring distance (http://www.gregegan.net/ORTHOGONAL/ORTHOGONAL.html http://www.gregegan.net/ORTHOGONAL/ORTHOGONAL.html) It's amazing to think how the universe is ruled by such minute details.
- bollu 6y ago
  Thanks! Are there more Greg Egan books about relativity? And does he spell out the mathematical details somewhere?
  - mherdeg 6y ago
    One of my favorites is (spoiler alert) Incandescence, which presents the utterly novel concept of an alien civilization whose faculties, environment, and thought processes are totally un-human. The characters in this story very gradually discover the principles of relativity from scratch. For me reading this book, the process of understanding what it is that they are understanding ends up conveying very little useful information of how humans understand physics -- a bizarre and thought-provoking experience.
    - rishav_sharan 6y ago
      Greg Egan books expand the brain in weird ways. They can be a bit of a chore till you hit a point where the narrative has grabbed you, but once that happens, it an amazing ride. Reading his books make me feel stupid and clever at the same time.
- Zecc 6y ago
  > minute details Pun unintended?
moonchild 6y ago
> A distance in space-time is now the square-root of minus the squares of the distances in each of the dimensions of space, plus c square times the squared distance in time. In case anyone was having trouble parsing this, that's: √(-(∆x² + ∆y² + ∆z²) + c²∆t²) Or: √(c²∆t² - ∆x² - ∆y² - ∆z²)
kaetemi 6y ago
So, we travel through time at the same speed _that light travels through time_? Did I get that right?
- db48x 6y ago
  No. Light travels through space at the speed of light. Physical objects travel through time at the speed of light.
  - JPLeRouzic 6y ago
    >> Light travels through space at the speed of light. That seems a tautology isn't? Perhaps you wanted to say something else?
    - MayeulC 6y ago
      I am not sure one could take the same perspective as light, but that reference frame probably obeys the same four-vector rules. Therefore, if I got this right, for an external observer, light does not travel trough time, only space. However, in its own reference frame, light should travel trough time at the speed of light (and doesn't move in space in that reference frame). I wonder what happens to the energy in that frame...
    - db48x 6y ago
      >> Light travels through space at the speed of light. > That seems a tautology isn't? Perhaps you wanted to say something else? Very nearly, but it is exactly what I wanted to say. Light travels at a fixed speed _through space_ which we happen to call "the speed of light". A physical object which is motionless in space travels at a fixed speed _in the time dimension_ which happens to have the same size as the speed of light. This has consequences. A physical object which has some speed in space still has a 4-velocity vector whose length is the speed of light. That means the space velocity and time velocity must combine into a vector whose length is c. Naturally, if the space velocity is non-zero, the time velocity must be less than c. This is called time dilation. > Therefore, if I got this right, for an external observer, light does not travel trough time, only space. However, in its own reference frame, light should travel trough time at the speed of light (and doesn't move in space in that reference frame). Not quite. Taken to the extreme, an object with a space velocity equal to the speed of light would have no velocity at all in the time direction. This can be a confusing statement, because we must make a distinction between times as measured by an observer and times measured by the moving object. If a photon could measure events (and most cannot), it would measure no time elapsing between it's own creation and it's own destruction. It will be emitted by an electron in one place (perhaps the coronasphere of the sun) and simultaneously absorbed by another electron in another place (perhaps a rhodopsin molecule in someone's retina). An observer, such as the owner of the retina, would of course still measure some time between the emission and the absorption; the events would not appear to be simultaneous. This is called relativity of simultaneity.
alextheparrot 6y ago
How fast can something orbit the earth? My standard model of “Thing going really fast” has been some nuclear powered craft that just shoots out into the abyss, but after a bit of googling apparently our fastest craft is/will be Juno, which is staying within the solar system. This will apparently go 250,000km/hr or 0.000232c partially based on Jupiter’s gravity. It is interesting to think about just “living fast”, where our lives are still only ~80 years, but the time elapsed on earth is on the order of many multiples of that. We would exist in a way that we orbit the earth at a much larger fraction of light speed, while still allowing us to interact with the earth’s resources on normal time scales. Imagine taking a vacation back to earth for a year, but popping back to the space station where it has only been a few hours. Maybe I’ll write a novel on that or find one already written, there seems to be a lot of depth in exploring that. [0] https://www.missionjuno.swri.edu/earth-flyby/story/how-fast-can-juno-go https://www.missionjuno.swri.edu/earth-flyby/story/how-fast-...
- wlesieutre 6y ago
  Parker Solar Probe is supposed to get up to 690,000 km/hr on a close flyby of the sun!
- ShamelessC 6y ago
  The movie Interstellar explores these concepts quite well. it's not 100% accurate but Kipp Thorne was consulted for the visuals on the black holes in the film at least.
- jazzyjackson 6y ago
  I saw a really moving animated short that used this as part of its plot, but I forget the name (japanese) and haven't been able to google it since. A couple of high school sweethearts promise they'll love each other forever, but there's a war on and the boy quickly gets recruited to fight at the edge of the solar system (I remember a scene where they stand at some railroad tracks and watch a monumental silver space craft fly overhead, one of the battle cruisers), but before he gets there the boy and girl continue their relationship as pen pals, at first sending messages back and forth from mars base (45 minute delay, but not aging any faster), but as the boy travels closer to the front of the war, his velocity increases and he ages less than the girl waiting for him back on earth, and of course more time passes by between messages. Eventually it's the boys 20something birthday and he still has love for the girl, while the girl has already grown up and moved on with her life.
austincheney 6y ago
The speed of light, c, is the speed of light in a vacuum. The speed of light is variable given the medium by which that light traverses. Also, when we speak of traversal the implication is closing distance across a space. Space is also variable. According to the Special Theory of Relativity space is curved and that curvature varies according to the forces acting upon it. The general assumption is that such a curvature is uniform according to the force imposed upon at a given point relative to that force, but there is no reason to make such an assumption. More simply space is curved and that curvature may vary in unexpected ways regardless of whether an observer is outside or effected by the force imposing the curvature. That said the safest prediction is to say we all travel at various speeds of light given enough space and duration compared to a competing observation.
- pdonis 6y ago
  > According to the Special Theory of Relativity space is curved No, it's the General Theory of Relativity that says that spacetime is curved. Spacetime is flat in Special Relativity. > curvature varies according to the forces acting upon it No, in GR spacetime curvature varies according to the matter and energy that are present. Gravity is not a force at all in GR.
  - cygx 6y ago
    Gravity is not a force at all in GR. Rather, the gravitational force is a pseudo-force like the Coriolis force.
    - pdonis 6y ago
      That's one way of viewing it, yes.
- irjustin 6y ago
  downvoted because time traveling through a medium vs traveling through curved space are not anywhere near the same underlying principles[0]. The implication being suggested is - if I was trapped in a block of plastic or... underwater, I would experience time different compared to someone standing on land just because "c" is slower. Clearly not the case. To be specific, when "the speed of light" is discussed like this, it usually means the "speed of causality"[1] which is the underlying meaning of "c" in relativity. The speed of causality through a medium (i.e. water) is the same as a vacuum if spacetime curvature is the same. [0] https://www.youtube.com/watch?v=CiHN0ZWE5bk https://www.youtube.com/watch?v=CiHN0ZWE5bk [1] https://www.youtube.com/watch?v=msVuCEs8Ydo https://www.youtube.com/watch?v=msVuCEs8Ydo
milchek 6y ago
Non-physicist here, so excuse the layman rambling, but to me this was always something I just assumed when asking myself how fast is time? I just figured it was a constant because it is, it doesn't slow down or speed up - well, except for when you're doing really tedious work of course!). So, I just always assumed that if it's a constant, then it must be equal to the fastest or slowest constants we know. That always leads me to this (probably) crazy rabbit hole I can never quite articulate or resolve that no doubt countless others have wondered about - what if it's not moving at all? What if time is 'solid'? I guess the best way to explain what I mean here is like a flip book. On each page there is a drawing, if you look at one page you just see a static drawing. If you look at the book, it's just a static book. But if you flip through the pages, it animates and appears to be moving. What if our experience of time is just like this 'flip book' experience, where in actuality, our experience of time is that we are moving through it, but time is not moving at all? I'd love to know if there have been any other studies or discussions about this? Because if time is a constant, it could also be 0, no?
- XorNot 6y ago
  This is in the same vein as the thought-experiment about the One-Electron Universe [https://en.wikipedia.org/wiki/One-electron_universe https://en.wikipedia.org/wiki/One-electron_universe]. The idea is, maybe everything in the universe is just 1 electron looping through 4D space over and over, interacting with itself at various points of the loop. Comes about from the observation that antimatter can be described perfectly as just regular matter moving backwards in time.
  - milchek 6y ago
    Thanks for linking that, I had not heard of it before - very cool. The theory from Yoichiro Nambu about production and annihilation of particle-antiparticle pairs really struck me as interesting! That said, I'm a real novice with all of this so I need to do some further reading.
- FeepingCreature 6y ago
  Non-physicist answer: If you switch to viewing the universe as purely a mathematical construct, all these questions will fall away. :) The notion of ontologically privileged time or even space is based on us elevating our model of the universe into objective fact for no reason. Anything that the brain would notice even in a timeless, purely mathematical universe is thus an unneeded addition purely on Occam grounds. For instance, we may ask the question: "would my brain, if you computed its output given its inputs, output data like 'I exist, I am conscious, I notice the flow of time passing, the present moment is ontologically privileged to me'? It seems obvious that it would; that is, the mathematical answer to the question of "what does my brain output" is that sentence. But a mathematical question cannot be ontologically privileged. We see thus that these notions of ours are necessarily vacuous; any conception of 'time', 'now' and 'me' must be relative, depending purely on the surrounding mathematical/causal structure, or else not exist at all - since even if they did exist, our noticing them is not dependent on their existence, and thus Occam bids us to discard them.
sheerun 6y ago
Little misleading title: each particle moves with exactly speed of light through spacetime; the time is just the direction in which it moves through it (relativity says time is individual for each particle).
jonny383 6y ago
I'm becoming more and more convinced that we are living in a simulation, and time dilation (as you approach c) is an effect of the limits of computing power. Consider that the faster you travel, the more collisions (or potential collisions) between particles need to be decided. Is the limiting factor of c directly related to the available computational power?
- ganzuul 6y ago
  It might be a limit to precision too if infinite accuracy requires infinite space. For low energy an approximation might suffice, and that would go some ways to explain quantum effects.
- zemnmez 6y ago
  If we were living in a simulation, why would our perception of relative time be absolute relative to the simulation? Why wouldn’t the simulation just simulate slower at whatever level of abstraction it operates on, causing us to be none the wiser, like the AI in a frame-locked videogame?
  - jonny383 6y ago
    Human perception isn't a consideration of the simulation, we are just a side-effect. You can either simulate the entire universe in infinite detail taking infinite time, or you can optimize to focus on the more important bits in finite time.
- mensetmanusman 6y ago
  But then it doesn’t matter, because what’s at the top of the simulation tree?
foreigner 6y ago
What a terrible bit of teaching. >physics is all about equations No. Physics is about the nature of reality. Mathematics is a tool we use to describe the world.
- geertj 6y ago
  At present math is the only tool that works though. I would even go a bit further and say that math isn’t only a tool but it’s a device that provides a deeper understanding and intuition. A metaphor that jumps to mind is that of woodworking. Woodworking tools are not just necessary to shape the wood. They also allow you to feel the grain and other properties of the wood, providing for a deeper understanding.
koonsolo 6y ago
Which is the reference point for speed? Is it the centerpoint of the universe mass? Is it relative and calculated with acceleration and deceleration? Something else?
- lalaithion 6y ago
  There is no reference point for speed. It's all relative. Hence the theory of "relativity".
  - koonsolo 6y ago
    I still don't get it. Let's say you jump on an airplane with an atomic clock, and fly around the earth. According to the theory, your time will go slower. So the clock on the plane will be behind the clock on earth. But what if you fly against the turning of the earth. That way the clock on the earth goes slower, and so when arriving back, the clock on earth is behind? Every time I look this up, I never seem to be able to wrap my head around that. EDIT: Found the answer to my question: https://en.wikipedia.org/wiki/Time_dilation#Reciprocity https://en.wikipedia.org/wiki/Time_dilation#Reciprocity . Basically when you accelerate and decelerate, your time gets influenced, and so the person experiencing the most acceleration will have the slower time.
kovacs_x 6y ago
there is no time. only the infinity of now. :)
smusamashah 6y ago
I think if we first understand what 'time' and 'space' really are than rest becomes easy to understand. To me time is the rate at which things change or its just a measure of change. Everything is changing state, electrons are moving, bodies are moving from one point to another and also the clock moves the same way. The diagrams that explain gravity show 'space' as a 2d plane with planets weighing it down making other small objects come toward it. That's a misleading representation I think in a sense that it depends on weight/gravity. I understand space as a sponge. Heavy objects with huge mass are like a squeezed lump in that sponge effecting space around it. Larger the mass, more it squeezes, more it effects what's around it. When an object moves through space, it effects time as well i.e. rate at which change happens within that object. At very fast speeds, speed of light, it's already moving through space changing space, change within that object is reduced to nothing making time move slow for it. That all I think it is.
- karmakaze 6y ago
  I like the squeezed sponge visual.
  - smusamashah 6y ago
    The cloth sheet example with heavy object in the center making other objects come towards it never made sense to me. This example would never work in zero gravity. Someone gave this sponge visual on stackexchange I think and since then it made so much sense. Also, this makes time travel totally senseless. You can not travel back in time without reversing all input you ever received from external factors. Only thing we can probably do is stop or slow down time for something.
de6u99er 6y ago
Ther was an experiment where scientists built two ultra precise clocks. One of them stayed on earth while the other was sent to space orbiting the earth at much higher speed than the one left behind. Once the clock from space was returned scientist observed that time on the clock in space progressed slower relative to the on the one from earth. Which proves Einstein's predictions based on General Theory Of Relativity. This is being considered when e.g. sending probes to Mars.
- MichaelApproved 6y ago
  It was also done with planes flying around the world. https://en.m.wikipedia.org/wiki/Hafele–Keating_experiment https://en.m.wikipedia.org/wiki/Hafele–Keating_experiment > The Hafele–Keating experiment was a test of the theory of relativity. In October 1971, Joseph C. Hafele, a physicist, and Richard E. Keating, an astronomer, took four cesium-beam atomic clocks aboard commercial airliners. They flew twice around the world, first eastward, then westward, and compared the clocks against others that remained at the United States Naval Observatory. When reunited, the three sets of clocks were found to disagree with one another, and their differences were consistent with the predictions of special and general relativity. —— Also, GPS wouldn’t work without understanding relativity because the clocks in the satellites move faster (I think?) than the ones here on Earth.
  - imglorp 6y ago
    And even locating a clock to a higher elevation, because gravitational curvature has the same effect as relative motion. https://arxiv.org/pdf/1710.07381.pdf https://arxiv.org/pdf/1710.07381.pdf Can't find the reference now but even moving the clock a few meters is a measurable drift.
- toxicFork 6y ago
  It's also considered with the GPS :) http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps....
- lapetitejort 6y ago
Ericson2314 6y ago
The thing that annoys me a bit about these explanations is that one can do a classical spacetime too, where every timeslice is just that, a slice through the space. I would rather spend a bit of time formalizing that classical spacetime, and then get into how special relativity is different. Much more apples to apples.
shoes_for_thee 6y ago
Okay so Movement through space (in any direction) is movement against time. Is positive kinetic energy the direction opposite time?
ngcc_hk 6y ago
Try this: 1 dim relativity https://www.youtube.com/playlist?list=PL04B3CE7DCE67CA5D https://www.youtube.com/playlist?list=PL04B3CE7DCE67CA5D
redshirtrob 6y ago
Serious question: Is there a reason to preface this post with "This transcript will not make much sense without the equations that I show in the video," rather than include the equations in the post directly? I ask because I cannot view the video for some reason. The embedded video errors, and so does the direct YouTube video. Which means this post cannot function as standalone content, by the author's own admission. Cynical me things this is a ploy to drive traffic to a YouTube channel. Non-cynical me hopes the equations in the video include snazzy graphics and animations a la 3b1b. Either way it left me frustrated.
- lovehashbrowns 6y ago
  I believe it's because the video is sponsored. Cynicism aside, this makes sense because having a blog isn't as lucrative as having a successful YouTube channel. This content (both the video and blogpost) is incredibly well-done, so I tend to be okay with these type of things.
  - redshirtrob 6y ago
    Thanks. I was asking because I couldn't view the video and really wanted to read the post. Looks like it's available now.
Zamicol 6y ago
I hope someone knowledgeable is still reading comments this late: I was thinking about this last night. How does this fit into GR? I understand how it fits into SR, but in GR space is just curved. Unlike velocity time dilation, under Einstein space isn't moving. In GR it's the curve that's "equal" to gamma, the dilation factor in SR. This is evident where escape velocity (root((2GM)/r)) replaces the (v^2) term in the Lorentz factor to give gravitational time dilation in the Schwarzschild solution (t' = t(root(1-(2GM)/(rc^2))). Mind you, I've always thought everything is moving at c through spacetime, but I understood that from SR. I just don't see how it fits with GR.
username505 6y ago
Imagine still thinking ‘You’ travel anywhere - as opposed to the distance between objects and the matter there within expanding and contracting to infinitismally small and large (real-time/life-size) proportions... hmmm lol begs the question do you move through life or does life pass you by?
deleted 6y ago
[deleted]

354 points by laurex 6y ago

> Now, of course there is a difference between time and space, so that can’t be all there is to space-time. You can move around in space either which way, but you cannot move around in time as you please. Not really. The instant you move in space, you also move in time, and since everything is moving and there is no fixed frame of reference, technically you can never move back to the same spot where you started. So even though it might appear to us, at our perception scale, that we can move back and forth in space, in reality whenever we move, space changes and so it's just like moving in time, there is no going back.

CGamesPlay 6y ago

What really cemented this for me was actually looking at spacetime diagrams. Light moves at 45 degree angles on a spacetime diagram, but EVERYTHING moves at the same speed on these diagrams. I have seen several good YouTube animations demonstrating the Lorentz transformation, here's one that might help clarify what I mean: https://www.youtube.com/watch?v=k73psdcmzEY https://www.youtube.com/watch?v=k73psdcmzEY

Zamicol 6y ago
Nice find on that YouTube link. Thanks.
quickthrower2 6y ago
What I’m still confused about is if you travel through time at a certain speed and time is like any other dimension doesn’t there have to be a underlying concept of “this the real damn time” underneath it?
- pas 6y ago
  There is, the damn spacetime, under all it's completely tangled together, you can't separate them, because relativity. It's a very useful "illusion" for everyday life, to think of time as something completely distinct and always present... but it seems, our best experiments and thus models suggest otherwise. Similarly with quantum stuff, there doesn't seem to be hidden variables, it's truly a probability distribution.
  - quickthrower2 6y ago
    Thanks in this case I am trying to understand what is meant by “moving through spacetime”
    - pas 6y ago
      Sitting at home, orbiting a star, falling into a blackhole, or being a photon, an electron, or a GPS sat. All are moving through spacetime, of course on different trajectories. Especially when looking at them with the lens of General Relativity.
    - nybble41 6y ago
      > what is meant by “moving through spacetime” AIUI the equations describe a path through spacetime. There is no "speed" or "movement" along this path, or at least none with an actual physical interpretation. The path simply has a certain shape. The equations define the path as a parametric function, and you could in principle calculate the change in 4D spacetime coordinates with respect to the change in that parameter, but it would have no physical meaning. To keep things simple, in a canonical representation of the path this ratio (the "speed" through spacetime with respect to the arbitrary scalar parameter—though "speed" is not really the right term since the parameter does not represent time) is held constant and defined as the speed of light. To put things in more familiar (or at least less spacetime-y) terms, these very different parametric equations both describe a 2D unit circle with respect to an arbitrary parameter r with range (-∞, ∞): p(r) = [cos(r), sin(r)] q(r) = [abs(2 - t) - 1, ((2 - t) / abs(2 - t)) * sqrt(1 - (abs(2 - t) - 1)**2)] The "speed" of p(r) with respect to r is the magnitude of the derivative [-sin(r), cos(r)], a unit vector, and thus equal to one for any value of r. The magnitude of the derivative of q(r) is something rather more complicated, and not even well-defined for most values of r. However, they both describe the same circle. The parameter r is not part of the path; only the set of [x, y] coordinates counts, and p(r) and q(r) describe the same sets of 2D points.
- CGamesPlay 6y ago
  There is, it's called the "spacetime interval". It's relative to the observer, because everything in spacetime is, but it's the "true" measure of separation in spacetime.

nitrogen 6y ago

Today I want to answer a question that was sent to me by Ed Catmull who writes: "Twice, I have read books on relativity by PhDs who said that we travel through time at the speed of light, but I can’t find those books, and I haven’t seen it written anywhere else. Can you let me know if this is right or if this is utter nonsense." I wonder, was this question asked by the Ed Catmull (https://en.wikipedia.org/wiki/Edwin_Catmull https://en.wikipedia.org/wiki/Edwin_Catmull)? ---- Edit: to make this comment less useless, I'd like to point any laypeople like myself who are interested in the nature of the universe to the PBS Space Time show/YouTube channel: https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g I'm pretty sure they have videos related to this question.

acidbaseextract 6y ago
The moment I saw that name I wondered the same thing. It would be cool to be running in the same internet circles as Ed Catmull. I'm in awe of his contributions to both computer graphics and Pixar, and I really liked his management book, even if he definitely did some sketchy stuff with wage fixing.

antognini 6y ago

Here is another explanation. In Newtonian physics lots of common quantities are represented by vectors: position, momentum, force, the magnetic potential, etc. Other important quantities are represented by scalars: time, energy, power, the electric potential, etc. One of the deep insights of special relativity is that these scalar and vector quantities are actually unified into a single, new entity called a "four-vector". Each four-vector has three spatial components and one temporal component. So in the two lists I wrote above, each scalar quantity gets paired with the corresponding vector quantity: time & position; energy & momentum; power & force; the electric & magnetic potentials. In Newtonian physics a rotation in three-dimensional space will transform one component of your position vector into another (say, some of your x-component becomes a bit of y-component). In special relativity observers moving at different velocities are related by a similar kind of transformation of their four-vectors --- a bit of the time component mixes with the spatial component and vice versa. But what happens if you do this with velocity? Velocity is a vector. It's corresponding temporal quantity is a bit weird: change in time per change in time. But it's a little more subtle than that, because it's actually the change in coordinate time per change in proper time. Basically this is the ratio between how fast you observe a clock tick in an observer's reference frame relative to how fast they see it tick within that reference frame. But the really weird thing about this particular four-vector is that it always has a magnitude of exactly the speed of light. No matter how fast you go, the magnitude of your four-velocity does not actually get larger. All you do by going faster is just mix some of the time component of your four-velocity into the spatial components. (This is why the clocks of moving observers tick more slowly.) And when you slow down, you just mix some of the spatial components into the temporal component. And when you're at rest the spatial components are all at zero, so the only component of your four-velocity is in the temporal direction. So at rest your four-velocity points directly in the future with a magnitude equal to the speed of light. This is what is meant by the statement that we travel through time at the speed of light.

vnorilo 6y ago
This is great, thanks! This is the first time I hear it explained like this and it just somehow clicked immediately. I now have this (probably wildly incorrect, total layman here) vision of the time dimension of velocity being "a normal" simultaneously to all the spatial dimensions and acceleration as rotation of the four-vector away from the time axis... sorry for any terminology errors.

ta1234567890 6y ago

> If you want to calculate a distance in space, you use Euclid’s formula. A distance, in three dimension, is the square-root of the of the sum of the squared distances in each direction of space. Here the Δx is a difference between two points in direction x, and Δy and Δz are likewise differences between two points in directions y and z. I really dislike it when explanations about reality become explanations about a model of reality. Can the original question be answered without assuming some model is the perfect reflection of reality, or just without using the model?

lokedhs 6y ago
What would that even mean? The model is a description of reality. When you talk about the model, you are referencing a mathematical description of reality. In a similar way, the ratio π is the mathematical description (or model, if you like) of a circle's circumference relative to its diameter. I'm not sure how to describe this relationship without invoking the mathematical description of said relationship.
- guerrilla 6y ago
  > The model is a description of reality No, not neccessarily. It's purpose is to make predictions. It need not be an accurate description of reality. We know this from our own internal model of reality and from classical mechanics, neither of which seem to be accurate descriptions of reality but both of which are very good models for predicting outcomes.
  - saagarjha 6y ago
    There really is no reality, just models that describe our observations more and more accurately.
    - i-am-curious 6y ago
      Of course not. We can prove things that are absolutely true. For example, we can show 2+2, with suitable definitions of 2, + and 4, always equals 4 anywhere in the universe.

Xlythe 6y ago

If you like hard sci-fi, Greg Egan has a written a few books that cover what life would be like if there were 2 dimensions of time (http://www.gregegan.net/DICHRONAUTS/DICHRONAUTS.html http://www.gregegan.net/DICHRONAUTS/DICHRONAUTS.html) or if the dimension of time was sumed instead of subtracted when measuring distance (http://www.gregegan.net/ORTHOGONAL/ORTHOGONAL.html http://www.gregegan.net/ORTHOGONAL/ORTHOGONAL.html) It's amazing to think how the universe is ruled by such minute details.

bollu 6y ago
Thanks! Are there more Greg Egan books about relativity? And does he spell out the mathematical details somewhere?
- mherdeg 6y ago
  One of my favorites is (spoiler alert) Incandescence, which presents the utterly novel concept of an alien civilization whose faculties, environment, and thought processes are totally un-human. The characters in this story very gradually discover the principles of relativity from scratch. For me reading this book, the process of understanding what it is that they are understanding ends up conveying very little useful information of how humans understand physics -- a bizarre and thought-provoking experience.
  - rishav_sharan 6y ago
    Greg Egan books expand the brain in weird ways. They can be a bit of a chore till you hit a point where the narrative has grabbed you, but once that happens, it an amazing ride. Reading his books make me feel stupid and clever at the same time.
Zecc 6y ago
> minute details Pun unintended?

moonchild 6y ago

> A distance in space-time is now the square-root of minus the squares of the distances in each of the dimensions of space, plus c square times the squared distance in time. In case anyone was having trouble parsing this, that's: √(-(∆x² + ∆y² + ∆z²) + c²∆t²) Or: √(c²∆t² - ∆x² - ∆y² - ∆z²)

kaetemi 6y ago

So, we travel through time at the same speed _that light travels through time_? Did I get that right?

db48x 6y ago
No. Light travels through space at the speed of light. Physical objects travel through time at the speed of light.
- JPLeRouzic 6y ago
  >> Light travels through space at the speed of light. That seems a tautology isn't? Perhaps you wanted to say something else?
  - MayeulC 6y ago
    I am not sure one could take the same perspective as light, but that reference frame probably obeys the same four-vector rules. Therefore, if I got this right, for an external observer, light does not travel trough time, only space. However, in its own reference frame, light should travel trough time at the speed of light (and doesn't move in space in that reference frame). I wonder what happens to the energy in that frame...
  - db48x 6y ago
    >> Light travels through space at the speed of light. > That seems a tautology isn't? Perhaps you wanted to say something else? Very nearly, but it is exactly what I wanted to say. Light travels at a fixed speed _through space_ which we happen to call "the speed of light". A physical object which is motionless in space travels at a fixed speed _in the time dimension_ which happens to have the same size as the speed of light. This has consequences. A physical object which has some speed in space still has a 4-velocity vector whose length is the speed of light. That means the space velocity and time velocity must combine into a vector whose length is c. Naturally, if the space velocity is non-zero, the time velocity must be less than c. This is called time dilation. > Therefore, if I got this right, for an external observer, light does not travel trough time, only space. However, in its own reference frame, light should travel trough time at the speed of light (and doesn't move in space in that reference frame). Not quite. Taken to the extreme, an object with a space velocity equal to the speed of light would have no velocity at all in the time direction. This can be a confusing statement, because we must make a distinction between times as measured by an observer and times measured by the moving object. If a photon could measure events (and most cannot), it would measure no time elapsing between it's own creation and it's own destruction. It will be emitted by an electron in one place (perhaps the coronasphere of the sun) and simultaneously absorbed by another electron in another place (perhaps a rhodopsin molecule in someone's retina). An observer, such as the owner of the retina, would of course still measure some time between the emission and the absorption; the events would not appear to be simultaneous. This is called relativity of simultaneity.

alextheparrot 6y ago

How fast can something orbit the earth? My standard model of “Thing going really fast” has been some nuclear powered craft that just shoots out into the abyss, but after a bit of googling apparently our fastest craft is/will be Juno, which is staying within the solar system. This will apparently go 250,000km/hr or 0.000232c partially based on Jupiter’s gravity. It is interesting to think about just “living fast”, where our lives are still only ~80 years, but the time elapsed on earth is on the order of many multiples of that. We would exist in a way that we orbit the earth at a much larger fraction of light speed, while still allowing us to interact with the earth’s resources on normal time scales. Imagine taking a vacation back to earth for a year, but popping back to the space station where it has only been a few hours. Maybe I’ll write a novel on that or find one already written, there seems to be a lot of depth in exploring that. [0] https://www.missionjuno.swri.edu/earth-flyby/story/how-fast-can-juno-go https://www.missionjuno.swri.edu/earth-flyby/story/how-fast-...

wlesieutre 6y ago
Parker Solar Probe is supposed to get up to 690,000 km/hr on a close flyby of the sun!
ShamelessC 6y ago
The movie Interstellar explores these concepts quite well. it's not 100% accurate but Kipp Thorne was consulted for the visuals on the black holes in the film at least.
jazzyjackson 6y ago
I saw a really moving animated short that used this as part of its plot, but I forget the name (japanese) and haven't been able to google it since. A couple of high school sweethearts promise they'll love each other forever, but there's a war on and the boy quickly gets recruited to fight at the edge of the solar system (I remember a scene where they stand at some railroad tracks and watch a monumental silver space craft fly overhead, one of the battle cruisers), but before he gets there the boy and girl continue their relationship as pen pals, at first sending messages back and forth from mars base (45 minute delay, but not aging any faster), but as the boy travels closer to the front of the war, his velocity increases and he ages less than the girl waiting for him back on earth, and of course more time passes by between messages. Eventually it's the boys 20something birthday and he still has love for the girl, while the girl has already grown up and moved on with her life.

austincheney 6y ago

The speed of light, c, is the speed of light in a vacuum. The speed of light is variable given the medium by which that light traverses. Also, when we speak of traversal the implication is closing distance across a space. Space is also variable. According to the Special Theory of Relativity space is curved and that curvature varies according to the forces acting upon it. The general assumption is that such a curvature is uniform according to the force imposed upon at a given point relative to that force, but there is no reason to make such an assumption. More simply space is curved and that curvature may vary in unexpected ways regardless of whether an observer is outside or effected by the force imposing the curvature. That said the safest prediction is to say we all travel at various speeds of light given enough space and duration compared to a competing observation.

pdonis 6y ago
> According to the Special Theory of Relativity space is curved No, it's the General Theory of Relativity that says that spacetime is curved. Spacetime is flat in Special Relativity. > curvature varies according to the forces acting upon it No, in GR spacetime curvature varies according to the matter and energy that are present. Gravity is not a force at all in GR.
- cygx 6y ago
  Gravity is not a force at all in GR. Rather, the gravitational force is a pseudo-force like the Coriolis force.
  - pdonis 6y ago
    That's one way of viewing it, yes.
irjustin 6y ago
downvoted because time traveling through a medium vs traveling through curved space are not anywhere near the same underlying principles[0]. The implication being suggested is - if I was trapped in a block of plastic or... underwater, I would experience time different compared to someone standing on land just because "c" is slower. Clearly not the case. To be specific, when "the speed of light" is discussed like this, it usually means the "speed of causality"[1] which is the underlying meaning of "c" in relativity. The speed of causality through a medium (i.e. water) is the same as a vacuum if spacetime curvature is the same. [0] https://www.youtube.com/watch?v=CiHN0ZWE5bk https://www.youtube.com/watch?v=CiHN0ZWE5bk [1] https://www.youtube.com/watch?v=msVuCEs8Ydo https://www.youtube.com/watch?v=msVuCEs8Ydo

milchek 6y ago

Non-physicist here, so excuse the layman rambling, but to me this was always something I just assumed when asking myself how fast is time? I just figured it was a constant because it is, it doesn't slow down or speed up - well, except for when you're doing really tedious work of course!). So, I just always assumed that if it's a constant, then it must be equal to the fastest or slowest constants we know. That always leads me to this (probably) crazy rabbit hole I can never quite articulate or resolve that no doubt countless others have wondered about - what if it's not moving at all? What if time is 'solid'? I guess the best way to explain what I mean here is like a flip book. On each page there is a drawing, if you look at one page you just see a static drawing. If you look at the book, it's just a static book. But if you flip through the pages, it animates and appears to be moving. What if our experience of time is just like this 'flip book' experience, where in actuality, our experience of time is that we are moving through it, but time is not moving at all? I'd love to know if there have been any other studies or discussions about this? Because if time is a constant, it could also be 0, no?

XorNot 6y ago
This is in the same vein as the thought-experiment about the One-Electron Universe [https://en.wikipedia.org/wiki/One-electron_universe https://en.wikipedia.org/wiki/One-electron_universe]. The idea is, maybe everything in the universe is just 1 electron looping through 4D space over and over, interacting with itself at various points of the loop. Comes about from the observation that antimatter can be described perfectly as just regular matter moving backwards in time.
- milchek 6y ago
  Thanks for linking that, I had not heard of it before - very cool. The theory from Yoichiro Nambu about production and annihilation of particle-antiparticle pairs really struck me as interesting! That said, I'm a real novice with all of this so I need to do some further reading.
FeepingCreature 6y ago
Non-physicist answer: If you switch to viewing the universe as purely a mathematical construct, all these questions will fall away. :) The notion of ontologically privileged time or even space is based on us elevating our model of the universe into objective fact for no reason. Anything that the brain would notice even in a timeless, purely mathematical universe is thus an unneeded addition purely on Occam grounds. For instance, we may ask the question: "would my brain, if you computed its output given its inputs, output data like 'I exist, I am conscious, I notice the flow of time passing, the present moment is ontologically privileged to me'? It seems obvious that it would; that is, the mathematical answer to the question of "what does my brain output" is that sentence. But a mathematical question cannot be ontologically privileged. We see thus that these notions of ours are necessarily vacuous; any conception of 'time', 'now' and 'me' must be relative, depending purely on the surrounding mathematical/causal structure, or else not exist at all - since even if they did exist, our noticing them is not dependent on their existence, and thus Occam bids us to discard them.

sheerun 6y ago

Little misleading title: each particle moves with exactly speed of light through spacetime; the time is just the direction in which it moves through it (relativity says time is individual for each particle).

jonny383 6y ago

I'm becoming more and more convinced that we are living in a simulation, and time dilation (as you approach c) is an effect of the limits of computing power. Consider that the faster you travel, the more collisions (or potential collisions) between particles need to be decided. Is the limiting factor of c directly related to the available computational power?

ganzuul 6y ago
It might be a limit to precision too if infinite accuracy requires infinite space. For low energy an approximation might suffice, and that would go some ways to explain quantum effects.
zemnmez 6y ago
If we were living in a simulation, why would our perception of relative time be absolute relative to the simulation? Why wouldn’t the simulation just simulate slower at whatever level of abstraction it operates on, causing us to be none the wiser, like the AI in a frame-locked videogame?
- jonny383 6y ago
  Human perception isn't a consideration of the simulation, we are just a side-effect. You can either simulate the entire universe in infinite detail taking infinite time, or you can optimize to focus on the more important bits in finite time.
mensetmanusman 6y ago
But then it doesn’t matter, because what’s at the top of the simulation tree?

foreigner 6y ago

What a terrible bit of teaching. >physics is all about equations No. Physics is about the nature of reality. Mathematics is a tool we use to describe the world.

geertj 6y ago
At present math is the only tool that works though. I would even go a bit further and say that math isn’t only a tool but it’s a device that provides a deeper understanding and intuition. A metaphor that jumps to mind is that of woodworking. Woodworking tools are not just necessary to shape the wood. They also allow you to feel the grain and other properties of the wood, providing for a deeper understanding.

koonsolo 6y ago

Which is the reference point for speed? Is it the centerpoint of the universe mass? Is it relative and calculated with acceleration and deceleration? Something else?

lalaithion 6y ago
There is no reference point for speed. It's all relative. Hence the theory of "relativity".
- koonsolo 6y ago
  I still don't get it. Let's say you jump on an airplane with an atomic clock, and fly around the earth. According to the theory, your time will go slower. So the clock on the plane will be behind the clock on earth. But what if you fly against the turning of the earth. That way the clock on the earth goes slower, and so when arriving back, the clock on earth is behind? Every time I look this up, I never seem to be able to wrap my head around that. EDIT: Found the answer to my question: https://en.wikipedia.org/wiki/Time_dilation#Reciprocity https://en.wikipedia.org/wiki/Time_dilation#Reciprocity . Basically when you accelerate and decelerate, your time gets influenced, and so the person experiencing the most acceleration will have the slower time.

kovacs_x 6y ago

there is no time. only the infinity of now. :)

smusamashah 6y ago

I think if we first understand what 'time' and 'space' really are than rest becomes easy to understand. To me time is the rate at which things change or its just a measure of change. Everything is changing state, electrons are moving, bodies are moving from one point to another and also the clock moves the same way. The diagrams that explain gravity show 'space' as a 2d plane with planets weighing it down making other small objects come toward it. That's a misleading representation I think in a sense that it depends on weight/gravity. I understand space as a sponge. Heavy objects with huge mass are like a squeezed lump in that sponge effecting space around it. Larger the mass, more it squeezes, more it effects what's around it. When an object moves through space, it effects time as well i.e. rate at which change happens within that object. At very fast speeds, speed of light, it's already moving through space changing space, change within that object is reduced to nothing making time move slow for it. That all I think it is.

karmakaze 6y ago
I like the squeezed sponge visual.
- smusamashah 6y ago
  The cloth sheet example with heavy object in the center making other objects come towards it never made sense to me. This example would never work in zero gravity. Someone gave this sponge visual on stackexchange I think and since then it made so much sense. Also, this makes time travel totally senseless. You can not travel back in time without reversing all input you ever received from external factors. Only thing we can probably do is stop or slow down time for something.

de6u99er 6y ago

Ther was an experiment where scientists built two ultra precise clocks. One of them stayed on earth while the other was sent to space orbiting the earth at much higher speed than the one left behind. Once the clock from space was returned scientist observed that time on the clock in space progressed slower relative to the on the one from earth. Which proves Einstein's predictions based on General Theory Of Relativity. This is being considered when e.g. sending probes to Mars.

MichaelApproved 6y ago
It was also done with planes flying around the world. https://en.m.wikipedia.org/wiki/Hafele–Keating_experiment https://en.m.wikipedia.org/wiki/Hafele–Keating_experiment > The Hafele–Keating experiment was a test of the theory of relativity. In October 1971, Joseph C. Hafele, a physicist, and Richard E. Keating, an astronomer, took four cesium-beam atomic clocks aboard commercial airliners. They flew twice around the world, first eastward, then westward, and compared the clocks against others that remained at the United States Naval Observatory. When reunited, the three sets of clocks were found to disagree with one another, and their differences were consistent with the predictions of special and general relativity. —— Also, GPS wouldn’t work without understanding relativity because the clocks in the satellites move faster (I think?) than the ones here on Earth.
- imglorp 6y ago
  And even locating a clock to a higher elevation, because gravitational curvature has the same effect as relative motion. https://arxiv.org/pdf/1710.07381.pdf https://arxiv.org/pdf/1710.07381.pdf Can't find the reference now but even moving the clock a few meters is a measurable drift.
toxicFork 6y ago
It's also considered with the GPS :) http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps....
lapetitejort 6y ago

Ericson2314 6y ago

The thing that annoys me a bit about these explanations is that one can do a classical spacetime too, where every timeslice is just that, a slice through the space. I would rather spend a bit of time formalizing that classical spacetime, and then get into how special relativity is different. Much more apples to apples.

shoes_for_thee 6y ago

Okay so Movement through space (in any direction) is movement against time. Is positive kinetic energy the direction opposite time?

ngcc_hk 6y ago

Try this: 1 dim relativity https://www.youtube.com/playlist?list=PL04B3CE7DCE67CA5D https://www.youtube.com/playlist?list=PL04B3CE7DCE67CA5D

redshirtrob 6y ago

Serious question: Is there a reason to preface this post with "This transcript will not make much sense without the equations that I show in the video," rather than include the equations in the post directly? I ask because I cannot view the video for some reason. The embedded video errors, and so does the direct YouTube video. Which means this post cannot function as standalone content, by the author's own admission. Cynical me things this is a ploy to drive traffic to a YouTube channel. Non-cynical me hopes the equations in the video include snazzy graphics and animations a la 3b1b. Either way it left me frustrated.

lovehashbrowns 6y ago
I believe it's because the video is sponsored. Cynicism aside, this makes sense because having a blog isn't as lucrative as having a successful YouTube channel. This content (both the video and blogpost) is incredibly well-done, so I tend to be okay with these type of things.
- redshirtrob 6y ago
  Thanks. I was asking because I couldn't view the video and really wanted to read the post. Looks like it's available now.

Zamicol 6y ago

I hope someone knowledgeable is still reading comments this late: I was thinking about this last night. How does this fit into GR? I understand how it fits into SR, but in GR space is just curved. Unlike velocity time dilation, under Einstein space isn't moving. In GR it's the curve that's "equal" to gamma, the dilation factor in SR. This is evident where escape velocity (root((2GM)/r)) replaces the (v^2) term in the Lorentz factor to give gravitational time dilation in the Schwarzschild solution (t' = t(root(1-(2GM)/(rc^2))). Mind you, I've always thought everything is moving at c through spacetime, but I understood that from SR. I just don't see how it fits with GR.

username505 6y ago

Imagine still thinking ‘You’ travel anywhere - as opposed to the distance between objects and the matter there within expanding and contracting to infinitismally small and large (real-time/life-size) proportions... hmmm lol begs the question do you move through life or does life pass you by?

deleted 6y ago

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