11 ms·

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-

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.
  - kadoban 5y ago
    Gp is talking about forward-only time travel though, you don't need anything exotic for that at all, just lots of energy and an efficient way to turn it into thrust.
  - xondono 5y ago
    The Alcubierre drive “beats” the speed of light, for the effects described 0.7-0.9c are more than good enough (if slightly unpractical). We can’t do that either for now, but is way easier on the feasibility scale.
    - lovegoblin 5y ago
      > The Alcubierre drive “beats” the speed of light The Alcubierre drive is only a thought experiment that requires "exotic matter" (aka fairy dust) to work.
      - xondono 5y ago
        I know, I was making reference to this > 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.
      - baja_blast 5y ago
        Papers have been published that have found ways to create warp drives without the need for exotic matter: https://arxiv.org/pdf/2006.07125.pdf https://arxiv.org/pdf/2006.07125.pdf
  - xyzzyz 5y ago
    No, for forward time travel you don’t need any sort of exotic matter. You just need a really fast rocket.
- zamadatix 5y ago
  At 1G constant acceleration (both speeding up and slowing down) you can make it on a short vacation to the Andromeda galaxy and back in a lifetime (or 5 million years from the perspective or Earth).
  - eesmith 5y ago
    It looks like about 57 years. Assuming constant acceleration to the 1/2-way point, flip, deceleration, and using http://www.projectrho.com/public_html/rocket/slowerlight3.php http://www.projectrho.com/public_html/rocket/slowerlight3.ph... : Time elapsed (in starship's frame of reference, "Proper time") T = (c/a) * ArcCosh[a*d/(c^2) + 1] (given acceleration and distance) year = 365.25*24*3600; c = 3E8; a=9.8; d=1.25*1_000_000*(c * year) from math import acosh T = (c/a) * acosh(a*d/(c*c) + 1) print(T/year) => 14.3 years each quarter => 57 years round trip The speed at flip would be 99.99999999993978% c - good thing intergalactic space is mostly empty.
    - wruza 5y ago
      Bad news CMB shifts into infrared, visible, uv, xray and then hard gamma.
      - deleted 5y ago
        [deleted]
      - eesmith 5y ago
        Oh, what a shame. I was about to install some rockets on my RV and head off to Andromeda. Guess I don't need to put my newspaper subscription on hold now.
      - smegger001 5y ago
        since we are talking insane speeds and energies required to reach them why not just say you are in a bigger ship with more radiation shielding a dozen or so meters of water in the hull and a layer of lead a foot thick aught to stop most of it
mjburgess 5y ago
If distances are dilating, why isn't that /less/ ground?
- 3pt14159 5y ago
  It basically depends on your perspective. To someone on earth watching the spaceship speed up to the speed of light, the spaceship looks like its contracting. To the person on the spaceship, the rest of the universe looks like it is approaching the speed of light, and hence, contracting.
- shrimp_emoji 5y ago
  The distance doesn't dilate; time does. The distance contracts. E.g., to a photon moving at 1c, the whole universe has a contracted length of 0 meters, and it crosses the whole universe instantly. To us, observers at <1c, the whole universe has a non-contracted length of <a lot> and the photon takes <a long time> to cross the whole universe. By the time the photon's 0-second journey across the entire universe has finished (whatever that means), we're all extremely old. :D This is the time dilation meme of slowly-aging space travelers but taken to the extreme.
  - redwood 5y ago
    Thanks for the clarification
  - tobtoh 5y ago
    So if I was in a space battle and the enemy 'jumps to light speed'(1) to make a quick escape ... they would actually be easier to target with a laser because they 'slow down' from my perspective? (1) 'light speed' as in the speed of light, not as in a sci-fi context of hyperspace jump/FTL jump.
  - micw 5y ago
    > E.g., to a photon moving at 1c, the whole universe has a contracted length of 0 meters, and it crosses the whole universe instantly. Since the universe expands with >1c, I wonder if the photon actually crosses the whole universe. And if not, how it would look like from the prespective of the photon?
yawaworht1978 5y ago
I know this is true, but I never got my head around this, maybe someone can help. So if a photon is emitted from the sun, it's passing earth immediately? Then why does sunlight need 7 or 8 minutes to get to earth? And let's say, i travel one light-year at the speed of light, that should be instantaneous, right? The odometer would show 1 light-year, my watch would should 0 seconds and some decimals. How much would I have aged by the end of the journey? If I travelled at one percent of speed of light, same distance, i suppose 100 years would elapse for me, how much would elapse on earth? Odometer would still show one light year? And what if I left earth at 50percent of light speed , traveled 1 light-year away and did a turn and came back to earth at same speed. For me, it would be 1 year, but if I had a twin brother who was waiting on earth, would i now be a year younger than him? And how is this possible?
- nycdatasci 5y ago
  Einstein can help: https://books.google.com/books/download/Relativity.pdf?id=3H46AAAAMAAJ&output=pdf https://books.google.com/books/download/Relativity.pdf?id=3H... His book was intended to help people understand these exact questions, without getting into any complex math. Regarding "how is this possible?": This was experimentally verified in the 70s in the Hafele–Keating Experiment. Read more here: https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experiment https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experim...
  - hollander 5y ago
    That Google link doesn't work. https://www.gutenberg.org/ebooks/5001 https://www.gutenberg.org/ebooks/5001
    - nycdatasci 5y ago
      Thanks!
- justusthane 5y ago
  > So if a photon is emitted from the sun, it's passing earth immediately? Then why does sunlight need 7 or 8 minutes to get to earth? From the photon's perspective, it passes earth immediately. From our perspective, it takes 7 or 8 minutes. > And let's say, i travel one light-year at the speed of light, that should be instantaneous, right? The odometer would show 1 light-year, my watch would should 0 seconds and some decimals. How much would I have aged by the end of the journey? You would have aged as much as your watch says you would have aged. Zero seconds. >If I travelled at one percent of speed of light, same distance, i suppose 100 years would elapse for me, how much would elapse on earth? I don't know how to do the math, but a very, very long time would have passed on earth. > And what if I left earth at 50percent of light speed , traveled 1 light-year away and did a turn and came back to earth at same speed. For me, it would be 1 year, but if I had a twin brother who was waiting on earth, would i now be a year younger than him? And how is this possible? Yes, you would be younger than him, and it's possible because that's just how relativity and time dilation work. It's even practically measurable in "real" life: http://www.leapsecond.com/great2005/tour/ http://www.leapsecond.com/great2005/tour/
godelski 5y ago
I'm pretty sure this doesn't work out because you have to slow down. So thinking about it, if you are traveling at that speed because of your inertial reference frame it is equivalent that everyone else around you is moving at (or near) the speed of light and they are moving slowly through time. This is the classic twin paradox and there is a resolution to it, which is that you can't instantaneously turn around.[0] Or in our case, we have to turn around to slow down. [0] https://www.youtube.com/watch?v=0iJZ_QGMLD0 https://www.youtube.com/watch?v=0iJZ_QGMLD0
- firebaze 5y ago
  It does work out, in fact you could go arbitrarily far in space (and, mandatory, in external time) if you had infinite energy to spend. The acceleration and deceleration phase is almost negligible. Edit: the faster you go, the slower your own (inertial) time passes. That means the external time passes faster, and the factor grows to infinity the closer you get to C. In fact, subjectively there is no speed limit. As you go faster, anything around you ages faster, but you yourself won't encounter any speed limit.
  - phkahler 5y ago
    But motion is all relative. Who is considered to be moving faster?
  - godelski 5y ago
    But in your inertial reference frame the people on Earth are moving at (near) the speed of light. So they are the ones that should be staying young. Or similarly the planet you are traveling to is actually speeding towards you and you are staying still. This is why the twin paradox is a paradox, because of the reference frames.
    - raattgift 5y ago
      You're right that the apparent symmetry is broken by acceleration(s!), and to show that I'd point to Michael Weiss's twin paradox equivalence principle analysis at https://www.desy.de/user/projects/Physics/Relativity/SR/TwinParadox/twin_gr.html https://www.desy.de/user/projects/Physics/Relativity/SR/Twin... rather than rewriting it. There is a subtlety not explicitly raised in the writeup, mainly that in General Relativity metrics do not superpose cleanly, in the sense of getting another solution to the Einstein Field Equations. We do not worry about this in the ultrasimplified twin-paradox model where the spacetime is flat in the sense that the Riemann tensor vanishes everywhere. However, if we want to consider the behaviour of gravitational waves with amplitudes outside of the weak https://en.wikipedia.org/wiki/Linearized_gravity https://en.wikipedia.org/wiki/Linearized_gravity limit, we are in a world of calculational pain. Physicalizing this subtlety, if our travelling twin is travelling in our neighbourhood of the galaxy, it is probably in for a bumpy ride due to gravitational waves from nearby binary stars https://news.berkeley.edu/2021/02/22/binary-stars-are-all-around-us-new-map-of-solar-neighborhood-shows/ https://news.berkeley.edu/2021/02/22/binary-stars-are-all-ar... . We cannot easily extract how bumpy by adding in the uniform pseudogravitational field proposed by Weiss. On the other hand, we probably cannot quantify the effects of gravitational waves at all by simple adapatation of the other strictly Special Relativity analyses at the related Weiss link, https://www.desy.de/user/projects/Physics/Relativity/SR/TwinParadox/twin_vase.html https://www.desy.de/user/projects/Physics/Relativity/SR/Twin... (which lists among other the resolution in the minutephysics youtube link you provided above). You're also right that the problem is one of reference frames. We are not obliged to use that of one twin as the spatial origin. In principle any will do, but some choices have advantages driven by features deliberately excluded from the Special Relativity twin paradox. Let's consider the "(s!)" tacked on at the end of acceleration. We have not only that of the travelling twin's spacecraft engine, but also that which drives the expansion of the universe. From within our galaxy we observe a highly spatially homogeneous and isotropic arrangement of extragalactic luminous matter (and cosmic radiation, locally) without distortions in the shapes of distant spiral galaxies that imply a spatially non-flat universe. The metric expansion of this, retaining bulk isotropy, gives us a preferred foliation (Wald's 1984 textbook develops this pp 92-93, but alternatively we could use Weyl's principle). Each twin is free to use a "cosmic fluid" observable (like the dipole-free temperature of the cosmic microwave background, which expands adiabatically), even while accelerating, to determine the https://en.wikipedia.org/wiki/Scale_factor_%28cosmology%29 https://en.wikipedia.org/wiki/Scale_factor_%28cosmology%29 . For example, each twin could consider the dipole pattern dT/T = v/c where T in the twin's proper time. Each twin can thus determine whether it is the relativistic traveller or not, even if it only wakes up occasionally and only long enough to look at a snapshot of the CMB. The travelling twin thus sees a clear breaking of the Copernican principle along the direction of its travel. Or more precisely, with respect to the bulk flow of matter and radiation in the universe, the non-travelling twin can conclude that it is effectively a Eulerian or comoving observer, while the relativistically-travelling twin cannot. Moreover, the twins (and any third party) can use "cosmic fluid" observables to determine the scale factor when the twins are together at the start of the travel, and when they (or at least one and the other's remains) are together again at the end. In this approach there is no paradox at all, there is only the consequences of one twin with a worldline with sections where the proper time is at a higher tilt to the cosmic time than the other twin's. We also avoid the difficulties in attaching a pseudogravitational field to a spacetime where there are gravitational waves of reasonably large amplitude, or relativistic stars and other massive compact objects. We head into the land of apparent paradox by stripping out evidence of an expanding universe. We must also eliminate evidence of the aging of galaxy clusters through gravitational collapse (including the rate of star formation and the change in abundance of heavy elements). Indeed, we have to arrive in a setting in which neither twin can determine that it has departed from a point at which some reasonable generalization of the Copernican principle applies. Indeed, the usual formulation of the apparent paradox gets rid of everything but the twins, so that one cannot even use Rindler/Unruh-like observables in flat spacetime, and this really emphasizes the "Special" in Special Relativity. In that setting, as I said above, relying on the equivalence of being in uniform acceleration (even if it's instantaneous) and being immersed in a uniform (pseudo)gravitational field, is a reasonable way to eliminate the apparent paradox.
deleted 5y ago
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wruza 5y ago
Also, it doesn’t take too much acceleration to make that trip. Comfortable earthlike 1g (~ 10m/s/s) is enough to build up a decent speed in a very reasonable time. Energy is the issue though.
- jandrese 5y ago
  It's a fantasy as long as you are still living under the tyranny of the rocket equation.
  - tenuousemphasis 5y ago
    I don't think anybody thinks such a ship is possible with chemical rockets.
    - reddiky 5y ago
      Just need Astrophage /s
      - DesiLurker 5y ago
        Ha, got the Hail Mary ref.
    - jandrese 5y ago
      It's worse than that. It's not possible with any type of rocket. If you are accelerating by pushing mass in an opposite direction then you'll never build a ship that can accelerate at 1G for years at a time. Unless we invent a reactionless drive the idea of traveling between solar systems remains a pipe dream.
      - paraschopra 5y ago
        Why? Can you expand?
- seanmcdirmid 5y ago
  You need to spend equal amounts of energy and time on both acceleration and deceleration. It doesn’t help to go fast if you can’t stop at where you are going.
nostromo 5y ago
You can even do so without going near the speed of light if you build a very massive ship and utilize time dilation of gravity. Arvin Ash has a cool video on this that is probably too complicated to describe in a HN comment: https://www.youtube.com/watch?v=PA66ah9b0U4 https://www.youtube.com/watch?v=PA66ah9b0U4
- deleted 5y ago
  [deleted]
- infofarmer 5y ago
  Your username adds an especially eerie context to your comment.
- smnrchrds 5y ago
  As long as we are talking about how far you can go in one lifetime, how would human body react in a spaceship with that much mass? Wouldn't the gravity crush any humans to death. 10% of the sun's mass is more than 30,000 times earth's mass. Add to that the fact that the diameter is only 620 m, and the gravity becomes 1.4e13 g.
  - sombremesa 5y ago
    The solar system is traveling in space and is quite massive but I haven’t been crushed to death yet.
    - smnrchrds 5y ago
      You are far away from the masses. We are alive because we only experience 1g on earth. 10g is enough to kill a human. I don't even know what 14,000,000,000,000g would do, but I imagine we would get added to the neutron star.
      - IIsi50MHz 5y ago
        1g towards Earth's center. Well, center of mass. :adjusts bow tie: Welllll…combination of that and angular momentum. Combined with the masses of the sun, and the moon, the other planets, and all the other masses of the universe. Fortunately, orbits factor in, too. Everything that orbits is essentially in free fall, wellll…until they get tooo eliptical and the oscillating accelerations get really noticeable. And it all affects time. Time is a bunch of wibbly-wobbly…stuff. (( Ok, ok: Matt Smith might not have been that wordy as The Doctor. ))
fuzzfactor 5y ago
>Why can’t I go faster than the speed of light? I assume people know how bad the penalty can be for going merely 10 MPH over the speed limit. Don't worry about an individual photon except as part of an image in this example. The light from the source hits the subject and is reflected toward your eyes at the speed of light. That's why they call it the speed of light, and radio signals do it too between their source & receiver. So now imagine you could travel faster than the speed of light to a planet a number of light-years away and you are going to get there from here. Once you leave Earth orbit you will be able to accelerate up to and beyond c in the safest most gradual way directly toward your destination. While still in orbit you look down on the traffic in your hometown, and everything is still moving at normal speed no matter how far it is down there, since you are a steady distance away from what it is you are looking at. As you accelerate away from Earth and approach the speed of light itself you're beginning to catch up with the light that was reflected off your home planet quite a bit earlier than the light which is simultaneously being seen by those back in orbit. So at half c you look out the window and it looks like everyone back in your hometown is moving at half speed. But naturally time marches on down there. You just can't be so sure any more. At that speed if you left 2 years earlier you will only be able to know additional things about your home which happened no more recently than 1 year ago at that point. Of course you can't communicate with them about this in real time because of how long it takes the radio signal to get back & forth so you don't bother. You keep going and reach full light speed which finally matches the rate the images are being reflected away from the Earth at, so now look back home and everyone appears to be standing still on Earth, as expected. Even though as far as you know they are still carrying on like normal. OTOH, approaching the destination planet at the speed of light, that's pretty fast, but you have to realize their alien traffic is actually only moving half as quickly as it looks from your craft, whilst you are speeding so rapidly in their direction. Don't let that fool you, the aliens are only half as advanced as they look. If you want to really see something, go faster than the speed of light and the planet you are approaching will be moving more than twice as fast as normal, and looking Earthward all you can see would be things moving backwards. One thing that's happening is that you are always seeing images of these two planets where the light source originated from two different suns. Once the distances get far enough, it's possible to launch a mission to a destination that actually no longer existed any more for quite some time before launch, only who knew? In that case the earlier you make your reservations the more unwise it could be.
sudhirj 5y ago
Yup. If Musk builds a continuously accelerating rocket, he could die at the end of the universe instead of on Mars.
hota_mazi 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 It his, it just depends on the observer. An external, stationary observer will never see you go further than 100 light years, but yourself? Assuming you are able to make your ship go any arbitrary speed approaching c, you could be traveling billions of light years. It's just that when you stop (if you manage to stop), the universe around you will have aged billions of years, while you will only be a few years older.

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.
- kadoban 5y ago
  Gp is talking about forward-only time travel though, you don't need anything exotic for that at all, just lots of energy and an efficient way to turn it into thrust.
- xondono 5y ago
  The Alcubierre drive “beats” the speed of light, for the effects described 0.7-0.9c are more than good enough (if slightly unpractical). We can’t do that either for now, but is way easier on the feasibility scale.
  - lovegoblin 5y ago
    > The Alcubierre drive “beats” the speed of light The Alcubierre drive is only a thought experiment that requires "exotic matter" (aka fairy dust) to work.
    - xondono 5y ago
      I know, I was making reference to this > 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.
    - baja_blast 5y ago
      Papers have been published that have found ways to create warp drives without the need for exotic matter: https://arxiv.org/pdf/2006.07125.pdf https://arxiv.org/pdf/2006.07125.pdf
- xyzzyz 5y ago
  No, for forward time travel you don’t need any sort of exotic matter. You just need a really fast rocket.
zamadatix 5y ago
At 1G constant acceleration (both speeding up and slowing down) you can make it on a short vacation to the Andromeda galaxy and back in a lifetime (or 5 million years from the perspective or Earth).
- eesmith 5y ago
  It looks like about 57 years. Assuming constant acceleration to the 1/2-way point, flip, deceleration, and using http://www.projectrho.com/public_html/rocket/slowerlight3.php http://www.projectrho.com/public_html/rocket/slowerlight3.ph... : Time elapsed (in starship's frame of reference, "Proper time") T = (c/a) * ArcCosh[a*d/(c^2) + 1] (given acceleration and distance) year = 365.25*24*3600; c = 3E8; a=9.8; d=1.25*1_000_000*(c * year) from math import acosh T = (c/a) * acosh(a*d/(c*c) + 1) print(T/year) => 14.3 years each quarter => 57 years round trip The speed at flip would be 99.99999999993978% c - good thing intergalactic space is mostly empty.
  - wruza 5y ago
    Bad news CMB shifts into infrared, visible, uv, xray and then hard gamma.
    - deleted 5y ago
      [deleted]
    - eesmith 5y ago
      Oh, what a shame. I was about to install some rockets on my RV and head off to Andromeda. Guess I don't need to put my newspaper subscription on hold now.
    - smegger001 5y ago
      since we are talking insane speeds and energies required to reach them why not just say you are in a bigger ship with more radiation shielding a dozen or so meters of water in the hull and a layer of lead a foot thick aught to stop most of it

mjburgess 5y ago

If distances are dilating, why isn't that /less/ ground?

3pt14159 5y ago
It basically depends on your perspective. To someone on earth watching the spaceship speed up to the speed of light, the spaceship looks like its contracting. To the person on the spaceship, the rest of the universe looks like it is approaching the speed of light, and hence, contracting.
shrimp_emoji 5y ago
The distance doesn't dilate; time does. The distance contracts. E.g., to a photon moving at 1c, the whole universe has a contracted length of 0 meters, and it crosses the whole universe instantly. To us, observers at <1c, the whole universe has a non-contracted length of <a lot> and the photon takes <a long time> to cross the whole universe. By the time the photon's 0-second journey across the entire universe has finished (whatever that means), we're all extremely old. :D This is the time dilation meme of slowly-aging space travelers but taken to the extreme.
- redwood 5y ago
  Thanks for the clarification
- tobtoh 5y ago
  So if I was in a space battle and the enemy 'jumps to light speed'(1) to make a quick escape ... they would actually be easier to target with a laser because they 'slow down' from my perspective? (1) 'light speed' as in the speed of light, not as in a sci-fi context of hyperspace jump/FTL jump.
- micw 5y ago
  > E.g., to a photon moving at 1c, the whole universe has a contracted length of 0 meters, and it crosses the whole universe instantly. Since the universe expands with >1c, I wonder if the photon actually crosses the whole universe. And if not, how it would look like from the prespective of the photon?

yawaworht1978 5y ago

I know this is true, but I never got my head around this, maybe someone can help. So if a photon is emitted from the sun, it's passing earth immediately? Then why does sunlight need 7 or 8 minutes to get to earth? And let's say, i travel one light-year at the speed of light, that should be instantaneous, right? The odometer would show 1 light-year, my watch would should 0 seconds and some decimals. How much would I have aged by the end of the journey? If I travelled at one percent of speed of light, same distance, i suppose 100 years would elapse for me, how much would elapse on earth? Odometer would still show one light year? And what if I left earth at 50percent of light speed , traveled 1 light-year away and did a turn and came back to earth at same speed. For me, it would be 1 year, but if I had a twin brother who was waiting on earth, would i now be a year younger than him? And how is this possible?

nycdatasci 5y ago
Einstein can help: https://books.google.com/books/download/Relativity.pdf?id=3H46AAAAMAAJ&output=pdf https://books.google.com/books/download/Relativity.pdf?id=3H... His book was intended to help people understand these exact questions, without getting into any complex math. Regarding "how is this possible?": This was experimentally verified in the 70s in the Hafele–Keating Experiment. Read more here: https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experiment https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experim...
- hollander 5y ago
  That Google link doesn't work. https://www.gutenberg.org/ebooks/5001 https://www.gutenberg.org/ebooks/5001
  - nycdatasci 5y ago
    Thanks!
justusthane 5y ago
> So if a photon is emitted from the sun, it's passing earth immediately? Then why does sunlight need 7 or 8 minutes to get to earth? From the photon's perspective, it passes earth immediately. From our perspective, it takes 7 or 8 minutes. > And let's say, i travel one light-year at the speed of light, that should be instantaneous, right? The odometer would show 1 light-year, my watch would should 0 seconds and some decimals. How much would I have aged by the end of the journey? You would have aged as much as your watch says you would have aged. Zero seconds. >If I travelled at one percent of speed of light, same distance, i suppose 100 years would elapse for me, how much would elapse on earth? I don't know how to do the math, but a very, very long time would have passed on earth. > And what if I left earth at 50percent of light speed , traveled 1 light-year away and did a turn and came back to earth at same speed. For me, it would be 1 year, but if I had a twin brother who was waiting on earth, would i now be a year younger than him? And how is this possible? Yes, you would be younger than him, and it's possible because that's just how relativity and time dilation work. It's even practically measurable in "real" life: http://www.leapsecond.com/great2005/tour/ http://www.leapsecond.com/great2005/tour/

godelski 5y ago

I'm pretty sure this doesn't work out because you have to slow down. So thinking about it, if you are traveling at that speed because of your inertial reference frame it is equivalent that everyone else around you is moving at (or near) the speed of light and they are moving slowly through time. This is the classic twin paradox and there is a resolution to it, which is that you can't instantaneously turn around.[0] Or in our case, we have to turn around to slow down. [0] https://www.youtube.com/watch?v=0iJZ_QGMLD0 https://www.youtube.com/watch?v=0iJZ_QGMLD0

firebaze 5y ago
It does work out, in fact you could go arbitrarily far in space (and, mandatory, in external time) if you had infinite energy to spend. The acceleration and deceleration phase is almost negligible. Edit: the faster you go, the slower your own (inertial) time passes. That means the external time passes faster, and the factor grows to infinity the closer you get to C. In fact, subjectively there is no speed limit. As you go faster, anything around you ages faster, but you yourself won't encounter any speed limit.
- phkahler 5y ago
  But motion is all relative. Who is considered to be moving faster?
- godelski 5y ago
  But in your inertial reference frame the people on Earth are moving at (near) the speed of light. So they are the ones that should be staying young. Or similarly the planet you are traveling to is actually speeding towards you and you are staying still. This is why the twin paradox is a paradox, because of the reference frames.
  - raattgift 5y ago
    You're right that the apparent symmetry is broken by acceleration(s!), and to show that I'd point to Michael Weiss's twin paradox equivalence principle analysis at https://www.desy.de/user/projects/Physics/Relativity/SR/TwinParadox/twin_gr.html https://www.desy.de/user/projects/Physics/Relativity/SR/Twin... rather than rewriting it. There is a subtlety not explicitly raised in the writeup, mainly that in General Relativity metrics do not superpose cleanly, in the sense of getting another solution to the Einstein Field Equations. We do not worry about this in the ultrasimplified twin-paradox model where the spacetime is flat in the sense that the Riemann tensor vanishes everywhere. However, if we want to consider the behaviour of gravitational waves with amplitudes outside of the weak https://en.wikipedia.org/wiki/Linearized_gravity https://en.wikipedia.org/wiki/Linearized_gravity limit, we are in a world of calculational pain. Physicalizing this subtlety, if our travelling twin is travelling in our neighbourhood of the galaxy, it is probably in for a bumpy ride due to gravitational waves from nearby binary stars https://news.berkeley.edu/2021/02/22/binary-stars-are-all-around-us-new-map-of-solar-neighborhood-shows/ https://news.berkeley.edu/2021/02/22/binary-stars-are-all-ar... . We cannot easily extract how bumpy by adding in the uniform pseudogravitational field proposed by Weiss. On the other hand, we probably cannot quantify the effects of gravitational waves at all by simple adapatation of the other strictly Special Relativity analyses at the related Weiss link, https://www.desy.de/user/projects/Physics/Relativity/SR/TwinParadox/twin_vase.html https://www.desy.de/user/projects/Physics/Relativity/SR/Twin... (which lists among other the resolution in the minutephysics youtube link you provided above). You're also right that the problem is one of reference frames. We are not obliged to use that of one twin as the spatial origin. In principle any will do, but some choices have advantages driven by features deliberately excluded from the Special Relativity twin paradox. Let's consider the "(s!)" tacked on at the end of acceleration. We have not only that of the travelling twin's spacecraft engine, but also that which drives the expansion of the universe. From within our galaxy we observe a highly spatially homogeneous and isotropic arrangement of extragalactic luminous matter (and cosmic radiation, locally) without distortions in the shapes of distant spiral galaxies that imply a spatially non-flat universe. The metric expansion of this, retaining bulk isotropy, gives us a preferred foliation (Wald's 1984 textbook develops this pp 92-93, but alternatively we could use Weyl's principle). Each twin is free to use a "cosmic fluid" observable (like the dipole-free temperature of the cosmic microwave background, which expands adiabatically), even while accelerating, to determine the https://en.wikipedia.org/wiki/Scale_factor_%28cosmology%29 https://en.wikipedia.org/wiki/Scale_factor_%28cosmology%29 . For example, each twin could consider the dipole pattern dT/T = v/c where T in the twin's proper time. Each twin can thus determine whether it is the relativistic traveller or not, even if it only wakes up occasionally and only long enough to look at a snapshot of the CMB. The travelling twin thus sees a clear breaking of the Copernican principle along the direction of its travel. Or more precisely, with respect to the bulk flow of matter and radiation in the universe, the non-travelling twin can conclude that it is effectively a Eulerian or comoving observer, while the relativistically-travelling twin cannot. Moreover, the twins (and any third party) can use "cosmic fluid" observables to determine the scale factor when the twins are together at the start of the travel, and when they (or at least one and the other's remains) are together again at the end. In this approach there is no paradox at all, there is only the consequences of one twin with a worldline with sections where the proper time is at a higher tilt to the cosmic time than the other twin's. We also avoid the difficulties in attaching a pseudogravitational field to a spacetime where there are gravitational waves of reasonably large amplitude, or relativistic stars and other massive compact objects. We head into the land of apparent paradox by stripping out evidence of an expanding universe. We must also eliminate evidence of the aging of galaxy clusters through gravitational collapse (including the rate of star formation and the change in abundance of heavy elements). Indeed, we have to arrive in a setting in which neither twin can determine that it has departed from a point at which some reasonable generalization of the Copernican principle applies. Indeed, the usual formulation of the apparent paradox gets rid of everything but the twins, so that one cannot even use Rindler/Unruh-like observables in flat spacetime, and this really emphasizes the "Special" in Special Relativity. In that setting, as I said above, relying on the equivalence of being in uniform acceleration (even if it's instantaneous) and being immersed in a uniform (pseudo)gravitational field, is a reasonable way to eliminate the apparent paradox.

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wruza 5y ago

Also, it doesn’t take too much acceleration to make that trip. Comfortable earthlike 1g (~ 10m/s/s) is enough to build up a decent speed in a very reasonable time. Energy is the issue though.

jandrese 5y ago
It's a fantasy as long as you are still living under the tyranny of the rocket equation.
- tenuousemphasis 5y ago
  I don't think anybody thinks such a ship is possible with chemical rockets.
  - reddiky 5y ago
    Just need Astrophage /s
    - DesiLurker 5y ago
      Ha, got the Hail Mary ref.
  - jandrese 5y ago
    It's worse than that. It's not possible with any type of rocket. If you are accelerating by pushing mass in an opposite direction then you'll never build a ship that can accelerate at 1G for years at a time. Unless we invent a reactionless drive the idea of traveling between solar systems remains a pipe dream.
    - paraschopra 5y ago
      Why? Can you expand?
seanmcdirmid 5y ago
You need to spend equal amounts of energy and time on both acceleration and deceleration. It doesn’t help to go fast if you can’t stop at where you are going.

nostromo 5y ago

You can even do so without going near the speed of light if you build a very massive ship and utilize time dilation of gravity. Arvin Ash has a cool video on this that is probably too complicated to describe in a HN comment: https://www.youtube.com/watch?v=PA66ah9b0U4 https://www.youtube.com/watch?v=PA66ah9b0U4

deleted 5y ago
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infofarmer 5y ago
Your username adds an especially eerie context to your comment.
smnrchrds 5y ago
As long as we are talking about how far you can go in one lifetime, how would human body react in a spaceship with that much mass? Wouldn't the gravity crush any humans to death. 10% of the sun's mass is more than 30,000 times earth's mass. Add to that the fact that the diameter is only 620 m, and the gravity becomes 1.4e13 g.
- sombremesa 5y ago
  The solar system is traveling in space and is quite massive but I haven’t been crushed to death yet.
  - smnrchrds 5y ago
    You are far away from the masses. We are alive because we only experience 1g on earth. 10g is enough to kill a human. I don't even know what 14,000,000,000,000g would do, but I imagine we would get added to the neutron star.
    - IIsi50MHz 5y ago
      1g towards Earth's center. Well, center of mass. :adjusts bow tie: Welllll…combination of that and angular momentum. Combined with the masses of the sun, and the moon, the other planets, and all the other masses of the universe. Fortunately, orbits factor in, too. Everything that orbits is essentially in free fall, wellll…until they get tooo eliptical and the oscillating accelerations get really noticeable. And it all affects time. Time is a bunch of wibbly-wobbly…stuff. (( Ok, ok: Matt Smith might not have been that wordy as The Doctor. ))

fuzzfactor 5y ago

>Why can’t I go faster than the speed of light? I assume people know how bad the penalty can be for going merely 10 MPH over the speed limit. Don't worry about an individual photon except as part of an image in this example. The light from the source hits the subject and is reflected toward your eyes at the speed of light. That's why they call it the speed of light, and radio signals do it too between their source & receiver. So now imagine you could travel faster than the speed of light to a planet a number of light-years away and you are going to get there from here. Once you leave Earth orbit you will be able to accelerate up to and beyond c in the safest most gradual way directly toward your destination. While still in orbit you look down on the traffic in your hometown, and everything is still moving at normal speed no matter how far it is down there, since you are a steady distance away from what it is you are looking at. As you accelerate away from Earth and approach the speed of light itself you're beginning to catch up with the light that was reflected off your home planet quite a bit earlier than the light which is simultaneously being seen by those back in orbit. So at half c you look out the window and it looks like everyone back in your hometown is moving at half speed. But naturally time marches on down there. You just can't be so sure any more. At that speed if you left 2 years earlier you will only be able to know additional things about your home which happened no more recently than 1 year ago at that point. Of course you can't communicate with them about this in real time because of how long it takes the radio signal to get back & forth so you don't bother. You keep going and reach full light speed which finally matches the rate the images are being reflected away from the Earth at, so now look back home and everyone appears to be standing still on Earth, as expected. Even though as far as you know they are still carrying on like normal. OTOH, approaching the destination planet at the speed of light, that's pretty fast, but you have to realize their alien traffic is actually only moving half as quickly as it looks from your craft, whilst you are speeding so rapidly in their direction. Don't let that fool you, the aliens are only half as advanced as they look. If you want to really see something, go faster than the speed of light and the planet you are approaching will be moving more than twice as fast as normal, and looking Earthward all you can see would be things moving backwards. One thing that's happening is that you are always seeing images of these two planets where the light source originated from two different suns. Once the distances get far enough, it's possible to launch a mission to a destination that actually no longer existed any more for quite some time before launch, only who knew? In that case the earlier you make your reservations the more unwise it could be.

sudhirj 5y ago

Yup. If Musk builds a continuously accelerating rocket, he could die at the end of the universe instead of on Mars.

hota_mazi 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 It his, it just depends on the observer. An external, stationary observer will never see you go further than 100 light years, but yourself? Assuming you are able to make your ship go any arbitrary speed approaching c, you could be traveling billions of light years. It's just that when you stop (if you manage to stop), the universe around you will have aged billions of years, while you will only be a few years older.