8 ms·

Ask HN: Time dilation and streaming data

4 points by uptownhr 11y ago

Let's say a spacecraft is traveling away from earth @ close to the speed of light. There is computer on the spacecraft, that is streaming data from earth. How would this data feed look as the space craft travels further and further away from earth? If this was a video stream(that did not buffer), how would the video play? Would the video start playing faster and faster?

trcollinson 11y ago
I will take a stab at this, maybe a proper physicist will correct me. If you are on the ship moving away from earth at nearly the speed of light but, we'll say, with a constant relative velocity, your time would slow down as observed by earth. Also assuming that the streaming video signal is coming from earth at the speed of light, relative to earth, and as observed by someone on earth, it would take the stream quite a long time to get to the ship (though it would only take the normal amount of time to leave earth). This is because if you are moving at 99.9% c and the video is streaming at c then relative to the ship the video is streaming at .1% c. Thus, it takes a long time to get there. However, since it's not buffering and just playing as it streams in, and because time has slowed for those on the ship, I will posit that the video would play at relatively the normal speed to the observers on the ship as both their time and the stream itself have slowed significantly.
- SamReidHughes 11y ago
  What if the ship is motionless and the Earth is moving at 99.9% c?
  - trcollinson 11y ago
    Again, I am not a physicist, hopefully one will come and confirm or deny. Because I would say at first blush, it would act the same. But upon further thought I came up with the following. If the ship were not moving relative to the earth which was moving as a constant velocity away from the ship at near the speed of light, the inhabitants of Earth would seem to age more slowly compared to those on the ship (or the ships passengers would be aging very quickly when observed by those on earth). With that if earth streamed the video it would seem to leave earth as a constant c and yet it would have to overcome the relative speed of the earth moving away from the ship. Thus, it would still come to the ship at 0.1% the speed of light but because time would seem faster to the ship passengers I posit that the video would play relatively slowly.
    - SamReidHughes 11y ago
      > If the ship were not moving relative to the earth which was moving as a constant velocity away from the ship at near the speed of light If the ship is not moving relative to the Earth, then the earth is not moving relative to the ship. The trouble I have with your first explanation is that the ship and Earth are moving away from each other -- it's not like one is the stationary one -- it can't be true that one experiences time more slowly, in some absolute sense, than the other. Whatever phenomenom happens to video streaming should happen in either direction.
- TheLoneWolfling 11y ago
  > However, since it's not buffering and just playing as it streams in, and because time has slowed for those on the ship, I will posit that the video would play at relatively the normal speed to the observers on the ship as both their time and the stream itself have slowed significantly. I don't think this is correct. If you're traveling near the speed of light, light from behind you is drastically redshifted from your point of view, which implies that the wavelength is longer from your point of view. So, for example, if Earth sends out a 1 Hz signal, you'll see it at much less than 1 Hz. So it shouldn't play at normal speed. It should be slowed down.
deleted 11y ago
[deleted]
SamReidHughes 11y ago
If anybody posts a link to Wikipedia or some other outside reference, that's cheating.
TheLoneWolfling 11y ago
Distance shouldn't matter. Assuming you can pick up the (extremely) redshifted signal. Also assuming it's headed directly away from Earth. What'll happen is that it'll be slowed down by a constant factor dependent on speed. How? Depends on the video player. The factor should be, if I remember my relativity right, sqrt((1 + v/c) / (1 - v/c)). The formula for relativistic doppler shift of wavelength. Note that "what factor does the video slow down by" is equivalent to saying "if you have two points a certain time apart in the video, what factor is it stretched out by on reception". Which is equivalent to redshift in wavelength terms where the time is just wavelength / c. There's also a gravitational effect, but it should be relatively negligible.

4 points by uptownhr 11y ago

trcollinson 11y ago

I will take a stab at this, maybe a proper physicist will correct me. If you are on the ship moving away from earth at nearly the speed of light but, we'll say, with a constant relative velocity, your time would slow down as observed by earth. Also assuming that the streaming video signal is coming from earth at the speed of light, relative to earth, and as observed by someone on earth, it would take the stream quite a long time to get to the ship (though it would only take the normal amount of time to leave earth). This is because if you are moving at 99.9% c and the video is streaming at c then relative to the ship the video is streaming at .1% c. Thus, it takes a long time to get there. However, since it's not buffering and just playing as it streams in, and because time has slowed for those on the ship, I will posit that the video would play at relatively the normal speed to the observers on the ship as both their time and the stream itself have slowed significantly.

SamReidHughes 11y ago
What if the ship is motionless and the Earth is moving at 99.9% c?
- trcollinson 11y ago
  Again, I am not a physicist, hopefully one will come and confirm or deny. Because I would say at first blush, it would act the same. But upon further thought I came up with the following. If the ship were not moving relative to the earth which was moving as a constant velocity away from the ship at near the speed of light, the inhabitants of Earth would seem to age more slowly compared to those on the ship (or the ships passengers would be aging very quickly when observed by those on earth). With that if earth streamed the video it would seem to leave earth as a constant c and yet it would have to overcome the relative speed of the earth moving away from the ship. Thus, it would still come to the ship at 0.1% the speed of light but because time would seem faster to the ship passengers I posit that the video would play relatively slowly.
  - SamReidHughes 11y ago
    > If the ship were not moving relative to the earth which was moving as a constant velocity away from the ship at near the speed of light If the ship is not moving relative to the Earth, then the earth is not moving relative to the ship. The trouble I have with your first explanation is that the ship and Earth are moving away from each other -- it's not like one is the stationary one -- it can't be true that one experiences time more slowly, in some absolute sense, than the other. Whatever phenomenom happens to video streaming should happen in either direction.
TheLoneWolfling 11y ago
> However, since it's not buffering and just playing as it streams in, and because time has slowed for those on the ship, I will posit that the video would play at relatively the normal speed to the observers on the ship as both their time and the stream itself have slowed significantly. I don't think this is correct. If you're traveling near the speed of light, light from behind you is drastically redshifted from your point of view, which implies that the wavelength is longer from your point of view. So, for example, if Earth sends out a 1 Hz signal, you'll see it at much less than 1 Hz. So it shouldn't play at normal speed. It should be slowed down.

deleted 11y ago

[deleted]

SamReidHughes 11y ago

If anybody posts a link to Wikipedia or some other outside reference, that's cheating.

TheLoneWolfling 11y ago

Distance shouldn't matter. Assuming you can pick up the (extremely) redshifted signal. Also assuming it's headed directly away from Earth. What'll happen is that it'll be slowed down by a constant factor dependent on speed. How? Depends on the video player. The factor should be, if I remember my relativity right, sqrt((1 + v/c) / (1 - v/c)). The formula for relativistic doppler shift of wavelength. Note that "what factor does the video slow down by" is equivalent to saying "if you have two points a certain time apart in the video, what factor is it stretched out by on reception". Which is equivalent to redshift in wavelength terms where the time is just wavelength / c. There's also a gravitational effect, but it should be relatively negligible.