

Ask HN: Time dilation and streaming data - uptownhr

Let&#x27;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?<p>If this was a video stream(that did not buffer), how would the video play? Would the video start playing faster and faster?
======
trcollinson
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
What if the ship is motionless and the Earth is moving at 99.9% c?

~~~
trcollinson
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
> 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
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.

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

