# Is time dilation relative, or absolute?

I explain my question through an example.
If we brought down a GPS clock, would it show the normal time, because it was accelerated before launch, but it has been slowed down by time dilation?
Or we have to accelerate those clocks in order to compensate for the time loss, till the signal from the ground (bended by Earth's gravity field) catches the fast moving satellite?
In the later case, i would say time dilation is relative from our viewpoint, but not absolute, a fast moving human would still age normally.
In the first case i would say that time dilation is absolute.

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Pengwuino
Gold Member
I can't completely understand the situation you've presented, so I'll throw in something that might answer your question anyways. An object on Earth, an object 2000km above Earth, and one 4000km above Earth will all see different different rates of time change with respect to each other.

English isnt my native language, so my composition may not the best.
Ok, they will all SEE different rates of time change. But what we see, isnt necessary the truth. What our instruments see, isnt necessary the truth.

Will they only see the time shift, due to the time loss, till the signals travel between them?
(And those signals also bended by gravity, and takes them longer time to reach a fast moving object.)
Or can we say, that time is slowed down, and if i boarded a fast moving spaceship, i could return to Earth hundred years later as a young man?

The GPS satellite would show normal time because it is programmed to correct for time dilation. Otherwise if it were not it would be inaccurate as a navigational tool.

Time dilation is always relative for inertial reference frames. Inertial reference frames are things moving in a straight line and not undergoing accerlation.

Time dilation is absolute for accerlation and things in a gravitational field.

"Time dilation is always relative for inertial reference frames."

So if a rocket moved with a speed 0.9 c, and another were not moving, would they see, that each other's clocks slowed down?
But as they both see the same thing, that means those clocks didnt really slowed down if i interpret things correctly.
But in a weak gravity field, a clock really slows down.

Both rockets would see each others clocks as slowing down if they were both moving in a straight line at a constant speed, and they would both be right. I know that does seem to be a contridiction, but if they were to meet up with each other one of the rockets would have to slow down, and then speed up, so since time dilation is absolute for accerlation one of the rockets clocks would get behind the other. When things speed up or slow down, only one of the rockets clocks slows down. But when they both move at a constant speed both rockets clocks slows down.

In any kind of gravity field time slows down. The stronger the gravity field the greater the time dilation.

Dale
Mentor
if i boarded a fast moving spaceship, i could return to Earth hundred years later as a young man?
Yes.

ghwellsjr
Gold Member
"Time dilation is always relative for inertial reference frames."

So if a rocket moved with a speed 0.9 c, and another were not moving, would they see, that each other's clocks slowed down?
But as they both see the same thing, that means those clocks didnt really slowed down if i interpret things correctly.
But in a weak gravity field, a clock really slows down.
Speed is also relative so if a rocket moved with a speed of 0.9 c in relation to another one that were not moving, don't you agree that the second one is moving relative to the first one at a speed of 0.9 c in the opposite direction? Don't you agree that they would each see the other one moving at 0.9 c (in opposite directions)? Would you then say that since they both see the same thing, that means they really aren't moving?

HallsofIvy
Homework Helper
Or can we say, that time is slowed down, and if i boarded a fast moving spaceship, i could return to Earth hundred years later as a young man?
Well, that depends upon how old you are now!

Cleonis
Gold Member
I explain my question through an example.
If we brought down a GPS clock, would it show the normal time, because it was accelerated before launch, but it has been slowed down by time dilation?
Well, the acceleration is not a factor.

The example of satellites is somewhat awkward, because heuristically we distinguish between velocity time dilation effects and gravitational time dilation effects.

In the particular case of satellites: for a satellite at an altitude of about 2000 kilometers the velocity effect and gravitational effect cancel. There is an altitude where satellite-borne clocks remain in sync with clocks on Earth, but as I said that is due to cancellation of effects.

We can simplify the case by remaining at very low altitude. The International Space Station (ISS) is at an altitude of just a couple of hundreds of kilometers. Therefore there is comparatively very little gravitational effect.

For a clock onboard the ISS less proper time elapses than for a clock on Earth. The longer the duration of the stay in orbit, the more accumulation of difference in elapsed proper time.

Perhaps in the near future there will be nano-second-accurate time keeping devices that are light enough to be approved for taking along for a stay on ISS.
Because the clock is taken back to Earth the comparisons will be direct comparisons; both before departure and after returning to Earth the clocks will be right next to each other.
This way what you compare is not difference in rate of proper time. You just compare clock readings at a start point (before departure) and at an end point (after rejoining the clocks). That way you find difference in amount of elapsed proper time. There is no dependency on point of view.

The following things will be seen:
- A clock that was twice as many days in orbit will show twice as much difference in elapsed proper time. (as compared to Earth clocks)

- The journey to orbit, and the descend to Earth, is without effect. That is, if you could send one clock for 1000 days, and another clock ten times back and forth, each time for a stay of 100 days, then at the end those two clocks will show the same difference in elapsed proper time (as compared to Earth clocks).

http://www.metaresearch.org/cosmology/gps-relativity.asp [Broken]

Well, that rather interprets things in Lorentzian relativity, with Earth's gravity field as a preferred frame.

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http://www.metaresearch.org/cosmology/gps-relativity.asp [Broken]

Well, that rather interprets things in Lorentzian relativity, with Earth's gravity field as a preferred frame.
The Earth is only preferred for effects of the Earth's gravitation such as gravitational time dilation.

If I see it correctly, that site is not "mainstream" and it doesn't really interpret things like Lorentz did, even if it pretends that it does - IOW, handle with care!

In connection with your earlier statements, a truly Lorentzian view would be that real physical effects occur to everything including our measurement instruments. As a consequence, what our instruments see, isn't exactly the truth*. Consequently time dilation between inertial frames is not a mere illusion. In that view the observed symmetry ("relative time dilation") is an illusion that is broken by non-inertial motion ("absolute time dilation").

* the truth would then be something like a "God's eye" view

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