# Time Dilation+Gravity

## Main Question or Discussion Point

The faster you go, the slower time goes relative to someone who is not moving as fast. The more gravity is acting on you, the slower time goes relative to someone who is not under as much gravity.
If the two previous statements are correct, could you go back in time by going really fast and being under a lot of gravity? I don't know if the effects would combine or add to eachother, but if they do, could you go back in time?

Related Special and General Relativity News on Phys.org
could you go back in time by going really fast and being under a lot of gravity?
Almost certainly not...there are lots of related threads on time travel here....Note that the individual "going faster" and/or "deeper in a gravity well" is locally unaware of any change in the passing of time.

You might also read about blackholes,whiteholes and EPR bridges (wormholes) in Wikipedia. If and when relativity and quantum mechanics are "unified" we might have further insights...so far the most interesting locations, blackholes and singularites, are rather obscured since we can can neither experimentally measure effects nor do we have unified mathematical (theoretical) descriptions...

You can also explore "future" and "past" realms inside blackhole horizons.

Might also read about relativistic degenerated neutron stars only about three times as big as the stellar black holes of which we really don't know whether they exist. Rotating neutronstars; the remains of supernova explosions are really there and the discussion whether they exceed 1,44 Msolar is less interesting than the fact that they are really small, rotate fast and indeed are of the same candidacy as the socalled fancied black holes...

Maybe you don't have to travel faster than "c" to cross the universe.

As you accelerate towards the speed of light, sure time dilation does occur.
But also, most people forget that space also contracts by the same factor.

Speed is distance divided by the time taken to travel it.

So at near light speeds from the travellers perspective, your trip of millions of light years may for you, only be a few million kilometers and only take a couple of hours.

Unfortunately though, when you arrive, those loved ones left behind would have died many millions of years ago.

As you accelerate towards the speed of light, sure time dilation does occur.
But also, most people forget that space also contracts by the same factor.

Speed is distance divided by the time taken to travel it.
Wrong.

For an observer on Earth, there is a time dilation on a spaceship (and vice versa). So the flight to Andromeda galaxy could take (by earth clock) millions of years while for an austranaut it took only few years.

Inside a spaceship, distance to Andromeda is contracted and time flows as usual.

So you're right, austranaut benefits from time dilation OR space contraction, but from one at a time. I disagree with the world ALSO.

Dmitry.

Thanks, you are absolutely right.
I stand corrected by my wording.

I used the word "also" twice in my post.
By "also most people forget" I did not include physicists.
By "space also contracts by the same factor" is correct but for different observers.
I used the "also" only to link the effects.

I'm new to this forum and I'm only just learning how careful one has to be with one's choice of words here.

Thanks again and cheers.

So does any one what would happen?
I understand that you wouldn't feel like you are moving any slower though time (or even backwards). In someone else's perspective, you would though...right?
If you go .95c and are somehow under extreme gravity, would you go back in time relative to someone else? I haven't gotten a definitive answer yet.

Janus
Staff Emeritus
Gold Member
So does any one what would happen?
I understand that you wouldn't feel like you are moving any slower though time (or even backwards). In someone else's perspective, you would though...right?
If you go .95c and are somehow under extreme gravity, would you go back in time relative to someone else? I haven't gotten a definitive answer yet.

The definitive answer is: No you would not. All combining time dilation due to velocity and gravitational time dilation will do is make you age more slowly than you would have by either alone.

Another point that should be made is that gravitational time dilation isn't due to strength of gravity but due to difference in gravitational potential. IOW, how much work it would take to move from one potential to the other. For example, a clock sitting at the center of the Earth would experience zero gravity, but would run slower than a clock on the surface which experiences 1g, since it would take work to lift the clock from the center of the Earth to the surface.

If you go .95c and are somehow under extreme gravity, would you go back in time relative to someone else? I haven't gotten a definitive answer yet.
I suspect the two time dilation factor for velocity and the time dilation factor for gravity would add the same way they add for two velocities,

*****u + v
w = ------------
****1 + uv/c^2

(The asterisks replace spaces)

where u is the dilation factor due to gravity (calculated by substituting the escape velocity for v in the Lorentz Transformation) and v is the velocity of the spaceship. w is the resultant velocity.

Another point that should be made is that gravitational time dilation isn't due to strength of gravity but due to difference in gravitational potential. IOW, how much work it would take to move from one potential to the other. For example, a clock sitting at the center of the Earth would experience zero gravity, but would run slower than a clock on the surface ....
Quantitatively, how great should the gravitational potential be to make the clock as slow as a thousandth and a millionth compared to a quite far way clock? And is this attainable on a surface of a very heavy star ?

I suspect the two time dilation factor for velocity and the time dilation factor for gravity would add the same way they add for two velocities,

*****u + v
w = ------------
****1 + uv/c^2

(The asterisks replace spaces)

where u is the dilation factor due to gravity (calculated by substituting the escape velocity for v in the Lorentz Transformation) and v is the velocity of the spaceship. w is the resultant velocity.
Hi Skeptic2
Nice to see you have inspiration for finding new formulas... u needs to have the same dimension as v, and so distance divided by time. Or is u the escape-velocity itself?
greetings Jan