# Time and gravity

1. Sep 19, 2013

### seb7

Time slows down near gravity (or under acceleration);

so if a 100 seconds pass in space, somewhere wheres there is no (near zero) gravity, how many seconds passed on earth over the period?

ie. whats the percentage difference does earth experience against time that is not effected by gravity?

2. Sep 19, 2013

### Staff: Mentor

First, there is absolutely nowhere in the universe that is unaffected by gravity. Since the range of gravity is infinite, at best you can say that out there in a huge interstellar void the effect is very very small.

The effect of the Earth's gravity on time dilation is very small, but very measurable. The clocks on board GPS satellites tick at a rate of about 40 microseconds faster per day than a clock here on Earth thanks to the lessened effect of gravity at their orbital distance. Way off in an interstellar void there would be an even larger difference, but it would still be very small.

http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html

3. Sep 19, 2013

### tiny-tim

hi seb7!
no, time dilation does not depend on acceleration, only on speed (or gravity)
time dilation due to gravity is approximately 1 - U = 1 - 2gM/rc2 = 1 - 2gr/c2

4. Sep 27, 2013

### seb7

Hi, yes I understand GPS satellites tick faster, but they are hardly out of the earths gravitational field.
So, whats the answer to 1 - U = 1 - 2gM/rc2 = 1 - 2gr/c2 ?
I'm interested to know what, as percentage, is time on earth slowed, compared to a place without gravity.

5. Sep 27, 2013

Staff Emeritus

6. Sep 27, 2013

### Chronos

In fact, we know that gravity in the early universe was 'denser' than it is in the present universe [re: the Permutter supernova study]. Gravitational time dilation is incredibly small near earth due to its 'tiny' mass. In a gigantic, cosmological void, it is virtually negligible.

7. Sep 27, 2013

### seb7

So we have no idea of time without gravity? So that could mean time doesn't exist without gravity.

8. Sep 27, 2013

### Staff: Mentor

Would space exist without gravity? Would either space or time exist without matter? There is no way to answer these questions because the universe has gravity, time, space, etc. What would happen if one didn't exist is unknowable since we do in fact have them.