# Thread: Clocks, light and gravity

1. The following questions pertain to general relativity:

1) Does a clock within a gravitational field advance at a different rate compared to an identical, relatively stationary clock outside the field?

2) Does light travel more slowly inside a gravitational field than outside, as measured by an observer either inside the field or outside the field?

3) Does light undergo a red shift as it climbs out of a gravitational field?

My natural intuition of all of these questions is nil.

2.

3. Originally Posted by geo_man
1) Does a clock within a gravitational field advance at a different rate compared to an identical, relatively stationary clock outside the field?
Yes.

Originally Posted by geo_man
2) Does light travel more slowly inside a gravitational field than outside, as measured by an observer either inside the field or outside the field?
No. Light will always be observed to move at the speed of light; the wavelength, rather than the velocity, will appear to change depending on the frame of reference.

Originally Posted by geo_man
3) Does light undergo a red shift as it climbs out of a gravitational field?
Yes.

Originally Posted by geo_man
My natural intuition of all of these questions is nil.
I know what you mean, but the effects are observable and fit the predictions of the theory.

4. Originally Posted by geo_man
The following questions pertain to general relativity:

1) Does a clock within a gravitational field advance at a different rate compared to an identical, relatively stationary clock outside the field?

2) Does light travel more slowly inside a gravitational field than outside, as measured by an observer either inside the field or outside the field?

3) Does light undergo a red shift as it climbs out of a gravitational field?

My natural intuition of all of these questions is nil.
1 yes. It ticks more slowly
2 yes, as measured by an observer outside the field. (Within the field time itself slows down therefore EVERYTHING slows down). However, an observer within the field would still measure the local speed of light to be c.

5. Thank you for your replies.

This raises the following questions:

1) In interpreting red shifts of astronomic objects, how can we separate the effects of gravity from those of velocity?

2) When we view highly red-shifted distant galaxies, how do we know the shift we see isn't due to the cumulative effect of the light climbing all the way out of the galaxy?

Galaxies have immense mass, and although gravity may be weak within the majority of its volume, there still is a slight red shift as the light travels farther and farther away.

It is also well known that gravity fields can bend light. How do we know that the entire universe isn't so distorting space-time that we're see the same galaxies multiple times the farther out we look? Maybe one of those galaxies we see in the distance is really our own at an earlier point in time.

6. Some of the redshift of quasars (super massive black holes) may indeed be due to gravitational time dilation.

7. Originally Posted by geo_man

This raises the following questions:

1) In interpreting red shifts of astronomic objects, how can we separate the effects of gravity from those of velocity?
It can be difficult unless we have a good idea of the mass of the object (for stars this can be calculated from there spectrum and luminosity.)
2) When we view highly red-shifted distant galaxies, how do we know the shift we see isn't due to the cumulative effect of the light climbing all the way out of the galaxy?

Galaxies have immense mass, and although gravity may be weak within the majority of its volume, there still is a slight red shift as the light travels farther and farther away.
Just as light red-shifts climbing out of one galaxy, it would blue shift falling into ours, so the effect tends to cancel out. Also, since the light leaving the core of a galaxy would shift more than light leaving the outer edges, if significant red-shifting took place, we would note a difference in red-shift across the galaxy.

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