# Clocks, light and gravity

• September 15th, 2010, 10:21 AM
geo_man
Clocks, light and gravity
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.
• September 15th, 2010, 11:18 AM
drowsy turtle
Re: Clocks, light and gravity
Quote:

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.

Quote:

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.

Quote:

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

Yes.

Quote:

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.
• September 15th, 2010, 11:19 AM
granpa
Re: Clocks, light and gravity
Quote:

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.
• September 15th, 2010, 12:36 PM
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?

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.
• September 15th, 2010, 12:46 PM
granpa
Some of the redshift of quasars (super massive black holes) may indeed be due to gravitational time dilation.
• September 15th, 2010, 01:19 PM
Janus
Quote:

Originally Posted by geo_man