# Time Dilation & Light

1. Jan 10, 2009

### Denton

Sources of great mass slow time down relative to an outside observer, so why is it that light itself is not slowed down, not in velocity but simply due to the fact that time has slowed down near the mass?

2. Jan 10, 2009

### Naty1

good question, no good answer.
One of the many great mysteries of physics! Why should mass curve space??
You could say that atomic structure requires stable EM fields...but then why did they evolve that way? either "somebody" planned it just right or there are an infinite number of combinations....and only those universes which "work" are able to evolve...and perhaps give birth to others....

3. Jan 10, 2009

### A.T.

It is!
Exactly like this. The distant observer will measure the light near the mass to travel at less than c, using his own fast clock. It will appear "slowed down" to him. But the local observer near the mass with a slow clock will measure the light passing him at c.

4. Jan 11, 2009

### LucasJ

Is that right? I thought one of the core ideas behind relativity was the constancy of the speed of light, meaning that no matter where you are at in the universe, you would always measure the speed of light (in a vacuum) to be 3x10^8 meters/second. Is there something I've misunderstood?

5. Jan 11, 2009

### atyy

Anything wrong with this handwaving?

c=f.l (speed = frequency.wavelength)

Time is related to frequency. If time slows, frequency decreases, but if wavelength also increases, speed can stay the same.

6. Jan 11, 2009

### A.T.

That's correct for light near you. But the OP is asking about an outside observer measuring the speed of light in a gravity well.

Not only frequency is affected. If you send a light signal from outer space to a mirror stationary to you in a gravity well, it will return with the same frequency. But it still will need longer then if there was no gravity. So it will appear to you, that it was slowed down.

Last edited: Jan 11, 2009
7. Jan 11, 2009

### Naty1

I disagree witht he first part of post #3...it's potentially rather confusing....but it's dependent on context....part two properly explains the way to think about lightspeed observations: local observers see light at "c"; distant observers see things differently depending on their reference frame.

8. Jan 13, 2009

### Denton

Ah yes, speed remains constant and its constituents change to stabilise it. Thus we see red shifting occurring at high gravitational potentials.

Then comes to the other question. What is so special about the velocity of light that it must remain constant, and that its frequencies and wavelengths are but insignificant.

9. Jan 13, 2009

### Naty1

Exactly!!! and also enigmatic: why should gravitational potential (a) curve space and (b) change time?? We have descriptions to explain what happens, but not WHY!!...did it have to be this way?? Might other universes have different relationships??

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