# Ignorant Question

1. Jun 1, 2004

### OneEye

All right. So, given that the speed of light varies in a gravitational field,

Why, then, does the Michelson-Morley experiment not show a variation in the speed of light?

I mean, we are in a gravitational field. Right?

2. Jun 1, 2004

### chroot

Staff Emeritus
The speed of light never differs locally from c. If light passes through some measurement apparatus, that apparatus will always say it's going c. Let's say you're in a space station, significantly outside some object's gravitational field. You can infer via time dilation that the light deep in the gravitational field is travelling more slowly than light out of that field, but you cannot measure it directly. If you put a measurement apparatus down in the field, it'll measure the light going c there, too. Of course, you'll measure the apparatus as running slowly!

- Warren

3. Jun 1, 2004

### franznietzsche

If you are removed from a gravitational field you will view an object within the field as being affected by time dilation. This however would not lead to the observation that lgiht was moving more slowly because the distances are stretched so that the light takes more time (on your clock) to cover a greater distance (you measurments) giving c. The person inside the field measures a smaller distance and a smaller time, still getting c.

4. Jun 1, 2004

### chroot

Staff Emeritus
Isn't that exactly what I just said? You can infer that the speed of light goes slower than c inside a gravitational field, but you cannot ever measure it directly.

- Warren

5. Jun 1, 2004

### franznietzsche

you can't infer that. You would be wrong.

you can measure it by sending a signal in to someone inside the field, them replying (say with a mirror) and you then use the total time measured by you, and the equations of general relativity to calculate the spacetime interval transversed by the signal from which you can calculate the speed, and you will get c.

6. Jun 1, 2004

### chroot

Staff Emeritus
Sorry, I'm not wrong. Can you show me your math?

- Warren

7. Jun 1, 2004

### chroot

Staff Emeritus
And I hope you're aware that the interval is invariant! :rofl:

- Warren

8. Jun 1, 2004

### robphy

Another way to say this is that
"at every event in spacetime, there is a light cone."

9. Jun 2, 2004

### OneEye

I guess what I'm thinking is this: Imagine that we had an apparatus which extended from Mercury to Mars. Would we not be able to measure the deformation in space-time caused by the various gravitic fields involved, and thus come up with a direct measurement of the variation in c which is caused by gravitic effects? I understand that such deformations will be shared by the measruing device, but if the device is long enough to bridge several such deformations, then should we not be able to directly measure thse deformations?

(Sorry about my plebeian terminology here. Please, if you can, tolerate the sci-fi language and decode it into something sensible.)

10. Jun 2, 2004

### honestrosewater

How often (in time or distance) would you measure the light (to track its position)? Would you only measure it at beginning and end, or several times along the path?

EDIT- Here is a link I found that seems to address your question quite nicely, if I indeed understand the root of your question. The explanation is short and nontechnical ;)
http://www.hitxp.com/phy/rel/gr/261102.htm

The key thing to understand is the difference between inertial and accelerated frames. And why no global inertial frames exist in GR, only local inertial frames.

BTW I hope I got that right It's been a while, and I'm no expert.

Okay, this is the 3rd and last time I will edit this post- promise. The reason I think the problem lies in an understanding of accelerated and inertial frames is because your idea of making the experiment larger suggests to me that you want to move towards a more "global" view. Perhaps I am mistaken.

Last edited: Jun 2, 2004
11. Jun 2, 2004

### honestrosewater

UhOh, I hope math is only required of challengers :uhh:

Last edited: Jun 2, 2004
12. Jun 2, 2004

### loseyourname

Staff Emeritus
What happens in a singularity, such as a black hole? There is no movement, so it would seem to be fallacious to speak of light as having any velocity. However, if you attempted to calculate the velocity, given zero passage of time, you would get an indeterminate answer.

13. Jun 4, 2004

### turin

There are in fact reflectors that have been placed on the moon. What do you suppose they are for? The surface of the moon is at a stark difference in gravitational potential to the surface of the earth.

14. Jun 5, 2004

### OneEye

Bon. But you neglected to post the most important piece of information: What have these reflectors told us?

15. Jun 5, 2004

### Creator

The answer to your question is YES, OneEye. And this measurement has already been done several times. Shapiro was the first to detect it (in 1970's?) bouncing radar beams off Venus (when it was in solar opposition), and likewise signals have been bounced off Mars, and also from the Voyager spacecrafts. All passed EM signals through the the solar gravitational field (planetary fields are far to small for detection) and all show a time delay that is consistent with Gen. Relativity (usually in tens of microseconds). This effect is typically referred to as the 'Shapiro time delay' and is well known in scientific literature and Nasa in general.

Creator

Last edited: Jun 5, 2004
16. Jun 5, 2004

### OneEye

Excellent. Thanks for the info. How, exactly, this is to be understood - the fact that c, in a vacuum, is inconstant over large distances - is a different question. I appreciate how Dr. Einstein would explain this phenomenon. But I have long suspected the Michelson-Morley experiment as being too narrow in scope to really answer the broader questions.

I don't mean to start anything. I'm happy with the information you've given. But it's certainly provocative in more directions than strict orthodoxy would allow.

I must say though, as a postscript, that I am surprised at those correspondents who took a doctrinaire position in response to my question. I won't mention names (too lazy to look it up), but at least one person said that, No, no such experiments could not possibly yield such results, since relativity did not allow the observation of variations of c in a vaccuum.

I am surprised that anyone was willing to answer so hastily. I had hoped for better.

Last edited by a moderator: Jun 5, 2004
17. Jun 5, 2004

### turin

I was hoping that would send you on a little search for more information about it (I'm lazy, too ). Anyway, AFAIK, they have been used to determine a precise distance to the moon. I don't know any more detail than that.

18. Jun 9, 2004

### Mark

Ok, please excuse this question if it is pathetic. But I thought light slows down when it changes to more dense surfaces. But then I was told that $c$ was constant...

Second, If light stops in a black hole, does it not need to decelerate to stop - or to die for say, wouldn't that involve light changing speeds

I mean, I understand that $c$ is constant, but can time dilation explain these two concepts?

19. Jun 9, 2004

### selfAdjoint

Staff Emeritus
c is a constant in relativity. Light in a vacuum travels at c. Light travelling in other media is interacting with the matter in the media, and the time for these interactions has to be added to its transit time, thus it takes longer to pass through.

Light does not stop in a black hole; it falls in. Once in. it can't get out.

20. Jun 10, 2004

### OneEye

But should it not be, "c is constant in Special Relativity, but varies with acceleration in General Relativity?"

I don't mean to quibble - perhaps I am just not getting the terminology right.

And I had thought that light never crossed the even horizon of the black hole, because the curvature of space was such that the "escape velocity" over the event horizon was greater than c.

Or is that just a science fiction version of reality?

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