# Is light slower when traveling inside a gravity field?

This question is not about phase velocity changed which causes refraction, but about the real time itself being slower by the gravity of any object (from general relativity).

If so, would this mean any light traveling inside a glass would be slower than in the air because it is directly affected by gravity of glass? Does this effect also contribute to refraction? How much this effect cause the light to slow down?

This is a more complicated question that you probably realise.

This first point to make is that the speed of light is always locally $c$, that is, if you measure the speed of light at your location you will always get the result $c$. The problem comes when you measure the speed of light at some location distant from you.

To measure the speed of light locally we use a coordinate system and measure time and position to locate spacetime points $(t, x, y, z)$. Locally the coordinate system is just the good old flat space coordinates as plotted on graph paper by generations of school children studying physics, and we determine the speed of light or anything else by measuring $dx/dt$ (assuming the object is moving in the $x$ direction). The problem comes when we extend our coordinate system, for example near to the event horizon of a black hole. If we monitor a light ray heading towards a black hole then our coordinates won't match the coordinates of another scientist hovering near the event horizon. That means if we calculate $dr/dt$ to get the light speed we will get a different (slower) value than the other scientist.

But does this mean the speed of light is really less than $c$? I think it depends on what you mean by the word really. For example suppose we give that other scientist a mirror and measure the time for the light to be reflected back to us. If we divide twice the distance to the other scientist by the time then we'll get a velocity of less than $c$. But it could be argued that this is because the distance to the other scientist is actually greater than we think it is, so the light travelled farther than we think and it is still travelling at $c$. There is some discussion of this in The bigger the mass, the more time slows down. Why is this?.

Which is all very well, but doesn't answer your question. I think most physicists wouldn't pay to much attention to anything that is coordinate dependant, and wouldn't regard the $dx/dt$ calculated using your coordinates as especially physically significant.

• The reason that the local speed of light is $c$ is because one meter is defined as the distance traveled by light in vacuum during a time interval of 1/299,792,458 of one second. So you DON'T (can't) measure the speed of light locally. You assume speed of light to be a CONSTANT and use this speed to define length.
– D-K
Dec 12 '13 at 12:14
• @D-K I'm not sure I understand this: sure there's an assumption but I think John and the OP are assuming GR holds, in which case it is motivated by John's paragraph 2 - the tangent spaces to any spacetime manifold are Minkowskian. Practically speaking, you don't need much length anymore to measure the speed of light, so practical violations of John's second paragraph are only going to happen in very high curvatures. Even then, we talk in principle: wouldn't we'd simply imagine being very small creatures, small enough such that their space were accurately modelled as a tangent space? Dec 12 '13 at 12:40
• @D-K I am, of course, assuming freefalling little creatures. Dec 12 '13 at 13:07
• Maybe there are some misunderstanding here. Yeah, in the presence of gravity, speed of light is constant if the frame is small enough and freelly falling and could be smaller than c if not. My understanding is that, $c$ is not only speed of light, but also part of the nature of spacetime, as $c$ appears in the field equation. In this sense, speed of light, or $c$ is always a constant. So my understanding to the original question is whether this $c$ is a constant or not. In GR, of course it is. Perhaps in GR we are not allowed to ask this question.
– D-K
Dec 12 '13 at 14:29
• What I really mean by the constancy of speed of light is related to the frequency of light and pertinent to quantum gravity theories, for which people haven't got a well-accepted one and GR needs to be modified. See this article： symmetrymagazine.org/breaking/2009/02/19/… Now I think I was off topic.
– D-K
Dec 12 '13 at 14:30

There is no experimental evidence on whether light travels slower in a gravity field. Some quantum gravity theories require light to be slower in an intensive gravitational field while others not so. So, it is to be determined by experiments or astronomical observations.

Light travels in glass as fast as in vacuum. Because microscopically, glass is nothing but huge amount of electrons and nuclei in vacuum. The reason that its measured speed is slower is a result of interactions between photons and the material.

• There is no experimental evidence on whether light travels slower in a gravity field - I suspect many will disagree with you on this. Dec 12 '13 at 8:24
• Doesn't gravitational lensing pretty much imply light is being slowed in the gravitational field? Dec 12 '13 at 8:26
• @Brandon Enright Gravitational lensing does change the propagation direction of light, but not the speed of light.
– D-K
Dec 12 '13 at 8:40
• The Shapiro delay is a well-established phenomenon. en.wikipedia.org/wiki/Shapiro_delay Oct 4 '15 at 17:56

The following articles may be helpful. This is actually an active scientific topic. http://www.symmetrymagazine.org/breaking/2009/02/19/most-extreme-gamma-ray-blast-also-probes-quantum-gravity

http://www.sciencemag.org/content/323/5922/1688.abstract

http://www.sciencemag.org/content/early/2013/11/20/science.1242353.abstract

• These seem to be about attempts to measure dispersion rather than the speed of light. Dec 12 '13 at 11:08