The measurement of the speed of light?

As I speed-read something about light’s velocity I noticed that they used mirrors to get it travel measurable distances. Is the reflected photon as fast as the original? Will the constant get more accurate when ever LISA Pathfinder starts it’s mission? I may show total ignorance, but shouldn’t the light in Bose-Einstein condensate bend in Earth’s gravity? It’s getting to me!

Locally, photons always travel at c - no exceptions.

Light does bend near a gravitating mass. In fact it bends at exactly twice what Newton’s laws would predict.

Note that under our current measurement system, one cannot really measure the speed of light in vacuum, since the meter is now defined in terms of how far light travels in vacuum in a specific amount of time. You’ll be measuring the distance, instead.

Small correction.
“Under our current measurement systems used to measure locally on Earth.”

we do actually have time dilation tests that use our own sun.

Yes. All photons travel at the exact same speed, c.

No. The speed of c is known, absolutely.
That’s like asking if we could get a more accurate answer to 2+2 than 4.

Yes, gravity can change the direction of light. But not the speed – light doesn’t slow down while bending.

I don’t believe that this is meaningfully true. Currently the meter is defined by the speed of light. So we have switched from measuring the speed of light to measuring the length of a meter. But still perform the same experiment.

Either way you think of the measurement, though, we do already know it really, really well.

That is, of course, true for a local observer, but because time slows near a gravitating mass light will appear to move slower with respect to a faraway observer’s frame.

I’m going to throw a monkey wrench in here and point out that only the round-trip speed of light can be measured. Any system of synchronizing clocks and then moving them apart and back together runs into relativistic effects that mess up the clock’s synchronization. Einstein’s relativity is formulated on light traveling at a single speed in all directions, but that’s axiomatic. It’s also possible to formulate a internally consistent mathematics in which light travels at different speeds toward and away from the observer, as long as the average speed over the round trip is still c. Such a model exchanges the relativistic weirdnesses that we have come to accept due to long exposure with another set of relativistic weirdnesses that are unfamiliar.

So, the average speed of a photon over a round trip is c, and we all chose to believe that it is c on the way out and c on the way back because that is intuitive and makes the math easier in many respects. But not because we can measure its speed on a one-way trip.

Eh? A symmetric setup would do this just fine. Say I have a device that is a precise chronometer, a light source, and a light detector. It records the precise (local) time that a light pulse is emitted. Separately, it records the precise (local) time that when any light pulse is detected.

Now say I have two of these. Synchronize them at location 0, and move each (arbitrarily slowly if you’d prefer) a distance D away, to locations +D and -D. Each one sends photons to the other, recording the emission times and detection times. At the end of the day, compare notes to determine the speed of light in either direction.

As I understand it, in isotropic relativity you can show that clocks that are separated and brought back together are still synchronized when moved slowly (in the limit of zero velocity), but you can’t prove that they stayed synchronized at every point along the loop. The ‘drift’ of one clock can be computed, but only if you already assume a particular one-way value for the speed of light (which you’re trying to measure). Assuming that the clocks are synchronized along the whole loop is equivalent to assuming that the speed of light is the same in all directions, which means that you have already defined the one-way speed of light as c.

When reflecting do photons stop and then gain instantly the speed of light or how do they keep up the speed?

There is no time at which the photon is not moving. You could argue, though, that there’s a short time during which the photon does not exist.

OK - is there any possibility that the reflected photon isn’t the original one? How can you observe the phenomena?

What do you mean by “the original one”? Particles don’t have serial numbers on them.

Interesting. Lots of stuff lose kinetic energy when colliding with another mass. Photons don’t?

When light goes through a medium that is not a vacuum, and it’s speed is substantially slower, is it still called “c”? or is it not the speed of light anymore because the medium is messing with it?

Two different things unfortunately called by the same name. The local speed of light is c and the constant that represents the speed of photons in a vacuum is also c. If you think of the latter as C or Einstein’s constant it helps to keep them separate.

How long? (Or, if you prefer; how short?)


When a photon collides with an object, the photo “disappears” and the energy is transferred to the object. Generally speaking, when a photon collides with an object and is reflected, the photon is absorbed and then reemitted. Usually an electron is bumped up to a higher energy by the colliding photon, and then releases a photon as it falls back to a lower state.