# Speed of light question.

## Main Question or Discussion Point

Hey,

When the speed of light is measured, what is it's speed measured relative to? If I sent two speed of light particles in opposite directions, would the one particle be travelling at twice the speed of light, relative to the other one? I have a sneaky feeling this violates a law, which is something like addition of velocities or something like that. This is all a bit misty in my mind, could someone please help me sharpen it up.

_Mayday_

Related Other Physics Topics News on Phys.org
No, one particle would not be travelling faster than the speed of light. That's what special relativity is all about. I know that's not explaining the situation but special relativity is exactly about that fact.

I think that is why the "ether" could not be found. In the Michaelson-Morley experiment no matter in what direction you measured the speed of light, in your case it would be in the opposite direction, the speed of light remained constant. Einstein took that results and then reasoned out the theory of relativity.

rbj
Michaelson measured the speed of light (in terms of the standard of length and standard of time of the day) by measuring the round trip that light took to a mirror on some mountain about 22 miles away. he used a rotating mirroe contraption that, in the short period of time that it took for light to make the round trip, the mirror moved an amount and the return beam reflected off in a different direction than it would have if either the mirror did not move or if lightspeed was infinite. there is an adjustment made by measuring the ratio of the speed of light through air vs. a vacuum and that can be done in a lab with a vacuum chamber.

before 1960, the meter was defined as the distance between scratch marks on a platinum-iridium bar. and the second is (today) defined as a certain number of periods of some Cesium radiation. given those definitions, they had to measure the distance to the mountain in terms of that meter stick and the speed of the rotating mirrors interms of that time standard. so it would have been in terms of that meter and that second that they measured the speed of light against.

today, the meter is defined to be the distance that light travels in 1/299792458th second. so you can't use that definition of a meter to "measure" the speed of light, because of the circularity of that definition. you will always get 299792458 m/s.

dst
He isn't asking what the numerical value of the speed of light is.

He's asking what the velocities of two photons would be if they are going in opposite directions.

Here a third observer is necessary. A third observer will see the speed of both photons as +c and -c respectively.

The photons and indeed all massless particles HAVE NO REFERENCE POINT. That is to say, there is no meaning in asking what the photons will measure as. It does not violate velocity addition because photons do not actually have a frame of reference. You cannot take a photon as a point of reference either since it is always going at c - this violates energy conservation. Suppose you are in one universe with you and a photon; the photon is moving at velocity c towards you. Change your reference point such that YOU are going at c towards a stationary photon - and you have infinite KE from the POV of the photon, which is of course, absurd (or is it!?) according to SR's kinetic energy/mass equation. And that also removes the concept of any absolute reference point for the universe. It's strange that these are intricately linked but I guess that's the way it is.

Shut up and calculate, as they say.

Last edited:
What regulates the speed of light in space? Near a black hole does light slow down, speed up, change directions, or something else?

dst
What regulates the speed of light in space? Near a black hole does light slow down, speed up, change directions, or something else?
It appears to slow down to us but it's actually "changing directions" from what I gather. More specifically, it's following the path of least time which will obviously be in a straight line - the black hole curves the "straight path" but it's still the path of least time.

Is there a 'media' which light travels? If there is, is it manipulated by gravity?

dst
Well $$c^2={\epsilon_0\mu_0}$$ where the two parameters are the permittivity and permeability of "free space" - in other words, how well empty space responds to propagating electric/magnetic fields.

Thanks dst, and everyone else for your time. I can imagine this questions pops up quite often! Thanks again!

Doc Al
Mentor
relative speed

When the speed of light is measured, what is it's speed measured relative to? If I sent two speed of light particles in opposite directions, would the one particle be travelling at twice the speed of light, relative to the other one? I have a sneaky feeling this violates a law, which is something like addition of velocities or something like that. This is all a bit misty in my mind, could someone please help me sharpen it up.
Rather than discuss photons, which have no meaningful reference frame, let's shoot particles (with non-zero mass) in opposite directions at 0.99c with respect to the earth. Using the relativistic addition of velocity formula, the relative speed of one with respect to the other would be about 0.99995 c, still less than the speed of light.

Of course, according to someone on Earth the particles separate at almost twice the speed of light. But that's not their speed relative to each other. (Rindler uses the term mutual velocity for this separation rate.)

He isn't asking what the numerical value of the speed of light is.

He's asking what the velocities of two photons would be if they are going in opposite directions.

Here a third observer is necessary. A third observer will see the speed of both photons as +c and -c respectively.

The photons and indeed all massless particles HAVE NO REFERENCE POINT. That is to say, there is no meaning in asking what the photons will measure as. It does not violate velocity addition because photons do not actually have a frame of reference. You cannot take a photon as a point of reference either since it is always going at c - this violates energy conservation. Suppose you are in one universe with you and a photon; the photon is moving at velocity c towards you. Change your reference point such that YOU are going at c towards a stationary photon - and you have infinite KE from the POV of the photon, which is of course, absurd (or is it!?) according to SR's kinetic energy/mass equation. And that also removes the concept of any absolute reference point for the universe. It's strange that these are intricately linked but I guess that's the way it is.

Shut up and calculate, as they say.
Careful, he asked what would the velocities be relative to each other and furthermore he did not say photons but mentioned particles.

Michaelson measured the speed of light (in terms of the standard of length and standard of time of the day) by measuring the round trip that light took to a mirror on some mountain about 22 miles away. he used a rotating mirroe contraption that, in the short period of time that it took for light to make the round trip, the mirror moved an amount and the return beam reflected off in a different direction than it would have if either the mirror did not move or if lightspeed was infinite. there is an adjustment made by measuring the ratio of the speed of light through air vs. a vacuum and that can be done in a lab with a vacuum chamber.

before 1960, the meter was defined as the distance between scratch marks on a platinum-iridium bar. and the second is (today) defined as a certain number of periods of some Cesium radiation. given those definitions, they had to measure the distance to the mountain in terms of that meter stick and the speed of the rotating mirrors interms of that time standard. so it would have been in terms of that meter and that second that they measured the speed of light against.

today, the meter is defined to be the distance that light travels in 1/299792458th second. so you can't use that definition of a meter to "measure" the speed of light, because of the circularity of that definition. you will always get 299792458 m/s.
Careful, that was not the Michelson-Morley experiment you are referring to but an experiment to measure the speed of light.

dst
Careful, he asked what would the velocities be relative to each other and furthermore he did not say photons but mentioned particles.
Pray tell what "speed of light particles" are if not "photons"? (don't be a smartass and say gluon, graviton or any other type)

I'm pretty sure the OP knows that you can't have mass-ful particles going at the speed of light so by deduction that leaves one possibility.

Pray tell what "speed of light particles" are if not "photons"? (don't be a smartass and say gluon, graviton or any other type)

I'm pretty sure the OP knows that you can't have mass-ful particles going at the speed of light so by deduction that leaves one possibility.
Let's keep with the original point,(as I see it), we are given two particles going at the speed of light, will they be going greater than c relative to each other? answer is "no". As doc al seems to satisfy you in the process of keeping with the overall point, suppose then the particles are going at .99c then the result would be similar. The relative velocity would not be greater than c. Again noting, that is the point of Einstein's theory of special relativity.