Light Q&A

[dleviwing]

A professor's response to students questions.
IS THE SPEED OF LIGHT CONSTANT?
No. The speed of light is different in different substances. For example the speed of light in water is 3/4 of the speed of light in a vacuum. This is usually expressed as the refractive index of water being 4/3.

OK then, is the speed of light in a vacuum constant?

In some ways that depends on what you mean by constant. There are at least 3 possible meanings to this.

1. Is the speed of light the same in all directions at any one location?
This is what is tested by the MichelsonMorley experiment and the answer is "yes" for inertial frames of reference. For non-inertial frames of reference the answer is generally "no" as demonstrated by the Sagnac experiment.

2. Is the speed of light in vacuum the same at all times?
Yes, because it is defined that way. It is defined to be 299,792,458 m/s. However if it was defined differently it might conceivably vary by ~1 part in 10^10 per year but if it did so it would have to be related to variations in other "constants".

3. Is the speed of light in vacuum the same at all places?
In terms of common conception the answer is no. Light is observed to bend when it travels very close to the sun. Such bending is due to a variation in the speed with distance near massive bodies. However some physicists say that space-time is bent and that light travels at a constant velocity. This leads to situations a bit like Dr Who's telephone box where there is more space on the inside than it looks like there ought to be. However this is just a way of looking at things and it is equally valid to say that the speed of light depends on gravitational potential and that space is Euclidean.

There is another aspect to the non-constancy of the speed of e/m waves which is worth mentioning also. I read once that one of the great scientists (I forget who, sorry, but he was referring to radio waves from an aerial) stated that it actually started at greater than c. For a long time I didn't understand this but now I do and this is the answer. At the point of emission the speed of e/m waves are actually 1.732c (sqrt(3)c) because in the wave equation the time part of the wave is balanced against the 3 spatial dimensions at once. It is only after a wave has traveled multiple wavelengths and can be approximated by a 1D plane wave that it slows down to c. I suspect that this effect is the one that was used in the sending of Mozart (or whoever it was) at faster than c.

DOES LIGHT HAVE MASS?
Much confusion has been caused by the fact that physicists changed the definition of mass that was used by Einstein. Many older books and books for the layman use Einstein's usage however there was a good reason for the change. Let us use the terms relativistic mass for the property which increases with velocity and rest mass for the property which refers to the mass of an object at rest. Then by the term "mass" Einstein meant relativistic mass and modern usage means rest mass.

So when we ask the question "Does light have mass?" then by Einstein's definition it does but by the modern definition it has no meaning because light cannot be at rest. If it were, then it certainly would not have mass.

That might seem to be the end of the matter but there are a couple of fine points still to consider. Firstly, it is possible that light consists of particles traveling at very near the speed of light and that they do have a tiny rest mass. From astronomical observations the maximum that this mass could be is 5*10^-60 grams (or 3*10^-27 ev as physicists like to measure in) and so for all practical purposes it is zero.

If a photon is believed to be a particle traveling at c then it has no rest mass because there is no frame in which its centre of mass is not moving. However if it is seen as a spherical shell of electromagnetic waves then it does indeed have a rest mass which is calculated by E = m c^2 in the rest frame in which it was emitted because that is the centre of mass of the expanding spherical shell.

Another aspect of rest mass that is not always mentioned by physicists is that it contains the relativistic mass of the components. This means that mass (meaning rest mass) is NOT a conserved property. As an example, if we consider the rest mass of the solar system then that is the mass as measured in a frame at rest with respect to the COM (centre of mass) of the solar system. In such a frame the combined mass is the total of the relativistic masses of the bodies in the solar system which may be viewed as being the rest masses plus the mass equivalent of the total kinetic energy. Likewise in an atom the rest mass is the sum of the relativistic masses of the components. This leads to the inevitable question, which won't be answered here, as to whether all mass ultimately derives from dynamic properties.

[subversion]

A reference please?

[dleviwing]

http://van.hep.uiuc.edu/van/qa/qaform.htm
http://arxiv.org/
http://www.scieng.flinders.edu.au/cp...ssphysics.html

[baudrunner]

Dave,

What is light?

I already know that light travels slower in water than in air. Why does it do that?

What is a laser beam?

Are you not contradicting yourself when you describe light as particle, photon perhaps, when you have already broadly asserted your position on the subject of reality and EM waves?

[dleviwing]

Baud;
The Q&A is a cut and past from a forum of students asking physics questions on the University of Illinois web-site. They are not my questions nor are they my answers.

I don't believe you are serious with your questions "What is light?" and "What is a laser beam?" but if you are; simply review your own posts or ask Sub.

[mkirkpatrick]

A reference please?

Chill out bro,you have friends
here you know,just relax,take a load off,chew some gum,and get in touch with your higher self,be cool dude okay.

best all michael.

[Guille]

Dave,

Your starter is a great post. I have another big question:
Is light a wave, a particle, or both, or none (something else)?

[Kismet]

I think light can be treated as both! particle and wave. because it is a eletromagnetic radiation. if you only talk about the visible light it has a certain wave length. And because energy is quantumize E = hv, so the light particle is defined.

[Guille]

I think light can be treated as both! particle and wave. because it is a eletromagnetic radiation. if you only talk about the visible light it has a certain wave length. And because energy is quantumize E = hv, so the light particle is defined.

But it is totally illogical. A particle and a wave are contradictory, it is one or the other, like 'to be or not to be' well, 'to be a particle or to be a wave'. Can't be both... That's why I included the possibility that it is none, we are lying by describing it as both, we won't be able to to describe it as any, therefore it must be something else! But what? I leave every member to think about it.

[subversion]

Chill out bro,you have friends
here you know,just relax,take a load off,chew some gum,and get in touch with your higher self,be cool dude okay.

best all michael.

Thank you michael, but I'm not upset, I was just asking for a reference, that's all.

But it is totally illogical. A particle and a wave are contradictory, it is one or the other, like 'to be or not to be' well, 'to be a particle or to be a wave'. Can't be both... That's why I included the possibility that it is none, we are lying by describing it as both, we won't be able to to describe it as any, therefore it must be something else! But what? I leave every member to think about it.

It is something else, something which is both, or neither, depending on how you look at it. Remember what Godel showed us and do not fear what is consistently contradictory. If you do not fear consistent contradiction, you may find the TOE. That's just some advice from somebody who may know. I will let you consider for yourself what merit is in my words.