Why is the Speed of Light Constant?

[Applemush]

I've always been terribly confused as to the logic behind why the speed of light is a constant. Say for a moment that you have a photon that's traveling along a radio wave at the speed of light. Then you have another photon traveling along a gamma wave at the speed of light. Now, assuming that they both have to travel the same distance under the same conditions, and started at the same time, why will they arrive at the same time. I always thought that the photon traveling along the radio wave would arrive first. This is because the periods on the radio wave are shorter, and the wave itself is "straighter". Wouldn't this mean that the radio wave is a more direct path-which implies less distance. And if that were true then the photon that was on the radio wave would arrive first? Please understand that I have an extremely limited background in physics, and no understanding of any of the theories on light. This is just a question that I emailed to my school chemistry teacher (we don't have a physics one) and they couldn't answer.

-Sincerely
Applemush

 

[Integral]

Photons do not follow a wave like path. One way to visualize a photon is as a wave packet. A envelope which contains the waves composing the frequency of the photon. So the photon travels straight while oscillations occur at the frequency determined by the energy of the photon.

 

[WarPhalange]

Do louder noises travel faster than quieter ones? Nope. And yet they have a bigger amplitude in their "wave".

A wave is just something that has an oscillating amplitude basically. Look up "wave particle duality".

 

[mgb_phys]

As integral said photons are not like waves on a string.

A usefull picture is a flashing light on a police car, suppose the blue light flashes twice per second as the car comes toward you, alongside is a fire engine with a red light flashing once per second - but they are both going at the same speed.
Now imagine painting a mark on the ground next to the car each time the light flashes, the distance between the marks is the wavelength. The spacing depends on the frequency and speed, the gaps between the fire engine's marks would be twice as large. The frequency is half as much and it's wavelength is twice as long.

 

[Ignea_unda]

Wow, that is an ingenious way to describe it, mgb_phys!

 

[Norman Graves]

Photons do not follow a wave like path. One way to visualize a photon is as a wave packet. A envelope which contains the waves composing the frequency of the photon. So the photon travels straight while oscillations occur at the frequency determined by the energy of the photon.

Perhaps you could elaborate on the word packet. Is this a substantive packet, ie does it have material form. If so how can it not have mass. If not then what exactly is the means by which energy is transmitted? What is it that is oscillating?

 

[Defennder]

It's a little hard to explain here. You might want to pick up an intro freshman physics textbook for a clearer explanation.

 

[Norman Graves]

The problem is that the freshmen physics textbooks do not really address the question.
Either the photon must have substantial form, in which case how to explain that it has no mass, or the environment in which it is moving is the cause of its existence, but this is to re-invent the ether by another name.

 

[DaveC426913]

The problem is that the freshmen physics textbooks do not really address the question.
Either the photon must have substantial form, in which case how to explain that it has no mass, or the environment in which it is moving is the cause of its existence, but this is to re-invent the ether by another name.

Why are these the only two options you allow?

Energy is (rest)massless, yet it travels through vacuum.

 

[Norman Graves]

Why are these the only two options you allow?

Energy is (rest)massless, yet it travels through vacuum.

The question is how does it do so?

 

[Landru]

I've been looking up "photon frequency" on Google and what I've come up with is that the frequency of light is really the vibrating of the photon, that this vibrating is a form of energy, the measured wavelength of light is really a measure of how quickly the photon is vibrating, and that degree of this vibration is determined by whatever is emitting the photons.

Is this right? If so, is the vibration of the photon the same phenomena as the vibrations of a heated atom?

 

[russ_watters]

No. Photons are not like normal particles. They can't be viewed as descrete particles that are vibrating. But they have a frequency/wavelength.

 

[Norman Graves]

So what is vibrating ?

 

[HallsofIvy]

So what is vibrating ?

No. Photons are not like normal particles. They can't be viewed as descrete particles that are vibrating. But they have a frequency/wavelength.

"Something vibrating" is a very simplistic "macro" concept of waves. Light (more generally, electro-magnetic fields) satisfies the wave equation. That is sufficient.


But, russ, according to quantum theory, not just photons but any sufficiently small "particles" have wave properties.

 

[russ_watters]

I realize that -- though isn't it any particle of any size has wave properties?

 

[Norman Graves]

That tells me what photons are not.

My question is what are photons?

It comes back to the question: what is vibrating, is it the photon that has substance or is it the environment?

The former poses the problem of how then the photon has zero mass, and the latter faces the problem that it is the ether by another name - and the ether has been shown not to exist.

 

[cryptic]

My question is what are photons?

I think photons are vibrating "virtual" electron-positron pairs because of the fact that high energy photons decay in an electron and a positron, if passing strong electric field, for example highly charged atomic nucleus. Vice versa electron and positron convert into photons.

 

[DaveC426913]

My question is what are photons?

They cannot be described in natural, human-scale terms. We do not have macroscopic counterparts to phenomena at the atomic, subatomic or photonic scale.

We will find that the only meaningful answer to your question is the mathematical equation that describes it. Any attempt to convert it to more comfortable words will, by definition, cause some degree of loss in the translation from equation to descriptive "model".

 

[navneet023]

I've heard that Einstein stated that nothing can exceed light in terms of speed as it will require infinite energy to further increase the speed once the speed of light is achieved.My doubt is originating from the fact that light itself is made up of particles called photons(which i assume have mass...no matter how small)...so how come these particles called photons travel at such a high speed...nd why there can't be any other particle wid mass a millionth fraction of these photons and speed more than light...?

 

[navneet023]

I think photons are vibrating "virtual" electron-positron pairs because of the fact that high energy photons decay in an electron and a positron, if passing strong electric field, for example highly charged atomic nucleus. Vice versa electron and positron convert into photons.

but won't that mean ki photon photons have mass...a finite mass...?

 

[Varnick]

Photons have 0 mass, and thus can move at C. Anything with mass cannot.

 

[navneet023]

Photons have 0 mass, and thus can move at C. Anything with mass cannot.

sorry friend...but i don't agree with ur fact that photons have zero mass...nothing is zero...not even vacuum...so photons can't hav zero mass...

 

[Varnick]

Photons do have 0 mass, this also (to my knowledge) is what allows for the infinite range of EM fields. http://en.wikipedia.org/wiki/Photon

 

[Norman Graves]

What do you mean by virtual electrons and virtual positrons. How do these differ from real ones? Do they have substance? Do they have mass? Do they have momentum?

 

[schroder]

Photons have 0 mass, and thus can move at C. Anything with mass cannot.

It has always struck me as a bit of a circular argument, or play on words, to say that a photon has zero mass. It is far more correct to say that it has zero rest mass. Theoretically, it has no mass when at rest, but when is a photon ever at rest? By its very nature as a “particle” of pure energy it is always moving at the speed of light (in a vacuum) and under those conditions it does have relativistic mass and it has momentum.
It is similar to saying that a pure mass at absolute rest has no kinetic energy. Perhaps we should say that a photon possesses “kinetic mass”?

 

[navneet023]

It has always struck me as a bit of a circular argument, or play on words, to say that a photon has zero mass. It is far more correct to say that it has zero rest mass. Theoretically, it has no mass when at rest, but when is a photon ever at rest? By its very nature as a “particle” of pure energy it is always moving at the speed of light (in a vacuum) and under those conditions it does have relativistic mass and it has momentum.
It is similar to saying that a pure mass at absolute rest has no kinetic energy. Perhaps we should say that a photon possesses “kinetic mass”?

bingo...i was trying to think of the same thing..but this rest mass nd kinetic mass thing...i was not getting to it...now thjat u have stated...i'd like to ask...a moving photon has some mass... m=m(initial)/sqrt(1-v^2/c^2)

so does it mean ki the mass of a moving photon is also zero??

m not sure if m correct wid the eqn..but i think its similar to that wer the relativistic mass depends on initial(rest ) mass...

 

[HallsofIvy]

I think photons are vibrating "virtual" electron-positron pairs because of the fact that high energy photons decay in an electron and a positron, if passing strong electric field, for example highly charged atomic nucleus. Vice versa electron and positron convert into photons.

I've heard that Einstein stated that nothing can exceed light in terms of speed as it will require infinite energy to further increase the speed once the speed of light is achieved.My doubt is originating from the fact that light itself is made up of particles called photons(which i assume have mass...no matter how small)...so how come these particles called photons travel at such a high speed...nd why there can't be any other particle wid mass a millionth fraction of these photons and speed more than light...?

sorry friend...but i don't agree with ur fact that photons have zero mass...nothing is zero...not even vacuum...so photons can't hav zero mass...

You have both made assertions, that photons are "electron-positron pairs" and "photon have non-zero mass" without citing any evidence. Since there exist a large body of experimental evidence that photons have no mass and so cannot be "electron-positron pairs", what experimental evidence can you cite for your positions?

 

[DrGreg]

bingo...i was trying to think of the same thing..but this rest mass nd kinetic mass thing...i was not getting to it...now thjat u have stated...i'd like to ask...a moving photon has some mass... m=m(initial)/sqrt(1-v^2/c^2)

so does it mean ki the mass of a moving photon is also zero??

m not sure if m correct wid the eqn..but i think its similar to that wer the relativistic mass depends on initial(rest ) mass...

In relativity there are a number of different definitions of mass. In particular,

- rest mass = invariant mass = proper mass which excludes kinetic energy

- relativistic mass which includes kinetic energy

Most modern physicists mean invariant mass when they say "mass". But there are some people who mean relativistic mass when they say "mass".

A photon has zero invariant mass but non-zero relativistic mass.

The equation

$$m_{rel} = \frac {m_{inv}} {\sqrt{1 - v^2 / c^2}}$$​

is valid only for objects moving slower than light. For photons both the top and bottom of the fraction would be zero which makes it meaningless.

An equation that is true for all particles, including photons, is

$$m_{rel}^2 = m_{inv}^2 + p^2 / c^2$$​

where p is momentum.

 

[malawi_glenn]

sorry friend...but i don't agree with ur fact that photons have zero mass...nothing is zero...not even vacuum...so photons can't hav zero mass...

So according to your logic, nothing can have zero electric charge either?

 

[Norman Graves]

I have only found one serious attempt to compare the speed of light at different frequencies. (Lovell B, et al (1964) Relative Velocity of Light and Radio Waves In Space Nature 202, 377 (25th April 1964)

The results were consistent with light traveling slower than radio waves, but the differences were within the range of experimental error, so the result was inconclusive.

It would make a very interesting experiment were someone to try it again over a wider frequency range.

 

[Varnick]

Varying the frequency of light will alter its wavelength, not its speed. In the special case of electromagnetic waves moving through a vacuum, then v = c0 , where c0 is the speed of light in a vacuum, and this expression becomes:

$$f = \frac{c_o}{\lambda}$$

 

[navneet023]

So according to your logic, nothing can have zero electric charge either?

yes...electric charge can't be zero...how can u say its zero or not..we say smthng is not there just coz we r not able to detect it...its zero with respect to us...our calculation system...nd not even zero...so small that we can't get any instrument so as to measure it...

 

[Ich]

I have only found one serious attempt to compare the speed of light at different frequencies.

There's a sticky " FAQ: Experimental Basis of Special Relativity". And it's there for a reason.

 

[navneet023]

In relativity there are a number of different definitions of mass. In particular,

- rest mass = invariant mass = proper mass which excludes kinetic energy

- relativistic mass which includes kinetic energy

Most modern physicists mean invariant mass when they say "mass". But there are some people who mean relativistic mass when they say "mass".

A photon has zero invariant mass but non-zero relativistic mass.

The equation

$$m_{rel} = \frac {m_{inv}} {\sqrt{1 - v^2 / c^2}}$$​

is valid only for objects moving slower than light. For photons both the top and bottom of the fraction would be zero which makes it meaningless.

An equation that is true for all particles, including photons, is

$$m_{rel}^2 = m_{inv}^2 + p^2 / c^2$$​

where p is momentum.

even then...since the invariant mass of a photon is zero..wont it lead to smthng like m(rel)=p/c?

and how r we supposed to calculate this momentum, p?

 

[Norman Graves]

Why are these the only two options you allow?

Energy is (rest)massless, yet it travels through vacuum.

Michelson and Morely proved that the ether does not exist. What this shows is that space is empty - it contains nothing. If space contains nothing and photon exist in space then the photon must be something - here are no other values in the set.