Explanation required: photons

[sr.vinay]

I've a few elementary doubts on the concept (let's call it that) of photons.

First of all, we say the 'rest' mass of photons is zero, when actually photons don't exist at 'rest'.

And, in another thread, one of them had pointed out that the mass to travel at the speed of light must be identically zero. Would that mean that the mass of photons is zero even when they're travelling at 'c'?

Do we define a boundary for photons? As in, there is a certain boundary for each photon, outside which the energy level is minimal?

[Sisyphus]

It's probably best just to say that photons do not have mass, period.

[sr.vinay]

One of their properties is energy. The other, momentum. Momentum without mass. Energy without a definite boundary. Doesn't add up, don't you think?

[Sisyphus]

What about it doesn't seem to add up?

[sr.vinay]

The momentum without mass. And, how does 'energy' travel through 'vacuum'?

[swansont]

One of their properties is energy. The other, momentum. Momentum without mass. Energy without a definite boundary. Doesn't add up, don't you think?

 

Classically, perhaps not. But this was a concept that was present at the dawn of both relativity and quantum mechanics. Welcome to the 20th century.

[mannzzu]

The momentum without mass. And, how does 'energy' travel through 'vacuum'?

what do you feel vaccum is here

[sr.vinay]

Well, could you welcome me to the 21st century by elaborating on the modern concept of a photon?

[mannzzu]

by vinay"The momentum without mass. And, how does 'energy' travel through 'vacuum'?"

 

 

vacuum is set of particles which have all properties like charge, spin,field etc>but it so happens that they cancel out!! (funny!isn't it? i don't know why?)

so they can travel in vacuum

[Mr Skeptic]

Don't all waves carry momentum?

[ercdndrs]

Yeah, light has momentum, I saw somewhere an experiment where a very powerful laser was shone onto a piece of foil; the foil was blown back like if instead of light, wind was coming out of the laser (to explain what it looked like). I can't find a specific site for this, though, so you'll just have to take my word for it.

 

If wikipedia is to be believed, photons have a constant momentum. If a material absorbs a photon, the photon's momentum is transferred to the material. If a different material reflecs a photon, 2x it's momentum is transferred to the material (momentum is conserved)

 

To quote Wikipedia: "Light (visible, UV, radio) is an electromagnetic wave and also has momentum. Even though photons (the particle aspect of light) have no mass, they still carry momentum. This leads to applications such as the solar sail. The calculation of the momentum of light is controversial (see Abraham–Minkowski controversy)."

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Also, a photon has energy; if a photon was somehow stopped one would think it's energy would be converted to mass

[swansont]

To quote Wikipedia: "Light (visible, UV, radio) is an electromagnetic wave and also has momentum. Even though photons (the particle aspect of light) have no mass, they still carry momentum. This leads to applications such as the solar sail. The calculation of the momentum of light is controversial (see Abraham–Minkowski controversy)."

 

The Abraham–Minkowski controversy is a very specific topic, and the controversy is not about whether light has momentum or the calculation of that momentum in free space.

[sr.vinay]

Also, a photon has energy; if a photon was somehow stopped one would think it's energy would be converted to mass

 

I had asked this doubt somewhere else. The answers I received weren't exactly satisfying.

Can you perhaps elaborate?

[toastywombel]

I had asked this doubt somewhere else. The answers I received weren't exactly satisfying.

Can you perhaps elaborate?

 

The energy and momentum of a photon depend only on its frequency (ν) or equivalently, its wavelength (λ):

where k is the wave vector (where the wave number k = |k| = 2π/λ), Ω = 2πν is the angular frequency, and ħ = h/2π is the reduced Planck constant.[13]

Since p points in the direction of the photon's propagation, the magnitude of the momentum is

 

Even if you cannot understand all the math, the only thing you need to know is that a photon's momentum is not dependent on its mass, which is zero.

 

We are past Newtonian Physics when it comes to this stuff.

 

http://en.wikipedia.org/wiki/Photon

 

"a single photon always has momentum (since it is determined, as we have seen, only by the photon's frequency or wavelength - which cannot be zero)."

[swansont]

I had asked this doubt somewhere else. The answers I received weren't exactly satisfying.

Can you perhaps elaborate?

 

If you "stop a photon," you don't have a photon anymore — the notion is contradictory. Its energy has been deposited somewhere, and that would result in an increase in mass.

[toastywombel]

If you "stop a photon," you don't have a photon anymore — the notion is contradictory. Its energy has been deposited somewhere, and that would result in an increase in mass.

 

Well, stopping a photon, would mean that it is stopped relative to the observer, so you could have an photon that is at rest, relative to you if you were able to travel at c, but then of course, you would have to be a photon to do that. So if a photon is at rest, relative to you, either the photon is now matter or you or now a photon.

[Sisyphus]

Well, stopping a photon, would mean that it is stopped relative to the observer, so you could have an photon that is at rest, relative to you if you were able to travel at c, but then of course, you would have to be a photon to do that. So if a photon is at rest, relative to you, either the photon is now matter or you or now a photon.

 

It doesn't really make sense to talk about the reference frame of a photon. It wouldn't be at rest relative to anything, even other photons, or for that matter even itself. When people say it's not a valid frame, they don't just mean it's physically impossible, but that it's nonsensical.

[toastywombel]

It doesn't really make sense to talk about the reference frame of a photon. It wouldn't be at rest relative to anything, even other photons, or for that matter even itself. When people say it's not a valid frame, they don't just mean it's physically impossible, but that it's nonsensical.

 

Well true, because observation and reference frames depend on light, but I was really just emphasizing the point that a photon cannot be at rest.

[Mr Skeptic]

Well true, because observation and reference frames depend on light, but I was really just emphasizing the point that a photon cannot be at rest.

 

What if you have a standing wave?

[ercdndrs]

Also, I'm pretty sure light has the same speed in any frame of reference

[PaulS1950]

To say a photon is zero mass disagrees? with this, from wikipedia.

I do not trust wiki as an absolute reference but I would like to know what the experts think of this:

 

Experimental checks on photon mass

The photon is currently believed to be strictly massless, but this is an experimental question. If the photon is not a strictly massless particle, it would not move at the exact speed of light in vacuum, c. Its speed would be lower and depend on its frequency. Relativity would be unaffected by this; the so-called speed of light, c, would then not be the actual speed at which light moves, but a constant of nature which is the maximum speed that any object could theoretically attain in space-time.[18] Thus, it would still be the speed of space-time ripples (gravitational waves and gravitons), but it would not be the speed of photons.

 

A massive photon would have other effects as well. Coulomb's law would be modified and the electromagnetic field would have an extra physical degree of freedom. These effects yield more sensitive experimental probes of the photon mass than the frequency dependence of the speed of light. If Coulomb's law is not exactly valid, then that would cause the presence of an electric field inside a hollow conductor when it is subjected to an external electric field. This thus allows one to test Coulomb's law to very high precision.[19] A null result of such an experiment has set a limit of eV.[20]

 

Sharper upper limits have been obtained in experiments designed to detect effects caused by the Galactic vector potential. Although the galactic vector potential is very large because the galactic magnetic field exists on very long length scales, only the magnetic field is observable if the photon is massless. In case of a massive photon, the mass term would affect the galactic plasma. The fact that no such effects are seen implies an upper bound on the photon mass of eV.[21] The galactic vector potential can also be probed directly by measuring the torque exerted on a magnetized ring.[22]. Such methods were used to obtain the sharper upper limit of 10−18eV. given by the Particle Data Group[23]

 

These sharp limits from the non-observation of the effects caused by the galactic vector potential have been shown to be model dependent.[24] If the photon mass is generated via the Higgs mechanism then the upper limit of eV from the test of Coulomb's law is valid.

 

The formulas didn't copy - you can find them here:

http://en.wikipedia.org/wiki/Photon

 

Does this mean that the mass of an electron can be above zero - if only quantifiable in the terms used?

[swansont]

What the wikipedia article is saying is that if the mass were nonzero, it could be measured form various effects. But the experimentally determined upper limit on the mass is a very small number, consistent with zero. Since it's an experiment, you will never get something that's identically zero; there will always be experimental uncertainty.

[PaulS1950]

So, at least as far as science is concerned the mass of a photon is exactly zero?

I just seemed to suggest that the mass was very low, on the order of

.000000000000000000000003 unit(s) as compared to zero.

What units are we supposed to use for mass?

I am sorry to ask so many questions but I am a 60 year old newbee and it takes all that I have to get some of these principals.

Paul

[Mr Skeptic]

We know for sure that the mass of the photon is absurdly low, to the limits of our measurements. Everything we know theoretically points to their masses being zero, but that can never be proven since we can't ever measure zero.

[PaulS1950]

Mr Skeptic, Thank you for that very precise answer. It really doesn't tell me anything other than what I assume to know. I believe your answer means that for all "real world" uses photons are assumed to have zero mass. (its easier than writing all those zeros anyway)

Is mass measured in units of grams? (continuing to wrap my head around this)