Does light always travel at lightspeed?

Because it's ten years old.

 

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I don't think scientist have yet discovered how much mass photons actually have, or if they have any at all. They say it may have very little or no rest mass. I myself don't see how they could have very little rest mass because of this, but it hasn't been completely ruled out that I know of. I have read about this being an issue that scientist are trying to solve. I think there is a good chance that they do not have mass, it just hasn't been completely proven yet.

 

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Photons have no mass. They can't travel at less than light speed.

(Edit - Snap!)

 

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Pete is correct. Prof. Layman is wrong, he has no physics education and typically will just guess at answers.

Welcome to the forum

 

Then you would be assuming that having little mass, would then make them travel slower than the speed of light because the mass increase. This was not shown to prove that they do not have mass via experimentation. It is an issue that was actually brought up to try and show that they do not have mass, but this was not enough to be able to actually prove it. So real scientist actual think that they have very little or no mass, since they have not proven that they have no mass.

 

You shouldn't listen to either of these guys, they are guessing the answer with no real scientific basis. They just aim to force their opinions on science onto everyone else so they accept them as scientific fact.

 

The best way to figure out who is right and who is wrong is by citing sources:

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

Yes, indeed, the photon is massless and Prof.Layman is just guessing/making stuff up as he goes along.

 

Then I guess you have resolved the problem of applying GR to photons? And have shown that the combinitation of GR and quantum electrodynamics clearly shows that the photon cannot have mass? Saying that proves the photon is massless has no mathmatical justification, and is only in the realm of psuedoscience!

"The fact that no such effects are seen implies an upper bound on the photon mass of m < 3×10−27 eV/c2.[24]"

"If the photon mass is generated via the Higgs mechanism then the upper limit of m≲10−14 eV/c2 from the test of Coulomb's law is valid."

This does not say that it is equal to zero, only that it is less than a value that is larger than zero!

 

Hi, max, and welcome to the site. Ignore the overblown remarks above.

Since photons propagate through free space as waves of radiant energy, the question of their mass in the context of your question is moot. That is, it's not equivalent to a particle of non-zero mass being accelerated to light speed, which, as you note, is theoretically impossible. We get around this discussion by calling it a zero mass particle, realizing that, while propagating, it exists merely as a wave.

 

Empirically speaking, this is correct. See Wikipedia.
It is impossible to directly measure the mass of light as exactly zero.

I addressed max's question in the theoretical context of relativity and QED (not that I pretend to understand QED), because that's how his question was framed.

 

In the interest of the questioner you should take your grandstanding to another thread.

 

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When the agreement said that one's posts could remain for years to come, I wasn't expecting this long.

Most forums do annual spring cleaning.

 

That's criminal!
I often refer to ancient threads I was involved in. There are some absolute gems in the archives that should be preserved forever.

For example, here's a thread about relativity of simultaneity (Layman, take note!) from 2004: [thread=38896]Different SR Paradox?[/thread] (I've learned a lot since then. Not much more physics, though.)

 

No, max, don't trust them. The internet is evil. I know you probably are reading a book or something that ask this very same question, but does not give an answer and you would like to know what that answer is.

Trust the book, it was most likely written by a real scientist. Don't let them draw you into their crank theories, of how General Relativity shows that photons have mass. The book would talk about how quantum mechanics and general relativity are not compatable theories. They cannot know then that general relativity proves that the mass of a photon is zero. No one has figured a way to show this mathmatically, or experimentally.

And see, it also probably explains wave particle duelity, and doesn't say that they know that it is just a particle or just a wave. Don't let them tell you that you don't know anything, trust the words of where you got these ideas, don't give in to the falsehoods of the internet. woo, woo....

 

Who said General Relativity shows that photons have mass?

I'm led to believe that there are very good mathematical reasons in quantum electrodynamics (a theoretical model which is extraordinarily well supported by experiment) for photons to have zero mass. Look up 'gauge invariance' in your favourite QED textbook.

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It is certainly good advice to be careful of the sources you trust, and particularly to be suspicious of advice you receive on anonymous Internet forums.

 

At what point should prof. Layman be banned from posting this crap in the science section, purposely trying to confuse someone with real questions?

 

And, people just wake up one day wondering why the mass of a photon is not said to be zero because of General Relativities mass increase problem. Maybe you should ask him where he got this question, I would be willing to bet that where he got it from says the same thing I am. I sure would like to know, I would love to give it a rereading.

You are the one that is causing confusion, I don't know why you guys are following me on these forums saying I am wrong about everything, when I have clearly and consistantly provided quotes that say I am correct from links said to prove me wrong.

Maybe you should take another look at post #28? I quoted it straight from the wiki page. Those are the expected mass of photons from experiments. They do not say the mass is zero. Only crackpot theorist on the internet have started saying that is zero, because people come online asking this question.

How does someone that not know anything about it, even come up with this kind of question? It is a question that I have read about in laymans text. They say that they do not know the answer, but yet you do, oh surprise surprise. Why is he so timmed in saying he doesn't know anything about physics if this is his first post on science forums?

If they found the value to be zero in experiments then the wiki would say that they found it was zero in experiments, it does not say that the experiments say it is zero. They say they are a value that is less than a number this is not zero. This is an obvious troll following.

If you trolls wanted to follow me some more to get me banned by saying everything I say is wrong, maybe you should try saying something that is actually right for a change.

 

The question is a about Special Relativity, not General Relativity, and is asking if a photon has zero mass.
In Special Relativity, light does have exactly zero mass, because (as the questioner points out) it could not otherwise travel at light speed.
In QED, I'm led to believe there are more rigorous reasons related directly to photons that I don't understand.

I've actually said that you are right in this thread. Empirically, the mass of photons is known to be very very small, possibly zero. It can never be directly experimentally proven to be exactly zero, because every experiment has a limit of precision.

The very same source you quoted (ie Wikipedia) says that it is exactly zero: " The mass of the photon is believed to be exactly zero, based on experiment and theoretical considerations described in the article."

 

There is not now nor has there ever been such a problem. Indeed, explaining how massless particles can be affected by gravity is one of the main reasons GR exists

 

A photon propagates as a wave.

Prove this is false, or face the charges of trolling.