How could a photon travel at lightspeed if it has mass?

[acesuv]

I've heard that photons have a mass. If a photon were to be stationary, would it have mass? If not, then where does the mass of a photon come from?

I know that if an object has stationary mass (forgive me if this isn't the correct term), then it takes an infinite amount of energy in order to achieve light speed.

I suppose that the mass of the photon is actually energy which is acquired somehow?

 

[jtbell]

I've heard that photons have a mass.

Where did you hear/read this? I'm not aware of any results indicating that photons have a nonzero invariant mass (a.k.a. "rest mass"). All I've ever seen are upper limits.

 

[pervect]

This thread is a good example of WHY it's less confusing to say "photons have energy" than "photons have relativistic mass". These two statements mean exactly the same thing, but I believe one is significantly less confusing to the lay person and significantly less likely to lead to confused followup questions and other assorted confusions.

 

[OmCheeto]

I've heard that photons have a mass. If a photon were to be stationary, would it have mass? If not, then where does the mass of a photon come from?

I know that if an object has stationary mass (forgive me if this isn't the correct term), then it takes an infinite amount of energy in order to achieve light speed.

I suppose that the mass of the photon is actually energy which is acquired somehow?

It was explained to me, on my first internet science forum, on March 13, 1997, thusly:

If photons have a small rest mass, they can no longer move at the speed we call "c". I know its confusing that in this situation "c" can no longer be described as the "velocity of light", but the situation is completely consistent and satisfactory, and is open to various experimental tests, which yield the limit of about 10-20 eV for the photon mass.

hmmm... I wonder if anyone has kept track of how many times this question has been asked.

hmmm... It sure took me a long time to get a humour award.

If it were more than zero, according to einstein, it's mass would be infinite, it would suck up the whole universe, and we would all be dead.
(this is nonsense...moderator)

(ref)

 

[Naty1]

If a photon were to be stationary, would it have mass?

photons always move at 'c' locally.

If not, then where does the mass of a photon come from?

as noted, photons have energy [and momentum], not mass.

I know that if an object has stationary mass (forgive me if this isn't the correct term), then it takes an infinite amount of energy in order to achieve light speed.

The term you seek is 'invariant mass', also commonly called called 'rest mass'. The 'infinite energy' is an imprecise way to explain that mass cannot travel at speed 'c'...only slower. There is not sufficient energy anywhere to make that happen. A simpler view is that any observer [which requires mass] sees local light at speed 'c' no matter how fast they are going.

So even a really fast observer still sees light buzzing by at good old 'c'...In other words, if you were able to accelerate and move at, say, 0.7C, which is possible, you would still observe light passing you by at the usual 'c'.