how do photons travel at light speed if they have mass?

Question

thought I'd ask this now since all the astrophysicists seem to be awake.

When a photon is thought of as a particle, it has mass as far as I recall. But when you think of it as a wave, it goes at light speed (at least in a vacuum). And both must be true at the same time. Right? (??) So surely at light speed, its mass would become infinite?

Answer 1

Photons have zero REST mass - in other words if a photon were to be stationary it would have zero mass - all zero rest mass objects can travel at the speed of light.

Photons do have mass when moving - energy E=hf, higher the frequency (f), higher the energy (E), and as E=mc2, the higher the energy (E) the higher the mass (m). So Xray photons are more energetic and therefore more massive than radio wave photons because they have a higher frequency.

As far as I am aware only zero rest mass particles can travel at at the speed of light (c) because as you point out above, anything with mass travelling at high velocity would indeed become more massive, so any object with non-zero rest mass cannot achieve c as it would become infinitely massive and require an infinite amount of energy to get to c.

Finally, one really neat thing to think about is that any object capable of travelling at the c (zero rest mass objects) does not experience time!!

Answer 2

This is a truism.
Photons travel at the speed of light BECAUSE THEY ARE LIGHT! How could they travel at any other speed? I don't recall any slacker photons travelling slower.

Also, if something has NO MASS, it ISN'T, and it ISN'T GOING ANYWHERE. How could a force be exerted on something with no mass? Remember E=mc²? Mass and energy are one and the same, so even energy has mass, even if it's very, very small.

Try looking up Tachyons. They are supposed to go faster than photons, that might be interesting.

Answer 3

They don't have mass. They have zero rest mass and zero size. When they move, they express the energy of their motion as a wave of light of a particular wavelength. The only time they behave as particles is when you view them through a double-slit experiment and when you observe them as being particles via other means and observations

When you are not observing them, they behave as both a wave and particle at the same time. That's the law of Superposition of State.......until you observe a system, the characteristics of a system and all the possibilities of those characteristics exist simultaneously. When you make the observation, then all other possibilities collapse into the observed characteristic.

Light particle can't become infinitely massive as they have no mass to begin with :):).

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mattpa, what's wrong with trying or being able to visualise quantum mechanics. Yes you're correct in that photons do have a relativisitc mass in motion, but the influences of quantum mechanics are also just as prevalant and important. If you have the mind to be able to visualise quantum mechanics, and understand its implications, then there's nothing wrong with that. Both relativity and QM are needed to explain the characteristics of moving and resting photons.

Answer 4

Photons travel at the speed of light because they ARE light. They have mass, one proof is the fact that they are pulled into black holes. Gravity only effects objects with mass. I have four other proofs of photon mass but this is not the time or place to discuss it.

Answer 5

Dont try and visualise quantum mechanics, you are doomed to failure and it will always lead you astray.
Photons move at light speed because they have zero rest mass and that is what counts for relativity. Photons have energy and so have relativistic mass but this is not the same as rest mass.

Answer 6

Congrats. This is a really top class question.
So many different theories exist over wave particle duality that I volunteer the answer .... they travel with infinite mass meaning the answer was in your question.

Answer 7

A photon is a massless particle. No mass = no conflict with relativity.

Answer 8

Who told you that things of mass can not travel at the speed of light? In space, there would be no friction, so why not.

Answer 9

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Answer 10

Because light is made up of photons.