Does light have mass?


Temporal Novice
I just thought of an interesting paradox: Light can't have mass because it would require infinite energy for it to move that fast, right? Yet have you ever heard of the solar sail? It's a giant piece of plastic in outer space that moves because of light pressure: i.e. the photons push against the sheet to make it move. Now, the only way that can happen is if the photons have mass so they can transfer momentum. But we previously stated that light can't have mass!

The only conclusions I can draw from this is that our original statement that light can't have infinite energy is wrong: LIGHT DOES HAVE INFINITE ENERGY! From this, wouldn't it be possible to extract this energy and use it to propel other masses to light speed?

I don't know if I'm making a mistake or what, but it seems pretty damned interesting to me!

Let me know what you think,
Solokin1 (a.k.a the former progboy1)
Friend Solokin 1 Of EarthTR125.0121

Friend light is made out of an array of particles, not just photons, that is but the beginning of the equation. But rather by an intricate collection of subluminic particles, these when taken as a whole provide the compound photon with a sort of relativistic mass. This mass can only affect solar sails because these sails are made from a material that is almost weightless and in the vacuum of space it can be imparted with momentum by our collection of subluminals.

Soon, I hope, our scientists will find out that all sorts of energies are in essence elements, however I do think that by our mind set at material things we will start looking into that avenue of thought in long while.

Until later becomes now.
Actually that is not why light seems to be affected by gravity: Since by Einstein's rubber sheet analogy we see that the sun creates a dimple in space. Now if you have a light beam the light beam will have to follow the rubber, and so be bent because the rubber itself (or space) is bending. Light does not bend directly because of gravity, but rather because gravity bends space/time itself.

I'm still puzzled whether or not light has mass.


Only to keep the spirit alive,

Then if they hane zero mass, why extremely low temperatures seem to slow it down?

I have always asked myself the same question. There must be a variable within the photon pack that keeps eluding us, maybe something that lingers between reality and un-reality.

Bare with me for a second, it hass been proposed to develop solar sails in order to propel vessels across space, I know that it is the transferred momentum of the photon particle that moves the object, but How can a massless object transfer momentum. Even the air around us can be meassured in terms of certain physicality.

Until later becomes now.
Electro Magnetic fields carry momentum:

and this article discusses Magnetic Flux and proposes background oceanic matter as the carrier of momentum (I think this is what they are getting at but this article goes a little beyond my understanding at this stage!):

Very interesting if you have time to read.

Part of the reason I think it seems odd that massless particles can transfer momentum is related to our learning of classical mechnaics in high school. Newton's second law, f=ma (force = mass by acceleration) can also be formulated as F=dp/dt, ie: force equals the change in momentum over time. We tend to assume if m is zero, then there is no force. F=ma is not the only solution (See the first link above).

Forces can be transmitted along electro-magnetic fieldlines. The originating particle only needs to be energised, it does not require the presence of mass to exist (we know this because light does indeed exist), and it's energy can be transferred to any particle that comes into range of the electromagnetic field generated by the massless particle.

This is my understanding, which may be warped!
Transient I just noticed I used a bit of circular logic re: photons being massless or not. What I intended to get at, is that the maths certainly supports massless energy transfer, therefore it is possible that photons are massless. I haven't considered your question re light slowing down at very low temperatures and why this might be.

Interesting question.

What makes you think that temperature has any effect on the speed of light?
The speed of propagation of light through a vacuum is equal to its wavelength time its frequency.
Temperature is related to the kinetic energy of molecules, where KE_avg = (3/2)kT, where k is Boltzmann's constant and T is absolute temperature(Kelvin).

good question keeb. I don't necessarily think it does I was just responding to transient's question. My understanding of thermo-dynamics is at a basic level only. I am willing to be educated!

Transient, what experiment or source are you referring to that you say "seems" to indicate speed of light slowing down at low temperatures?

If this is just your reading of it, elucidate us with your reasoning.
Right on, Wedge. I meant to be responding to Transient as well. I just finished a physics course this semester covering light, e/m, special relativity, and basic quantum mechanics. I would sure like to see a link to his information.

There have been many experiments done demonstrating that certain crystals can slow light down significantly. Perhaps this is what he meant.

Actually, light slows down all the time. The water and glass refract light is because the light slows down as it passes through the material. The reason we see it as bending is because the beam does not enter the material at the same time, thus part of the beam of light begins to slow while the rest doesn't until it also reaches the interface.

I hope that made sense. I'll try and find a diagram to post.

Well, I'm not a physicist, but my dad just happens to be a laser research physicist. I'll ask him to explain some of this to me at some point, and to pass on what he says.

However, from my understanding at the moment, what light has is no rest mass. I'm rather fuzzy on the details, but what that means is that if a photon were to stop moving then it wouldn't have any mass, rather than it actually not having any mass when it's going at lightspeed.

Also, it's not true to say that it's impossible to travel at lightspeed with mass. It's impossible to accelerate up to lightspeed from a slower speed, but you can be created moving at lightspeed without making the equasions self-destruct.