How could light not have any mass?

Mass is made up of atoms.

Light is made up of photons, which are not atoms.

There is mass in a campfire (hence the smoke and ash). But the light emitted from the fire has none.

The more interesting question is why do atoms have mass? That's the one that physicists today are struggling with. Photons, which are the particles which make up light waves, are massless, and that's just how it is. Gravitons, the particles which are theorized to transmit gravity, would also be massless. Maybe someday there will be an explanation, perhaps through string theory, but for now, we just have to accept that photons have no mass.

So far as the campfire thing, here's the tricky part: what we see as fire isn't just a bunch of photons. The flames are really parts of the air and other vapors that have been superheated and are glowing. There is a chemical reaction going on in the fire, and the energy released in this reaction heats the air to the point of glowing. You could supercool the air, but then it wouldn't glow anymore, since it needs to be very hot to emit visible light.

Wow. That one is a whopper!

I work in Biophysics but am not an expert on this stuff.

Well you first have to understand that nothingness, even in space or a perfect vacuum; is something. It is a kind of medium that we don’t completely understand, but is a kind of balanced zero energy domain (there are balancing reactions that go on but are crazy fast). That I guess can be described as electromagnetic/gravitational in nature, but you can think of it as the blank canvas for the universe.

This same stuff, that nothingness is made from (people often call this ‘space-time’); is what probably makes up all forms of matter and energy. You see matter is just a stable construct of energy, of electromagnetic energy and some other types of energy for the nucleus of atoms. Matter interacts with the universe in such a way as to have a gravitational pull, is affected by gravity and possesses inertia. All three of these things are probably due to the general gravitational (best way you could describe it) ‘context’ of the universe.

Light on the other hand can be described as an energetic ripple in space time, something akin to magnetic and gravitational ‘disturbances’ in our aforementioned ‘blank canvas’ of space-time. It is itself an electromagnetic wave, there may be many such energetic disturbances in space-time but light is just one we happen to see. Who knows there might be gravity waves as well, which have no mass but interact with matter? You must also keep in mind that we have to distinguish between matter and energy; as well as electromagnetic, gravitation and nuclear forces because we don’t understand the universe. If we did we would probably see all as the ‘same’ thing, but just in different ‘forms’.

So as far as we can tell light has zero mass at REST, which means it has no inertia that is why it is the limit for speed. It seems that our space-time sort of ‘holds’ matter back more the faster it goes, this causes an increase in mass(inertia) and time dilation.

What is really freaky is that we don’t know how fast we are going on our planet, the earth revolves around the sun, the solar system revolves around the center of the galaxy and the galaxy is moving ……. you get the idea. It all depends on your frame of reference, but a constant for any frame of reference is the apparent speed of light.

Although you are correct that light does have a kind of ‘mass’ at light speed, since it has momentum and can ‘push.’ But this doesn’t necessarily mean that it has ‘mass’ per se as it interacts with matter as outlined above.

So we might just say that light, perhaps all forms of mass-less energy phenomena probably moves at the same rate; it is the speed limit of the universe(for these things).

The fire is low temperature plasma, or light emitting ionized gas; if you cooled it you should get either liquid and/or solid material covering a much smaller volume. If you did it instantaneously (which would be almost impossible) you might get a captured liquid/solid flame that would emit light for maybe a few picoseconds (?).

Dave gave exactly the right answer.

Light can't have mass -- if photons had any mass, they couldn't travel at the speed of light (because having any mass would require an infinite amount of energy to get them to the speed of light).

There are other massless "particles" besides photons...

:)

The photons actually have 0 invariant mass. Invariant

mass is rest mass or intrinsic mass.

However, the really suprising thing is that photons do

carry momentum - so we have "particles" that have

zero mass but can collide with other atomic

particles and affect them.

It is the nature of the beast..

Energy has no mass..

when a particle is said to be massless they are referring to its mass at rest, which implies that the particle can never be at rest. welcome to the wierdness of quantum mechanics.