Is light “matter”?
Sure. Light is a form of energy and Einstein showed that matter and energy are equivalent.
That’s what the famous E=mc2 is all about.
(Wish I knew how to do exponentials on my keyboard.)
If you’re hot, that’s good.
If you’re cool, that’s good.
I don’t get it.
If that’s true, then shouldn’t light have mass?
It does have mass. What it does not have is a rest mass. That is, a photon at rest is massless because it contains zero energy. But this is an idealization. A photon cannot be completely at rest because the Uncertainty Principle forbids it.
Check out the Photons thread from GD. http://www.straightdope.com/ubb/Forum7/HTML/000455.html
E=mc[sup]2[/sup] does not say that matter and energy are equivalent. It says that there is a relationship between mass and rest energy.
I would not qualify light as matter. The classic school definition of matter is something that has mass and takes up space. I don’t know of a technical QM def of matter, but I would suggest a QM def might be something like “something that is limited by the Pauli exclusion principle”. By either def, light doesn’t fit the bill well.
The issue of “relativistic mass” is hit pretty heavily in the Photons thread and in the link provided by tanstaafl.
According to “The History of Physics” which I am referring to at this moment, Einstein called his famous equation Mass-Energy Equivalence.
Just a quote from a long chapter:
“No one noticed the equivalence of mass and energy until Einstein demonstrated it theoretically in 1905.”
My understanding is that it is more than a relationship. It is an equality, different aspects of the same thing.
If you’re hot, that’s good.
If you’re cool, that’s good.
I don’t get it.
For starters, Wally, mass and matter are not synonyms.
If you haven’t already, you should read the page linked by tanstaafl. Einstein’s views on relativity weren’t set in stone when he first wrote about it. When he first published the paper “Does the inertia of a body depend upon its energy content?”, general relativity hadn’t been developed yet. The interpretation of E=mc[sup]2[/sup] has changed somewhat since 1905, esp due to the fact that the idea of “relativistic mass” does not mesh well with general relativity.
Well, Dude, methinks my book is out of date.
I guess I should take some comfort in what Richard Feynman said:
“Nobody understands Quantum Physics. You just get used to it.”
I’m trying. I’m trying.
If you’re hot, that’s good.
If you’re cool, that’s good.
I don’t get it.
I like the Feynman quote. It is comforting. 
Your ref might not be that old. General education and reference on the subject is way behind the physicists in this sort of thing. I was taught the whole “relativistic mass” thing in high-school, and there are hints of it in EB. My attack on relativistic mass is starting to border on a crusade these days, partially because I think it is a supposed simplification that actually complicates the issue.
SPOCK. HELP. US.
Typer
"If you want to kiss the sky, you’d better learn how to kneel "
Physicists usually divide “everything” into either matter or radiation. In this sense, light is definitely the latter. The mass-energy equivalence describes how much mass can be converted into how much energy and vice versa, not that the two things are identical. Therefore, IMHO, light is not matter.
“The inability of science to grasp Quality, as an object of enquiry, makes it impossible for science to provide a scale of values.”
Robert Pirsig
…which leaves you with the question of why light is bent by a gravitational field.
John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams
Not really. In general relativity curved space-time geodesics are considered inertial paths. Light is simply following this natural path. If light acted gravitationally as if it had mass, black holes wouldn’t be thought to have an event horizon. They’d be thought to have a spectrum of levels at which different wavelengths of light would be grabbed.