light

Discussion of theories involving time as a dimension, time travel, relativity, branes, and so on, usually applying to the "real" universe which we live in.

light

Postby papernuke » Sat Nov 04, 2006 5:45 pm

Light beams are only photons, which are just atoms that react to your eyes so you can see. So, if they're atoms, then wouldn't it go faster than light and go back a few years or so and die? Also, if light is still matter, then how can it go at the speed of light?
"Civilization is a race between education and catastrophe."
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Postby houserichichi » Sun Nov 05, 2006 3:30 am

Photons are not atoms, they are fundamental particles just like the electron (in the sense of being fundamental, that is). Atoms are made of nucleons (protons and neutrons) which in turn are made up of other things called quarks which, as far as the Standard Model of particle physics goes, are fundamental too.

It travels at light speed because it has no mass. All massless (rest mass) particles travel at light speed.
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Postby papernuke » Sun Nov 05, 2006 3:37 am

But if they're fundamental particles, then wouldn't they still have mass?
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Postby quickfur » Sun Nov 05, 2006 7:52 am

Icon wrote:But if they're fundamental particles, then wouldn't they still have mass?

No. Not all fundamental particles have non-zero(rest) mass. Some, like electrons and quarks, have non-zero rest mass, but others, like photons, don't.

There is a difference between rest mass (mass when something is stationary) and relativistic mass (which is rest mass + kinetic energy contributed by Einstein's equation, m=E/c<sup>2</sup>).

Photons have zero rest mass, but they are never stationary, since they always travel at light speed, so their relativistic mass is never zero.
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Postby houserichichi » Sun Nov 05, 2006 6:03 pm

Most fundamental particles do, in fact, have mass. In fact the only massless particles I know of are the photon, gluons (which don't technically exist in their singular form just like quarks), and the graviton (which hasn't even been found yet).

Not to be nitpicky but relativistic mass is an outdated concept and rest mass is more commonly referred to as invariant mass in modern literature. When one speaks of the "mass" of a particle they (should be) referring to the invariant mass of the particle which, in the case of the photon, is zero. At any rate, it's just a side quibble.
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Postby papernuke » Mon Nov 06, 2006 12:22 am

But if light is just a bunch of photons, and it never rests, then how come when you shine a light somewhere, it dosen't go all the way there? It eventually fades.
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Postby houserichichi » Mon Nov 06, 2006 12:56 am

Because photons have to pass through air to eventually get to your eyes. They get absorbed into the particles that build up the atoms that build up the molecules of air and get re-emitted sometimes as other particles. Also, treating light as a wave, the wave smashes into the atoms and gets bounced off in a different direction. Eventually the light rays are so far apart that they're no longer concentrated enough to be visible to the eye.

That's how I simplistically understand it, anyway.
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Postby blazes816 » Mon Feb 26, 2007 4:56 am

Thank you so much. I read The Elegant Universe and this was the biggest thing that I couldn't seem to understand.
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Postby papernuke » Tue Feb 27, 2007 4:44 am

I read that book too! i have it!! :D
"Civilization is a race between education and catastrophe."
-H.G. Wells
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Postby blazes816 » Tue Feb 27, 2007 11:33 pm

I think everyones read that book. :lol:
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Postby PWrong » Mon Mar 05, 2007 1:35 pm

So they should, it pwns. It's my 2nd favourite non-fiction book after the God Delusion.
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