View Full Version : What is light?
12-16-2000, 12:04 AM
I've been reading Stephen Hawking's "A Brief History of Time," and it's really made me wonder about some things. First of all, just what the hell is light? I mean, I understand about wavelengths and everything, but wavelengths of what? I also have questions about the speed of light, but I'll save them for another thread.
12-16-2000, 12:07 AM
Ho boy, one express ticket to Great Debates please :-)
12-16-2000, 12:08 AM
Oops. Is this in the wrong forum?
12-16-2000, 12:21 AM
Not yet, but I'll lay even odds that it will be by this time tomorrow.
Simple answer. Light is composed of streams of photons. Photons are particles that move in waves, or maybe they're waves that move like particles. From there on it it gets complicated.
12-16-2000, 12:41 AM
Originally posted by Elvis
I've been reading Stephen Hawking's "A Brief History of Time," and it's really made me wonder about some things. First of all, just what the hell is light? I mean, I understand about wavelengths and everything, but wavelengths of what?In the nineteenth century, it was presumed that light was waves of "aether" (not the anesthetic), a universal medium that was responsible for electromagnetism. But the aether theory was dealt a serious blow by the famous Michelson-Morely experiment, which failed to show any evidence that the measured speed of light is effected by the Earth's motion. This lead to Relativity.
Meanwhile, the nature of light became the province of the various theories that are collectively known as Quantum Mechanics. It is now accepted that light (and other subatomic particles as well) are of a nature that everyday notions of "particles" or "waves" simply aren't accurate. The "quanta" of light have properties of both: they always occur in discrete bits, thus seeming particle-like; but the quanta cannot be localized like classic particles, they can only be described in probablistic terms that mathematicaly resemble wave functions.
If this still leaves you scratching your head, join the club. QM is very counter-intuitive, and while the math yields usefull calculations, what it "means" is still the subject of endless debate.
12-16-2000, 01:06 AM
Don't let anyone tell you QM is complex. You might discover that it is, or you might discover that it is a very elegant theory. Never let anyone try to think for you.
That said, I think I can explain it. A photon can be described as a particle we can never slow down enough to get a good look at. We can predict where it 'is' to within a certain tolerance. If you can visualize the possible space as being 2-dimensional, you can use height to define the odds of it being at a certain spot, higher 'hills' representing higher probablities. You can thereby represent a photon, and most subatomic particles including electrons (I'll describe electrons below), as a 'hill' of probability on a 'plain' of spacetime. We don't know exactly where it 'is', but we do know what the probabilities say, so we can create the 'hills', which look like waves.
Well, photons also act like waves. But, remember, these are waves in probability. There is no 'water' here. But the interference patterns from the famous double-slit experiment look like what happens in water under similar conditions. However, we can also detect the individual photons, showing further that the waves are in probability, not in spacetime.
Electrons show how interesting the probability waves can act. Imagine the 2-dimensional spacetime plain and the third dimension, height, as the probability of a particle occupying that spacetime. An atom can be represented as a very slender, very high, mountain (the nucleus, with the well-defined protons and neutrons) with lower 'rings' (doughnuts, as I call them :)) around it (the electron shells). That's right: There are no individual electrons here. Just places where electrons might be, if we looked. We can look anywhere in those rings and have a certain probability of seeing an electron. That is called 'collapsing the waveform', because we 'force' the waveform to 'choose' a specific place in spacetime (we make the electron occupy a certain place on the plain) by looking at it. Electrons have mass and charge, and can be isolated (we isolate them in particle accelerators all the time). We know they can act as particles. But they can also act as waves.
You can learn a lesson here: Saying anything 'is' something can bite you in the ass later. Calling a photon a particle fails to do justice to reality. Calling a photon a wave has the same problem. Saying they act as waves in probability, particles in spacetime seems to jibe with the experimental results better.
I hope that wasn't too hard to follow or too far from reality. The above explanation comes from how I see the Universe. Very interesting topic, if you ask me.
12-16-2000, 10:43 AM
try howthingswork.com search for 'light'
12-16-2000, 12:19 PM
IIRC, light was described as a magnetic field inducting an electric field inducting an magnetic field...
This doesn't get into the particle nature of light, but it gives a good explanation of the way that light propogates.
12-16-2000, 03:19 PM
Thanks all. I think that I'm starting to understand.
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