Sidian

i'm not straight on what exactly the current leading theory on light's composition is.

Do they say light is a photon simply because it works well with other quark theory, or is there more evidence then that. I know we have equations and stuff that work nicely with light being a particle but there seems to be difficulties.

It seems utterly rediculous to me that light would be a tiny particle that travels through/around atoms in glass and plastic (hard liquids) but not through other materials.

Also when you start thinking about distances to where light is coming from, the numbers get preposterous.

So what then is light? It doesn't behave like a vibration through a medium (like sound i mean). In fact it acts like a particle some of the time but then not other times. Someone gimme something current and if possible easy to read.

Oolon Colluphid

Light of course is both wave and particle, in that if you look for it as if it is one or the other, that’s what you’ll find. The most useful (though mind-bending) answer to it is David Deutsch’s: that it is always a particle, but that the photons in neighbouring universes interfere with each other to produce the apparent wave effects. This is indicated because the interference patterns are produced even if yopu fire the photons one at a time. See Chapter 2, ‘Shadows’, of <a href="http://www.qubit.org/people/david/FabricOfReality/FoR.html" target="_blank">The Fabric of Reality</a>.

Why? Alpha particles are happy to pass through or past the molecules in air, but can be stopped by a piece of paper; beta particles will ignore paper but can be stopped by a lead sheet. (IIRC, this is O Level physics from 1983 I’m remembering, so the details may not be spot on, but I think the principle is!) Whether a particle can pass through a material, or gets bounced back off the atoms/molecules in it, is simply down to the size of the particle, the energy it possesses, and the size of the atoms in the material. If we could see beta particles, paper would look as transparent as glass. Why is light ridiculous?

And there’s plenty of transparent crystal solids. Go look in a jeweller’s window. Also, the materials don’t have to be ‘hard liquids’ either. Plastics may not have a crystal structure, but can still be solids -- they’re polymerised hydrocarbons etc, with proper molecular bonds (ie not just hydrogen bonds), not free-floating bits as in liquids.

Why? I take it you mean stars? It is down to the quantities of photons produced. A candle similarly produces photons, but don’t expect to see it ten miles away. Stars, being vast nuclear furnaces, produce rather a lot, yet there are many many stars that are too dim to be seen with the naked eye, many that are barely detectable even with large telescopes, and presumably lots more we can’t 'see' at all. What's the problem?

Cheers, Oolon

[ April 15, 2002: Message edited by: Oolon Colluphid ]</p>

Sidian

I've spent most of my time just reading these boards. I very much enjoy alot of the discussions here. I'll definately be coming back and reading other stuff.

I noticed that people tend to be very specific (which is great!) when they answer, refute and debate. So here goes with a better look at what I wanted to explain/ask. (it's long sorry)

Polaris, the north star, is 680 light years away. Light travels at 186,000 miles per SECOND (theoretically) which puts the north star 3,988,673,280,000,000 miles away. To be able to look up at night sky and see the north star, light must be reaching all that way to the very spot you are standing. Now step one foot to the left, is the star still there? Of course. Again step to the left another foot, and again. What about stepping backward and foward? Still see the star? This may seem rediculous since we used this star for navigation for so long, but it baffles me if light is a particle.

It is logical that light from this star is not hitting just planet earth, but is evenly "firing" photons in all three dimensional vectors. For us to be seeing the north star from VARIOUS POSITIONS on earth, Polaris must be firing enough protons to place one in every square foot(we'll just stick with square feet since smaller measures are REALLY bad) at a distance of 680 light years.

We can use the equation for the surface area of a sphere to figure out just how many photons would be required to place JUST ONE in each square foot. 680 light years is about 3,988,673,280,000,000 miles and there are 5280 feet in a mile. That brings us to 21060194918400000000 feet. This value is the radius of the sphere around Polaris that earth would sit on. The equation for surface area of a sphere is SA = 4*(pi)*r^2. For our sphere, we come up with about 5.5734 * 10^39 square feet. To put one photon in each square of this sphere firing from the center of the sphere (polaris) would require 5.5734 * 10^39 photons.

Now consider this, the figure above represents only the number needed for ONE photon. For our eyes to see Polaris, we need many, and we need them to be coming constantly. So to put multiple photons (we're talking thousands per second atleast I'm sure) per foot increases that number dramatically, and if we were to take an estimated measure of a certain amount of time (and stars theoretically live to be billions of years old) the number becomes preposterous. It seems rediculous for light to be a particle of any size. Even if every atom of a star had millions of it's own photons to be drained. There are farther stars as well.

If you want to add more complications, think about one more spin on the "experiment". Add sunglasses for style. Sunglasses can be made to block a good 40% of the light (photons supposedly) traveling toward your eyes. Stepping in each direction (one foot at a time), you can still see the north star with your shades on. So not only is the number for photons that we were discussing extreme (impossible?) It also only represents 60% of the light that's coming to us.

Another question, how do we know how fast light travels? I didn't think we would have anyway of measuring this considering we have no idea how fast we ourselves (our planet) is moving through space. IF light is a constant speed (theory) how can we possibly measure it from a moving platform (earth) with some unkown speed. It seems rediculous.

I'd have an easier time believing light travels instantly and our measures of light are actually measures of how fast our electonic measuring devices' transistors can fire.

And I still have no idea why light seems to act like a particle when it bounces but like a wave all other times.

As for "traveling" through objects, I could almost see light traveling through crystals making sense if it was a particle, almost like a strainer for spaghetti or something. But it also goes through plastics (non crystal). I can't imagine how 6 inches of heterogeneous plastic's atoms are more of a barrier then fractions of a millimeter of pounded flat gold or other metals.

BAFFLED!

[ April 15, 2002: Message edited by: Sidian ]</p>

Boro Nut

Check out a scale model of an atom and ask yourself what solid means.

Coragyps

We know very accurately how fast the Earth is travelling, and in a variety of frames of reference. You don't really think NASA could get a probe to Jupiter without a good speedometer, do you?
As for the speed of light, there are lots of ways to get a measurement. The first accurate way was done about 1700 by Ole Romer, IIRC. He found that eclipses of Jupiter's moons were 16 minutes later than scheduled if Jupiter was near the Sun in the sky instead of opposite the Sun. The difference is the amount of time it takes light to cross the diameter of Earth's orbit - 300,000,000 km divided by 1000 seconds = 300,000 km/sec. Romer's method wasn't super-precise, but close.

No foolin'. Me too, but it does. And atoms act like waves, and fast-pitched baseballs do too - the wavelength gets awful short, though.
More on your Polaris question later...

tronvillain

Well, it may seem ridiculous to you, but apparently you just have a problem with large numbers. The number of photons would be incredibly large regardless of whether a photon is a wave or a particle, since a photon is simply a quanta of light. You seem to have trouble concieving of the existence of such a large number of photons, but that's your problem, not ours.

Now, the sun fuses 6.11E11 kilograms of hydrogen to helium every second, which converts 4.34E9 kilograms of mass to energy every second, resulting in an energy output of 3.9E27 joules. Now, since the average energy of a photon from the sun is 1.18E-19J, the number of photons you mentioned would amount to 6.60E20J, or 0.0000169 percent of the sun's total output.

As for the speed of light, it has been measured by a wide variety of <a href="http://www.what-is-the-speed-of-light.com/roemer-speed-of-light.html" target="_blank">methods</a> and was calculated by Maxwell.

Coragyps

Like Tron said.
I get a per-second output of 10^45 photons from the Sun, assuming an average wavelength in the yellow, which is about right. And Polaris has about 100,000 times the luminosity of the Sun - so we have enough photons. The eye, after all, can detect a single photon if you're dark-adapted enough.

Freethinking Ferret

"IF light is a constant speed (theory) how can we possibly measure it from a moving platform (earth) with some unkown speed."

The fact that the speed of light is a constant for all frames of reference has also been well tested: <a href="http://www.what-is-the-speed-of-light.com/morley-speed-of-light.html" target="_blank">(nice reference page, tronvillain!)</a> or <a href="http://www.drphysics.com/syllabus/M_M/M_M.html" target="_blank">(another page about the Michelson-Morley experiment)</a>.

ohwilleke

To nit pick, experiment actually shows that it takes two to four photons to register in the human eye (although they need not come at exactly the same time).

Mageth

Light travels in waves. By the time the light reaches us from a star (and from the sun, for that matter), it strikes the earth as a "wavefront," and is "flat" for all practical purposes. That's why no matter where you go, there they are.

Here's a <a href="http://starlight.jpl.nasa.gov/interf/whatis.html" target="_blank">link</a> that describes an interferometer, a clever device that takes advantage of this phenomenon to determine information about a star not available through conventional telescopes.