What is LIGHT? Particle or Wave or Both!

High frequency electromagnetic radiation, such as light, has some similarity to waves in the macroscopic world, such as ocean waves or sound waves in the air, and some similarity to small bits of matter such as billiard balls on a table. When attempting to make analogies to the world of our common experience, we must call it both. When we think about light more as a particle, we usually use the word, photon.

At what frequency is LIGHT no longer light?

This is mostly likely defined based on when the eye doesn't much pick it up anymore because the frequency is either too high or too low. Generally the frequencies go from red to blue with red the lowest.

Brandon: where do radio waves, microwaves and gamma rays fit in to all of this spectrum?

Radio waves are very low frequency, microwaves are higher, light is higher, and gamma rays are even higher. It may be helpful to remember that the higher the frequency, the larger the wavelength, since the two are related by the equation fw = c, where f = frequency, w = wavelength, and c = the speed of light.

Instead of saying that light is both wave and particle, would it be more accurate to say that light is neither wave nor particle, but something else entirely? Something which simply possesses characteristics of those other things?

Yes, that's exactly correct. We say it is both a particle and a wave only in an effort to compare it to familiar, every day objects.

Does light possess other characteristics which are uniquely its own, and which have no analog to particle or wave?

Why would light happen to possess an appearance of particle/wave duality, instead of having no similarities at all?

I think every wave phenomenon at certain frequencies possesses it. It is only high frequency EM radiation that has particle like behavior.

Why is this?

Maybe the concepts of Particle and Wave are themselves not fundamental properties, but mere shadows of a deeper, possibly dimensional phenomenon.

It has always been my impression that there is no solid matter in the Universe at all, and that everything is some form of field phenomena, which in turn, is some type of tightly packed dimensional boundary. (pure speculation on my part of course)

Interesting thoughts there Ros: "some sort of field phenomena" - novel.

Not sure which this you mean. Wave/particle duality is probably significant in all high frequency wave phenomena because of the basic nature of the universe. Low frequency waves behave the same as the waves at the seashore or sound in the air which are, of course, examples of low frequency waves.

The Physics answer is that light is both wave and particle. What we call the wave/partical duality is well defined. Incidently this has nothing at all to do with the "frequency".

The problem (as with much of science) is that it takes some background knowledge to even understand what this means.

For anyone who wants to take the time required to gain a decent laymans understanding of this... I highly recommend the book "The Search for Schrodingers Cat" by John Gribbon.

Let me attempt to give as good an explaination as possible in a short space on this forume.

Light as a Wave
We say that light is a wave because light does things that classical particals can't do. The most important example of this is diffraction, where, when you shine light through two narrow slits, you get a repeating pattern of light and shadow.

If you picture shooting bullets (or any object you have experience with) through two openings, you would expect the bullet holes to cluster around one or two spaces. If you think of light as bullet like particles, there is simply no way to explain the pattern that emerges.

However, waves (including water and sound and slinkies) show the same pattern. If a wave meets another wave (or part of the same wave) and the two waves are "out of phase" the waves will cancel each other out.

So, this is very good evidence that light is a wave. If you consider that light can be "out of phase" with other light and cancel out, you can explain the light and dark pattern. Furthermore it is very easy to see and explain the exact same patter in water (and this is done in thousands of high school classrooms every year.)

Furthermore, Maxwell came up with equations that describe light that were just versions of equations for any other kind of waves. For this reason, scientists were convinced that light was simply a wave untill the early 1900's.

Light as a Particle
We say that light is a particle because it does things that a classical particle simply can't do. The best example of this is the "photoelectric effect". This is where light hits an atom and adds a specific amount of energy.

This behavior doesn't happen with a wave, but it can be easily explained with a particle.

This photoelectric effect was described in a billiant paper by Albert Einstein, and the fallout is a very interesting story in itself (which I leave to tantalize, but won't discuss further here).

Science is all about the Experiment
So, science runs on experiments. If you know a lot about waves, and run an experiment on light that shows light does something that only waves do (but not particles), we can say that light must be a wave.

Likewise, if you know a lot about particles and run an experiment on light that shows that light does something that particles can do (but not waves) that you can say that light must be a wave.

But if you show that light does both-- acts as only a wave can in some experiments, and only as a particle can in others-- what can you say. Well, this is why we talk about the wave/particle duality.

I want to emphasize that there is no confusion here. Scientists have run the experiements and know when light acts like a wave and when it acts like a particle. It is just a matter of reaching this understanding of light based on the experiments.

The key is the experiments.

Into the Rabbit hole
Modern science took a very strange turn when DeBroglie (as a very smart grad student) asked the possibly obvious question-- If light, which up to that point had been thought of as a wave, could in some experiments act like a particle... could things like electrions, when had been thought of like a particle, act like a wave?

You may have noticed I slipped the word "classical" in my explaination above. The reason I did this is that it turns out that you can reproduce the double-slit diffraction experiment with electrons (and other "particles"), that is you can shoot a stream of electrons through the equivalent of narrow closely spaced "slits" and get a pattern of "light" (many electrons) and "dark"... just like with waves (including light). Again this can not be explained with our understanding of everyday "particles".

This question started the revolution that is Quantum mechanics. You should be happy that it did, the computer you are reading this on depends on technology developed with the understanding we achieved on this very strange topic.


It comes down to experiments
I want to say very clearly that any intelligent discussion must be based on experiments.

If this discussion is just throwing around terms and pop-culture misunderstandings of science, it is not worth happening.

If we talk about double-slit experiments, electron scattering, barriers and the photelectric effect, we can have a pretty interesting understanding.

But you are going to have to trust the physicists on this (unless you want to run the experiments and develop the mathematical theory again on your own.)

Light is something which depending on how you observe it you it can interact as a particle or a wave, that does not inherently make it one or the other.

I am not sure only high frequency EM radiation has a particle like behaviour, once it carriers more energy that a basic quantum level if it interacts with matter and transfers that energy then it will raise that matter to a higher quantum energy state.

If light where at incredibly high energy levels (think 10^ 50 joules) it might distort spacetime like a blackhole travelling at lightspeed.

Radio engineers and The FCC defines high frequency as 5 million hertz to 50 million hertz, VHF = very high frequency, 50 million hertz to 500 million hertz, UHF = ultra high frequency, 500 million to 5000 million hertz. There are perhaps ten higher decades to get to the top of the gamma EM, which also can be called high frequency. The particle idea is mostly applied to visible light which is about 3 decades above UHF. Neil