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As I remember long ago, in my physics classes, I always had a great trouble understanding the concept of waves. Our professor used to explain, as if everything in this world is made up of waves. However, with any logic, a normal person would always conclude that probably world is created of very small point like particles.

So what are waves exactly? I think it's just a type of disturbance that is able to make particles jump up and down? So how come they become an entity?

I would appreciate explanation. (A simple explanation, in lay-man language. :) )

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  • $\begingroup$ Do you mean "how come waves look like particles" or do you want a more general answer including sound waves etc? $\endgroup$
    – bountiful
    Nov 11, 2012 at 15:18
  • $\begingroup$ I mean, why waves exist really ? Is wave a separate entity that make particles move... or particles movement give rise to waves ? $\endgroup$
    – Vishwas
    Nov 11, 2012 at 15:22
  • $\begingroup$ Related: physics.stackexchange.com/q/40985/2451 $\endgroup$
    – Qmechanic ♦
    Nov 11, 2012 at 16:47

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The first thing you need to get to grips with is that particles are waves. This can be shown with a simple experiment called the double slit experiment, which I will attempt to explain.

Imagine a water wave travelling across a tank. Then imagine you place a wall in the middle of the tank, and place two thin slits in it. If you create a wave (by dropping a stone etc) on one side of the wall, it will travel through the two slits and interfere like this.

The double slit experiment does the same thing, but for light. If you have a wall with two slits in it and shine a beam of light through the slits onto a flat screen behind, you can see a similar interference pattern on the screen.

This shows that light acts as a wave.

Now imagine that rather than a beam of light you can create a steady stream of electrons. Electrons are a small "fundamental" particle ("fundamental" means they cannot be broken down into smaller components). If you point your electron stream at your two slits you will see a very similar interference pattern as before! Until this experiment was done it was believed that electrons were solid particles (like billiard balls), but this showed that they also act as a wave!

Since we have shown that particles can also show wave-like properties, can we show that waves can have particle-like properties? It was shown by Einstein and Arthur Compton that light can in fact be shown to be made up of particles, due to the fact that light must have momentum.

This is known as wave-particle duality.

As I said at the beginning, waves and particles are the same thing. There are some "waves" like electromagnetic waves which make particles move. These are only called "waves" because it is easier to model and calculate that way. It is possible to describe the interaction as 2 (or more) particles (but it is considerably more difficult).

I hope this answers your question.

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  • $\begingroup$ Particles are waves ? I can depict particles, in my mind ( Say, like a small point on paper, so that it concludes that many such particles together constitute a paper ). But how should i depict a wave, taking the same example of a paper ? How can i say there are many waves that make that piece of paper ? $\endgroup$
    – Vishwas
    Nov 13, 2012 at 6:06
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    $\begingroup$ This answer is full of mistakes. It is plain wrong that a particle was a wave. Double slit experiments prove that photons, electrons... are just particles. and this why we call them "particles". The wave-particle duality is a misnomer which is abandoned in modern and rigorous literature. fophillips seems to confound an electromagnetic wave (a true wave) with a wavefunction (a mathematical function)... $\endgroup$
    – juanrga
    Nov 16, 2012 at 17:55
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    $\begingroup$ As stated in the above link (and in the references cited therein) the double slit experiment proves that particles are never waves: "however individual particles are always particles and never waves." $\endgroup$
    – juanrga
    Nov 16, 2012 at 17:58
  • $\begingroup$ Downvoted, because of the same point @juanrga made. You are confusing waves with wavefunctions. Wavefunctions are a mathematical representation of a probability. Individually, the electrons wavefunctions are basically interfering, to provide the interference pattern (since at some points the probability is zero, and at other points it's ~ 1.) This doesn't mean that the electron itself is a wave. $\endgroup$
    – Kitchi
    Nov 22, 2012 at 13:31
  • $\begingroup$ Moreover, @fophillips is clearly unaware that quantum particles, as any other quantum system, can be in quantum states (e.g. mixed states) not described by any wavefunction. It is an unfortunate fact that the OP accepted such misguided answer. $\endgroup$
    – juanrga
    Nov 22, 2012 at 18:53
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Your teacher is wrong. Universe is not made of waves but of particles. Let me quote the CERN site:

The theories and discoveries of thousands of physicists over the past century have resulted in a remarkable insight into the fundamental structure of matter: everything in the universe is found to be made from twelve basic building blocks called fundamental particles, governed by four fundamental forces.

Of course those elementary particles are quantum particles not Newtonian particles. Waves such as the electromagnetic waves or the water waves described in your above link are a macroscopic approximation to the collective behaviour of lots of quantum particles.

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  • $\begingroup$ But these particles also behave like waves, and are governed by their wavefunction. $\endgroup$
    – bountiful
    Nov 14, 2012 at 16:24
  • $\begingroup$ In my answer I remark how collections of particles behave as waves under certain approximations. Each individual particle is not a wave, contrary to what you believe (but don't worry I will be not rating negatively your post as you did with mine). $\endgroup$
    – juanrga
    Nov 14, 2012 at 20:33
  • $\begingroup$ But individual particles have been shown to behave like waves. Performing the double slit experiment one photon at a time shows this. $\endgroup$
    – bountiful
    Nov 15, 2012 at 19:07
  • $\begingroup$ A particle is described by its wavefunction, a probability amplitude describing its quantum state. When a system is observed the wavefunction "collapses" into a sharply peaked function at some point, which appears to be like a classical particle. Then again Quantum Field Theory sort of renders the difference between the two moot. $\endgroup$
    – bountiful
    Nov 16, 2012 at 9:45
  • $\begingroup$ A collection of photons can behave as a wave. A single photon behaves as a particle either in the double slit experiment or elsewhere. This is why we say that the photon is a particle and the corresponding discipline is named "particle physics". In my answer I emphasized that "particle" in quantum theory does not mean classical particle but you continue confounding concepts... $\endgroup$
    – juanrga
    Nov 16, 2012 at 17:43

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