# Thread: What's this wave particle duality?

1. I'm reading up on optics, and I know that there is a wave-particle duality to light, or so the current theory holds. But I'm not convinced there is a duality, or perhaps I just don't understand it.

A wave is still made up of infinitely small particles like photons, and, in other circumstances, electrons and protons. So a wave is still made up of particles, just as a wave of water is made up of many particles of hydrogen-oxygen molecules.

Am I missing something?

2.

3. It's sort of like "what's the sound of one hand clapping". Two electrons can interfere with each other to create an interference pattern like two wave fronts crashing together. How do you create a water wave with only one molecule of water?

There's other, more subtle answers I'm sure. But that's the gist of it.

4. Two electrons are still two particles. So why the duality?

5. Two particles that can interfere like two waves. See this diagram.

Well actually you don't get the pattern unless you fire lots and lots of electrons. But you don't have to fire them all at once. You can fire them two by two.

6. So each particle acts like a wave, as opposed to it acting as a particle. Am I correct?

7. Sometimes they act as waves. Sometimes they act as particles.

For instance, electrons are particles because atoms can only gain or lose an integer multiple of an underlying base charge. And when they gain or lose this integer multiple, they also gain or lose an integer multiple of a certain mass. So we conclude that there's a particle with consistent mass and charge. An atom can lose or gain some number of electrons, but can't gain half an electron. So electron's are particles.

They just also happen to be waves.

Hence wave/particle duality.

8. Yes, I know they're particles - that makes sense to me. But they are also waves. To me, all waves are made up many particles, so there's really no duality.

For example, a water wave is made up of many H2O particles, and the wave acts like a wave, but its component particles also act like, well, particles.

So when is a wave NOT a particle? I'm slowly getting the hang of it, but I'm still confused.

9. But in this case a single electron is acting like a wave. Which is why I likened it to one hand clapping at the top of the thread. You need many particles to form a wave. This is because waves are energy transport mechanisms, not matter transport mechanisms. As a wave passes, a water molecule will change vertical position but generally not longitude/latitude. Which means if you have a wave it's moving faster than the individual particles are. If you have a bathtub full of water in the back of your pickup truck doing 60 on the highway, and form a wave, that wave is traveling 60 + it's wave velocity, which is probably like 3 or 4 miles per hour.

So the classical understanding of waves is insufficient in the microscopic world. Because the question is what is making the wave? Electrons are atomic and indivisible (well, not really, but that's a whole can of worms and not as relevant as you might think). How do you make a wave with just a single particle?

10. Ah, I think I get it.

So it acts like a particle, like, say, a pebble on the beach. Hence a particle.

Then it, by itself, acts like a wave of water, like the surf breaking on the beach - even though it's one pebble, and a pebble should not be acting like a wave, because, well, it cannot. Except it does.

Have I got it?

11. Yep

12. Thank you!

13. Wave-Particle duality comes from the problem that things on a quantum level exhibit both wave and particle properties,

For Example,
If you fire an light beam at a Young Slit's model (http://www.chemistrydaily.com/chemis...DoubleSlit.png) it behaves like a wave it defracts through the slit and a wave diffracting through multiple slits will collide constructively (2 dull diffractions will combine making a brighter wave (see the lower middle left of the link) And of course it should act like a wave, we all know light travels in waves right?

But if you take that same light beam, you can fire it at certain metals and essentially knock off an electron,
Example: If I take a light beam with 10eV of power and fire it at a sheet of iron for half a second, I will knock off an electron, If I take the same beam of light but only at 9.9999eV power and fire it at the sheet for half a second I won't knock an electron off.

If light really was just a wave you would expect that by firing the beam for a whole second would knock an electron off, because you've put double the power into the sheet right? Well this is where W-P duality comes in, because you can fire that 9.999eV beam at the sheet for a month non-stop and not remove a single electron, because according to the Particle Function light travels in packets of energy called Photons, and the above example is like throwing pea's at a window to break it all day, you can throw millions of peas and never break it, but you can throw a rock and smash it every time

and of course the Particle Function doesn't explain how light can diffract and behave like a wave, Hence the Wave Function and the Wave-Particle Duality theory which is simply that electromagnetic forces exhibit both wave and particle properties in different (and sometimes the same) circumstances

14. Any thoughts on how to resolve this duality?

15. Originally Posted by LegalEagle
I'm reading up on optics, and I know that there is a wave-particle duality to light, or so the current theory holds. But I'm not convinced there is a duality, or perhaps I just don't understand it.

A wave is still made up of infinitely small particles like photons, and, in other circumstances, electrons and protons. So a wave is still made up of particles, just as a wave of water is made up of many particles of hydrogen-oxygen molecules.

Am I missing something?
Photons are particles. Wave-particle duality is a bit of a misnomer.

But they are not classical particles. They are not "little marbles". Their motion is described probabilistically, and the probabilities are described using wave mechanics.

If you would like a nice overview, by the guy who developed the theory and won a Nobel Prize, written for a general audience, get yourself a copy of QED by Richard Feynman.

16. Couldn't you use Einstein's quote?

The first 11 seconds of his presentation.

17. Originally Posted by C_Sensei
Couldn't you use Einstein's quote?

The first 11 seconds of his presentation.
Einstein's quote is very profound, but it has nothing to do with the question that was posed. Right answer. Wrong question. This belongs in the thread.

18. Sometimes he said they're equivalent, but I have heard a recording where he says they are identical...in the first 11 seconds of this link he says that they are

different manifestations of the same thing.
Therefore, should we be surprised to find this 'duality'?

19. Well, if a lot of energy is a little bit of mass and vice versa, then why shouldn't a pebble sometimes be a wave and vice versa?

Maybe pebble equals wave c sqaured.

20. You can fire the electron one by one and still get an interference pattern. You can see a particle act like a particle instead of a wave by looking at the photoelectric effect. I don't think anyone understands why, but maybe I don't get it.

21. Originally Posted by LegalEagle
Well, if a lot of energy is a little bit of mass and vice versa, then why shouldn't a pebble sometimes be a wave and vice versa?
It could. But wavelength is inversely proportional to mass/energy. For something the mass of a pebble, its wavelength would be much much much smaller than its size. For a one gram pebble, its "wavelength" would be some 4 x 10^15 times smaller than the classical radius of the electron.

22. Originally Posted by Wildstar
You can fire the electron one by one and still get an interference pattern. You can see a particle act like a particle instead of a wave by looking at the photoelectric effect. I don't think anyone understands why, but maybe I don't get it.
Depends on what you mean by "understand". We have a rather elegant mathematical formalism that models wave/particle duality. But obviously we can't explain why nature follows the rules of this mathematical formalism.

Note that "wave/particle duality" is a term that people used a long time ago, before they understood things like quantum electrodynamics.

Since the development of qed, physicists have had a perfectly well-defined notion of what a photon "is". Unfortunately, it's impossible to explain this notion to anyone who doesn't have a significant amount of mathematical training. So when writing physics articles for the general public, one is forced to say that a photon exhibits behaviors that are both wave-like and particle-like.

The one exception is Richard Feynman, who I think did a very good job of explaining to a general audience what a photon really is, and where wave/particle duality comes from, in his book QED.

In fact, I think it's very safe to say that no physics education would be complete without knowing the contents of that book.

23. Originally Posted by salsaonline

The one exception is Richard Feynman, who I think did a very good job of explaining to a general audience what a photon really is, and where wave/particle duality comes from, in his book QED.

In fact, I think it's very safe to say that no physics education would be complete without knowing the contents of that book.
I'll second that sentiment. In fact I recommended this book earlier in this thread.

It is a wonderful little book, and requires very little background on the part of the reader.

24. wiki might help:

In physics and chemistry, wave–particle duality is the concept that all matter and energy exhibits both wave-like and particle-like properties. A central concept of quantum mechanics, duality addresses the inadequacy of classical concepts like "particle" and "wave" in fully describing the behaviour of small-scale objects.

makes sense - the way i see it the scientists who put this together had to use 'classical' physics as their starting point, so as soon as waves and particles became the same thing in their eyes they had to join the two together................

i might be really wrong here but wave-particle duality makes real sense - if it is a wave then surely the particles that make up the wave are behaving like a wave............. :?

25. Originally Posted by fatman57
wiki might help:

In physics and chemistry, wave–particle duality is the concept that all matter and energy exhibits both wave-like and particle-like properties. A central concept of quantum mechanics, duality addresses the inadequacy of classical concepts like "particle" and "wave" in fully describing the behaviour of small-scale objects.

makes sense - the way i see it the scientists who put this together had to use 'classical' physics as their starting point, so as soon as waves and particles became the same thing in their eyes they had to join the two together................

i might be really wrong here but wave-particle duality makes real sense - if it is a wave then surely the particles that make up the wave are behaving like a wave............. :?
The problem with your analogy is two-fold

1) The "wave-like" behavior can be seen to build up by the actions of discrete particles over long periods of time. It is NOT a matter of the "particles making up a wave" in many cases. For instance you might look in Silverman's book More Than One Mystery or the photos in this Wiki article that show the classic wave interference pattern being buildt up one particle at at time.
http://en.wikipedia.org/wiki/Double-slit_experiment

2) Despite the story that is told to freshmen and other undergraduates there is no duality. Photons, electrons, etc, are particles. But they are quantum particles and they are described by quantum mechanics -- they are NOT little marbles. It is their quantum behavior that results in what is observed as "wave-like" behavior.

Again, the reading of Feynman's little book QED is highly recommended.

Also, if something regarding quantum mechanics "makes real sense" then you can be pretty certain that it is wrong.

"There was a time when the newspapers said that only twelve men understood the theory of relativity. I do not believe that there ever was such a time. There might have been a time when only one man did, because he was the only guy who caught on, before he wrote his paper. But after people read the paper, a lot of people understood the theory of relativity in some way or other, certainly more than twelve. On the other hand, I can safely say that nobody understands quantum mechanics." – Richard P. Feynman in The Character of Physical Law

26. Originally Posted by DrRocket
1) The "wave-like" behavior can be seen to build up by the actions of discrete particles over long periods of time. It is NOT a matter of the "particles making up a wave" in many cases.
thats what i meant but you rephrased it to portray a better example of the observed matrix.

27. For those too lazy to read "QED", here is brief explanation of what quantum mechanics is all about and where wave/particle duality comes from.

In quantum mechanics, we can no longer know what the outcome of an experiment will be. It only makes sense to ask what the ''probability'' of a particular outcome is. These probabilities are absolute squares of certain complex numbers called ''amplitudes'' associated to each possible outcome.

Now the fundamental difference between quantum behavior and classical behavior is illustrated by the following example: Suppose that a particular outcome, A, of an experiment can happen in two ways, with amplitudes z and w (so that the corresponding probabilities are ||z||^2 and ||w||^2. In classical physics, we would predict that the probability that A happens is just the sum of the probabilities of the two ways that A can happen--that is: ||z||^2+||w||^2. Strangely, nature, for reasons unknown, does not appear to work this way. Instead, to get the probability for the outcome A, we have to add the amplitudes first, and only then square it. That is, the true probability for A is given by ||z+w||^2. This means that the two amplitudes associated to the two possible ways A can happen can constructively or destructively interfere with each other. This gives rise the wave-like behavior of particles observed in situations such as the double-slit experiment.

28. Originally Posted by salsaonline
For those too lazy to read "QED", here is brief explanation of what quantum mechanics is all about and where wave/particle duality comes from.

In quantum mechanics, we can no longer know what the outcome of an experiment will be. It only makes sense to ask what the ''probability'' of a particular outcome is. These probabilities are absolute squares of certain complex numbers called ''amplitudes'' associated to each possible outcome.

Now the fundamental difference between quantum behavior and classical behavior is illustrated by the following example: Suppose that a particular outcome, A, of an experiment can happen in two ways, with amplitudes z and w (so that the corresponding probabilities are ||z||^2 and ||w||^2. In classical physics, we would predict that the probability that A happens is just the sum of the probabilities of the two ways that A can happen--that is: ||z||^2+||w||^2. Strangely, nature, for reasons unknown, does not appear to work this way. Instead, to get the probability for the outcome A, we have to add the amplitudes first, and only then square it. That is, the true probability for A is given by ||z+w||^2. This means that the two amplitudes associated to the two possible ways A can happen can constructively or destructively interfere with each other. This gives rise the wave-like behavior of particles observed in situations such as the double-slit experiment.
1) That is an excellent explanation.

2) Anyone "too lazy to read QED" doesn't deserve the benefit of your explanation.

So, to those who are "too lazy: Get off your duff and go read the book.

29. particles emit wave particle duality because they are non-quantitative material compacted into quantitative densities. see my post on how gravity works. http://www.thescienceforum.com/how-g...rks-19843t.php ive thought about it for hundreds of hours and i think that im getting very close to exactly how it works.

30. Originally Posted by gravityguru
particles emit wave particle duality because they are non-quantitative material compacted into quantitative densities. see my post on how gravity works. http://www.thescienceforum.com/how-g...rks-19843t.php ive thought about it for hundreds of hours and i think that im getting very close to exactly how it works.