# Quantum Physics (I): Wave-Particle Duality 量子力學 （一）波粒二象性steemCreated with Sketch.

in #cn6 years ago (edited)

What is stuff made of? This is a question that was asked by many many great philosophers and scientists. Let me clarify: the question isn't: what is this wooden chair made of? (Wood, duh!). The question is: are there fundamental building blocks of the Universe? Today, thanks to Rutherford's scattering experiment, we know that stuff is made of atoms, or particles.

It is actually amazing how the world has so many complex objects and yet fundamentally they are all just a bunch of particles, all of which governed by the same laws of Physics. Physicists are really a bunch of particles trying to understand themselves! Atoms and their constituents are not all there is in the Universe, though, waves can also carry energy but they need not contain any particles (at least before we think about wave-particle duality). Waves are pretty!

Before we talk about the main topic, Wave-Particle Duality, which, as the name suggests, implies an object can be both a wave and a particle at the same time, let's make sure we understand what waves and particles are and why it is amazing that things can have wave and particle characters both at the same time.

Atoms were long thought to be particles. We can think of them as tiny tiny balls, so tiny that for most purposes we can think of them as points. The size of an atom is around 0.1 nanometre, or 0.0000000001 metres. We are all familiar with how particles behave: if we throw a ball at a solid wall, it will bounce back. If we throw a ball through a big enough gap on the wall, it will pass straight through as though the wall is not there whatsoever.

Waves, on the other hand, have very different behaviours. A very characteristic behaviour of waves is that they diffract when they pass through a gap. Diffraction is when a plane wave passes through a gap and comes out circular. This is very different to particle behaviour: as we just mentioned, a particle passing through a gap will just pass through the gap! It will not diffract or do other funny things. Let's look at some pretty pictures of wave phenomena. An example of a wave is water waves. We can generate them by dropping something into a pond of water:

If a wave passes through some gap, it diffracts.

Another everyday example of a wave is visible light; more specifically, visible light is an electromagnetic wave that propagates through empty space at the speed of light at around 300,000,000 metres per second. Other examples of electromagnetic waves include microwaves, UV and X-rays. Electromagnetic waves exhibit wave-like behaviours such as diffraction. A particularly interesting phenomenon takes place when a wave is passed through a double slit. A double slit looks something like this:

Waves passing through one gap interfere with those passing through the other, resulting in an interference pattern as shown in the picture below.

The point is that particles and waves are fundamentally different concepts; for instance, one cannot explain the interference pattern using a particle description. The understanding in the scientific world before the early 20th century was that certain things behave as particles and certain things behave as waves. For example, light was thought to be a wave while an electron was thought to be a particle. In the early 20th century, Einstein proposed that light must consist of particles in order to explain the photoelectric effect.

(I may write another post on the photoelectric effect if people are interested, but the point now is that the photoelectric effect only makes sense if light consists of particles, which we call photons. Einstein was awarded the Nobel Prize in Physics for explaining the photoelectric effect, although many would think that it was for his theory of relativity.)

Wait a minute. So is light a wave or a particle? In fact, both. The current understanding is that light is a wave and a particle. Under some conditions, it behaves as a wave, and under others, it behaves as a particle. This is wave-particle duality. The obvious question to ask is then, although it took several years before someone asked it: Can particles behave as waves? The answer is yes! Electrons, which we long thought to be particles, can actually diffract! A beautiful illustration of the interference pattern of electrons through a double slit is given below:

We can imagine firing electrons at the double slit one by one. Electron diffraction means that as we fire an electron through a double slit, it goes through both gaps at the same time and interferes with itself, producing the final interference pattern. By the way, electron diffraction is illustrated in the double slit picture above. Notice that the screen has multiple lines of electrons, i.e. an interference pattern! If electrons were particles, we would only expect there to be two lines of electrons on the screen.

If you are shocked after reading this, welcome to the amazing quantum world! I was so amazed when I heard about wave-particle duality and this eventually led me to study Physics at uni. There are many other interesting ideas in Quantum Physics that go against our natural intuition. This is unsurprising: after all, the quantum physics happens on scales so small that we do not experience it in everyday situations. However, very precise experiments can be done these days and quantum effects can be very clearly seen.

If you've read this far, thank you very much! :D Wave-particle duality is a central concept in Quantum Physics that encapsulates many ideas in Quantum Physics. This article has only covered a subset of the ideas. If you have enjoyed reading, please leave a comment to encourage :) Other comments and questions are most welcome! :D

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6 years ago (edited)

Very nice explanations. Really! (And believe me, I have already seen a bunch on this topic.) What are you working on exactly, as a half-physicist?

[I don't like self-advertising myself, but you may be interested by a quantum mechanics series I started months ago (and that progresses very very slowly) and where I have already addressed the photoelectric effect, for instance (but I haven't touched the particle-wave duality so far).]

6 years ago (edited)

Hi lemouth. Thanks for your kind words! I have recently graduated from a Maths/Physics degree, currently taking a break. So delighted to meet someone who writes about Physics! I shall check out your quantum mechanics series. I am still new and have started several 'series', although at present each of them only has one post LOL
Glad to hear that you have written an article on the photoelectric effect; I encourage those who have just read the post above to read it!

Information, the fundamental thing that all else is made from is information. Check out Physicist Thomas Campbell's work in this field.

I've always thought the uncomprehensible-ness of the wave-particle duality comes from the fact that we have defined a "particle" and a "wave" from what we have experienced in our immediate reality - we can see how waves work when we interact with water, and we generally believe that a particle acts in the way of a ball - a tennis ball bouncing around a room for example. Then at no point have we seen water act like a ball or vice versa - these bahaviours seem to be incompatible with each other. We have then applied these concepts when trying to understand the quantum world in some sort of top down metaphor, which ultimatly leads to an explanation that goes against our natural intuition. If we get rid of these preconceived ideas that we enforce onto quantum behaviour and take a "bottom up" approach, I think the concept is easier to understand. That is to say that an electron behaves in a certain way - in some circumstances it creates an interference pattern and in some circumstances it does not, these are its properties . These are the facts and our starting point in our understanding. Our similies and metaphors for understanding our universe can be limiting in some cases and we need to accept new fundamental concepts in order to grasp the phenomena what we observe. This is how I've always wrapped my head around this. Great article and a fascinating subject!

@bramlyapple I think you got this! Indeed Nature does not have to conform to our limited intuitions. We should perhaps think of an electron as sort-of-a localised wave packet that resembles both a wave and a particle, and hence has particle-like as well as wave-like behaviours. Indeed, the de Broglie formula tells us how to relate the momentum of a particle and its wavelength, so a wave description and a particle description are not mutually exclusive at all.

Amazing article.

Your post reminds me of my quantum physics class.

Well, I think everything has a dual nature. But it's mostly observable in smallest particles and electromagnetic waves.

Very nice to read it, and images and animations used are just outstanding.

Keep it up bro @john811. :)

What you send out is what you recieve ;)

@john811 there is always a huge leap in quantum physics

Indeed Quantum Leap is a thing!

Thank you for this very interesting article. It has been advertised on our chat channel (and upvoted).

The steemSTEM project is a community-supported project aiming to increase the quality and the visibility of STEM (STEM is the acronym for Science, Technology, Engineering and Mathematics) articles on Steemit.

6 years ago (edited)

Thanks so much @steemstem ! I'm quite new to steem. Please could you let me know how in other ways I can participate in steemSTEM activities? Also, what are the criteria for a post to be advertised by you?

Join us in the steemSTEM chat channel, links above~ We are current starting a science fair contest~

Sure. Thanks for letting me know biuiam!

Well that was a mind bender! Well done.

Well done to the amazingly clever scientists that did all the work for us :)

Awesome post!! better cite some source, for photos and contents, steemSTEM would prefer post with reference and source~

Hi biuiam, thanks for the kind reminder! All the images that I used are marked as 'free to use or share' by google. I will make sure that is 100% clear in the future. I will also cite sources when there are specific results that I quote. Glad you liked this post. I hope you will only be more satisfied with the future ones!

wow ,lovely article doc

Thanks lolcat :) Glad you enjoyed it!

good information

👍👍👍👍

wave particle duality does not exist. Define a wave. A wave is a process of transmitting force over a distance through a medium. A wave does not exist.
A wave is a process not a noun. Check out "Ken L. Wheeler" also check out www.thunderbolts.info

You can think of wave as the process of transmitting energy through a medium, this is perfectly fine. Or, you can think of the wave as a manifestation of the energy being transmitted. These may just be different ways of defining the wave. However, I think the definition I used is more conventional, as for example we normally think of water waves on an ocean as a physical thing, rather than a process. Interesting perspective though and thanks for your bringing this up @brainkerr !

6 years ago (edited)

A wave is what you get when projecting a point revolving around a circle on its axis ! At least that the trigonometric definition.

A signal spectrum is modelized as sum of wave with different frequencies ( point moving at different speed around the circle), using fourrier series.

amazing!! following you for more.

Many thanks alejandr0!

Definitely worth an upvote and a resteem :]

@smartdeveloper You're so kind!

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good education
thank you

You're welcome!

Thanks for your thorough post! If I may, I would like to hear what you think of the following:

Important features about the double-slit experiment are: the slit width, and the velocity of the particle (when the particles are larger such as a molecule, they must pass through smaller slits at slower speeds in order to produce an interference pattern).

What that tells us is that: diffraction is gravitational lensing. If the slit is large, the vast majority of the particles will pass through the slit at a distance from the walls of the slit that is too large to be substantially gravitationally lensed by the local gravity when passing through the slit. If the slit is sufficiently small, any slight variation in the flow pattern of the particle from the exact center of the slit will cause a local gravity variation on the particle that will arise in the observed diffraction. This would lead to a wave such as we see. If the particle is high in mass, it must travel slower so as to be exposed to the local gravity for longer so as to go from a particulate result to an interference pattern.

In this way, light is a wave of particles, in the same way as a wave in the ocean is a wave of particles. Other particles can also produce interference patterns when they are small enough in mass and/or travel slow enough to have the actual local gravity of the slit influence the trajectory of the particles.

-Steve

Hi Steve,

Sorry for the late reply. I must say what you said is very interesting, though I think if diffraction is gravitational effects then the observed diffraction pattern will not be vertical lines but a continuum instead.

Dicken

6 years ago (edited)

Hi Dicken, thanks for the response!

I agree, that's how it would seem but maybe there is something missing in the way that small particles interact in this way; like within certain ranges of the electromagnetic field of the slit material--within each "ring" of the field--they are gravitationally lensed similarly akin to orbitals. Not to say that that is what is going on, but there could be something missing in a particle interpretation based on gravity that is not immediately obvious. We happen to interpret interference patterns as the result of waves, but that does not preclude them from being the result of particles traveling together as waves on such a small scale level. Even single slits produce interference patterns, after all. Just some thoughts, still contemplating that exact thing myself.

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