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Quantum Introduction The Quantum Casino Quantum Entanglement Quantum Decoherence Reality Is Relative The Block Universe
The Arrow of Time The Anthropic Principle The Mathematical Universe Is the Universe
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"Truth is stranger than fiction; fiction has to make sense."
- Leo Rosten

Quantum Mechanics: An Introduction

At the smallest, subatomic level, quantum mechanics has made remarkable discoveries about the behaviour of elementary particles. It would seem logical to assume that by studying these fundamental building blocks of our universe we might gain some of our best insights into the true nature of reality.

The predictions of quantum mechanics are very much driven by experimental results. Quantum mechanics does not have much to say about "why" things happen, it can just be used to predict "how" things behave, that behaviour being based on the well-established results of experiments. And the most famous of those experiments is the double-slit experiment.

In September 2002, the double-slit experiment was voted "the most beautiful experiment" by readers of Physics World. Richard Feynman is said to have remarked that it contains everything you need to know about quantum mechanics. But perhaps the most useful property of the experiment is that it shows just how weird quantum reality can be!

In the double-slit experiment an electron gun is aimed at a screen with two slits, and the positions of the electrons is recorded after they pass through one of the two slits, making little dots on the screen. It is found that an interference pattern is produced on the screen just like the one produced by diffraction of a light or water wave passing through the two slits. There are bright bands ("constructive interference") and dark bands ("destructive interference").

Double-slit experiment

This is quite strange: electron particles are interfering with each other as if they were waves. However, things turn much more weird if we only emit one electron at a time. We wait until an electron makes a dot on the screen before emitting another electron, so there is only ever one electron in the system at a time. A pattern slowly builds on the screen, dot-by-dot, as each individual electron hits the screen. What we see is quite incredible: the accumulation of dots on the screen eventually produces a pattern of light and dark bands - an interference pattern emerges even though there is only one electron in the system at any one time! The electron appears to be interfering with itself. So in some strange way it is as if the electron goes through both slits at once!!

How can this be? Maybe half the electron is going through one slit and half through the other slit? But if we put a small detector screen on the other side of one slit we only detect whole electrons sometimes passing through (sometimes passing through the other slit). It's as if the electron does indeed pass through both slits at once, but when we make an attempt to detect it, it suddenly decides to act like a single particle which has gone through just one slit!

It is only in the last ten years or so that quantum mechanics has at last been able to shed some light on what is happening in the double-slit experiment (this will be considered later in the page on Quantum Decoherence), but before we get to that stage there is a fair bit of theory to be covered. And it's best to start at the beginning.

Quantum mechanics could be said to have started in 1900 when Max Planck made the discovery that light, which was considered to be purely wave-like, was in fact composed of energy which came in discrete packets (called "quanta"). In the Planck formula, the energy of the packets, e, is proportional to the light frequency, f, the constant of proportionality being Planck's constant, h:

This result suggested that waves (light) were in fact composed of particles. The converse of this result came in 1923 when Louis de Broglie (pronounced to rhyme with "destroy") suggested that matter (particles) behaves as a wave (as is evident in the double-slit experiment), the wavelength, , being inversely proportional to the particle's momentum, p. Here's the derivation:

We now know that absolutely everything in the known universe is made out of these strange particle/wave entities which obey these two formulae for quantum behaviour, given above.

How can we make sense of this strange wave/particle duality? Would it be possible to combine these two results in a single equation, and in the process reveal more insights into the true nature of reality at the quantum level?

Yes, Erwin Schrödinger did it in 1926. And we'll see that in the next page on The Quantum Casino.

The Heisenberg Uncertainty Principle

Now let us imagine that we want to measure the position of a particular particle. In order to do this, we must "see" the particle, so we have to shine some light of wavelength, , on it. This means there is an inevitable uncertainty about the position of the particle (due to the resolving power of the light used), the uncertainty in the particle's position being:

When the light particle (photon) hits the particle under observation it inevitable alters its momentum (speed) according to the result of Louis de Broglie (given above):

and on combining these two equations we get:

which is the Heisenberg Uncertainty Principle.

The Heisenberg Uncertainty Principle tells us that the more accurately we determine a particle's position, , the less accurately we can know its momentum, . This Uncertainty Principle equation has been used to generate the animation below:

The animation shows the relevant spreads in the uncertainty for position and momentum (for a light wave, and the light wave's corresponding photon particle). From the result of de Broglie (considered above) we know that if we have a precise value for a particle's momentum, p, then we have a precise value for the wavelength of the wave, , and hence a precise colour of our light wave. A precise value for particle momentum is shown in the above animation when the momentum value becomes a single vertical point. You will see that when that occurs, the corresponding uncertainty in the particle's position spreads out to infinity, i.e., when we have light behaving as a normal light wave (with a colour) we can have no idea where the corresponding particle is. In the wave/particle duality model this shows the case when the light is acting like a wave, not a particle.

Conversely, the situation when we have a precise position for the particle, e.g., when we shine the light wave onto a screen and detect the proton, is shown in the above animation when the position value becomes a single vertical point. You will see that occurs, the corresponding uncertainty in the particle's momentum spreads out to infinity, i.e., we can now no longer determine the wavelength or the colour of the light wave. In the wave/particle duality model this shows the case when the light is acting like a particle, not a wave.

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Comments are now closed on this page.

Hi Andrew, thanks for these wonderfully entertaining pages - I'm in the process of reading them all. I noticed a small typo on this page: s/proton/photon/g (1 occurrence). - John, 31st July 2007
Thanks so much for calling these pages "entertaining". That's probably the most satisfying thing you could have said. If I've succeeded in presenting physics in an entertaining manner (rather than dry and dull technical papers) then I'm very happy. Thanks also for the typo spot. I've fixed the text now. Typo and broken link reports are always very welcome. - Andrew Thomas, 31st July 2007
I'm no maths/physics student but surely if you measure a particles position TWICE you can determine it's momentum. If particle z is at location a,b,c (3d co-ordinates) and then measured one tick later at a2,b2,c2 why isn't it's momentum obvious? - Neil Barnes, 11th September 2007
That's a really good question, Neil, but you're thinking in terms of classical physics. What you suggest is perfectly correct for large, "ordinary objects" which are large enough so that quantum effects are not evident. But down at the smallest scales, things just don't work like that. When you start dealing with the smallest particles, they behave very differently, counter-intuitively. After all, how would we measure the position of the smallest, elementary particle? We could only do that by interacting with it, by getting the original particle to collide with another (measuring) particle - thus inevitably modifying the very position measurement we were trying to obtain! Even with large objects we have to modify them when we take a measurement. For example, when we take the temperature of an object we have to remove a small sample of heat from the object. Or when you check the air pressure in your car tyres you have to take a bit of air out of the tyres. These effects are so small with large objects that we do not notice them (so your proposal would be fine in that case), but we can't ignore these effects when we deal with quantum objects.

So these elementary particles behave very differently to "ordinary objects". They behave according to quantum mechanical rules: sometimes they behave like a wave, sometimes like a particle. The more we constrain one property (say, the position property) to act like a particle, localised in space, the more the other property (momentum) acts like a wave, spread out in space.

Wikipedia provides a helpful analogy with sound waves which resembles your "two points in space" idea: "It is meaningless to ask about the frequency spectrum at a single moment in time, because the measure of frequency is the measure of a repetition recurring over a period of time. Indeed, in order for a signal to have a relatively well-defined frequency, it must persist for a long period of time, and conversely, a signal that occurs at a relatively well-defined moment in time (i.e., of short duration) will necessarily encompass a broad frequency band." See So the more you try to pin-down the frequency of a wave, you need to stretch out your time measurements (the two position measurements you mention), and the more accurately you know the time, the less accurate is the frequency measurement.

But thanks for your idea. You'd be correct for everyday objects, just not for quantum mechanical objects. - Andrew Thomas, 11th September 2007
In the Double slit experiment-If we shoot only one electron at a time at the two slitted screen, and do not attempt to measure which slit it went thru, do we get a separate interference pattern from each such single electron, or only from the conglomeration of many electrons? - Paul Gayet, 3rd October 2007
I see what you're getting at, and the text was not clear about this. Each electron, after it has passed through the slits, makes only a single point on our detector screen: when we try to detect a particle, we see a particle. We see the interference pattern in the distribution of dots on the screen - that's when you see the light and dark areas. So we need many electrons making many dots. I've just amended the text to make that clearer. - Andrew Thomas, 3rd October 2007
Hi Andrew! I'm just a dilettante reader about quantum physics, but even so I want to thank you for stopping the final meltdown of all my neurons with your intelligible explanations.
I'm just curious: there is no info about who you are and what is your work and why you do this kind of mathematical charity ?!
All comments date from few months ago. Hv u started this site so recently?

Francisco Peres, 48, Curitiba - Brasil - Francisco Peres, 4th November 2007
Hi Francisco, thanks for your comment. I've been doing this site for three years but it only went live this year. I do it because I love it and I'm fascinated by these subjects. The underlying principles are usually surprisingly simple, and I think it's important to get that across. - Andrew Thomas, 4th November 2007
what the double slit experiment is telling us is that subatomic particles are able divide their energy and still maintain the properties of the original particle. What enables them to rejoin? - eleeo, 13th November 2007
Eleeo, you'll have to read the remaining pages! Interpretations of quantum mechanics suggest that there is no such object as the "original particle" to which you refer. Instead, before we detect the particle, we must consider it to be unreal - and we have to consider it as being potentially in all positions at once! Only after we detect the particle does it become "real", with a clearly-defined position. But that's all covered in the later pages. - Andrew Thomas, 13th November 2007
Hi there...just wanted to let you know this is a fascinating website. I am not a physicist, rather a ghost hunter who relies on scientific backing. Great resource for understanding how paranormal behavior is possible through quantum! - Rebel, 25th February 2008
Oh well, that wasn't quite my intention but that's certainly interesting! - Andrew Thomas, 25th February 2008
Hi this is Tiffany in Vancouver Wa. USA
I am so glad I found this web site because you present the material so that I can understand it.
It really gets me thinking alright.
A real good starting point for my study of Quantum Mechanical Concepts.
I am fond of saying that we are all here because we probably can be because I believe this to be true.
Well--that is all I have to say.
Tiffy - Tiffany, 19th March 2008
I am an engineer and I have at least 10 years of reading physics behind me, but this is one of the most accessible explanations I have read. Normally, I doze off in the middle.

And here I am, taking notes!...

1/f ))) - DJ Fadereu, 21st March 2008
Hi Andrew,

Your derivation of the uncertainity principle is very counter-intuitive. I'm not being able to understand how you derived -

p = h/lambda
=> delta p = h/delta lamba

this seems incorrect (from a calculus viewpoint)

Wouldn't an approach using operators be a more correct approach (although, it will be a more mathematically rigorous approach)? - Kedar, 17th May 2008
Hi Kedar, yes, you're right - it's not particularly rigorous. I'm trying to explain quantum mechanics to everyone, and show that even someone with school-level maths can follow the derivation of the Uncertainty Principle! Hence showing that quantum mechanics is nothing to be scared of. Thanks for your comment. - Andrew Thomas, 17th May 2008
Hi Andrew, Lovely effort. However, would be worth checking your technical details in authoritative texts, even beginning with an undergraduate text such as Tipler's "Physics".
E=hf and p=h/lambda
were known to be true for photons. de Broglie then POSTULATED that these relations hold also for material particles. See Tipler for an explanation of how he came to make these postulates.

For a material particle, the relationship between the REST MASS of a particle and its energy is a little complicated. Even if you use the relativistic mass, m, when, indeed,
then still
p = m u,
where u is the velocity of the particle. In particular,
See Tipler for more details. - Dick, 19th May 2008
Thanks for that, Dick. Like I said in my comment above, I'm not trying to be rigorous at all (yes, get a textbook I want rigour). I'm just trying to convey an understanding in the simplest form possible. - Andrew Thomas, 19th May 2008
Hi, Andrew, That's fine. But, you will get physics students and even potential physics students coming to your pages. Indeed, you see from Kedar's comment how important it is to get whatever technical details you give right. So, it's best then if you don't include any technical stuff at all or otherwise try and get it checked out by a physicist friend who knows his/her stuff. You should find something like Tipler's "Physics" quite simple and experts have contributed to it. For more advanced material, I would suggest you make reference to where you've got it from; it will help people who are so enthused by your pages as to want to delve further into the subject.

Cheers, Dick.

P.S. In response to Kedar's query:


p = h/lambda
=> delta p = delta(h/lamba) = h*delta(1/lamba)


delta(1/lamba) = -(1/(lambda^2))*delta(lambda)

Well, approximately, as this is only strictly true if delta is an infinitesimal! - Dick, 21st May 2008
Thanks for that. I will think about what you say. I would say, though, that I think there is nothing technically incorrect about any of the maths on the site. But do let me know if you find any error! Thanks. - Andrew Thomas, 21st May 2008
Errors! On this page, which I've taken some care to look at, as I pointed out in a previous email, for a material particle,

p = m u < m c

i.e., p = mc ONLY for massless particles such as the photon. p is not equal to mc for a material particle with non-zero rest mass.

Also for light of a definite wavelength, lambda, i.e., for a plane wave, Delta x is actually infinite, not lambda. And, in this case, Delta p is zero, as p has the definite value,

p = h/lambda

The rigorous way to derive the Uncertainty Relation is to do it for a wave packet. As I suggest, have a look at Tipler's "Physics" or similar simple text, if interested. Although in Tipler the general wave result for a wave packet is just quoted! But, it does give you an extra factor of 1/(2*pi). Mathematically, there's a further factor of 1/2 (see Wikipedia on the Uncertainty Principle). However, this might be a good case where it might have been best to just quote the result and used your wave pictures in momentum and in position to illustrate the Principle. Cheers:) - Dick, 22nd May 2008
Hi Dick, I'm very keen to correct errors and always welcome feedback. Thanks.

Considering your first point, you're right about the energy of particles - I have modified the text to make it clear I am dealing with photons. Good spot, thanks. For the second point, we are taking a measurement of position of the particle (using incident light), so delta x is finite not infinite. So the lambda refers to the wavelength of the *incident* light used in the measurement, and it is the resolving power of that light which gives the delta x. That incident light then affects the momentum of the initial photon (so delta p is not zero). So it's the wavelength of the *incident* light which connects the change in position AND momentum of the initial photon.

I think it's a really simple and ingenious derivation, and I stand by it. People have nagged me before about my maths so hopefully I have ironed out all the remaining errors on the site. But thanks for the details in your own comment and the reference to Tipler's book - people can follow that up if they want rigour. On this site, though, I keep it simple and clear. - Andrew Thomas, 22nd May 2008
Hi, Andrew, I don't want to go into it in detail. Remember that in Quantum Mechanics, the probability of finding a particle at a particular position, x, is proportional to (equal if psi(x) is a normalized wave function)


where psi(x) is its wave function.

For a wave with the definite wavelength, lambda,

psi(x) = exp(i(kx-wt)),

where k is the wave number, 2*pi/lambda, and w is the angular frequency. In which case,

|psi(x)|^2 = 1,

whatever the x. Thus, there is an equal probability of finding the particle anywhere. Hence, the uncertainty in the particle position is infinite.

Just look at your own picture for a pure sinusoidal wave in x. In particular, the wave has a definite wavelength. There you can see that the particle position is very uncertain, but on the other hand the plot for p is just a spike, i.e., p is a definite number and so there is no uncertainty in p!

Cheers:) - Dick, 23rd May 2008
Hi, Andrew, My apologies, as I had glanced over your work. Didn't realize that you were proposing a thought experiment to try to derive the Uncertainty Principle. The worry I have about the thought experiment is that it would also apply for a classical system, yet the Uncertainty Principle concerns the intrinsic quantum mechanical property of objects, their wave/particle duality. Unfortunately, the rigorous way of deriving it is quite mathematical. Cheers:) - Dick, 23rd May 2008
Hi Dick, thanks. Your comment (above) did get stored - my anti-spam software is over-zealous! - Andrew Thomas, 23rd May 2008
Hi, Andrew, Still worried about your thought experiment. What do you mean by stating that light of a particular wavelength, lambda, has resolution, Delta x approx= lambda?

Consider a plane wave incident on an infinitely massive point object. Then, by observing the scattered spherical wave, the object's position is determined with certainty as being at the centre of the sphere of the scattered wave. Thus, in this case, Delta x = 0.

As I said, I don't see where in your thought experiment the instrinsic quantum mechanical nature of the object being observed comes in.

Cheers:) - Dick, 3rd June 2008
Hi Dick, you're not happy, are you! The thought experiment I described was basically Heisenberg's Gamma Ray Microscope:

I agree about your second point, and that link I just posted says it well: "Looking closer at this picture, modern physicists warn that it only hides an imaginary classical mechanical interaction one step deeper, in the collision between the photon and the electron. In fact Heisenberg's microscope, although it was a big help in developing and teaching the quantum theory, is not itself part of current understanding. The true quantum interaction, and the true uncertainty associated with it, cannot be demonstrated with any kind of picture that looks like everyday colliding objects. To get the actual result you must work through the formal mathematics that calculates probabilities for abstract quantum states." - Andrew Thomas, 3rd June 2008
Much of this stuff i don't totally understand. I do know that i find it fascinating and enjoy reading about it. Do you have to be a genius to actually be able to get into this stuff? Or could an average guy like me get into college for it? - Victor Wilson, 31st August 2008
Victor, a fairly decent mathematics education would help, but even if you don't have the maths you should be able to follow the principles of quantum mechanics. Basically, there are several competing interpretations (i.e., guesses at what is going on "behind the scenes") but nobody on the planet knows which interpretation is correct (or, indeed, if none of the interpretations is correct). So as long as you can understand the principles behind the interpretations you know as much about the truth of quantum mechanics as any Nobel prize winner!

In fact, you could even propose your own interpretation of quantum mechanics, and - as long as it agreed with experimental observation - it would be just as valid as the existing interpretations.

I can recommend "Quantum" by Jim Al-Khalili as a good introductory book. - Andrew Thomas, 31st August 2008
I am actually inquiring of one of your pieces of art. I googled 'inside out' and a piece of yours (orange/brown)came up. I am organizing a conference for young women focusing on making the inside better so the outside can be its best (life/body) and I would like to use this piece of art on my posters and promo material. May I? - Jean, 6th October 2008
Yes, thanks for asking. Good luck with the conference. - Andrew Thomas, 6th October 2008
Hey Andrew,
I am astonished of how simple you explain quantum mechanics, I've never found something like that.
Now my problem is,
you say that delta(x) ~~ lambda
How do you get to this formula?
Thanks - Christoph, 10th October 2008
Thanks, Christoph. The relationship between the resolving power of a microscope and the wavelength of light is described here: It shows that there is a close relationship between delta(x) and the wavelength of the illuminating light. - Andrew Thomas, 10th October 2008
Thank you for this succinct and exceptionally clear explanation of the uncertainty principle.
It is now very clear to me where Heisenberg and De Broglie erred.
The uncertainty principle only applies to measurement using electromagnetic waves. It implies that there is a limit to the resolving power of any measuring instrument based on electro-magnetism that can ever be build.
However, Heisenberg and De Broglie commit a logical fallacy in thinking that a particle cannot exist with a distinct position and momentum at the same time just because those parameter cannot be measured with accuracy at the same time using electromagnetism!
There could exist other measuring systems that do not rely on EM waves, maybe using dark energy or neutrinos.

I would sincerely appreciate it if you can comment on my thought (info (at) biotele (dot) com)

Thank you - biotele, 30th October 2008
Hi Biotele. No, I'm afraid my description might have misled you as quantum mechanics says there is a fundamental limitation on our knowledge no matter what method we use. We can never know a particle's position and momentum at the same time by any method. I just used the example of electromagnetic radiation as it was easier to understand and not too mathematical. - Andrew Thomas, 30th October 2008
This is one of the few resources (that I'm aware of) which explains things in a such way so everyone can understand how things work at the microscopic level, I must say.

However, there are still some concepts that I don't understand.

Where does the uncertainty principle apply to? Elementary particles and photons?

Why is it impossible to measure both position and momentum precisely? Is it because the observer changes the nature of things?

"when we have light behaving as a normal light wave (with a colour) we can have no idea where the corresponding particle is" -- I thought light was made of electric and magnetic fields only. So where does the particle you're talking about come in?

About the gifs you provided what are those waves refering to? Is it the space where you might find the particle within their envelope?

According to de Broglie postulate do particles whether behave literally like a wave or appear to act like one? It doesn't make sense to me imagine that a particle following a wave path will produce the same results as a real wave (ie. radiation)...

This kind of questions are poorly described in almost every resource (articles, books, etc), even from lecturers. That's why quantum mechanics is not a easy subject to grasp, at least for me as I like to understand things in detail. - Tiago, 2nd November 2008
Hi Tiago, thanks. In the most general sense, the uncertainty principle applies to everything physical, because everything is made of particles. So we can never know the exact position and momentum of ANYTHING. However, the effect would only have a noticeable impact for particles.

You asked: '"Why is it impossible to measure both position and momentum precisely?" Well, if we knew the answer to that we would get a Nobel Prize. It's just the way the mathematics of quantum mechanics works. It's just the way the quantum world is.

You asked: "I thought light was made of electric and magnetic fields only. So where does the particle you're talking about come in?" Light can be considered as a particle OR a wave, the particle is a photon. You need to read about wave/particle duality:

Particles behave like a REAL wave, not as a particle following a wave. It's all very strange! But that's quantum mechanics. - Andrew Thomas, 3rd November 2008
Firstly, thanks for your reply.

Imagining we were the same size as the smallest particles... Would we be able to measure them (position and momentum) precisely?

Do the particles always behave like a wave when they aren't under any measurements? Is that why the quantum world is probabilistic?
Are their wave-behaviour related to their wavefunction? - Tiago, 6th November 2008
Hi Tiago, "Imagining we were the same size as the smallest particles... Would we be able to measure them (position and momentum) precisely?" That's very imaginative! I wouldn't imagine that would make a difference - we seem fundamentally limited in our knowledge no matter what method we use.

You asked "Do the particles always behave like a wave when they aren't under any measurements?" Well, it's true to say that before we detect a particle's position it acts as though it is in a superposition state, so yes, it behaves like a wave. You then asked: "Is that why the quantum world is probabilistic?" Well, not really, but when we detect the particle's position we get a random result, so that is where the probability comes in. You then asked: "Are their wave-behaviour related to their wavefunction?" Well, yes, the wave behaviour IS related to the wavefunction in that two wavefunctions for two different particles can add together to create constructive or destructive interference (just like a wave). And after we know the shape of the final wavefunction, Max Born said the probability of finding the particle at a certain position was equal to the square of the wavefunction. But the wavefunction is more than a simple probability wave as two wavefunctions can introduce interference effects (seen in the double-slit experiment) but probabilities can never be negative (and so can never produce interference). - Andrew Thomas, 6th November 2008
"we seem fundamentally limited in our knowledge no matter what method we use" - What does this uncertainty has really to do with then? Is it due to the wave-particle duality or we cannot measure anything precisely (ie. I can't tell you that my desk is exactly 1m wide, because it's precise real value goes to infinty (for instance, 1.00010000200001000300...m))?

What do you mean with superposition state? You're saying that before any measurements a particle is spread in space so it doesn't have a well defined position only probabilities where it might be; therefore the particle seems to exist in different places at the same time? Is that when quantum mechanics gets bizarre? :P

I thought the wavefunction would give you the probability of finding a particle in x at a/any given time... I can't understand the difference between the wavefunction itself and its square... - Tiago, 6th November 2008
Hi Tiago, at its most fundamental level quantum mechanics is impossible to understand from a "intuitive, common-sense" viewpoint. There's just nothing in our everyday experience which behaves the same as quantum mechanics, so the intuitive examples you provide (such as the length of the desk) are really not applicable. We can only express quantum behaviour through mathematic equations, and it is the mathematics which tells us that there is a fundamental uncertainty about our knowledge (through the uncertainty principle formula).

Moving on to consider the wavefunction, the wavefunction is a very peculiar entity which again has no real parallel in our everyday experience as we can only express it accurately by using mathematics - the phase of the wave is a complex number. This is described in more detail on the "Quantum Casino" page. But, yes, by squaring the wavefunction we convert the complex number to a real, normal number which we can take to be the probability of finding the particle. - Andrew Thomas, 7th November 2008
Well Andrew, I guess my problem is that I was trying to understand the quantum world from a "intuitive, common-sense" viewpoint.

So let me see if now I can understand it. The uncertainty principle is a built-in uncertainty in nature so to speak. It also comes from the fact that the observer plays an essential role in the behaviour of particles. If we aren't taking measurements about the position of a particle, the particle itself is spread out over space (wave behaviour), and as we don't know its position the particle seems to exist in every possible position where it might occur (superposition). Therefore the better we know about its momentum (I'm following the maths but this seems to make no sense at all lol), because its wavelength is related to where the particle might be found. However, if we try to measure the position of a particle, the wavefunction (wave behaviour) collapses and the momentum/wavelength (again this only makes sense using the maths lol... momentum of the particle or momentum of where it might be found?) is no longer certain.

Did I mention something wrong? Am I in a good way about the understanding of quantum mechanics? - Tiago, 9th November 2008
Tiago, yes, you've got it! It's a shame we can't explain it in easier terms, but that is basically the way it works. - Andrew Thomas, 10th November 2008
Hello everybody,
Could one say: the future is superposition, the now is the wavecollaps and the past has no Heisenberg uncertainty ? - Frans , 21st December 2008
Hi Frans, yes, you could certainly say that. What you are suggesting is basically the quantum mechanical arrow of time (the wavefunction only collapses in the forward time direction). Maybe have a read on the section on entropy on the Quantum Decoherence page. - Andrew Thomas, 22nd December 2008
Hi Andrew, I am a sophomore in college completing an independent study in physics/mathematics on quantum mechanics. Specifically, the double-slit up to the uncertainty principle.--I just wanted to say that your clarity is unrivaled and I legitimately understand some quantum theory because of it. Thanks a ton, you've been very helpful. - Christopher Giuliano, 18th January 2009
Thanks a lot, Christopher. I'm glad it helped. - Andrew Thomas, 19th January 2009
i m doing MSC in physics .this site helps me so much in QM so thanks making this wonderful site

- sapana, 23rd January 2009
The electron gun in Young's double slit experiment can presumably be aimed. 1)What happens as the gun is turned? 2)Laser light can be aimed precisely. What if you aim the gun to always pass through one slit? - Kingstilts, 24th January 2009
Hi Kingstilts, yes, you make a good point in that most descriptions of the double-slit experiment contain unstated assumptions regarding the type of light and the position of the slits.

Firstly, you have to use a laser as the light has to have a single wavelength (monochromatic) in order to produce the interference pattern. In practice this means that the slits have to be less than a millimetre apart so they can both be in the laser beam.

Thanks for your comment. - Andrew Thomas, 26th January 2009
What can one say Andrew other than Quantum Physics is amongst the most difficult of subjects to teach well and properly, and in this page and the 12 that follow (so far) you have not just met the challenge but have met and as we say in America, OWNED it.

Well well, done, very well done. Thank you. I hyperlink your home page often to those who wish to be introduced to the clearest (that I can find) introduction to this very intimidating, but shouldn't be intimidating, subject. Thanks again.

I particularly love the way you begin with the notorious (yet wonderful) double-slit experiment, then quickly segue into Uncertainty.

I have never read a clearer more concise condensation in my life. I call Uncertainty "The Wall," because when I learned it at age 22, it's non common-sensical result inflamed and killed what until then was a deep love for Math and Science, and not my bad professor's fault (though he was bad not uncommon given America's "Publish or Perish" higher education philosophy ... plus he ws Physics Dept. teaching 3rd-year Engineers Intro to Physics 2), but rather MY fault for not putting in what was probably the 30 extra minutes to understand what Heisenberg was really saying.

I have since returned, because it is a REAL fact of life, Quantum Physics. Without Quantum Tunneling in particular, there is no transistor, no Microchips, no Electronic Computers, or cell phones.

We all use Quantum Physics every day.

Take nothing for granted. - Greg Sivco, USA-NJ, 23rd February 2009
Thanks a lot, Greg. I quite like the idea of "owning" quantum physics! - Andrew Thomas, 24th February 2009
"Own" is American slang for "you totally understand the subject."

And yes you do own it. :-)

I'm a bit sad you don't get more hits. I assume your Marketing method is "word of mouth"?

I ask ....

Can we do better?

IMO, every college kid studying this subject for the first time should come here first to make this allegedly "difficult" subject understandable, and before their first class in the subject.

How can we help? - Greg Sivco, 23rd March 2009
Hi Greg, I get about 200 vistors a day which is a number I am more than happy with (the site ranks highly on Google for just about all its subject areas, and there's a lot of links on Wikipedia to the site). Most of those visitors appear to be just casual visitors who do not stay long. But about 5% take a real interest in the subject (people like yourself).

To be honest, I'm really only interested in that "5%". I'm not particularly interested in increasing my visitor numbers as this is very much a niche field so I'd only be sucking-in more casual visitors if I increased my visitor numbers. - Andrew Thomas, 24th March 2009
Hi Andrew,
I admire your knowledge greatly, but find it or science lacking only the simplist wisdom or truth.
If your looking for the truth of nature, One need only to remove any uncertainties or doubts.
Once removed, both mathematically and emperically, = or equal is all that remains.
Measure was the flaw.
And truth was much more simple than thought.
E = mc2 has been reduced to truth and I thought you would like to know.
Have a truly equal day,


Most people prefer theories and faiths over truth, because for most, truth has yet to be seen.
- Michael J Ahles, 2nd April 2009
OK Mike but don't you accept that there are theories out there that ARE true, and that one of the faiths may be correct? - Greg Sivco, 2nd April 2009
This is an excellent QM presentation.

In the 2 slit experiment you are claiming no other electrons in the system; what about the electrons of the material forming the 2 slits? What if it is the absence of matter (slits) that influence the spatial permittivity that causes the diffraction patterns? I don't really believe the test system has been thoroughly thought out! Physical reality is neither weird nor strange; just unknown.

Just thought I would give you all something to ponder other than accepting absurd mathematical paradigms for reality. - David, 6th April 2009
That's a good point about the electrons in the slits, David. Those electrons form part of the system as well, and the interaction of the beam with those electrons is all part of the dynamics of the system.

As you will see on the Quantum Entanglement page, we have to consider the slits, the beam, and the screen as just **one** entangled system. - Andrew Thomas, 7th April 2009
If visible light is reflected by particles of unknown location, is it natural to assume that the more discrete portions of the EM spectrum are invisible to us because of this. Because they are definite, real, unchangeable. Whereas the flip of wind that catches the leaf in the tree across the street, is visible to us because it is glowing with possibility, and our curiosity-driven brainputers attach significance and meaning in order to study an aspect of our own nature more closely. - Sol, 6th May 2009
I have a layman Q: as photon is massless, how doe a single photon has energy? According to E=1/2*M*V^2 or E= MC^2, a massless photon can not have energy.

As a photon is massless, then it shall not affect a particle when we use a photon to observe a particle. - Wang, 10th May 2009
I have another Q: are there any tests done to test if space/time is quanta or not? - Wang, 10th May 2009
Hi Wang, firstly, a photon is considered to be pure energy, so no problem there.

Secondly, yes, there are experimental attempts to show that space is quantised: - Andrew Thomas, 11th May 2009
Andrew, in the last link you provided, I think there is a small error. See if you notice it and confirm if I'm right, thanks:

"In these theories the vacuum is treated as an 'energy foam' in which microscopic quantum energy fluctuations occur on approximately the Planck length (10E-33 cm) and time (10E-19 GeV)"

The 'error" I believe is in the units of time. Shouldn't they be "sec" not "GeV"? - Steven Colyer, 11th May 2009
That's a good spot, Steven. They actually mean the Planck energy (1.22 10E19 GeV). - Andrew Thomas, 11th May 2009
I'm making my way through your entire presentation, which is, well, ace. I shall probably prod you at intervals and will do so right now.

There's actually an error -- rather, a misinterpretation -- towards the end of this particular page; you refer to the wavelength of the light as being, or perhaps as producing, its 'colour'.

This isn't so, if you will excuse me. The colour of a beam or patch of light is not defined by its wavelength, but by the effect a light-beam of that wavelength has upon my conscious mind.

This effect results from the conversion of photons by the retina into nerve impulses which are transmitted along the optic nerve to the primary optic cortex of my brain; these impulses are mingled with, and modified by, nerve impulses from other parts of my brain -- all of this being performed according to rules and processes which we understand quite adequately in physical terms -- and are then transferred, this time by a process about which we have not the faintest beginnings of an inkling of an idea, to 'my mind', where they produce the sensation of a colour.

This 'sensation' -- one of a group of entities called 'qualia' in neurology -- has a property which we cannot even describe, let alone define or measure, but which we call 'colour.'

We have, though, developed an accepted coding system, which allows us to label the conscious sensation thus produced as one (or more) of a series of names, such as 'red', 'blue', 'green' or 'pink' and even, imprecisely but understandably, as 'sort of greenish-brown.'

These are the names of colours, and the name I give to a colour depends upon the wavelength of the light which originally impinged upon my retina and produced it. Importantly, though, and unlike wavelength, colour cannot be directly described by me to you, nor by you to anyone else. The only way we can use those words to one another and be understood when we do so is simply and solely because, as each of us has grown up, we have become used to pointing at different objects and saying to one another "that's a red ball", or (of a cloudless sky) "that's blue" or of grass that it is "green."

There is no way of knowing whether what I experience when I describe the sky as blue is the same as what you experience when you see the same patch of sky. We use the word 'blue' purely as a conventional label, so that we can talk to one another about the way things look to each of us.

We cannot measure 'blueness', as we can measure the wavelength of blue light in nanometers.

Blueness (we might fairly say) is not amenable to physics. We might also describe it as being 'immaterial'.

And the interface between the material world of conventional physics which we access via our brains and other physical instruments, and the immaterial world which we access with our minds, is going to become crucial to our understanding of The Way Things Are, via the quantum world which, crucially, includes observation by the mind. - Martin Woodhouse, 22nd May 2009
Hi Martin, thanks for that. Yes, it's a perhaps a bit sloppy to directly equate wavelength with colour without stating that some form of human interpretation is required. - Andrew Thomas, 25th May 2009
Appears that the word "of" is missing after "position" in "Now let us imagine that we want to measure the position a particular particle.". - Hamish, 8th June 2009
Thanks a lot, Hamish. I'll fix that now. - Andrew Thomas, 8th June 2009
Most simplistic explanation I have encountered on the web! I honestly don't understand the mathematics involved, but I should probably mention I am 13 years of age... However, I am very interested in quantum physics and will surely come back and check this site out once more after I receive my high school diploma ;)
I'm always up to a good challenge and understand completely this material is not for me...
- Dan, 31st July 2009
Awesome, being able to understand Einstein's derivations sounds like fun! ;)
Thanks for everything, in the mean time, I've got some reading to do!
- Dan, 2nd August 2009
great site! I have a question, if we could for the double slit experiment, slow the speed of the electron or any other type of particle so it is travelling at much less than the speed of light, would we still get the interference pattern? - quincy, 25th September 2009
Electrons do not travel at the speed light. Only photons (radiation quanta) do. To date the largest object sent through a double-slit experiment are C-sub60 Buckminsterfullerene molecules, aka "buckyballs." They're working on dead viruses now, but the larger the object the harder it is to detect their wavelengths.

As far as varying speed is concerned, that's an interesting question and I haven't thought about it. My gut says it wouldn't matter, but I'm not sure.

- Greg sivco, 26th September 2009
Thank you Andrew for the way you approach this topic--I am still reading my way through the articles but it's already been very useful, including the comments and some links therein. I am a non-physicist with no calculus but a 40-year on and off interest in QM and related topics. Lately I've been looking for REALLY simple ways to convey the 'weirdness' to high school students--this really helps; it's a pleasure. Cheerio from the US! - deWitt, 2nd October 2009
According to Jim Al-KHALILI (QUANTUM PAGE 69) "The Uncertainty Principle is not as a result of the experimenter giving the electron (particle) an unavoidable random kick through the act of locating its position and hence changing its speed and direction. Rather it is a consequence of the nature of the wavefunctions that describe the electron's possible location and state of motion even before we look." - Neil Hudson, 23rd November 2009
Indeed, Jim Al-Khalili's book is excellent and he's quite correct. Although the mathematical workings I presented arrive at the correct answer, you should take this idea of "kicking" the particle using a photon with a pinch of salt. It's not really a valid way of considering the uncertainty principle. You have to just consider the maths, and the maths alone. No physical analogy is suitable. - Andrew Thomas, 23rd November 2009
It has been proposed that humans have reached the height of our knowledge of Quantum Mechanics. Some say that the human state is not physically capable of discovering anything more about the nature of the universe on a quantum scale.
I do not agree with this. In my opinion, as long as the human brain has logic, then there are no limits to what we can learn. In general, we do not give ourseleves enough credit for our intelligence, as the only life we can compare ourselves with are animals. I think by any standards, knowing about the basic unit of matter (the atom) and seeing impossibly distant galaxies using only our five senses, we have done exceedingly well.
Just because we don't understand something at first, it doesn't mean we won't eventually. Albert Einstein admitted that he was not hugely more intelligent than anyone else in his field, he just stuck at the problem for longer. Tenacity is the key, and as long as we 'stick at the problem' we will eventually master it. After all, what did they say about planes at first? They said it was impossible to fly like a bird, and yet we did it. They said it would be impossible to walk on the moon, and yet we did it.
These are testaments to out ingenuity, and as now, some speculate, we have almost reached the hight of our knowledge, and that we cannot learn much more- I say, we will do it!

post your opinions on this, thank you.
- Shaun, 10th January 2010
Hi Shaun, yes, I absolutely agree that there seems to be no end in sight for human progress, and the implications of this extraordinary progress and not widely appreciated (you might want to have a look at the final page of this website on "The Intelligent Universe" for more about this idea).

However, when it comes to quantum mechanics, there is the possibility that there are some things which we are *fundamentally* forbidden from knowing - no matter how advanced our technology. For example, the Heisenberg Uncertainty Principle described on this page is a good example of this: we are *fundamentally* prohibited from knowing both the position and momentum of a particle at the same time. That's a fundamental limitation on our knowledge - nothing to do with the inadequacy of our measuring equipment. So maybe there are some things which we can just never know?

However, I tend to agree with you that as our technology and understanding increases we will find that most of these apparently fundamental limitations to our knowledge will disappear. - Andrew Thomas, 10th January 2010