# How Do Physicists Know That Quantum Superposition Does Exist?

1. Oct 30, 2014

### pink laser

Quantum Physicists claim that a sub-particle ( let's say an electron ) can be in superposition ( like having spin up and spin down at the same time ) before being measured. But how do they know that it is in superposition without measuring it ?

What is the experimental set-up and the experimental results that give them this conclusion ?

Last edited by a moderator: Oct 31, 2014
2. Oct 30, 2014

When referring to superposition, it is related with the probabilities of finding any of the possible states, that is, before measuring any values, each of them has probability "p" to be found. That means that, if an electron would have probabilities of 1/2 before measuring the spin, it is on a superposition of states, and it is the measurement action what determines in which of these states we find the electron. Having found that, then the system will evolve in this state is nothing more is done to it, and if you measure again, you will have always the same value in your measurement of this quantity, because now you will have your electron in a very well defined state. I recommend you to look at the MIT opencourse in Quantum mechanics of professor Allan Adams. In the first lecture, he explains this concept very well with an experiment of measured "hardness" and "colour" of a particle. It is an awesome explanation !

3. Oct 30, 2014

### pink laser

I would prefer it explained in few paragraphs instead of having to watch an entire course of quantum mechanics just to answer this single question. I find the probabilistic explanation unsatisfactory and unclear.

4. Oct 30, 2014

### Staff: Mentor

You'll see superposition described this way ("the spin is both up and down") in the pop-sci press all the time, but it's not right. As far as the mathematical formalism of quantum mechanics is concerned, there is no direction of spin at all unless and until it is measured.

There are a number of experiments that suggest superposition. Probably the best starting point is the double-slit experiment conducted with single-photon sources (google for "double slit single photon"). Without superposition, it's very difficult to explain the interference pattern that appears.

Last edited: Oct 30, 2014
5. Oct 30, 2014

### Staff: Mentor

Wouldn't we all? :)
If it were that easy, it wouldn't have taken the combined brainpower of all the world's physicists the best part of a half-century to figure it out.

Einstein wasn't exactly thrilled by it either, so you're in good company. Unfortunately, the universe doesn't seem to feel any obligation to work in ways that we find satisfactory.

6. Oct 30, 2014

### naima

Are you looking for a device which does not measure anything but gives information?

Last edited by a moderator: Oct 31, 2014
7. Oct 30, 2014

### Staff: Mentor

What it is when not observed is anyones guess - forget the pop-sci rubbish.

Superposition is simply a reflection of the vector space structure of so called pure states.

Why the vector space structure? Who knows - nature is just like that.

The following however may help in understanding what's going on:
http://www.scottaaronson.com/democritus/lec9.html

Its not really the conclusion of any single experiment, but rather making sense of a lot of them from the double slit, atomic spectral lines, to the Stern-Gerlach experiment.

Thanks
Bill

Last edited by a moderator: Oct 31, 2014
8. Oct 30, 2014

### DrChinese

You *probably* won't like the Heisenberg Uncertainty Principle (HUP) either. :)

9. Oct 31, 2014

### georgir

Well, I sure don't. And I don't see how it can still stand, with examples like pretty much any particle in its own reference frame having completely well known position and momentum (both 0), or Bohmian mechanics in which all particles have completely deterministic positions, and from that naturally also momentum, etc...

10. Oct 31, 2014

### atyy

The Bohmian interpretation has probabilistic initial conditions. The Bohmian interpretation, to the extent that it is an exact interpretation, does not get rid of indeterminism. Rather it parameterizes it in a way that makes quantum mechanics look similar to classical statistical mechanics.

11. Oct 31, 2014

### Staff: Mentor

Neither the position nor the momentum is "known to be zero", even in a frame in which the particle is at rest. What is known to be zero is the expectation value of the average of the results if we do the experiment many times on identically prepared systems.

12. Oct 31, 2014

### Staff: Mentor

They can - but not at the same time. Then again there is no known experiment that can measure both simultaneously.

Thanks
Bill

13. Nov 7, 2014

### Pollock

Surely the principle of superposition is all in the mind.When a particle or system can have several equally probable values we don't KNOW which until we measure it.But that doesn't mean that it didn't have a definite value before the measurement;the electron must always have a spin either+/_ 1/2.Superposition is just an admission of our ignorance about a system.

14. Nov 7, 2014

### Staff: Mentor

You'd think so, wouldn't you? And you'd be in good company - google for "EPR paper quantum mechanics" to see just how good the company is.

But you'd also be wrong. Google for "Bell's theorem" and "Aspect experiment" to see developments in the decades since Einstein and company published the EPR paper.

Also search this forum for "bell's theorem", and be sure to read everything at our own DrChinese's web site: http://www.drchinese.com/Bells_Theorem.htm

Bottom line: certain quantum mechanical observations are inconsistent with any theory of the type that you describe.

15. Nov 7, 2014

### Staff: Mentor

Well actually its simply a reflection of the vector space structure of so called pure states that are a special case of general states in QM, which are in fact operators.

There is a very important theorem, called Gleasons Theorem, that shows if you have one part of QM, namely the mapping of the outcomes of experiments to so called POVM's then, first states exist, are the operators mentioned above, and secondly the statistical outcomes of observations is as per the so called Born rule (see post 137):

It would seem an inevitable consequence even if QM is partially correct.

As to it's meaning - do states just reside in the minds of theorists, does it exist out there like electric fields etc etc. There are all sorts of views - but none are experimentally distinguishable from the others. This sometimes causes heated arguments when discussing QM foundations :p:p:p:p:p:p.

Thanks
Bill

16. Nov 8, 2014

### carllooper

One could equally ask of an unobserved particle: "Surely the principle of position is all in the mind".

While the concept of superposition, as much as position, is something that does take place in the mind, that's only because the mind is as good a place as any to entertain such concepts. But in and of itself the concepts would have only mathematical (and perhaps philosophical) interest if they weren't actually allied to the practice of physics. There is a relationship between the concepts and that which can be physically observed (be it human or machine observation). And how one might exploit such concepts technologically.

The concept of super-position may very well express ignorance, but the concept of position is not any different. In any location where we do not intervene (ie. make a measurement) we can only imagine what might be there: be it a particle having a position, or a particle having super-position.

If we adopt position as our preferred concept it is not any more reasonable than if we had adopted super-position as our preferred concept. For what would be the reason for using the concept of position? Both concepts take place in the mind because there is no immediately visible interaction occurring at the nominated location in space. One could even argue there's no physics taking place at all.

However once you factor in time (duration, intervals etc.) the concept of 'no immediately visible interaction' is seen to be too tight a restriction on what might actually be physically visible at a particular location in time and space. Could measurements made elsewhere constitute a measurement of what is happening at a given point? If nothing else events occurring at other points in space and time can provide a concept on what is at least potentially visible at a particular location in space and time.

The concept of super-position (wave functions etc) emerges in this context. And they are particularly apt concepts precisely because we're not performing a localised interaction with the location in question. We are asking instead, what would happen if we made a measurement at that location BUT, at the same time requiring that measurement not occur there - ie. so it can take place elsewhere instead. And not only that but the concept should also be consistent with measurements that are actually made at the location in question. And the concept of super-position (etc.) does a pretty decent job of satisfying that rather difficult problem. At least a whole lot better than just the concept of position.

As to whether super-position exists, the answer is obviously yes. How can we be talking about it if it doesn't exist? It is a very well defined concept. If you'd asked that same question a 100 odd years ago the answer might be no. It hadn't been conceived in it's modern form. Interestingly the roots of the concept are not entirely modern. Indeed one can trace the roots back to some very old ideas in Ancient Greece. The Stoics were particularly interested in some of the root problems, and how such might be logically resolved. Strangely enough. Not because they were studying the rather strange world of quantum mechanics, but because there are macro-scale problems that have a related difficulty (to do with the probability of naval warfare and whether the Gods know the outcome and if they do is it pre-ordained, etc). The Stoics elaborated propositional logic to deal with such problems (imposed on them by stressed out naval commanders).

C

Last edited: Nov 8, 2014
17. Nov 8, 2014

### Matterwave

I'd like to add a caveat to this post. The Bell inequalities rule out any local hidden variable theories, but does not rule out non-local hidden variable theories (like de Broglie-Bohm theory). So one can still retain realism (that the electron is always either spin up or spin down) but then one must get rid of locality...which is perhaps an even harder pill to swallow for a modern physicist.

18. Nov 8, 2014

### Staff: Mentor

The original question has been answered and we're veering off into areas that go beyond physics. It's time to close this thread.

19. Nov 8, 2014

### Staff: Mentor

Thank you, that's an important qualification.