# One thing cannot be in two places at the same time

1. Jan 27, 2014

### batmanandjoker

One "thing" cannot be in two places at the same time

There are alot of people touting that an object in superposition is the same thing as an object being in various locations simoulaneously which mathematiclly is untrue. I think they are confusing the wave function of possible locations (until an object is observed its not techincley "real") instead of the POSSIBLE positions the object could be in. Someone said it before but in the double slit experiment only one mark is left on the backboard even when there is an interferance pattern when firing one photon at a time.

There has never been a collapsed particle in two places at the same time from the reasearch Ive done. I just want a conformation that about this. Also in the macro classical world Ive never encountered an object being in two or more places at the same time, my question is why since considering the laws of quantum mechanics applies also to the macro classical world. Can someone shed some light on this.

2. Jan 27, 2014

### atyy

An object that is in a superposition of different positions is said to be "simultaneously in those different positions" in the sense that when you measure its position, the object will collapse to any one of the positions of the superposition.

3. Jan 27, 2014

### batmanandjoker

This is an oversimplified explanation of something I already know.

4. Jan 27, 2014

### batmanandjoker

Why doesent this happen in the classical world.

Im quoting bhobba

"Its often said from the principle of superposition an object that can be in a state that is position A and one where its in position B can be in a superposition of those two states - which is true.

Unless its being observed, in which case it will be in either one or the other, then it doesn't have the property of position and cant be said to be anywhere. Its easy to fall into the trap, and I did in the past, to think of superposition as being literal ie it is literally in those two places. From posting on this forum I quickly had my misunderstanding corrected - and I had read a LOT of QM books - it doesn't have a property until its observed to have that property. You cant say anything about position until its observed to have it - in particular if its in a superposition of position you cant say its in a number of different positions at the same time.

I suspect the reason some people fall into this trap, and its true in my case, is Dirac's famous book explains the principle of superposition in a way that suggests its like that - if you read it carefully you can see that's not really what he is saying - but a cursory read does suggest it.

I am now of the view that the principle of superposition, as an actual principle, rather than simply a byproduct of the vector space structure of QM, is probably not the best way to look at it.

So yes - you can in principle have an object in a superposition of states any distance apart - but that doesn't mean its literally in those positions simultaneously - or even that the concept of position is appropriate in such a situation.

Thanks
Bill"

5. Jan 27, 2014

### atyy

In principle, we can make "Schroedinger cat" states. See http://www.nobelprize.org/nobel_prizes/physics/laureates/2012/haroche-lecture_slides.pdf (slide 19, 31).

What decoherence suggests is that because of the entanglement of the environment, measuring apparatus and experimental system, although it is in principle possible to cause the apparatus to collapse into a superpostion of positions (by measuring something other than position), in practice we simply do not control the environment well enough, and we can only collapse the apparatus into a definite position. A key assumption necessary for decoherence to make it hard for us to collapse the apparatus into something other than a definite position is the assumption that interactions are local, ie. nearby objects interact strongly, and distant objects interact weakly.

Last edited: Jan 27, 2014
6. Jan 27, 2014

### StevieTNZ

If you get your hands on GianCarlo Ghirardi's excellent book called "Sneaking a Look at God's Cards", he gives an accurate statement on how we should interpret an object being in superposition (pg. 88)

7. Jan 27, 2014

### rpt

I believe that the superposition theory is one way to explain the phenomenon of reduced space-time dimensions in quantum scales. Describing the quantum particles by a wave function as opposed to a point in the space is actually reducing the space dimensions of the object reduces. For example when the space-time dimensions are reduced to 3 from 4 the quantum object exists in a plane as opposed to a point in the 4D space-time. However the orientation of this 3D space-time is arbitrary.
This can explain most of the extraordinary observations people see in the quantum scales.
rpt

8. Jan 27, 2014

### Staff: Mentor

Sure, but how do you explain the interference if the particle doesn't go through both slits in some sense? It seems to me that your assertion that it is only in one place before it is measured is highly suspect.

9. Jan 28, 2014

### Staff: Mentor

Well, something has to go through both slits to explain the interference pattern. But it doesn't necessarily follow that that something had to be the particle.... Maybe it's some sort of "pilot wave"? Oops, I see someone else has gone down that path already.

And seriously, kidding aside, I read batmanandjoker's original post as just asking whether he's right to not take the pop-sci descriptions of superposition too literally. We can answer that question affirmatively without going into an interpretational discussion.

10. Jan 28, 2014

### Maui

At the quantum scale a system can be delocalized and its position corresponding to it being in several places at once. This is quantum computing in practice(most often practically implemented as spin states in superposition). There are technical diffucluties like making the gates operate faster than decoherence times, but these have been overcome in the last 5 years for small number of tested atoms for up to a microsecond.

Working with a group of theoretical physicists led by Steven Girvin, the Eugene Higgins Professor of Physics & Applied Physics, the team manufactured two artificial atoms, or qubits (“quantum bits”). While each qubit is actually made up of a billion aluminum atoms, it acts like a single atom that can occupy two different energy states. These states are akin to the “1” and “0” or “on” and “off” states of regular bits employed by conventional computers. Because of the counterintuitive laws of quantum mechanics, however, scientists can effectively place qubits in a “superposition” of multiple states at the same time, allowing for greater information storage and processing power.

http://news.yale.edu/2009/06/28/scientists-create-first-electronic-quantum-processor

Demonstration of Two-Qubit Algorithms with a Superconducting Quantum Processor

http://arxiv.org/pdf/0903.2030v2.pdf

You have to be an outright conspiracy theorist(which qm and its formalism almost requires by definition since that's how students are trained to think) to believe otherwise - namely that quantum systems act as if they do something which they actually don't. So, what you would normally get here in response to such questions is talk of coherent states(no reference to the real world or any familiar concept, in fact most will attack ANY attempt to connect coherence states with observed reality), decoupling from the environment( again no reference to the outside world for such systems), coherent times(the time while the cat is both alive and dead) - this argument seems to have been started by Schroedinger and his cat analogy and how it would be ridiculous of the physicist to think of coherent states as having a reality of their own. Time puts them in a tight corner though and more and more needs to be exlained away by yet more levels of conspiracy as technology advances. So if you are sticking with the commonly accepted jargon("coherent states" instead of using "systems which are in both states simultaneously") you will not get replies like "that's against logic and consistency", "show me the exact wording - which usually is crypted as coherence states", "that's loose language", etc. It's up to you what you want to believe at this point and since you said "thing" in the title, you may need a thourough revision of what is meant by the word.

Last edited: Jan 28, 2014
11. Jan 28, 2014

### haael

People often assume that the macroscopic limit of the quantum superposition is something mystical. I.e. if we did a superposition on a cat, we would get two images of semi-translucent cats, lit by a golden glow and chanting verses from Bhagavad-gita. It doesn't have to be so. The macroscopic limit of quantum phenomena might be something more usual and common.

One example: interaction of opposite charges. In the quantum realm, the interaction between electron and positon leads to such phenomena as annihilation and creation, that are non-classical. In the macroscopic limit, we no longer observe annihilation and creation, but we see opposite charge attraction instead. The macroscopic picture looks different than the microscopic, despite they are consequences of the same quantum process.

Similarily, superposition in macroscopic realm doesn't have to look the same as in microscopic. In the microscopic realm we observe probability amplitudes and indefiniteness of position. OK, but the macroscopic equivalent doesn't have to be the same. My personal suspicion is that superposition in macroscopic realm leads just to low temperature and nothing else. If we ever created a Schroedinger cat, we would just see a frozen cat and nothing else. No bilocation, no mysticism. Just a pile of ice in the shape of a cat.

12. Jan 28, 2014

### Staff: Mentor

Indeed.

I wrote this in another thread but I also think it's of relevance here.

QM is a theory about what happens when something is observed, not about when its not being observed. When its in a superposition of position, since its not being observed the theory says nothing about its position - the state is simply, like probabilities, something that helps us predict the likelihood of an outcome if it was observed. Probabilities tells us likelihoods if you flip a coin, but while its spinning up in the air its meaningless to talk about the outcome of flipping it - same here - its meaningless.

Here is the conceptual core of QM - to those who have just read popularizations forget about them - this is its core:
http://www.scottaaronson.com/democritus/lec9.html
'So, what is quantum mechanics? Even though it was discovered by physicists, it's not a physical theory in the same sense as electromagnetism or general relativity. In the usual "hierarchy of sciences" -- with biology at the top, then chemistry, then physics, then math -- quantum mechanics sits at a level between math and physics that I don't know a good name for. Basically, quantum mechanics is the operating system that other physical theories run on as application software (with the exception of general relativity, which hasn't yet been successfully ported to this particular OS). There's even a word for taking a physical theory and porting it to this OS: "to quantize."

The double slit experiment is a good example of the formalism. Normally it's used to motivate the QM formalism, but really it should be the other way around - QM should explain it - and it does:
http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

Of course the answer is expressed in the language of mathematics - sorry but physics is about mathematical models.

Basically QM is a variant on standard probability theory that allows continuous transformations between so called pure states:
http://arxiv.org/pdf/quant-ph/0101012.pdf

Again consider flipping a coin. Probability theory describes the frequency of outcomes - but not what causes each outcome. Same with QM - it describes the frequency of outcomes - but not what causes any outcome. We simply do not know what that is - or even if there is a cause - nature may simply be like that. But regardless the QM formalism is silent about it. We have interpretations that speculate about it - but until there is some way to decide experimentally they are simply conjectures.

Thanks
Bill

Last edited: Jan 28, 2014
13. Jan 28, 2014

My guess is that a lot of people who believe that QM is only a "mathematical phenomena" that will always emerge as a "statistical results" at the classical level, are up for a real treat in the coming years, as quantum machines gets bigger and bigger...

14. Jan 28, 2014

### atyy

I disagree. Quantum mechanics already applies on cosmological scales, as reviewed by http://arxiv.org/abs/hep-ph/0505249. And even distinguished experts on dBB assured me I can still be a good Copenhagenist https://www.physicsforums.com/showthread.php?t=667996&highlight=mukhanov :)

15. Jan 28, 2014

### DrChinese

Actually, I would say it IS mathematically true. You could say that there is a small probability it is in a lot of places. When you collapse to a position eigenstate, it has a high probability of being in a single place. What's the difference? It is still a probability.

Of course, you may insist that for something to be somewhere, it must have a probability of 1 to be there. But that is simply your own definition, so you are in effect denying the mathematical formulation. Ie it is a circular argument.

16. Jan 28, 2014

OMG atyy, you've just put my green brain in superposition of "tiny confusion" and "cosmic confusion"!

But if I have choice between, "there is no universe as a whole" / "the wave function of the universe is real", and this beautiful experiment above – I go for experiments, all the way! :grumpy:

()

17. Jan 28, 2014

### atyy

I think one can agree the experiments are beautiful in any interpretation!

18. Jan 28, 2014

### ChrisVer

Well, may I think of something else? Why is it difficult to realise the superposition, since we are talking about probabilities?
Even in classical mechanics we have probabilities (eg distributions in energies- Maxwell and Boltzmann). The only difference between the Quantum and the Classical case is that the second comes as our incapability of taking in account all the parameters, while in the first it's an intrinsic property (no parameters cause it). But nevertheless, the interpretation of probabilities are still the same- given a particle in a gas, before measuring its velocity you are still saying that its velocities follow the Maxwellian distribution. It doesn't mean that the particle is moving with all the possible velocities, but before measuring it you cannot find it out.

Am I somewhere wrong?

19. Jan 28, 2014

### bohm2

I can think of something that isn't spatial: thoughts/mental stuff. Mental phenomena are not located and extended in the usual way. I guess that is a "thing".

20. Jan 28, 2014

### StevieTNZ

The best way to put it is a superposition of positions of A and B is the potential that a system can be found in positions A or B. It doesn't exist at both positions A and B at the same time (while we consider it a superposition) in the classical sense.

I quote Ghirardi on page 88 of "Sneaking a Look at God's Cards":
(he speaks of the superposition |superposition>=|O> + |E>)