# How come my shoes are where I left em last night?

1. Jan 10, 2005

### trosten

How come my shoes are where I left them last night before I went to bed? I didnt observe them during the night since I was asleep. Did someone else observe them? :uhh:

How come large objects tend to keep their appearence even if no one is meassuering them.

I would prefer an answer from someone who has gone through all of the education system. :tongue:

2. Jan 10, 2005

### DrChinese

I think you are answering your own question, but you don't know it. Why wouldn't they be there is perhaps a better question.

I assume your example is supposed to be a reference to QM somehow. But the fact is that QM does not try to say that your shoes are not there when you are not looking at them. There is nothing for anyone to explain.

Science is about comparing evolving theory to actual experiments. You can perform an experiment and determine if your shoes are there the next morning. Whenever scientists perform experiments, they get answers consistent with the predictions of QM. That makes QM useful.

So if you are still wondering about your shoes, I suggest you check around the house a little more. I think you will find them. :)

3. Jan 10, 2005

### HallsofIvy

Staff Emeritus
Hey, I'm always wondering why my shoes AREN'T where I left them last night!

Quantum effects are "damped out" in large objects. Remember that there is a 1/m term in the uncertainty equation.

4. Jan 10, 2005

### jk

QM only applies to socks

You are missing the whole point of quantum mechanics. It makes no statements whatsover about shoes. It only conerns itself with socks.
Socks obey the Pauli Exclusion principle: no two socks can occupy the same state. Therefore you will never find a pair of socks in the same place.

5. Jan 10, 2005

### caribou

The answer is a process called decoherence by which particles in the air, in the floor and all sorts of other things are interacting with the particles in your shoes and this makes your shoes to stay where you left them. The same things makes the air and the floor stay where they are too.

So the room is "observing" your shoes and your shoes are "observing" the room.

6. Jan 10, 2005

### DrChinese

Unless, of course, they don't match...

LOL, this is great...

7. Jan 10, 2005

### trosten

This decoherence thing is that a thing that can be proved from QM? As far as I can tell from my books it isnt clear what causes the collapse of the wave function? It seems like whenever QM scientist do experiments they are able to collapse the wave function, so suppose that the wavefunction has been collapsed. Can other objects then keep the wave function from evolving? I realize that we should have one wavefunction for all objects in the experiment, but it seems noone does QM that way since the meassuring device is never part of the wavefunction.

About the sock problem. What if the two socks are in an entangled state as the electrons in the ground state of helium (singlet state) couldnt you then find find two socks at the same position? Since the antisymmetri gets stuck in the spin-part (of the socks . Whats stopping the socks (or electrons) from having a nonzero probability density of beeing at the same place. (this is also a serious question about the electrons in Helium

8. Jan 10, 2005

### arildno

Aaah..but does this also have something to do with the fact that I always lose at least one sock every time I use my washing machine??

9. Jan 10, 2005

### LURCH

In your shoes are where they are in the morning because that is where you observe them to be in morning. During the night, when no one is observing them, it would make no sense at all to make any statement about their position or their momentum. In the deterministic sense, they're not there at all.

10. Jan 10, 2005

### caribou

Decoherence is the way in which interactions between particles destroy coherence as particles become entangled with each other. It destroys quantum interference and makes things behave much more classically. It was experimentally confirmed in 1996 and occurs in nanoseconds at most but usually is occuring all the time.

More recent interpretations of quantum theory involve no collapse of the wave function and include the measuring devices in the overall wave function. Wave function collapse is then just seen to have no physical meaning, like the mathematical odds of a horse race changing from before the race to afterwards.

However, this splitting, uncollapsed wave function can be recombined and erase the result of an earlier measurement in an ideal experiment! But this isn't possible for anything much bigger than a few atoms in the real world.

11. Jan 10, 2005

### Gonzolo

Not necessarily one, but 1+2n, where is an integer, thus the "quantum" of it all.

12. Jan 14, 2005

### vanesch

Staff Emeritus
Decoherence is indeed the straightforward application of quantum theory without any collapse. The idea is the following: interactions cause entanglement between the interacting parts, and when you do a few straightforward calculations, you find that macroscopic bodies unavoidably interact with their environment (black body radiation of 3 kelvin, a few atoms in a high vacuum ...). It is btw amazing how FAST this entanglement takes place.

And now the trick is the following:

Suppose your "system" is in a superposed state (in what basis, you will ask ; I'll come to that):

|S> = a |Sa> + b |Sb>

And your environment is, well, in a state, |E0>

So the overall state of system + environment is a product state:

|S>|E0>

The interaction between system and environment will extremely rapidly lead to an entanglement:

|psi> = ( a |Sa> |Ea> + b |Sb> |Eb> )

and usually, because of the many degrees of freedom of the environment, |Ea> and |Eb> will be essentially orthogonal.

Now, there is a treatment in standard quantum statistics that demonstrates that ALL LOCAL OBSERVABLES on a system HAVE THEIR EXPECTATION VALUES FULLY DETERMINED BY THE LOCAL DENSITY MATRIX.

The local density matrix is the full density matrix in which we take the partial trace with respect to all the non-local states (here, the environment states).

You can easily work out that it is a diagonal matrix with |a|^2 and |b|^2 on the diagonal. The off-diagonal elements are 0, due to the near orthogonality of the environment states |Ea> and |Eb>.
This describes a STATISTICAL MIXTURE. So, when looking locally onto the system, we have a probabilistic mixture of the state |Sa> and of |Sb> and any interference has disappeared (the system decohered). It is as if we would have applied a collapse.

However, there's a hic in the above demonstration, in that we use density matrices, which themselves are based upon the Born rule. So although we do not need an explicit COLLAPSE anymore, somehow we keep the Born rule which turns coefficients of base vectors into probabilities.

So decoherence theory (which is nothing else but a rigorous application of quantum theory) gives only part of the answer : we still don't know where the Born rule comes from. But it gives already a lot of insight in "why a quantum world looks finally quite classical", even though it doesn't give a final answer.

Many World proponents try to derive the Born rule from the other postulates of quantum theory. I'm in the process of writing a publication on why I think this method is doomed to fail (but I can fail myself, of course ). In my opinion (and I think I have good arguments), you still need the Born rule somehow, and it is still a mystery. However, decoherence teaches us something else: if you apply the Born rule late enough in the procedure, collapse or no collapse will not make any difference in the results.
This is at the same time a blessing and a curse. It is a blessing, because it means that in practice, there's no point: if you apply Born's rule "late enough" it doesn't matter where you apply it. It is a curse, because it means that the question of "where is the Born rule physically applied" is not open to experimental enquiry.

That's why, in other discussions here, like on "quantum erasers" I try to insist on applying the Born rule only at the end of your calculation. You have then much less conceptual problems with collapsing states at a distance, or predicting decisions in the future and so on. Of course you get other conceptual problems, related to accepting that the universe is one big messy entangled state....
But when it matters in doing calculations, you always get the right result if you "collapse only at the very end".

There are still issues. One is: WHAT is the basis of the system in which we have to apply this "decoherence" thing ? It turns out that this follows from the structure of the interactions with the environment ; there is still a discussion on what exact criterion should be used, but in practice, you always find about the same "preferred base states". For instance, for a macroscopic body, it is in most cases if not all, the position basis. This explains why we don't see macroscopic objects "in two places at once".

cheers,
Patrick.

13. Jan 16, 2005

### jackle

100 years ago, scientists would just quote that a particle remains in constant motion until acted on by a force, so your shoes will remain at rest all night.

This is interesting, because it makes me ask myself how we would answer some of today's obvious questions in a 100 years time.

Do we really 'know' the answer or simply model the way it looks?

14. Jan 16, 2005

### DrChinese

All theory is a model. There are no theories that are not models. No single theory can answer all questions.

If you have a hard time accepting that, remember this: there are patterns and pattern exceptions, and theories attempt to explain patterns and the exceptions. To do this, you must define a pattern and this requires that you assume SOMETHING about the data you are trying to explain i.e. what qualifies as the pattern and what qualifies as the exception.

A simple example: Earth's gravity accelerates you at 9.8 m/s^2. Or does it? Are you falling towards the center of the earth at a high rate of speed right now? No, because the floor is stopping you. So gravity accelerates you in the absence of other couteracting forces. Now you must describe those forces too! It never ends.

One theory can have greater utility than another. One theory can have a different scope than another. And some theories can overlap in their scope. But ultimately the theory is really a shorthand for explaining patterns and pattern exceptions.

Utility is not a bad thing. A model is not a bad thing. And no one is saying that we know everything. I hope in 100 years, science will progress. Don't you?