# Is anything physically impossible in quantum mechanics?

1. Sep 10, 2009

### Loren Booda

Some things are statistically impossible in QM, I believe, but are things there physically impossible?

2. Sep 11, 2009

### Fredrik

Staff Emeritus
Two fermions in the same quantum state? You may need to explain what you mean by "physically impossible".

3. Sep 11, 2009

### Dmitry67

charge is always conserved (not just statistically)
electron does not decay
single photon can not switch into say neutron, even charge is conserved, the momentum is not
etc etc

4. Sep 11, 2009

### ytuab

In QM, the impossible things are

to rotate the particles by 2 pai and return them to their original forms, and
to use the relativistic theory without infinite "virtual particles and photons"....

Electrons and neutrons are actually existing particles in this real world.
But QM says that they won't return even if they are rotated by an angle of 2 pai.
(If the angle is 4 pai, they will return.)

(the 4pai rotation experiment)

Last edited: Sep 11, 2009
5. Sep 11, 2009

### alxm

I'm not sure about pai, but a rotation through 2 Pi is an identity operation for any integer-spin particle.

Because they're fermions.

6. Sep 11, 2009

Can you please explain what will happen to the electrons (and to fermions in general) on rotation by 2*pi. What do you mean by, "they wont return".

Last edited: Sep 11, 2009
7. Sep 11, 2009

### alxm

I don't know what he means either, but what actually happens is just a phase change:
$$| \psi \rangle \rightarrow -| \psi \rangle$$

8. Sep 11, 2009

### Loren Booda

All of your responses are welcome as a wake-up call to me.

9. Sep 11, 2009

### ytuab

Hi, alxm. Thanks for repling for me.

Surely, there are some experiments in which spinning neutrons went back to their original forms when they are rotated by an angle of 4π (not 2π).

But I think it is natural for us to doubt this fact when we hear this for the first time like Ancient Nomad.

Is it possible that there are some mistakes in the precondition?

For example, if the (spin) angular momentum is hbar (not 1/2hbar), these experiments only showed that spinning neutrons went back to their original forms when they are rotated by an angle of 2π.

Last edited: Sep 11, 2009
10. Sep 11, 2009

### Count Iblis

Quantum mechanics is unitary and that implies that cloning of arbitrary quantum states is not possible.

11. Sep 13, 2009

### alxm

I wouldn't think so, no. But there are many contexts where phase is unimportant.

But I think it is natural for us to doubt this fact when we hear this for the first time like Ancient Nomad.

No, the difference between bosons and fermions is quite well-understood and experimentally verified.