Is anything physically impossible in quantum mechanics?

[Loren Booda]

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

 

[Fredrik]

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

 

[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

 

[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.)

To confirm this, please see this thread,
https://www.physicsforums.com/showthread.php?t=328878
(the 4pai rotation experiment)

 

[alxm]

In QM, the impossible things are

to rotate the particles by 2 pai and return them to their original forms

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

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.)

Because they're fermions.

 

[Ancient_Nomad]

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.)

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 won't return".

 

[alxm]

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 won't even return".

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

 

[Loren Booda]

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

 

[ytuab]

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

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).
(in the thread of #4)

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.

 

[Count Iblis]

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

 

[alxm]

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).

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.

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

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