What Happens When Two Up-Up Fermions Collide?

[valleyman]

This morning I've been thinking about a simple problem whose I can't find out a satisfying solution. We all know that two "close" fermions can't exist in a up-up or down-down spin configuration because of vanishing state function as expressed by the exclusion principle.
But what does *practically* happen when I try to collide two up-up fermions, maybe with an accelerator? I've encountered several times the vague concept of a "Pauli repulsion force" in studies about atomic structure but is it a misurable force - i.e. real - or just an emergent property of the system itself? Or am I missing something obvious?

Thanks in advance,
Valleyman

 

[zenith8]

This morning I've been thinking about a simple problem whose I can't find out a satisfying solution. We all know that two "close" fermions can't exist in a up-up or down-down spin configuration because of vanishing state function as expressed by the exclusion principle.
But what does *practically* happen when I try to collide two up-up fermions, maybe with an accelerator? I've encountered several times the vague concept of a "Pauli repulsion force" in studies about atomic structure but is it a misurable force - i.e. real - or just an emergent property of the system itself? Or am I missing something obvious?

Thanks in advance,
Valleyman

The answer to this depends on what interpretation of QM you subscribe to, in particular whether you believe that the wave function represents a real objectively existing wave field (as in de Broglie-Bohm theory). If you do think this, then that does generally imply that Pauli repulsion is a real force.

If you think that the wave function represents 'knowledge' or 'information', then - realizing that it is very difficult for knowledge to push things around - your answer would be 'Splutter. Don't be ridiculous. How dare you ask such a stupid question. Of course it isn't a real force. Pauli proved this mathematically with some terribly complicated bit of quantum field theory that your tiny brain can never understand.'

On the other hand, I found http://www.tcm.phy.cam.ac.uk/~mdt26/PWT/lectures/towler_pauli.pdf" quite useful in sorting out my thinking on this.

 

[alphali]

i know that pulsars and neutron stars don't collapse because Pauli repulsion force but in bigger stars the gravitational force is stronger than Pauli repulsion force so the star collapses to forma a black hole so i guess its real. its what i know

 

[zenith8]

i know that pulsars and neutron stars don't collapse because Pauli repulsion force but in bigger stars the gravitational force is stronger than Pauli repulsion force so the star collapses to forma a black hole so i guess its real. its what i know

Pauli repulsion force/degeneracy pressure in stars was discussed extensively in https://www.physicsforums.com/showthread.php?t=364464".

 

[Bill_K]

But what does *practically* happen when I try to collide two up-up fermions, maybe with an accelerator?

All that happens is that since the spin part of the wavefunction is symmetric, then the space part must be antiysmmetric. So L=1, 3, ... This is not a "force", but does mean the particles remain farther apart.

 

[alphali]

i think it happens all the time in particle accelerators when they collide proton-proton beams.its like in the neutron star and black hole, if u have enough energy they will collide (in particle accelerator)and release energy and in case of a black hole the star is made of fermions and it collapses to form a black hole so all the particles are in one place but that only happens if the gravitational energy is enough to overcome exclusion principle.