I'm a master student in mathematics currently taking Quantum Mechanics and since the lecture notes provided by the lecturer aren't cutting it I'm reading "Quantum Mechanics and Path Integrals" by R. Feynman and A.R. Hibbs.
In this book there is one line I have been unable to understand. The line in question is:
"Actually this is wrong and, remarkably, electrons obey a different rule. The amplitude for an event in which the identity of a pair of electrons is reversed contributes 180 out of phase."
The line is in chapter "1-3 Interfering Alternatives" under "Some Illustrations".
Now the part I don't get is what an "event in which the identity of a pair of electron is reversed" actually is. The word "identity" is already super weird for me because in that context both of our electrons have the same spin so I don't get why their "identity" would be "reversed".
If anyone can help me it would be highly appreciated.