The whole thing is an abstraction. The nucleus isn't actually tiny ball shaped things mashed together, but rather cloudy stuff which would probably not be identical if we could actually see them. The quarks that make up protons and neutrons are considered elementary particles and identical, but they don't move around much unless energy is used to split them.
The electron however is an elementary particle that moves outside of the nucleus and can move from one atom to another. So the hypothesis is that if we could follow one electron from the big bang to the end of the universe, and this electron could move both forwards and backwards in time, it would potentially be enough with just one.
It probably doesn't hold up very well, but it's an interesting thought experiment.
It's one of those things which would be pretty much impossible to prove, but it holds well with the effects we currently see. Electrons can annihilate by colliding with positrons. But the collision we see could be a single electron changing from moving forwards in time to moving backwards in time. It holds that it's the same particle in the equations by cancelling out the minus sign of the charge with the minus sign in the time. So while we see a collision, the electron would just see itself changing charge and start moving backwards in time instead.
It's a beautiful hypothesis, and fills me with chills to think about the electron "experiencing" all of history an unimmaginable amount of times.
A big part of quantum mechanics is the fact that matter can show wave-like behaviour, which sort of breaks a bunch of "rules" that we have from classical physics. This only is relevant if we're looking at stuff at a teensy tiny scale.
Someone else has already mentioned that electrons are a fair bit smaller than protons and neutrons (around 1840 times smaller) and this means they tend to have a smaller momentum than protons or neutrons, which means they have a larger wavelength, which was easier to measure experimentally. That's likely why electrons were a part of this theory, because they're small enough that they're sort of a perfect way to study the idea of things that are both particle and wave, but also neither. In 1940, quantum mechanics and particle physics were super rapidly moving fields, where our knowledge hadn't congealed much yet. What was clear was that electrons get up to some absolute nonsense behaviour that broke our understanding of how the world worked.
I like the results of some of the worked examples here: https://www.chemteam.info/Electrons/deBroglie-Equation.html , especially the one where they work out what the wavelength of a baseball would be (because that too, could theoretically act like a wave, it would just have an impossibly small wavelength)
TL;DR:
electrons are smaller than protons/neutrons
Smaller = larger wavelength
Larger wavelength = easier to make experiments to see wave-like behaviour from the particle
Therefore electrons were useful in figuring out how the heck a particle can have a wavelength and act like a wave
Maybe, because we can measure the number of protons and neutrons with an ion accelerator? I don’t know if the something similar can be done with electrons.
You would need a positron to do that and all you might have done is reflect it backwards in time.
If you could "remove" it by placing it into another dimension, it might disprove the theory, but the causal domain might be larger then previous assumed.
This is one of those Math Theories that isn't technically a Science Theory. We can make a mathematical model, but it's untestable.
To destroy every other quantum state of the single electron, wouldn't you need to destroy it at its beginning state? The end state would be at/just after the heat death of the universe, so it wouldn't really make any difference then.
Na, we got those too. Muons, tauons and neutrinos. But the universe unfortunately hasn't imploded, meaning I have to go to work and pay taxes and shit.
It is an interesting theory, for sure. Instead of countless 3-dimensional particles, you have a single (or very few) 4-dimensional objects. You can imagine it like a sheet of fabric that is our present, with everything above the sheet being the future, everything below the past. When you want to sew a thread (our electron) through the sheet, you need to pierce the fabric, but to do it again, you first need to piece it the other way, giving you a positron. You can create or destroy arbitrary many of these, but you need create or destroy one of each every time. More interestingly, it is exactly determined which two will annihilate each other, as the allegorical loop of thread gets pulled tighter and tighter until it gets pulled though the sheet. The universe would be deterministic.
I'm sure there's a myriad of contradictions to modern QM and particle physics, but it's fun to think about nonetheless
One reason why that is probably not true is because there are less positrons but if it were true they should number the same as electrons, right?
But if electrons are moving along the same "time direction" as we are and positrons are moving in the opposite "direction" then wouldn't we expect there to be less protons? As we can't measure the protons that already "passed" us? And we would measure more electrons as a some/many/all of the existing electrons are traveling alongside us?
Positrons are different from protons. Both have a positive charge, but a positron is an elementary particle of a similar mass as an electron. They are rather rare in nature which OP was noting. Protons are made of three elementary particles, much heavier than positrons, and are, I imagine, present in nature in about the same order of magnitude as electrons.
That is one of the many thought you have when taking LSD. Another is that this is the dream of a super intelligence (similar to simulation theory).... well well