Sabine
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Why can elementary particles decay?
🕑 Added 2021-11-13 13:00:05 +0000 UTC[This is a transcript of the video.]
Physicists have so far discovered twenty-five elementary particles that, for all we currently know, aren’t made up of anything else. Most of those particles are unstable, and they’ll decay to lighter particles within fractions of a second. But how can it possibly be that a particle which decays is elementary. If it decays doesn’t this mean it was made up of something else? And why do particles decay in the first place? At the end of this video, you’ll know the answers.
The standard model of particle physics contains 25 particles. But the matter around us is almost entirely made up of only half of them. First, there’s the electron. Then there are the constituents of atomic nuclei, the neutrons and protons, which are made of different combination of up and down quarks. That’s 3. And those particles are held together by photons and the 8 gluons of the strong nuclear force. So that’s twelve.
What’s with the other particles? Let’s take for example the tau. The tau is very similar to the electron, except it’s heavier by about a factor 4000. It’s unstable and has a lifetime of only three times ten to the minus thirteen seconds. It then decays, for example into an electron, a tau-neutrino and an electron anti-neutrino. So is the tau maybe just made up of those three particles. And when it decays they just fly apart?
But no, the tau isn’t made up of anything, at least not according to all the observations that we currently have. There are several reasons physicists know this.
First, if the tau was made up of those other particles, you’d have to find a way to hold them together. This would require a new force. But we have no evidence for such a force. For more about this, check out my video about fifth forces.
Second, even if you’d come up with a new force, that wouldn’t help you because the tau can decay in many different ways. Instead of decaying into an electron, a tau-neutrino and an electron anti-neutrino, it could for example decay into a muon, a tau-neutrino and a muon anti-neutrino. Or it could decay into a tau-neutrino and a pion. The pion is made up of two quarks. Or it could decay into a tau-neutrino and a rho. The rho is also made up of two quarks, but different ones than the pion. And there are many other possible decay channels for the tau.
So if you’d want the tau to be made up of the particles it decays into, at the very least there’d have to be different tau particles, depending on what they’re made up of. But we know that that this can’t be. The taus are exactly identical. We know this because if they weren’t, they’d themselves be produced in larger numbers in particle collisions than we observe. The idea that there are different versions of taus is therefore just incompatible with observation.
This, by the way, is also why elementary particles can’t be conscious. It’s because we know they do not have internal states. Elementary particles are called elementary because they are simple. The only way you can assign any additional property to them, call that property “consciousness” or whatever you like, is to make that property entirely featureless and unobservable. This is why panpsychism which assigns consciousness to everything, including elementary particles, is either bluntly wrong – that’s if the consciousness of elementary particles is actually observable, because, well, we don’t observe it – or entirely useless – because if that thing you call consciousness isn’t observable it doesn’t explain anything.
But back to the question why elementary particles can decay. A decay is really just a type of interaction. This also means that all these decays in principle can happen in different orders. Let’s stick with the tau because you’ve already made friends with it. That the tau can decay into the two neutrinos and an electron just means that those four particles interact. They actually interact through another particle, with is one of the vector bosons of the weak interaction. But this isn’t so important. Important is that this interaction could happen in other orders. If an electron with high enough energy runs into a tau neutrino, that could for example produce a tau and an electron neutrino. In that case what would you think any of those particles are “made of”? This idea just doesn’t make any sense if you look at all the processes that we know of that taus are involved in.
Everything that I just told you about the tau works similarly for all of the other unstable particles in the standard model. So the brief answer to the question why elementary particles can decay is that decay doesn’t mean the decay products must’ve been in the original particle. A decay’s just a particular type of interaction. And we’ve no observations that’d indicate elementary particles are made up of something else; they have no substructure. That’s why we call them elementary.
But this brings up another question, why do those particles decay to begin with? I often come across the explanation that they do this to reach the state of lowest energy because the decay products are lighter than the original. But that doesn’t make any sense because energy is conserved in the decay. Indeed, the reason those particles decay has nothing to do with energy, it has all to do with entropy.
Heavy particles decay simply because they can and because that’s likely to happen. As Einstein told us, mass is a type of energy. Yes, that guy again. So a heavy particle can decay into several lighter particles because it has enough energy. And the rest of the energy that doesn’t go into the masses of the new particles goes into the kinetic energy of the new particles. But for the opposite process to happen, those light particles would have to meet in the right spot with a sufficiently high energy. This is possible, but it’s very unlikely to happen coincidentally. It would be a spontaneous increase of order, so it would be an entropy decrease. That’s why we don’t normally see it happening, just like we don’t normally see eggs unbreak. To sum it up: Decay is likely. Undecay unlikely.
It is worth emphasizing though that the reverse of all those particle-decay processes indeed exists and it can happen in principle. Mathematically, you can reverse all those processes, which means the laws of nature are time-reversible. Like a movie, you can run them forwards and backwards. It’s just that some of those processes are very unlikely to occur in the word we actually inhabit, which is why we experience our life with a clear forward direction of time that points towards more wrinkles.
Comments
I'd like to see an episode on sterile neutrinos, expaining what it takes for a particle to be its own anti-particle and if two particles are identical, why they should annihilate each other.
Check out my reddit post where I admit that maybe you are right about electrons not thinking: https://www.reddit.com/r/RadicalPanpsychism/comments/qx311z/we_are_all_in_the_matrix/
Hi, I still have difficulty to digest that decay is never driven by minimizing the energy of final products (offset by changes in binding energy of the whole structure), simply cause one reads about it all over the place. If I take a different example, e.g. beta+ decay (i.e up quark transformed to down quark in proton), are you suggesting that it is only about increase of entropy? Then why do not nuclei keep decaying via beta decay again and again? I've always thought beta decay happens to make products more stable, not just as a means to increase the entropy. Or was this video strictly about elementary particles in isolation? Thanks for any clues.
Very timely, for me. I was thinking about the decay of the neutron and was wondering how a fundamental particle, the Down quark, could "decay" if it was a fundamental particle and then I got your email about this explanation! Again, great timing.
There's still no viable explanation provided for how any of this could function in objects that are on sub-atomic scales.
Sabine was defending the idea that particles are elementary even though they decay. I will try to defend the opposite thesis that low mass particles are very complex even though they behave in a simple identical way for each kind of particle. The low mass particles can be very complex if they don't control their external behavior! If virtual particles acting as agents of the conscious Universe (I am thinking Mr. and Mrs. Universe) are exerting very strong top down control are controlling or at least correcting low mass particles to behave pretty much the exact same way for each type of particle, the particle will behave the same even if they are in fact very complex dreaming baby conscious universes! The low mass baby universes could have all kinds of things in it including neural nets, long genetic codes, and conscious agents of its own but those would be for developing its mind by dreaming not for controlling external movements! A high mass dark matter particle would serve as homunculus in a brain because universes evolve by natural selection to have low mass particles good for making bodies and machines and more mature high mass particles that are holodecks for a mind that interfaces with a enormous variety of bodies! The brain sends and receives electromagnetic code to and from a high mass dark matter holodeck particle surrounded by an electromagnetic wave focusing crystal!
How does all this happen without neural networks within or between these particles?
I don't know really. Personally I never had difficulties with this. It's taken me a while to understand why people ask this question in the first place.
With Radical Panpsychism https://www.reddit.com/r/RadicalPanpsychism/ universes evolve by natural selection to have low mass particles good for making bodies and machines and more mature high mass particles that are holodecks for a mind that interfaces with a enormous variety of bodies! It would serve no useful purpose for low mass particles to express their free will externally because they are too immature and also they could not serve as standard parts for bodies and machines! They can still direct dreams (internal free will) that can be masked by randomness and at a much reduced time perception since they are not responsible for their external behavior! High mass particles such as dark matter serve as holodecks for a mind and according to radical panpsychism theory can be experimentally detected in an awake brain expressing libertarian free will because when activated by the brain, dark matter holodeck particles gain a positive charge and can send and receive electromagnetic homuncular code to receive sense data and send free will codes to the brain!
I now understand why particles that decay are categorized as "elementary" due to their *observable* interactions. But I feel that the fact of decay is a clue to some deeper structure - which we can only model but not directly observe. What if .... - Particles are 4D but we only see intersection with our 3D space. The missing "structure" could be in the additional dimension? - The missing structure is simply too small to be detected with our instruments? - Decay is caused by interaction with unseen neutrino or dark matter or quantum vacuum structure?
Do physicists have trouble understanding this? The information will sink in for me later but the whole thing seems like a weird paradox at the moment.
