Why can elementary particles decay?

If a particle decays into other particles, how can it possibly be that they are elementary? Doesn't the decay mean that the particle must have been made up of the other particles? And why do particles decay in the first place? These are really good questions which I get a lot. At the end of this video you'll know the answers.

Correction to the illustration at 4 mins 25 seconds. The "e" on the left side should be a positron, not an electron.

0:00 Intro
1:30 Are the decay products already in the particle?
3:01 Are particles conscious?
3:50 Decay is an interaction
5:18 Why do particles decay?
7:13 Sponsor message

[yannrondeau-chartrand]

As soon as Sabine says “Einstein”, I like to say out loud: “That guy again?!” just to hear her respond to me: “Yes, that guy again”. It fills me with joy. Fight me.

[thomassturm]

"...we experience our lives with a clear forward direction of time which points towards more wrinkles." Thank you, Sabine, what a wonderful proverb!

[nedmerrill]

Always learn something new from Sabine. Today I learned that "decay" has a precise meaning in particle physics; you can't take decay's meaning from the English dictionary when discussing particle physics.

[piotrrashman]

"let's stick with the tau because you've already made friends with it"

i wouldn't say friends, more like a flleeting acquaintance

[philuribe]

It's Satruday, so another Sabine video. Yes, that gal again :)

[tobby]

I love how she answers a question i never even asked 😂. As soon as she put forward the idea that Taus could be composite particles of Neutrinos and Electrons because that's what they decay to, I'm like woah, "that's a very good point!" Then she systematically breaks apart that notion with the observational evidence. I love it!

[nas]

This is probably the best explanation of entropy I've ever seen. I think undergrad students should be introduced to entropy this way.

[boaz]

Sabine’s pragmatic take on science is refreshing and inspiring.

[richardaversa]

Sabine has become one of the best physics YouTubers, answering the deepest question about our universe with impressive clarity (and accuracy, I assume, though as a layman I wouldn't really know).

[jimmygravitt]

I saw my first video from this channel yesterday, and I have non-stop binged these videos ever since. This channel is awesome.

[gene]

Thanks for the video! Some questions:

1. Decay may be explained in terms of entropy, but what about certain weak interactions where T-symmetry is not conserved? If an interaction can happen one-way but not in reverse, is it fair to say that the decay products are fundamentally more elementary than the original?

2. I've previously seen decay attempted to be explained via interaction with quantum fluctuation, and therefore as random as the fluctuations themselves are? Or it this is explanation essentially unprovable/circular in nature? I always wondered whether a particle's decay modes could be altered by placing it an environment with less interactions with certain fluctuations, such as whether particles placed between two close plates (where the Casimir force would be observed) be less likely to decay.

[MrDrissoun]

I have seen many of Sabine's videos, but this one is just flabbergasting ! Few words, logical constructions, for each sentence, clear sketches, and everything flows, we just get it as she says it. Sabine should teach teachers. Bravo and thank you.

[greenaum]

Wow, thanks, Sabine! I'd love you as my science teacher. Though I'm not a student any more. You not only explain what there is, but also why it could not be something else, with evidence and logic. Great! This is all the stuff scientists have to do, eliminating every other possibility until you have the undoubted one. At least until somebody notices something anomalous and comes up with a better theory.

I think the fact that the menagerie of weird particles appear in atom smashers proves that it's possible for some to be elementary. A particle accelerator simply accelerates a particle (duh!) to ridiculous speed, then smashes it into something, possibly another particle going the other way. Their kinetic energy is stopped dead, lost, and from that energy condenses various particles, mostly by chance as I understand it, but corresponding to the energy involved in the collision.

So if a particle can condense purely from energy, surely it can evaporate back into it? Or re-configure into different particles that add up to the same energy. It's almost true to say energy = mass. In many ways that's true, like the way they affect gravity. At the heart of it, the true nature of reality must be very weird. What IS mass, what is energy, and why can, say, electricity used to power a particle accelerator, speed up a particle, which smashes into something, and then real solid matter appears?

Theoretically it would seem that with a big enough atom smasher you could end up producing gold out of pure vacuum. Or some other solid, real thing, that you could lift and carry. But yet energy isn't some yellow glowing thing like on Star Trek, energy is movement or heat or sound. Movement in particular... you move something fast enough, you stop it, and matter appears from what was previously motion. Motion is a very real and ordinary thing, but it can create matter!

On another utterly unconnected note... there is evidence to suggest that the Universe is actually 2-dimensional. Our supposed 3D existence is, somehow, embedded into the 2D as a hologram is imprinted in 2D film. That's not how it looks from here, but what does here *_really_* look like? Really really? Underneath the covers, where the gears are. I wonder if some scientists have a vision of it in their heads. An impressive feat for a load of sodium ions and proteins and everything else a brain is made of.

[anon]

I would enjoy a subsequent video explaining in more detail how we know tau, or other elementary particles, are indeed identical

[ravenlord]

Ah, the Standard Model particle zoo reminds me of the novel Animal Farm. "Some particles are more fundamental than others" :)

[lenin]

Loved your comment on the lack of internal states

[FFwachten]

Not hearing "that's what we"ll talk about Today" was the biggest dissapointment today.

[bobeasterly]

Great presentation. Always very interesting topics in physics to help answer our questions.
Small note: At 7:02 in the video, the 'CC' text states "...unlikely to occur in the word we actually inhabit, , , ". Sabine states it correctly, but the 'CC' should be "..unlikely to occur in the WORLD we actually inhabit, , ".

[finnjake]

She is a freaking genius!!! in explaining things. God I love her!

[shackamaxon]

You gave the best explanation of entropy I have ever heard. It is much clearer now why we experience time in one direction because the chances of something "unbreaking" are so small