Viewing a single comment thread. View all comments

Derice t1_j8hdqj8 wrote

Reply to comment by IonizedRadiation32 in Light traveling through a medium that slows it. Does the same photon emerge? by TheGandPTurtle

The Wikipedia article on this is quite good. If you want some intuition for this, you have encountered things that work kind of like this in your daily life: cups of water.

  1. Take two different cups of water, cup A and B. As long as they are kept separate you can kind of label them by the cup they are in.
  2. Pour them into the same container. Now you know that you have two cups of water, but it does not make sense to ask which is which.
  3. Pour the water out into the two cups again. You can not say whether the current cup A is the same as the first cup A.

That particles act like this has huge consequences for any physics that depend on how many different states are available to the system. Consider two distinguishable particles: 1 and 2, that each can be in one of two states: up or down. There are four possible states:

  1. 1 up, 2 up
  2. 1 up, 2 down
  3. 1 down, 2 up
  4. 1 down, 2 down

If they are indistinguishable there are only three states:

  1. Two particles are up
  2. One is up and one is down
  3. Two are down

This is a pretty big thing, and has macroscopic consequences! Distinguishable particles follow Maxwell-Boltzman statistics while indistinguishable particles follow either Fermi-Dirac or Bose-Einstein statistics. Identical particles of the first group are called fermions and the second bosons. Fermions have the property that two fermions can not be in exactly the same quantum state, and since protons, neutrons and electrons are fermions this is partly what gives matter "solidity". Bosons can all be in the same state, allowing for things like laser light and Bose-Einstein condensates.

28

IonizedRadiation32 t1_j8hkskc wrote

1, thank you for the detailed reply! I can't wait to understand this better.

2, at least for me, the water cup analogy doesn't quiiite work, because the reason they become indistinguishable when you mix them is because they are made of a bunch of the same stuff and it gets mixed, but subatomic particles are made from distinct units, so in theory even if you "mix" them you should be able to follow where each part goes, at least in theory, no?

1

Derice t1_j8ho1g1 wrote

> subatomic particles are made from distinct units, so in theory even if you "mix" them you should be able to follow where each part goes

Actually no. Subatomic particles are all excitations of the same underlying quantum field, and if we are using quantum field theory, they are not really things in themselves.
If you use quantum field theory to model e.g. sound waves you find that you can describe them with particles called phonons. However, if you have a sound wave in a material and pause time, no matter how much you zoom in on the sound wave you will never find it to be made of little balls flowing through the material.
In quantum field theory particles are less the water in my cup analogy, and more the abstract volume measurement of "a cup". You can add or remove 1 particle's worth of excitation, but when you do you do not add a "real thing", you add an amount of excitation to a real thing: the field.

11

jazekers t1_j8hm11l wrote

>subatomic particles are made from distinct units

Then we enter into the particle vs wave interpretation. If you think of them as rigid particles then you would indeed think that you could follow them (keeping out the fact that observing means interactions, which means altering the state). My particle physics professor said it like this "subatomic particles are spatiotemporal fluctuations of quantum fields", which is a very abstract but interesting way to put it.

A proton for example is made up of three quarks, kind of. In fact, it also contains virtual quark pairs that exist for a ridiculously short amount of time, being fluctuations in the strong nuclear field.

But some things are still conserved. Meaning that if I have two particles with one being spin up, and one being spin down. Then when I measure them I will still find one spin up, and one spin down. But that doesn't mean that the particle remained "intact" and rigid along the way. What is conserved is the total spin of the system. Not that of the individual particles.

3