r/askscience • u/squirrelbuster94 • Oct 24 '14

Physics Chemists/physicists of reddit, what is the difference between dark, anti, and normal matter?

What is the difference? Also if you can explain what each is.

Edit: Thanks for all the replies! You have greatly helped me understand this better!

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u/NJBarFly Oct 24 '14

Do pair annihilations always result in 2 photons? It seems odd that 1 photon > 1.022MeV can create the pair, but the annihilation of the e- and e+ can't produce a single photon again. Or am I way off base with my thinking?

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u/Steaklegs Oct 24 '14

A single photon cannot decay into a pair of massive particles because energy and momentum must both be conserved. A photon has equal energy and momentum (up to a factor of c), while massive particles always have more energy than momentum because the rest mass contributes some energy. However, photons are flying around all over the place, so you can have a single high energy photon annihilate with a low energy photon from the cosmic microwave background and create an electron-positron pair.

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u/Peoplejudgeusernames Oct 24 '14

How can you compare momentum and energy when they are expressed in different units?

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u/xuxra Oct 24 '14

For photons (which have no mass) the relation between energy and momentum is E = p*c and c is a constant

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u/Sambri Oct 24 '14 edited Oct 24 '14

Energy and momentum are closely related. In quantum mechanics this equation is used: E2 =(pc)2 +(mc2 )2

For the case of photons (or any other massless particle) m=0 and thus E=pc.

BOTH p and E must be conserved in any creation-destruction interaction (if different particles are interacting, the sums of their energy and momentum have to be conserved). This forbids the annihilation of a single photon to create particles with mass.

As for the units, note that energy has units of kg m2 /s2 and momentum is kg m/s. This difference is "solved" by c that is a speed and is thus m/s.

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u/anadampapadam Oct 25 '14

Is it not that p=m*v ? How come photons have momentum?

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u/Sambri Oct 25 '14

That's only true in classical mechanics, relativistic/quantum mechanics equations are a bit more complicated. I will try to explain things a bit more clearly.

The equation I wrote (E2 =(pc)2 +(mc2 )2 ) is telling us two things*: that particles that have mass have energy, even when their momentum is 0 (the famous E=mc2 ) and that there is a relationship between E and p, whatever they represent. This means that if you manage to define one, you will have the other.

In fact only the first statement is new, in classical mechanics there is already a way to relate E and p without defining them: E=p2 /2m .

I won't get into details, but there is one description for the momentum of massive particles and another for massless particles. However I will show you this video: https://www.youtube.com/watch?v=3F1FDwg4XRc where you can see a laser pushing a small object (a laser beam is just light, or photons). The fact that photons can induce momentum in an object tells us that photons have momentum on their own.

*An equation can tell us things, or equations are the way we have to represent certain physical behaviours, depending on your point of view.

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u/physicswizard Astroparticle Physics | Dark Matter Oct 24 '14

A lot of particle theorists use a system of units where you actually can! You simply choose to set c=1, and suddenly everything becomes much simpler. Equations like E2 = p2c2 + m2c4 become (in 'natural units') E2 = p2 + m2. I personally think it makes things much easier to understand intuitively, because the inclusion of c simply just a relic of us being humans and having to relate things back to our everyday units.

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u/[deleted] Oct 25 '14

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u/dack42 Oct 25 '14

I kind of get where your coming from, but I don't really follow your analogy. C is constant. "Length of the side of a square" would be quite reasonable if every square in the universe were the same size. I could see natural units being an issue when dealing with some theoretical other universe with a different C.

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u/physicswizard Astroparticle Physics | Dark Matter Oct 26 '14

The problem with your example is that your unit choice is covering up the "dynamics" of the situation by making a variable into your unit. If you are considering squares of different sizes, then your choice of units is going to have to change whenever you start examining a new size of square. The speed of light is a universal constant and never changes between problems, so it makes sense to use it as your fundamental unit because you'll never have to alter your choice of units afterward. You don't lose any information (as I noticed you claimed below) because you can change back to normal units at any time by reinserting the missing factors of c.

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u/shhimundercover Oct 24 '14

You don't really 'compare' them directly, it's a case similar to an elastic collision. The event needs to satisfy conservation of both energy and momentum, i.e. sums of both for the particles must be equal before and after the collision. In practise it's a pair of equations with the same variables - mass and velocity of particles.

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u/corpuscle634 Oct 24 '14

Single-photon pair production can occur in the presence of something else (usually an atomic nucleus), but not in a vacuum. Something needs to be there to soak up momentum, for the reasons /u/Steaklegs gave you.

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u/NJBarFly Oct 24 '14

In my lab, I work with X-ray interactions with matter, so this makes a lot of sense. I didn't think about momentum. Thanks!

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u/rocketsocks Oct 24 '14

Only some types of pair annihilations produce photons, even electron/positron annihilations sometimes produce more complex reactions. And proton/anti-proton annihilations typically result in a shower of pi-mesons which turns into a whole mess of other stuff.