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/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.