what happens when a particle and an anti particle collide

L-x

"I think it's possible in principle - just very very difficult."

very very difficult is an absurd underestimate. Producing and containing 1kg of antimatter is "possible in principle" in the same way that counting every particle in the observable universe is "possible in principle".

Karimspencer

"Producing and containing 1kg of antimatter is "possible in principle" in the same way that counting every particle in the observable universe is "possible in principle""

Scientists have found ways to calculate the number of atoms in the universe. I am guessing you can do so by finding the average volume of the atom and the volume of all matter in the observable universe(planets, stars....) and then you can divide the volume of that matter by the average volume of the atom to find the number.
I might be wrong and you must consider a lot of other things.

L-x

Scientists have indeed, and you're thinking along nearly the right lines: cosmological observations allow scientists to make a reasonable estimate of the energy density of matter in the universe, and calculating the volume of the observable universe is straightforward (though interestingly, due to the expansion of the universe, that volume isn't just 4/3 pi (age of universe times c)^3). In fact it's calculations like this which have lead to conclusions that the universe is full of mysterious "dark matter", because when cosmologists predict how much "stuff" we should be able to see, they get an answer that's about 5 times bigger than what we can actually observe through telescopes.

I didn't say calculate though. I said "count", as in getting the exact number. All I meant (in a slightly silly way, admittedly) is that someone obtaining that much antimatter is not going to happen in the remotely foreseeable future.

Karimspencer

I know ... it's unpractical and unreasonable how ever it's known that there are 2 * 10to the power 78 atoms in the universe.

Karimspencer

That's just an estimate

Karimspencer

And you don't find the volume of the universe , you have to find the volume of the matter in our universe.

And only 0.0000000000000000000042 % of the universe consists of matter.

mfb

This number is wrong by a factor of about 10^21 (source: The Cosmic Energy Inventory). This is approximately the same difference as between the size of a single atom and the orbital radius of earth.

Karimspencer

oh ok

jnorman

what about for virtual particle pairs created from the quantum foam? are they made from energy, and if so, where does this energy go? what is the difference between the particles made from virtual pair production and particles made from high-energy photons? thanks

L-x

I'd be fascinated to know how one could estimate the volume of matter in the universe without using the method I outlined above. I'd also be interested to know how you came up with that percentage.

Karimspencer

i googled it

L-x

The technical difference between virtual and real particles is that virtual particle are "off shell" meaning that their mass energy and momentum does not satisfy the relation E^2 - P^2 = M^2, however virtual particles should be thought of as a sort of mental abacus that help us calculate things about interactions, rather than as real particles that just happen to have strange properties.

mfb

Volume is not a meaningful quantity here - most of the matter is very thin interstellar plasma of variable density, and the remaining part is mainly plasma in stars. Just a small fraction is solid.

How to estimate the matter density? Mainly: observe it. The total amount of matter is then given by a multiplication with the size of the observable universe.

Simon Bridge

Um - as interesting as working out how many atoms there are in etc etc isn't it a bit off topic? I intended my statement to be an absurd understatement :) Even attempting to manufacture, without bothering to contain, the mere 1g that was my example would be silly.

L-x

That's precisely the point I was trying to make.