**Ask a monkey a physics question thread**

[MadMojoMonkey]

Anti Matter. Explain please?

Anti-matter is identical to matter in every respect except the sign of it's electric charge.

Any particle and its anti-particle have the same mass, same spin, same expected decay half-life, same magnitude of charge, even.
The only difference is that a + charged particles have - charge anti-particles, and vise versa.

Are they real or theoretical?

Positrons are anti-electrons.

A PET scan is a medical procedure in which Positron Emission within your body causes positron-electron annihilation events to release photons of a specific frequency, which are measured by an external sensor and fed to a computer to form an image out of (Tomography).

Where do positrons come from?
Certain "Weak Interactions" release them in a nuclear process called positron emission. It is possible for a proton to change into a neutron. This is a common occurrence in certain radioactive nuclei. They spit out part of their nucleus and what gets left behind is not in the lowest energy state, so it will release energy in various ways until it is in the lowest available energy state.

One way this happens is when the ratio of protons to neutrons is such that it takes more energy to have that 1 more proton and 1 less neutron than the cost to change a proton into a neutron. In this case, it is energetically favorable for this to occur. As the proton switches into a neutron, it goes from having a +1 charge to having a 0 charge. It emits a positron (and an electron neutrino) in the process, thus conserving charge and some other properties.

Another source of positrons is in pair production. Technically, pair production can occur when any neutral boson with enough energy turns into a particle / antiparticle pair. Mostly, though when people talk about pair production, they're talking about a photon turning into an electron / positron pair.
The rest mass of an electron (or positron) is ~511 keV (an eV is a measure of energy). If a photon has just over 1.022 MeV, then it has enough energy to "pay" the cost in rest mass for the particles. It needs to have a little more than this, so that the pair are created with some momentum. All of this happens "near" an atom.

So there are 2 well known and often used methods of creating anti-matter electrons. One of them is a medical scan which most people have heard of.

Where is all the anti-matter?
Dunno. It's an open question in physics as to why there isn't an equal amount of anti-matter as there is matter with only a fairly hand-waving explanation about some process which could have gone like [...] happened just prior to or during the cosmic inflation period, which physics has done nothing, really, (yet) to explain.

We are pretty sure that the universe isn't actually in balance, but with some regions dominated by matter and other regions dominated by anti-matter. If this were true, there would be bright flashing boundaries where these regions met, dominated by particle-antiparticle annihilation.

[Poopadoop]

Anti-matter is identical to matter in every respect except the sign of it's electric charge.

Any particle and its anti-particle have the same mass, same spin, same expected decay half-life, same magnitude of charge, even.
The only difference is that a + charged particles have - charge anti-particles, and vise versa.

Where does the idea that if they meet they will destroy each other come from, or is that just science fiction?

[MadMojoMonkey]

Where does the idea that if they meet they will destroy each other come from, or is that just science fiction?

Fiction? IDK. Depends what you mean.
Nothing I say in this thread is intentional fiction.

Science doesn't produce facts, it produces falsifiable statements. So if you say, "If a statement of science is shown to be false, then that statement was fiction," then yes. Everything in this thread is fiction. All the scientific "truths" of the past were later shown to be approximations. It stands to reason that all our understanding today is approximations. If you consider approximation to be fiction, that's cool. BUT if you think that a good description being off by a fraction of a percent from "perfect" is non-fiction, then nothing in this thread is (intentional) fiction. (I do make mistakes and don't understand everything as well as I think I do.)

Does that make sense?

Where I cannot rely on publicly available, peer reviewed data or my own personal experiments, I make it clear that those topics are theory based, and not observation-based.
If you go back and read what I said about virtual photons, this is plainly evident. (but at 13 pages, I don't really expect you to do this.)

***
particle-antiparticle annihilation is well observed and a fundamental mechanism of the PET scan. The annihilation creates 2 photons of specific frequency, moving in opposite directions. The scanner looks for 2 photons moving exactly away from each other, and measures the difference in time between those signals. It then backs out the distance to the common origin in space and time of those 2 photons to determine where the flows of bodily fluids are going inside your body.

[Poopadoop]

Fiction? IDK. Depends what you mean.
Nothing I say in this thread is intentional fiction.

Science doesn't produce facts, it produces falsifiable statements. So if you say, "If a statement of science is shown to be false, then that statement was fiction," then yes. Everything in this thread is fiction. All the scientific "truths" of the past were later shown to be approximations. It stands to reason that all our understanding today is approximations. If you consider approximation to be fiction, that's cool. BUT if you think that a good description being off by a fraction of a percent from "perfect" is non-fiction, then nothing in this thread is (intentional) fiction. (I do make mistakes and don't understand everything as well as I think I do.)

Does that make sense?

Where I cannot rely on publicly available, peer reviewed data or my own personal experiments, I make it clear that those topics are theory based, and not observation-based.

I was not attacking you personally. If you said it was true, I didn't see it and wasn't questioning it. I was asking the question based on what I'd heard about it from other sources.

It's funny that you feel the need to teach me about what science is though...you know I am a scientist, right?

If you go back and read what I said about virtual photons, this is plainly evident. (but at 13 pages, I don't really expect you to do this.)

If you said it 13 pages ago I certainly would not have remembered it when I posed the question, assuming I understood it when i read it in the first place.

***

particle-antiparticle annihilation is well observed and a fundamental mechanism of the PET scan. The annihilation creates 2 photons of specific frequency, moving in opposite directions. The scanner looks for 2 photons moving exactly away from each other, and measures the difference in time between those signals. It then backs out the distance to the common origin in space and time of those 2 photons to determine where the flows of bodily fluids are going inside your body.

Ok, so if I understand this correctly two photons are being created in the process. What is being destroyed (i.e., a molecule, an atom?). Again, not trying to be difficult I just want to understand.

[MadMojoMonkey]

I was not attacking you personally.

Me or anyone, as far as I can tell.
IDK what you're talking about with "attack" word.
You asked a question. I answered to the best of my ability.

It's funny that you feel the need to teach me about what science is though...you know I am a scientist, right?

My intent wasn't to teach you about science, it was to describe my approach to science and physics.
My intent was to provide future context so that your question was answered not only in this case, but in all cases.

If you said it 13 pages ago I certainly would not have remembered it when I posed the question, assuming I understood it when i read it in the first place.

Page 1 has some gold. Prob other pages, too.
I didn't realize you've been on FTR for so long.

Ok, so if I understand this correctly two photons are being created in the process. What is being destroyed (i.e., a molecule, an atom?). Again, not trying to be difficult I just want to understand.

The particle and antiparticle are annihilated and 2 photons are created with equal energy.