ask a physicist

[Scott DeWar]

For every particle, there is supposedly an anti particle. So where is all of the anti-matter and why isn't matter obliterated by the anti matter?

 

[Umbran]

What would happen if you shot a gun in space?

Okay, so let us assume you're floating out there in a space suit. Let us also assume you're using a pretty modern firearm. First off, yes, the gun will fire - cartridges these days are loaded with a charge that contains its own oxidant, so it doesn't need to be fired in air to burn.

So, you have gun, you pull the trigger. A small explosive goes off in the chamber. A bullet flies... and so do you. This is "conservation of momentum". A "standard human" is about 65 kilograms. A bullet from a .45 caliber round is about 13 grams. If my quick math is right, if the bullet takes off at a speed of 260 m/s, you take off at a speed of 0.05 m/s - about 2 inches per second.

If you are holding the gun up at shoulder level, you'll begin to spin as well, as the force of recoil is away from your center of mass.

The bullet will go on i a straight line until it runs into something.

 

[Scott DeWar]

And so the shooter will just spin on and drift at .05 m/s in the opposite direction from equal force. I was actually not sure of the bullet working in space, but yu have confirmed the chemical needs.

 

[Umbran]

For every particle, there is supposedly an anti particle. So where is all of the anti-matter and why isn't matter obliterated by the anti matter?

To our first best guess, the Big Bang should have created just as much matter as antimatter - particles are produced in matter-antimatter pairs. So, yes, we should see a lot of antimatter around, but don't. Why?

The answer is: We don't know yet. There are a few hypothesis.

One is that, back in the very early universe, when matter and anti-matter should have been made in equal amounts, there was some asymmetry we don't currently see that led to more matter than anti-matter being created. Then, there was a frenzy of matter and anti-matter annihilating, leaving us with the slight excess of matter we see now.

Another is that there's a *slight* difference in the decay rates of matter and anti-matter. If anti-matter decayed a bit more quickly than matter, we might then see an excess of matter today.

A third is that the Big Bang did create equal amounts of matter and anti-matter, but they are in regions widely separated - this leaves us with a problem of why stuff was created in uneven bubbles, instead of evenly. Part of that may be answered by the Anthropic Principle - in order for us to see a universe, it must be a universe we can live in. If the universe had just as much of both, they'd all annihilate, and leave nothing for us to be made out of! So, while move of the entire universe may have been created with a pretty even distribution, maybe there was at least one statistical anomaly (in an infinite universe, there *will* be some anomalies) where the matter and antimatter were created slightly separated, and we live in one. If we didn't, we wouldn't live at all....


https://en.wikipedia.org/wiki/Baryon_asymmetry

 

[freyar]

What would happen if you shot a gun in space?

Well, there's something in common between firing a gun in space and on earth: the bullet (and "exhaust" or whatever you call the byproducts of the exploding gunpowder) would come out of the muzzle in a straight line, and you'd feel the recoil and start moving in a straight line in the opposite direction but with equal momentum to the bullet. That's all that happens in space.

On earth, quite a few other things happen. First off, the bullet falls toward the ground, so it doesn't really quite move in a straight line after it leaves the gun. Air resistance also slows the bullet down, so it doesn't move quite as fast as it would in space. And that exhaust (and potentially turbulence of the bullet in the air) will create sound. If the bullet is moving fast enough, it will make a sonic boom. And, of course, you still feel the recoil, but unless you're on wheels, chances are that your friction with the earth means that the recoil affects you and the earth together; since the earth is so massive, the recoil speed is effectively zero. And then, after the flight of the bullet, all the bullet's momentum gets transferred to the earth and atmosphere, so the recoil stops.

So actually, firing the gun on earth is a lot more complicated than in space, I think.

EDIT: also, what Umbran said.

 

[GMMichael]

there was some asymmetry we don't currently see that led to more matter than anti-matter being created. . .
while move of the entire universe may have been created with a pretty even distribution, maybe there was at least one statistical anomaly (in an infinite universe, there *will* be some anomalies) where the matter and antimatter were created slightly separated, and we live in one. If we didn't, we wouldn't live at all....
https://en.wikipedia.org/wiki/Baryon_asymmetry

Well, this more-or-less answers my "why isn't the universe composed only of perfectly symmetrical spheres" question. It sounds like a pretty important physics question though: if the universe had/has an asymmetrical anomaly, what caused that anomaly???

I'd like to answer myself, and blame it on the quantum field, which was basically subatomic chaos, pure randomness, if my last reading was interpreted correctly. So...

1) How's quantum field theory coming along, and is the field(s) as chaotic as I've understood?
2) Could the quantum field exist before/during the big bang, and explain why the universe isn't spherically symmetrical?

and just for fun...

3) I've read that time isn't just some simple measurement that keeps going steadily forward. Does Time show up as a variable in current physics math, and how does it vary from the Time that we plug into high school math? Or getting-to-work-on-time math?

 

[Umbran]

Well, this more-or-less answers my "why isn't the universe composed only of perfectly symmetrical spheres" question. It sounds like a pretty important physics question though: if the universe had/has an asymmetrical anomaly, what caused that anomaly???

2) Could the quantum field exist before/during the big bang, and explain why the universe isn't spherically symmetrical?

I wasn't speaking of a spacial asymmetry, but a lack of symmetry in physical laws - like matter and antimatter were not exactly opposite in all ways.

But, the spherical symmetry question raises a point. It assumes that space is finite, and has an origin, a middle. This is probably not correct. If space is infinite, it cannot have a center.

Space is still spherically symmetric, if you choose yourself as the center, mostly. But only in a sort of statistical sense, on the large scale.

 

[Scott DeWar]

Re: Matter/anti-matter question

All very interesting theories, but I had to laugh at the blunt honesty of

The answer is: We don't know yet.

Read more: http://www.enworld.org/forum/showthread.php?466337-ask-a-physicist/page3#ixzz3jgDWsIde

 

[freyar]

For every particle, there is supposedly an anti particle. So where is all of the anti-matter and why isn't matter obliterated by the anti matter?

Umbran does a pretty good job breaking down the logical possibilities. I'd like to elaborate, though, especially because that wikipedia article is a bit disappointing compared to most of the physics wikipedia articles.

The most important point to remember is that we really don't know the answer to this question. It's also quite likely we won't for a long time, since there's very little way to test any of the possibilities --- there are some potentially related effects we can test experimentally, but those are really only good at ruling out options as opposed to pointing toward the correctness of one.

To our first best guess, the Big Bang should have created just as much matter as antimatter - particles are produced in matter-antimatter pairs. So, yes, we should see a lot of antimatter around, but don't. Why?

The answer is: We don't know yet. There are a few hypothesis.

One is that, back in the very early universe, when matter and anti-matter should have been made in equal amounts, there was some asymmetry we don't currently see that led to more matter than anti-matter being created. Then, there was a frenzy of matter and anti-matter annihilating, leaving us with the slight excess of matter we see now.

This is what people think happens for the most part (I'll explain why below), though how this happens is completely up in the air. What we do know is that there are several criteria that have to be met (called the Sakharov conditions) and that they are not satisfied (enough) by the Standard Model of particle physics. In fact, the Standard Model can almost cause a predominance of matter over antimatter, but it would require a certain behavior of the Higgs boson field in the early universe --- and in the Standard Model, the Higgs doesn't behave that way. To get it to work, you have to add other ingredients to change the behavior of the Higgs.

Of course, there are other possibilities. There could be an entirely new, undiscovered group of particles that are responsible for the excess of our matter. The roots of the excess could happen during inflation. Or another possibility is that dark matter is also somehow unbalanced between matter and antimatter, and that imbalance gets generated at the same time as the imbalance of normal matter. There are many different theories.

I should also mention that the excess amount of matter is very tiny. In the early universe, the amount of matter and antimatter was essentially equal. For roughly every 10 billion matter/antimatter pairs of particles, there was one extra matter particle. Then all the 10 billion or so pairs annihilated each other away, leaving behind the one matter particle.

Another is that there's a *slight* difference in the decay rates of matter and anti-matter. If anti-matter decayed a bit more quickly than matter, we might then see an excess of matter today.

There's a way this is correct and a way it's not correct, so I want to be very careful here.

If we're talking about the decay rates of "everyday" particles and their antiparticles (like protons/antiprotons, neutrons/antineutrons, and electrons/positrons), this is a logical possibility that just doesn't work out. Based on cosmic ray measurements, the lifetime of the antiproton is at least a million years, which wouldn't leave enough of an imbalance between protons/antiprotons. Furthermore, there's a mathematical theorem in particle physics that says the total decay rates of particles and their antiparticles must be the same, and any decay of a proton/antiproton generates the same amount of matter/antimatter. It doesn't work (incidentally, a violation of this theorem would be a super-big deal, meaning we'd have to redo basically all of subatomic physics).

On the other hand, it is possible that some very heavy undiscovered particle and antiparticle decay differently into matter and antimatter. That can happen and can create the imbalance we need. Of course, we've not discovered such a particle yet.

A third is that the Big Bang did create equal amounts of matter and anti-matter, but they are in regions widely separated - this leaves us with a problem of why stuff was created in uneven bubbles, instead of evenly. Part of that may be answered by the Anthropic Principle - in order for us to see a universe, it must be a universe we can live in. If the universe had just as much of both, they'd all annihilate, and leave nothing for us to be made out of! So, while move of the entire universe may have been created with a pretty even distribution, maybe there was at least one statistical anomaly (in an infinite universe, there *will* be some anomalies) where the matter and antimatter were created slightly separated, and we live in one. If we didn't, we wouldn't live at all....

This is an interesting idea, though I confess I've not seen any work related to it. I also can't immediately think of a way to implement it, either, since, if it's just a "local" fluctuation, it would have to be created during inflation. Anyway, suffice it to say that I'm not sure if there's a way to get this to work off the top of my head.

 

[freyar]

Re: Matter/anti-matter question

All very interesting theories, but I had to laugh at the blunt honesty of

It really is one of the big questions!