Matter/antimatter imbalenc - forked from AMA ask a physicist

[freyar]

Not so much correct, as complete. The fact that this transition must be mediated was something I was leaving out for sake of brevity. Concept first, then details!

Same reason I left out discussion of "spin" vs "helicity".

Fair enough, though the photon also has spin, so the electron and positron wouldn't necessarily have the opposite spin as each other, either. In fact, if the thing the photon hits doesn't flip spins and has negligible recoil, the electron and positron need to have the same spin.

 

[freyar]

So many quotes ... I've left off the attribute of the above.

I've always wondered ... if so much matter/antimatter combined, wouldn't that create a huge excess of energy? Where is that 10^10 factor of energy?

That is, particle/antiparticle annihilation doesn't destroy the energy of the particles. Depending on the energy of the particles, very roughly speaking, either, a photon is produced, or a photon and some other particles are produced, or maybe just some new particles depending on the exact energy of the event.

Or does the energy recycle back into particles, but, considering the whole cycle, inefficiently, leaving a growing excess of matter?

But then, the fraction seems too high! There would be so many creation/annihilation events in the early universe, one in 10 billion seems to to be too many.

Or do the events only happen in some narrow transition region, say, when matter and light decoupled? Or very very early, such that the relevant time where the asymmetry mattered was very short?

Thx!

TomB

All around you!

We are talking about events at a time when the universe was extremely dense, just a plasma of particles. Lots of particles and antiparticles tooth-by-jowl, so to speak. When the particles annihilate, what you typically get* is photons**. With things so dense, those photons get immediately absorbed by particles of matter***, and re-emitted.

Eventually, as the universe expanded, atoms formed, and the universe dropped to a density where photons could fly free without running into stuff. What photons were left that didn't run into stuff to be absorbed became what we now call cosmic microwave background radiation.



*You sometimes get particle-antiparticle pairs instead, but then those annihilate, too, because everywhere you go there's stuff to annihilate with, so eventually you get back to photons.

** Freyar may correct me if I am wrong - this may be happening before the electromagnetic force and weak nuclear force decoupled, so we would then be looking at W and B bosons, carrying the electroweak force of the time, but the rest is about the same.

***We note there that "matter" in this case may also include so-called "dark matter" - while today it doesn't interact much, back in the day or high density it may have been a major contender- so some of that energy may be tied up in stuff you can't see now.

Umbran has this covered in the basics, so I just want to fill in a couple of tomBitonti's particular questions and address the footnote.

Any matter/antimatter annihilation can create just about anything it has enough energy to make (oversimplifying a bit again). In particular, that can include dark matter, electrons, protons, etc, etc. The very early universe (after inflation, if you know what that is) was a very hot, very dense soup of particles, so these annihilations happened all the time as Umbran said. But, one peculiarity of an expanding universe is that it cools things down, so what annihilations have enough energy to make eventually gets to be lighter and lighter particles, and the heavier things basically go away. In the last stages, you have some left over protons and neutrons (antimatter is all gone, so they can't annihilate), electrons and positrons, and photons.* When the electrons and positrons annihilate (with a few electrons left over), the energy can only go into photons (like Umbran said). At the time, that meant almost all the energy was in photons.** However, the universe continued to expand and cool, and photons cool down much more quickly than protons and electrons, etc. So by now, the photons have cooled off a lot (to a temperature of less than 3 Kelvin (ie, 3 Celsius-sized degrees above absolute zero)) and make up very little energy compared to the matter. As Umbran said, these photons are the cosmic microwave background (CMB). A neat thing about the CMB is that you can see it. If you have an analog TV antenna and set your TV to a station that's not there, about 1% of the static snow is due to the CMB. The CMB is also immensely useful to understand the early universe, since it's nearly unchanged since quite early times. One thing it tells us is what the starting conditions were like for the formation of structures like galaxies, etc.

*There is also some dark matter, which either has no anti-dark-matter left over (and nothing to annihilate with) or else is too dilute to annihilate efficiently, and neutrinos, which don't interact with the other stuff I mentioned much.
** and neutrinos.

 

[Scott DeWar]

I know what I am going to ask is going to make my head spin, but this dizzy trip is starting to get to be fun - if I can survive the accompanying headache . . . .

Next Subject:
Neutrinos

1. You mentioned that do not react with some forms of matter. Why? Are they too high of an energy particulate?

1a. how do they move at such high velocities sometimes, but not all times?

Ref: At University of Missouri-Columbia Research Reactor, the core sits in a pool of water with a blue glow around it. It is explained that the glow is caused by neutrinos escaping at near 'C' velocities; however the neutrinos do not move that fast in open air.

2. What is a neutrino exactly, if that is known. I know of neutrino detection labs deep in the earth, the detectors set in these deep chambers and the slow gathering of data that is accomplished.

3. Has there been any purpose for these neutrinos as yet - such as the anti-electron has done.

4. hidden in all of these answers there might be an answer to this next question and I am sorry if I missed it, but, Is there an anti-neutrino?

4a. if there is an anti-neutrino, does it react the same as other antiparticles? That is annihilating and producing a gamma wave?

That should be enough for now, I think. I am giving my self that head spin and headache.

 

[freyar]

These I can answer pretty quickly before I get to the other stuff....

I know what I am going to ask is going to make my head spin, but this dizzy trip is starting to get to be fun - if I can survive the accompanying headache . . . .

Next Subject:
Neutrinos

1. You mentioned that do not react with some forms of matter. Why? Are they too high of an energy particulate?

It isn't an energy issue but a "what type of force" issue. We say we know of 4 fundamental forces: electromagnetic, strong nuclear, weak nuclear, and gravitational (technically, we can now add a 5th, the Higgs force, though it hasn't yet been observed in all the same ways the others have). For particle physics experiments, gravity is so feeble as to be non-existent, so let's not count that. While electrons feel the electromagnetic and weak forces, neutrons feel the strong and weak forces, and protons feel all three, neutrinos only feel the weak force. As you might guess from the name, the weak force doesn't affect things very strongly, which is due to the fact that it is very short range. So neutrinos just don't have the capability to interact with other particles easily.

Here's another way to put it. Imagine you are a fundamental particle. If you look around, other particles appear to be carrying targets sized according how easy they are for you to hit. If you're an electron or neutron, a proton looks really big. However, if you're a neutrino, other particles look super-tiny. To give you some perspective, if you had a beam of neutrinos, half of them would make it through a wall of lead more than a light year thick. This means neutrino detectors have to be very big to catch a very few out of a fantastical number of neutrinos.

1a. how do they move at such high velocities sometimes, but not all times?

Ref: At University of Missouri-Columbia Research Reactor, the core sits in a pool of water with a blue glow around it. It is explained that the glow is caused by neutrinos escaping at near 'C' velocities; however the neutrinos do not move that fast in open air.

In addition to interacting very weakly, neutrinos have a very small mass, no more than one-millionth that of electrons, which are the next lightest massive particles. At the energies we've ever observed neutrinos, they are so relativistic to be moving at a speed indistinguishable from c, the speed of light in vacuum. And most any neutrino produced has enough energy to be moving that fast. (Since neutrinos do have mass, it is technically possible to have one moving slowly, but I don't know of a way, for example, you could easily produce them in an experiment or astrophysical event and have them come out slowly.)

That blue light in the reactor is called Cherenkov radiation and deserves a bit of explanation. Light moves at c, "light speed," only in vacuum. In air, light moves a bit more slowly and even more slowly in water. What's happening in that reactor is production of a bunch of neutrinos with a lot of energy. A small fraction of these neutrinos hit electrons in the water so hard that the electrons start moving very close to c and even faster than light moves in water. Just like an object moving faster than sound in air creates a sonic boom, those electrons create a "light boom," which is the Cherenkov radiation.

2. What is a neutrino exactly, if that is known. I know of neutrino detection labs deep in the earth, the detectors set in these deep chambers and the slow gathering of data that is accomplished.

Bear with me, mini-lecture ahead. There are two basic types of fundamental particles, force-carriers like photons (called bosons), and matter particles like electrons (fermions). The fundamental matter particles are 6 types of quarks, electrons, muons (heavier version of electrons), taus (even heavier versions of electrons), and neutrinos. Now, as far as the weak nuclear force is concerned, these matter particles come in pairs: 3 pairs of quarks, electrons and electron neutrinos, muons and muon neutrinos, taus and tau neutrinos (yes, imaginative naming, I know). So there are actually 3 types of neutrino.

As for other properties of the neutrinos (all 3 types), we've already covered that they only interact through the weak force and that they have a very small mass. The other very important thing about neutrinos is that they can change type as they fly along. In other words, you could create a whole bunch of electron neutrinos and send them off to a detector. But by the time they reach the detector, some will have changed into muon neutrinos or tau neutrinos. This is what the big neutrino experiments are trying to understand, for the most part --- what is the precise physics that causes neutrinos to change type. This is one of the things we know about particle physics that tells us the Standard Model is incomplete, and it might be telling us that there are even other types of neutrinos that don't even feel the weak nuclear force (called "sterile" neutrinos), and these might be the dark matter we observe in astrophysics (or a part of it). Anyway, one of my colleagues at my university works on one of these neutrino experiments.

3. Has there been any purpose for these neutrinos as yet - such as the anti-electron has done.

I assume you're asking about technological uses. Neutrino technology is in very early stages since they're very slippery particles, but there are some ideas. As you've noticed, nuclear reactors produce lots of neutrinos, so a sophisticated neutrino detector could act like an "x-ray machine" to look inside of nuclear reactors where you can't send equipment due to the high radiation. This could help with monitoring non-proliferation agreements or the Fukushima accident site. There have also apparently been some experiments using neutrinos for communication purposes, but I don't know details.

4. hidden in all of these answers there might be an answer to this next question and I am sorry if I missed it, but, Is there an anti-neutrino?

4a. if there is an anti-neutrino, does it react the same as other antiparticles? That is annihilating and producing a gamma wave?

That should be enough for now, I think. I am giving my self that head spin and headache.

Yup, there are anti-neutrinos. Certainly, neutrinos and anti-neutrinos can annihilate and could even produce gamma rays or at least photons of some energy. But the odds of that happening are astronomically slim, so I'm pretty certain no one's ever seen that happen.

Another fun fact about neutrinos and anti-neutrinos: we talked a bit earlier about the spin of particles. Most matter particles, like electrons, can spin in two basic ways. However, neutrinos can only spin one way --- we call them left-handed. Anti-neutrinos can only spin the other way --- they are right-handed. This is another complicating feature for neutrino theory. Where are the right-handed neutrinos? Are they the sterile neutrinos? Do they exist at all? There are lots of models to explain this fact along with the way neutrinos change type, but we just don't have the data yet to say what's right.

 

[Umbran]

Fair enough, though the photon also has spin, so the electron and positron wouldn't necessarily have the opposite spin as each other, either. In fact, if the thing the photon hits doesn't flip spins and has negligible recoil, the electron and positron need to have the same spin.

And here, we see perhaps some fundamental difference in communication philosophies. All of what you say is entirely true, but not necessary for the point - that the example given did not lead to being able to separate matter and anti-matter in the overall universe!

The point is that one of the possible handles we have to separate the pair requires information to use. Putting this back in context of the patent - the target is a slab of metal, so we don't know, and cannot measure, the spin state change or recoil of the individual atoms in the interaction. The electron and positron spins are correlated, such that their creation adds no new net angular momentum to the system, but we still don't know what way they'll be pointing. So, separating on the basis of that is not simple.

I was also trying to stay away from the details of quantum mechanical spin, helicity, and magnetic moment, as those can be *really* confusing to laymen.

 

[Mustrum_Ridcully]

Moreover, you'd have to deal with how, *just by coincidence* that weird thing happens to be out where we cannot see it.

Maybe any spot where it would be visible would also be harmful to human life, or hinder the forming of biological life? Not quite satisfying, of course, but that's always the issue with theanthropic principle.

 

[freyar]

And here, we see perhaps some fundamental difference in communication philosophies. All of what you say is entirely true, but not necessary for the point - that the example given did not lead to being able to separate matter and anti-matter in the overall universe!

The point is that one of the possible handles we have to separate the pair requires information to use. Putting this back in context of the patent - the target is a slab of metal, so we don't know, and cannot measure, the spin state change or recoil of the individual atoms in the interaction. The electron and positron spins are correlated, such that their creation adds no new net angular momentum to the system, but we still don't know what way they'll be pointing. So, separating on the basis of that is not simple.

I was also trying to stay away from the details of quantum mechanical spin, helicity, and magnetic moment, as those can be *really* confusing to laymen.

Yeah, there are different communication styles, which is perfectly fine. But I thought we were answering the question about whether antiparticles have the opposite spin of their corresponding particles, right?

 

[freyar]

Maybe any spot where it would be visible would also be harmful to human life, or hinder the forming of biological life? Not quite satisfying, of course, but that's always the issue with theanthropic principle.

The problem is that it wouldn't be a big deal for human (or other intelligent) life unless the boundary were quite close. Even a supernova, which would be much more inimical to life, would have to be within 100 lightyears or so of earth to kill off life here. And that's just no distance at all in terms of cosmology.

 

[Scott DeWar]

These I can answer pretty quickly before I get to the other stuff.... . . . . edit lots of stuff . . . . .
That blue light in the reactor is called Cherenkov radiation and deserves a bit of explanation. Light moves at c, "light speed," only in vacuum. In air, light moves a bit more slowly and even more slowly in water. What's happening in that reactor is production of a bunch of neutrinos with a lot of energy. A small fraction of these neutrinos hit electrons in the water so hard that the electrons start moving very close to c and even faster than light moves in water. Just like an object moving faster than sound in air creates a sonic boom, those electrons create a "light boom," which is the Cherenkov radiation.

for lack of a better way for me to put that, that is so kool! further on you mention usin neutrinos -possibly- in a technological use of an "Xray for reactors" of sorts. That would have so many positive implications that is almost astounding!

Bear with me, mini-lecture ahead. There are two basic types of fundamental particles, force-carriers like photons (called bosons), and matter particles like electrons (fermions). The fundamental matter particles are 6 types of quarks, electrons, muons (heavier version of electrons), taus (even heavier versions of electrons), and neutrinos. Now, as far as the weak nuclear force is concerned, these matter particles come in pairs: 3 pairs of quarks, electrons and electron neutrinos, muons and muon neutrinos, taus and tau neutrinos (yes, imaginative naming, I know). So there are actually 3 types of neutrino.

that we know if anyway. you mention that sterile neutrinos might be dark matter for all that we know

As for other properties of the neutrinos (all 3 types), we've already covered that they only interact through the weak force and that they have a very small mass. The other very important thing about neutrinos is that they can change type as they fly along. In other words, you could create a whole bunch of electron neutrinos and send them off to a detector. But by the time they reach the detector, some will have changed into muon neutrinos or tau neutrinos. This is what the big neutrino experiments are trying to understand, for the most part --- what is the precise physics that causes neutrinos to change type. This is one of the things we know about particle physics that tells us the Standard Model is incomplete, and it might be telling us that there are even other types of neutrinos that don't even feel the weak nuclear force (called "sterile" neutrinos), and these might be the dark matter we observe in astrophysics (or a part of it). Anyway, one of my colleagues at my university works on one of these neutrino experiments.

I assume you're asking about technological uses. Neutrino technology is in very early stages since they're very slippery particles, but there are some ideas. As you've noticed, nuclear reactors produce lots of neutrinos, so a sophisticated neutrino detector could act like an "x-ray machine" to look inside of nuclear reactors where you can't send equipment due to the high radiation. This could help with monitoring non-proliferation agreements or the Fukushima accident site. There have also apparently been some experiments using neutrinos for communication purposes, but I don't know details.

perhaps for ftl communications for interstellar communications?? *shrug*

Yup, there are anti-neutrinos. Certainly, neutrinos and anti-neutrinos can annihilate and could even produce gamma rays or at least photons of some energy. But the odds of that happening are astronomically slim, so I'm pretty certain no one's ever seen that happen.

Another fun fact about neutrinos and anti-neutrinos: we talked a bit earlier about the spin of particles. Most matter particles, like electrons, can spin in two basic ways. However, neutrinos can only spin one way --- we call them left-handed. Anti-neutrinos can only spin the other way --- they are right-handed. This is another complicating feature for neutrino theory. Where are the right-handed neutrinos? Are they the sterile neutrinos? Do they exist at all? There are lots of models to explain this fact along with the way neutrinos change type, but we just don't have the data yet to say what's right.

re: right handed neutrinos,
if anti neutrinos are sterile neutrinos and ignoring 'the proverbial target' of other mass, and possible other normal matter neutrinos, could this be what anti matter of non neutrino typs are also doing? Maybe it is somehow out of some sort of quantum phase as the rest of the universe? thus able to 'miss' touching any other of matter?

on an aside,
**- [NEW QUESTION ALERT!! BEWARE!!!]-**
I see much talk of such as the electron and positron annihilating each other as with the anti-proton/proton and neutron/anti neutron pairs respectfully, but what if a positron makes contact with a proton?

 

[Umbran]

**- [NEW QUESTION ALERT!! BEWARE!!!]-**
I see much talk of such as the electron and positron annihilating each other as with the anti-proton/proton and neutron/anti neutron pairs respectfully, but what if a positron makes contact with a proton?

Note that "makes contact with" is kind of a misapprehension. We have this idea of "contact", like when we put a hand on a table, and we think that we are really touching, matter to matter. But, on the really small scale, we are not. The electron clouds of my atoms get close to the electron clouds of the table's atoms, and eventually the electric repulsion keeps them apart. Some photons are exchanged to do that, but the electrons never *touch*, per se. The electrons are so close to being mathematical points that the concept of 'touching" is of questionable meaning.

So, we while we tend to speak of them "colliding", really, they get close to each other, and start exchanging force-carrying particles (like photons), until something interesting happens.

In this case, they don't just annihilate into energy, if that's what you are asking. A particle annihilates with its own antiparticle. It isn't a general, "if you are 'antimatter' you annihilate with *anything* that is matter".

Oh, hey, look, someone has studied this a bit:
https://en.wikipedia.org/wiki/ZEUS_(particle_detector)
http://www.hep.ucl.ac.uk/undergrad-projects/3rdyear/photons-at-HERA/pshera.htm

The basic interaction at the energies of these experiments seems to be scattering. They are both positively charged, so getting them close enough to get the positron to go *into* the proton is really hard. Instead, the electrostatic repulsion has them just bounce off each other.

In these interactions, they exchange an energetic photon (that carries the electromagnetic force) in the scattering . Sometimes, that photon is big enough to crack the proton open, and you get a cascade of hadrons* (mostly mesons, I expect). Sometimes, the photon is big enough to produce particle-antiparticle pairs of its own (a meson, in this case), as discussed earlier, and that whams into the proton and you get a cascade of hadrons, slightly different than if the photon did it directly.


*Hadron = particle made of quarks and antiquarks - the proton and neutron are hadrons. There are mesons made of quark-antiquark pairs as well.