[Aeris (OP)]

Awhile ago, I posted a list of questions on this site about Antimatter and it's various properties. I was extremely satisfied with the answers I got, but there's still so much more about Antimatter that I still don't fully understand. Here's three new questions I have about Antimatter.

1. As just about everyone who’s familiar with antimatter knows, whenever antimatter comes into contact with regular matter, they both annihilate each other in a burst of energy. Why exactly does this happen though? Antimatter is identical to regular matter in just about every conceivable way, with the sole difference being its electric charge. How does this one, little thing cause antimatter to react so violently with regular matter?
2. A lot of people say that there is a lot more matter in our universe than antimatter, but how certain can we really be about this? Like, are we absolutely certain that there is way more regular matter in our universe than antimatter? What’s wrong with the idea of the universe having equal amounts of both, albeit in places far apart from each other?
3. Let’s say, hypothetically speaking (key word, hypothetically, NOT theoretically), I created a wooden, regular matter chair from a huge amount of energy without creating any antimatter whatsoever. It really doesn’t matter (no pun intended) how exactly I did this. All that matters is that I created lots of matter without creating even a little bit of antimatter. How many physical laws would I break from doing this? I created a ton of electrically neutral molecules, and since conservation of charge demands only that the net electric charge of the entire universe is 0, I have to assume that that law is safe for the most part. What about the other laws though? Which ones would render this imaginary process impossible, both in theory and in practice?

[Origin]

Awhile ago, I posted a list of questions on this site about Antimatter and it's various properties. I was extremely satisfied with the answers I got,

Good.

Antimatter is identical to regular matter in just about every conceivable way, with the sole difference being its electric charge.

That statement is incorrect.  No one said that in response to your earlier questions.
Why ask more questions when you seem to have ignored the answers to the last set?

[Halc]

As just about everyone who’s familiar with antimatter knows, whenever antimatter comes into contact with regular matter, they both annihilate each other in a burst of energy.

But they always show something like an electron/positron collision, which leaves nothing but photons. But it's not always the case. What if a positron meets a free neutron? Hard to even find a site describing that, but they're not going to annihilate each other.
A normal rock meets an antimatter rock and sure, it goes bang, but most of the material in both will survive and just be moving away fast.

Why exactly does this happen though? Antimatter is identical to regular matter in just about every conceivable way, with the sole difference being its electric charge.

No, it is not just a difference in charge, as was mentioned before. Yes, a charged particle's antiparticle has the opposite charge, but plenty of matter has no charge at all. Electrons and protons have opposite charge but neither is antimatter.  A neutron and antineutron do not differ in charge.

How does this one, little thing cause antimatter to react so violently with regular matter?

A small mass is equivalent to considerable energy, so when mass in annihilated, the resulting energy has to be proportional. The Hiroshima bomb converted less than a gram of matter into energy.

A lot of people say that there is a lot more matter in our universe than antimatter, but how certain can we really be about this?

We can't, but we can observe the lack of interaction at boundaries that would exist if the clumps were smaller than the observable universe. We don't see that, so if the antimatter is out there, it's further away than we can see.

What’s wrong with the idea of the universe having equal amounts of both, albeit in places far apart from each other?

That works.

Let’s say, hypothetically speaking (key word, hypothetically, NOT theoretically), I created a wooden, regular matter chair from a huge amount of energy without creating any antimatter whatsoever. It really doesn’t matter (no pun intended) how exactly I did this. All that matters is that I created lots of matter without creating even a little bit of antimatter. How many physically laws would I break from doing this?

Don't know. There's not a conservation law. I can dump mostly antimatter into a black hole and it's just gone, and it eventually fizzles away in Hawking radiation which is neither matter nor antimatter, thus illustrating the lack of conservation law. Still, I know of no way to create one without the other, unless I somehow sort it out and get rid of one in some way.
Look at the bomb I mentioned. It converted matter to energy without requirement of equivalent antimatter. Just do that in reverse. The laws of physics are time-symmetrical, so there's actually nothing illegal about the chair you made.

I created a ton of electrically neutral molecules, and since conservation of charge demands only that the net electric charge of the entire universe is 0

Black holes preserve charge, so yes on this. Charge is indeed a conserved quantity, but your chair didn't have a net charge, so it doesn't violate conservation laws.

[evan_au]

1. Antimatter is identical to regular matter in just about every conceivable way,

I would prefer to say "Antimatter is the opposite of regular matter in just about every conceivable way"- except probably gravitational attraction (and perhaps the direction that it travels through time).

As I recall, it was Feynman made the mind-bending comment that a positron is just an electron traveling backwards through time (a time-reversed & charge-reversed electron).
See: https://en.wikipedia.org/wiki/CPT_symmetry

2. What’s wrong with the idea of the universe having equal amounts of both, albeit in places far apart from each other?

That is still possible, but generally considered unlikely: On the boundary of matter-dominated space and anti-matter space you would expect a flood of gamma rays. This has not been seen.
- There is no known mechanism at the Big Bang that could separate matter & antimatter before they could annihilate
- but if gravitational attraction is different between matter and antimatter, that might do it. This experiment is still underway at the LHC, but with no accurate results as yet.

3. Which ones would render (creation of matter only) impossible, both in theory and in practice?

It's true that forming a neutral atom from electromagnetism obeys conservation of charge.
- But there are other quantities that are conserved in quantum theory (at least approximately)
- Conservation of baryon number: You have not created an equal number of protons and anti-protons
- Conservation of lepton number: You have not created an equal number of electrons and anti-electrons (positrons)

There are instances where these conservation rules are occasionally violated (specifically, via the Weak Nuclear Force), but no scenarios have been found that can account for the observed ratio of matter to antimatter
- or indeed, the ratio of (matter+antimatter) to electromagnetism: If matter & antimatter were created in equal quantities at the Big Bang, it is expected that they would annihilate before they could separate, leaving far more electromagnetism in the universe than we observe today.

See: https://en.wikipedia.org/wiki/Conservation_law#Approximate_laws

Oops! Overlap with Halc...

[Aeris (OP)]

Sorry everyone for the late reply. Had a very busy day. Gonna go through this one by one.

Origin

"That statement is incorrect.  No one said that in response to your earlier questions.
Why ask more questions when you seem to have ignored the answers to the last set?"

Care explaining to me why this statement is wrong? Just saying a statement is incorrect without actually explaining WHY it's incorrect holds about as much water as religious people saying God exists because they said he does (which is why I ended up dropping my religion years ago). Also, this statement has absolutely nothing to do with my old questions, it's related to an entirely new question.

Halc

"But they always show something like an electron/positron collision, which leaves nothing but photons. But it's not always the case. What if a positron meets a free neutron? Hard to even find a site describing that, but they're not going to annihilate each other. A normal rock meets an antimatter rock and sure, it goes bang, but most of the material in both will survive and just be moving away fast."

Hmmmm... that's actually really interesting. So an antimatter particle can only annihilate its regular matter counterpart, and not just any random regular matter particle? Does that mean that antimatter atoms and molecules won't react violently with regular matter atoms and molecules? If so, could that mean there's entire planets and galaxies out there in the universe made of antimatter that's safe from annihilation?

"No, it is not just a difference in charge, as was mentioned before. Yes, a charged particle's antiparticle has the opposite charge, but plenty of matter has no charge at all. Electrons and protons have opposite charge but neither is antimatter.  A neutron and antineutron do not differ in charge."

Ok... that still doesn't answer my question though. WHY does regular matter and antimatter annihilate each other upon contact?

"We can't, but we can observe the lack of interaction at boundaries that would exist if the clumps were smaller than the observable universe. We don't see that, so if the antimatter is out there, it's further away than we can see."

But like, would the energy bursts produced from those reactions REALLY be that noticeable to us? We can only observe a small percentage of the universe, and some have theorized that it's infinite in size.

evan_au

"It's true that forming a neutral atom from electromagnetism obeys conservation of charge.
- But there are other quantities that are conserved in quantum theory (at least approximately)
- Conservation of baryon number: You have not created an equal number of protons and anti-protons
- Conservation of lepton number: You have not created an equal number of electrons and anti-electrons (positrons)"

What do you mean approximately? Either those quantities are conserved, or those quantities are NOT conserved. I've never heard of any theory that would try to have it both ways (or neither of them).

Also isn't baryon number and lepton number already violated simply on the virtue of the universe having (at least, to our knowledge) more matter (protons and electrons) than antimatter (antiprotons and positrons)? Even if that didn't do it though, surely things such as nuclear explosions, nuclear fusion, hawking radiation and proton decay (assuming it turns out to be real) violate these numbers?

Thanks everyone for your answers    

 
   

[Halc]

So an antimatter particle can only annihilate its regular matter counterpart, and not just any random regular matter particle? Does that mean that antimatter atoms and molecules won't react violently with regular matter atoms and molecules?

The one is protons, neutrons and electrons and such, and the antimolecules are composed of anti-all-those-things, so they'll annihilate just fine if you can get them to get close enough. So an anti-rock sitting on Earth is going to completely blow up, taking its mass in regular matter with it, because the parts have nowhere to go that doesn't just meet more matter. But two rocks in space will band and just end up moving away from other in a hot expanding cloud of dust that won't interact hardly at all.

If so, could that mean there's entire planets and galaxies out there in the universe made of antimatter that's safe from annihilation?

If they're safe its because there's no regular matter nearby. There has to be a border where they meet somewhere, and you'd be able to see that. We don't see it, so if there is such a boundary, it's outside the visible universe.

But like, would the energy bursts produced from those reactions REALLY be that noticeable to us?

They probably wouldn't be bursts, but a continuous line of radiation. Yes, they'd be very visible, especially in the cosmic microwave background which would clearly show boundaries.

Ok... that still doesn't answer my question though. WHY does regular matter and antimatter annihilate each other upon contact?

It just does. There doesn't seem to be a why to it. That's what is observed, and science describes the behavior, and doesn't posit the reason for the behavior. There might be sites that flesh out a better answer than mine. Hey, I'm hardly a particle physicist here.
You create antimatter and it is stupidly difficult to prevent it from destruction immediately. They've done it, slowing it down to actually create anti-hydrogen molecules.

The sun creates antimatter (mostly positrons) at a pretty furious rate as part of the process of the creation of neutrons it needs for fusion reactions. Each created antimatter particle lives probably less than a nanosecond on average.

[Aeris (OP)]

If they're safe its because there's no regular matter nearby. There has to be a border where they meet somewhere, and you'd be able to see that. We don't see it, so if there is such a boundary, it's outside the visible universe.

But would you still be able to see it even if it was somewhere outside of the observable universe? The light from countless stars and galaxies is still able to reach our eyes for quite a long time even, though those planets and galaxies are far beyond us (so far beyond us that, even with hypothetical light-speed technology, we may still not ever be able to visit them).

It just does. There doesn't seem to be a why to it. That's what is observed, and science describes the behavior, and doesn't posit the reason for the behavior. There might be sites that flesh out a better answer than mine. Hey, I'm hardly a particle physicist here.
You create antimatter and it is stupidly difficult to prevent it from destruction immediately. They've done it, slowing it down to actually create anti-hydrogen molecules.

The sun creates antimatter (mostly positrons) at a pretty furious rate as part of the process of the creation of neutrons it needs for fusion reactions. Each created antimatter particle lives probably less than a nanosecond on average.

So... we know that matter and antimatter annihilate each other upon contact, but we do know WHY they do that? That's... very lame honestly. Also, why is it so difficult to prevent antimatter from immediately interacting and annihilating regular matter? Are they electrically attached to each other or something?

I actually didn't know that the Sun created antimatter particles. I especially didn't know that it NEEDS them for its constant nuclear fusion reactions. How exactly do they contribute to the Sun's energy generating reactions exactly? I'm curious to know.

[Halc]

But would you still be able to see it even if it was somewhere outside of the observable universe?

If you can see it, it is by definition inside the observable universe.

The light from countless stars and galaxies is still able to reach our eyes for quite a long time even, though those planets and galaxies are far beyond us (so far beyond us that, even with hypothetical light-speed technology, we may still not ever be able to visit them).

The observable universe is all locations in cosmological space at which a cause might have occurred which possibly had an effect on us now. This is far more than we can see. The observable radius is currently about 48 BLY. The maximum proper distance from us of any light that has ever reached here only has a radius of under 6 BLY. That's the maximum proper width of our past light cone in cosmological coordinates. The CMB is pretty much the furthest we can see with visible light, and that was emitted from a proper distance of perhaps 2 million LY away.

What you're talking about seems to be the event horizon, beyond which light emitted cannot reach Earth even given infinite time, and beyond which we cannot visit even given the fastest ship possible. That's currently about 16 BLY (proper) distance away, and is only there due to acceleration of expansion. If space expansion was not accelerating, there would be no event horizon at all and light/ships from here could reach any location in space given sufficient time.

So... we know that matter and antimatter annihilate each other upon contact, but we do know WHY they do that? That's... very lame honestly.

I said I don't know. It seems similar to asking WHY a positively charged particle is attracted to a negatively charged one. No matter how good the explanation, you can always ask 'why' to go a level deeper.

Also, why is it so difficult to prevent antimatter from immediately interacting and annihilating regular matter?

You try to prevent a fast moving object in the middle of other matter from touching literally anything. It has been done, but only with incredible effort.

I actually didn't know that the Sun created antimatter particles. I especially didn't know that it NEEDS them for its constant nuclear fusion reactions.

Yes. A hydrogen nucleus is just a proton, and helium needs two protons and two neutrons, and the universe didn't create many free neutrons. So two of the protons need to be changed into neutrons first, and that process creates positrons to preserve the charge.

At the top you see two protons colliding, producing a deuterium nucleus (a proton and neutron) plus the positron and a gamma ray.

For the record, there is also the CNO cycle that is a catalyst reaction more common in larger stars:

Exact same net effect. Using clock coordinates, Helium comes out at 11 O Clock. Two positrons are created during spontaneous decay of the respective unstable nuclei, one each at 3 and 9 O Clock respectively.

How exactly do they contribute to the Sun's energy generating reactions exactly? I'm curious to know.

Well a positron is going to find an electron very quickly and turn into high energy, so I think that counts as contributing to the energy.

[Eternal Student]

Hi.

I'm late to join this thread and it seems like you (all of you) have already sorted out a lot of ideas.

    Let's start with Q.1.

1. As just about everyone who’s familiar with antimatter knows, whenever antimatter comes into contact with regular matter, they both annihilate each other in a burst of energy. Why exactly does this happen though? Antimatter is identical to regular matter in just about every conceivable way, with the sole difference being its electric charge. How does this one, little thing cause antimatter to react so violently with regular matter?

    Firstly, anti-matter doesn't always interact with matter.   I think @Halc made this point first.  I'm just providing some more detail and/or re-phrasing the ideas.
   Annihilation requires a particle to meet it's own exact anti-particle partner.   So an anti-proton cold annihilate with an ordinary proton but if an anti-proton came into close proximity with some other particle of ordinary matter like a neutrino than nothing very much is likely to happen.  The anti-proton is one of a class of particles called Baryons while the neutrino is a different class of particle, a Lepton.  They can't be particle and anti-particle partners.
    I took a Lepton as one of my particles in a collision for the first example because they are quite fundamental particles,  the situation gets more complicated for particles that are composite particles.
   
    What really happens when a proton collides with it's exact anti-particle partner?
   The proton is not fundamental: It contains an evolving mixture of quarks, antiquarks and gluons. These quarks and gluons are collectively called partons and are held together by the QCD interaction, like marbles in a bag. While the marble metaphor is flawed in many ways, like most real-world imaginings of quantum behavior, it provides a helpful analogy to consider the various possible outcomes of particle collisions, so let’s go with it. 

   The most common outcome of a proton-antiproton collision is that the two hadrons simply break apart — the two bags of marbles break — weakly scattering the internal quarks and gluons. This is called a “soft” interaction.

[Taken from a Fermilab article:  https://news.fnal.gov/2014/02/seeing-double-in-proton-antiproton-collisions]

   If you read the rest of the short article you'll see that they are implying that quite often you get a shower of smaller particles of various types and only occasionally is there a direct interaction between two partons where new particles could emerge which may include something like an annihilation event where only photons emerge.

    This is quite different to the way most Science Fiction and popular Science articles describe matter and anti-matter reactions.  They give the impression that if you got an anti-particle anywhere near it's ordinary matter equivalent they would always and completely annihilate.  This is simply not the situation we observe, most interactions between protons and anti-protons are much "softer" than this.   However, in our ordinary everyday lives we have lots of ordinary matter around us.  So that if some anti-particle was dropped into an ordinary room in your house, then it can have millions of encounters with ordinary matter particles in a fraction of a second and then (by probability alone) it should interact.  It's not guaranteed that it interacts with the first particle of ordinary matter it encounters, the whole process is much more random than this.  However, within a very short amount of time we would reasonably expect to have nothing but some photons left behind.

    There are also some interesting structures that might exist, albeit very briefly, and involve a particle being in close vicinity of its anti-particle without annihilating:   https://en.wikipedia.org/wiki/Positronium. 

    Let's focus on the next part of the original question "Why exactly does this (annihilation) happen though?".   It doesn't seem that there is any "law" or imperative that forces annihilation's to happen when particle and anti-particle pairs come into contact.  It is likely to be more of a random process that can happen rather than something that must happen.  It's possible to imagine the phenomena on a macroscopic scale:  A random process like this should be governed by thermodynamics and statistical mechanics.   In the current state of the universe the density of photons is low and most of these photons are not of high energies.  So it is statistically likely and thermodynamically favourable for particle and anti-particles to annihilate and produce high energy photons.  This produces a higher entropy state.  This change may be so favourable that we almost always observe particle and anti-particle annihilation's and almost never observe a reverse process.  Meanwhile, we can imagine that in the early universe the situation was different, we had a higher density of photons and the average energy of these photons was also much higher.  In the early universe the reverse process (particle and anti-particle creation from photons) might have been thermodynamically favoured.   So, this macroscopic view is one way to explain WHY anti-particles tend to annihilate with their ordinary particle partners:  It's just random but now so thermodynamically or statistically favoured that we will almost always observe this process and not the reverse and the change is rapid when it happens.   [see video reference 2]

   The later part of the question states the following:   "Antimatter is identical to regular matter in just about every conceivable way, with the sole difference being its electric charge".    This has been adequately discussed by @Halc and @evan_au.   Every particle in the standard model of particle physics should have a corresponding anti-matter particle.  It's a bit messy since some particles may be their own anti-particle.

[Image taken from Wikipedia]

   There should be 61 fundamental particles if you include all the anti-particles.   These can combine to form various composite particles like protons (or entire atoms) and if all the fundamental particles in these composites are replaced with their anti-particle partners then the new composite particle is generally called an anti-proton (or anti-atom respectively).
  To the best of my knowledge we haven't observed the anti-particle of every fundamental particle and so the idea remains theoretical.  However, we are using anti-electrons  (positrons) routinely in medical P.E.T. scanners and CERN seem able to collect anti-protons and anti-electrons to construct complete anti- Hydrogen atoms.  So at least some anti-particles do seem to exist and behave as we expect.
   While Popular Science does describe anti-particles as being exactly the same as ordinary matter but with an opposite electrical charge, this is not really a sufficient characterisation of anti-particles.   It's true that anti-matter would have an opposite charge but many particles have 0 charge anyway and then -0 and +0 are just the same thing.  A better description of an anti-matter particle is that it should have the opposite charge AND behave like a mirror version under the Weak Force [see video reference 1].     The implication being that it behaves exactly the same under gravity, strong force and the electromagnetic force (noting that charge is negated).  So in this sense it behaves as we would expect ordinary matter to behave and all we can really notice is that the charge is opposite since most of us do not have the equipment in our house to probe and test a particle for interaction under the weak force.  If you did have that ability then anti-particles interact with W bosons to produce anti-particle equivalents of the same interaction between the W boson and its ordinary matter partner.   As previously stated, this remains theoretical and CERN are currently conducting experiments to try and determine if their anti-Hydrogen atoms really do respond to a gravitational field in the same way as we would expect an ordinary Hydrogen atom to respond.

This is an easily followed YT video about anti-matter.  I'm a bit too lazy to find more authoritative references for some of the things I've mentioned above (and also fairly sure that most readers wouldn't want to try reading more formal texts on the subject anyway).

Why is there more anti-matter in the universe?   by the Royal Institute, released on You Tube.  Duration approx. 7 minutes.

References:   [1]    at   0:40  to  1:00     Description of anti-particles.
[2]   at 1:40 to 2:30    A brief description of annhilation and particle synthesis as a reversible or equilibrium process in the early universe.


Best Wishes.

[Aeris (OP)]

Halc

"If you can see it, it is by definition inside the observable universe."

"The observable universe is all locations in cosmological space at which a cause might have occurred which possibly had an effect on us now. This is far more than we can see. The observable radius is currently about 48 BLY. The maximum proper distance from us of any light that has ever reached here only has a radius of under 6 BLY. That's the maximum proper width of our past light cone in cosmological coordinates. The CMB is pretty much the furthest we can see with visible light, and that was emitted from a proper distance of perhaps 2 million LY away."

"What you're talking about seems to be the event horizon, beyond which light emitted cannot reach Earth even given infinite time, and beyond which we cannot visit even given the fastest ship possible. That's currently about 16 BLY (proper) distance away, and is only there due to acceleration of expansion. If space expansion was not accelerating, there would be no event horizon at all and light/ships from here could reach any location in space given sufficient time."

Yeah, ignore what I said yesterday about that. I had misinterpreted what was said in this video
 

"I said I don't know. It seems similar to asking WHY a positively charged particle is attracted to a negatively charged one. No matter how good the explanation, you can always ask 'why' to go a level deeper."

I'm not so sure that kind of logic can really be applied to something like this. Like, say for a moment that matter and antimatter reactions are the result of the electromagnetic force doing some funky opposites attract junk or whatever (this is not at all the answer, but pretend for the sake of this response that it is). Where can you really go from there? I guess you could ask how the electromagnetic force came into existence in the first place, but at that point, you've pretty much got to accept the notion of either emergence from nothing or eternal existence.

Eternal Student

So... it's a case of probability then?

[alancalverd]

"I said I don't know. It seems similar to asking WHY a positively charged particle is attracted to a negatively charged one. No matter how good the explanation, you can always ask 'why' to go a level deeper."

You should never ask "why" in physics. The question presupposes a controlling  entity with an ulterior motive, for which there is no evidence.
Physics (and indeed all sciences, since all others are more or less complicated embodiments of physics) is about observing "what" happens and building a predictive mathematical model of "how" it happens. We take the universe at face value, though we scrutinise that face very carefully.
We observe that some particles are attracted to one another and that attraction is modelled by the mathematics we call electrostatics, which is based on the conservation of complementary charges. These are arbitrarily* labelled positive and negative and have been found to be quantised in units of + or - 1 electron charge.
Any "deeper" explanation is a fairy story.


*The definition, established hundreds of years ago, is that rubbing ebonite with cat fur gives a positive charge to the fur and a negative charge to the ebonite. No kidding - it's still the basis of electrostatics.

[evan_au]

What do you mean approximately? Either those quantities are conserved, or those quantities are NOT conserved.

Wikipedia has a subcategory of approximate conservation laws..
https://en.wikipedia.org/wiki/Conservation_law#Approximate_laws

The Weak Nuclear Force can (sometimes) violate some conservation laws that are obeyed by the other fundamental forces. But the cross-section of the Weak Force is so small that these events are rather rare.

WHY does regular matter and antimatter annihilate each other upon contact?

As Eternal Student suggested, I suspect it has a lot to do with entropy.

If we think of the "Heat Death of the Universe" as being the highest entropy state, with all energy spread thinly through a low temperature universe...
- An isolated electron or positron represents a concentrated local mass/energy of around 0.5 MeV/c², unaffected by the expansion of the universe.
- If an electron and positron collide in space and produce a pair of gamma rays, these will be eventually red-shifted away to lower energy by the general expansion of the universe
- Even faster, if an electron and positron collide in the core of a star, the pair of 0.5 MeV gamma rays experience many collisions, breaking down the high energy into many lower energy photons, until they are eventually emitted at the surface of the star as many photons with perhaps 1eV energy, travelling in 500,000 different directions. This has even higher entropy (and these, too will be red-shifted by the expansion of the universe).

why is it so difficult to prevent antimatter from immediately interacting and annihilating regular matter?

The LHC has a bottle to store anti-protons and positrons, using electric fields in an extreme vacuum.

Even trickier, they also have a bottle that can store neutral anti-hydrogen, using powerful superconducting magnets.
https://home.cern/science/physics/antimatter/storing-antihydrogen

[Eternal Student]

Hi again.

"If you can see it, it is by definition inside the observable universe."

    That's true, you just have to be careful about what you meant by being able to see it.    You may not be able to see distant objects as they are NOW.

"The observable radius is currently about 48 BLY. The maximum proper distance from us of any light that has ever reached here only has a radius of under 6 BLY. That's the maximum proper width of our past light cone in cosmological coordinates.

   Yes to the first and not quite to the second.   6 units might be the width of a cone in cosmological co-ordinates but it wouldn't be BLY as those units.   By cosmological co-ordinates you're probably talking about co-moving co-ordinates and those have distances in fairly arbitrary units.   Well.... they aren't entirely arbitrary,  1 unit of distance in those co-ordinates would be equal to a fixed number of light years (let's say 1 BLY) provided you kept time frozen.   I'm not usually able to freeze time, so then 1 unit of spatial separation in the co-moving frame is a time-dependant, ever increasing amount of actual physical distance.   (Minor note,  I'm finding more references claiming the radius of the observable universe is approx. 46.5 BLY  and not 48 BLY - but it's not worth quibbling over).

  (from Aeris)
(...Concerning previous discussion of anti-matter and matter annihilation....)
So... it's  a case of probability then?

   Well, it does seem to be.    However, you shouldn't just believe everything I've said.  The earlier post pulls a collection of different articles and ideas together.   It's an attempt to answer a direct question   "Why does matter annihilate with anti-matter?"   using what information is available.  You should check the sources of information yourself, make your own decisions and carefully note where any caveats or direct menton of a lack of evidence has been mentioned.  For example, I mentioned that although there should be an anti-particle for every fundamental particle in the standard model of physics, we haven't observed all of these and the idea does remain theoretical.
   As Alancalverd indicated, many people don't ask why and indeed there aren't any articles I can find that attempt to explain why.  So what you've got is a collection of things we do know about related situations and these have been stitched together to try and answer your question in what is hopefully a logical and consistent way.   It's the best I can offer:  There just aren't any definitive articles or texts written to explain WHY it happens (well, not that I could find) although there are many theories in which it CAN happen.   Presumably you haven't found it yourself with Google and/or you found quite a lot of conflicting information.

1.   The first thing to note should be that anti-matter just doesn't always annihilate with its ordinary matter partner when they come into contact.  That's a myth, Science Fiction or PopSci  idea.  Check reports from Fermilab about proton and anti-proton collisions (and there are also other establishments that work with anti-protons).

2.  From this we can reasonably conclude that there is no hard or immutable rule in nature that says matter must always annihilate with anti-matter when they come into contact.   It was obviously going to be complicated when an anti-matter particle like an anti-Neutron came into contact with an ordinary matter particle like a proton because these two composite particles do share some common quarks, so that some partial annihilation could have been possible but total annihilation wasn't.  However, we don't even have to consider anything that complicated.  Fermilab have been colliding protons into their direct anti-proton partners and still they didn't always undergo total annihilation.
     Halc mentioned that most articles only consider the most elementary particles like electrons and positrons annihilating.  However, if you dig hard enough you'll see that even these interactions don't seem to happen all the time.  In a hospital P.E.T. scanner it is the mainstream theory that positrons emitted by the radio-tracers in the patient do travel a short distance through the tissue before annihilation with electrons.  Now it should have gone past or indeed right through quite a few (thousand) orbital clouds of electrons around atoms during this travel. 
   ...The emitted positron travels in tissue for a short distance (typically less than 1 mm, but dependent on the isotope[51]), during which time it loses kinetic energy, until it decelerates to a point where it can interact with an electron.[52]   -  taken from https://en.wikipedia.org/wiki/Positron_emission_tomography
    From this we can conclude that even positrons and electrons don't always annihilate when they are in close proximity and we can already see that there are going to be problems deciding when they are "in contact" since everything at this scale is governed by quantum mechanics and particles only have a certain probability of being found at a certain place.  The PET scanner also indicates that velocity might be one factor that influences the likelihood of an annihilation event occurring.

 --- I've deleted the rest of what I was going to say, I've already said too much.  Let's just wrap this up:  The annihilation of particles and their anti-particles is not a certainty just because they are in close proximity.  It may be genuinely random or there may be a collection of factors that would entirely determine if the interaction occurrs.  However, if there is such a collection of factors it is currently not well understood and not realistically in our control, so that on a macroscopic scale,  annihilations are effectively a random process even if they are not a genuinely random process.  This is sufficient to apply some probability theory and thermodynamics and statistical mechanics follows (as in the earlier post).

Best Wishes.

LATE EDITING:  Fixed errors in the quotes from other people.  Most are from Aeris but some were incorrectly identified as being from other people.

[alancalverd]

That's true, you just have to be careful about what you meant by being able to see it.    You may not be able to see distant objects as they are NOW.

Indeed if you take a classical view of information, you can't ever see anything as it is, but only as it was when the light left it.

[evan_au]

even positrons and electrons don't always annihilate when they are in close proximity..

An electron and positron frequently form "positronium" for a short period before they annihilate.
- So annihilation is much more likely if the electron and positron are in the "same" location and velocity for some time before they annihilate.

The average lifetime of Positronium depends on the spins of the electron and positron:
- Parallel spins: 0.12 ns (nanoseconds)
- Opposite spins: 140 ns
- So the lifetime varies by 3 orders of magnitude, depending on the spins (ie their magnetic fields)

A medical PET scanner typically generates positrons with an average energy of 0.9MeV, so they are travelling at relativistic speeds. At these speeds, the positron is not near any particular electron long enough for annihilation to be likely with that electron.
It takes many elastic collisions with electrons and atomic nuclei (emitting electromagnetic radiation) before the positron slows down enough to form positronium, after which it will annihilate with confidence.
See: https://en.wikipedia.org/wiki/Positronium

[Halc]

LATE EDITING:  Fixed errors in the quotes from other people.  Most are from Aeris but some were incorrectly identified as being from other people.

But you still got it wrong, and I edited your post, putting my name on the words that were mine.
A good deal of the problem is that Aeris doesn't seem to be using the quote feature and is just putting other people's works in quotation marks which doesn't identify whose words they are.

BTW, great post, especially the part of how much trouble an anti-proton might have in its quest to be totally annihilated. It's far more informative than my answer since I was unaware of how precise an interaction was required for a full reaction, even if in stages.

If it's that much work do destroy an antiproton (negatron?), imagine how much work it is to put one together. I admire the guys that manage not only to do it, but to capture it as well. Now they have to make anti-helium, or better, anti-water, which seems easier to contain as rocket fuel. You can even store it in a nice big anti-fuel-tank and just figure out how not to touch the tank. Plumbing fittings are going to be interesting. You thought it was hard going from steel to copper pipes....

If you can see it, it is by definition inside the observable universe."

That's true, you just have to be careful about what you meant by being able to see it. You may not be able to see distant objects as they are NOW.

Indeed, and it has nothing to do with objects being distant. I cannot see my own keyboard as it is NOW. It's just a super-recent past. Observable universe is a relation between the current event (here and now) and points in space per cosmological coordinates.

"The observable radius is currently about 48 BLY. The maximum proper distance from us of any light that has ever reached here only has a radius of under 6 BLY. That's the maximum proper width of our past light cone in cosmological coordinates.

Yes to the first and not quite to the second.
6 units might be the width of a cone in cosmological co-ordinates but it wouldn't be BLY as those units.   By cosmological co-ordinates you're probably talking about co-moving co-ordinates and those have distances in fairly arbitrary units.

I didn't say comoving distance or comoving coordinates, I said proper distance. No light that we see today has ever been a greater proper distance from here than 6BLY. That means you're measuring the distance with a bunch of stationary (zero peculiar velocity) meter sticks end to end. So there very much are units to proper distance, and that was what I meant by that statement

1 unit of spatial separation in the co-moving frame is a time-dependant, ever increasing amount of actual physical distance.

Agree, I just wasn't using such units.

(Minor note,  I'm finding more references claiming the radius of the observable universe is approx. 46.5 BLY  and not 48 BLY - but it's not worth quibbling over).

I've always imagined that it was something very dependent on the current agreed upon values for the ratio of mass to dark energy, so I'm fine with the latest accepted figure.

My only comment to Aeris that that matter and antimatter are not inherently more attracted to each other. A hydrogen atom and an antihydrogen atom might have immeasurably small gravitational attraction between them, but that's it, same as two regular hydrogen atoms.

[Aeris (OP)]

Eternal Student

"Well, it does seem to be.    However, you shouldn't just believe everything I've said.  The earlier post pulls a collection of different articles and ideas together.   It's an attempt to answer a direct question   "Why does matter annihilate with anti-matter?"   using what information is available.  You should check the sources of information yourself, make your own decisions and carefully note where any caveats or direct menton of a lack of evidence has been mentioned.  For example, I mentioned that although there should be an anti-particle for every fundamental particle in the standard model of physics, we haven't observed all of these and the idea does remain theoretical."
   
"As Alancalverd indicated, many people don't ask why and indeed there aren't any articles I can find that attempt to explain why.  So what you've got is a collection of things we do know about related situations and these have been stitched together to try and answer your question in what is hopefully a logical and consistent way.   It's the best I can offer:  There just aren't any definitive articles or texts written to explain WHY it happens (well, not that I could find) although there are many theories in which it CAN happen.   Presumably you haven't found it yourself with Google and/or you found quite a lot of conflicting information."

Trust me, I have spent hours researching this stuff online and I would not be here asking this question if I didn't already find a satisfying answer anywhere else. My question's unbelievably specific as it is, so the fact that you were even able to give me a solid-enough answer is really quite impressive. No, I shouldn't just believe every word you say, but I honestly have very little reason to NOT believe a lot of what you so considering just how much more reliable you guys have been than other science forums I've visited in the past.

Halc

"But you still got it wrong, and I edited your post, putting my name on the words that were mine.
A good deal of the problem is that Aeris doesn't seem to be using the quote feature and is just putting other people's works in quotation marks which doesn't identify whose words they are."

Yeah, sorry about that. I'm only responding to answers the way I am because A) I'm responding to multiple people at once and this is quicker than creating 2-4 separate posts, and B) I admittedly don't know how to use the Quote option that well.

"My only comment to Aeris that that matter and antimatter are not inherently more attracted to each other. A hydrogen atom and an antihydrogen atom might have immeasurably small gravitational attraction between them, but that's it, same as two regular hydrogen atoms."

Interesting. Thanks for the info.

 

[Origin]

I admittedly don't know how to use the Quote option that well

All you have to do is highlight the part of the post you want to quote and then click on the blue "ACTIONS" box at the top right of the post and then click "Quote (selected)".   

[Eternal Student]

Hi again.

An electron and positron frequently form "positronium" for a short period before they annihilate.
- So annihilation is much more likely if the electron and positron are in the "same" location and velocity for some time before they annihilate.

   Thanks for the reply and the extra information.  I knew that positronium could sometimes be formed but not that it was frequently formed.
    It's still talking about annihilation as being "likely", i.e. some random process that takes a certain amount of time before it actually happens.

It takes many elastic collisions with electrons and atomic nuclei (emitting electromagnetic radiation) before the positron slows down enough to form positronium, after which it will annihilate with confidence.

    Taking the phrase "with confidence" to mean ... well not with any certainty at all..... just that it becomes more likely with increasing elapsed time (if I've understood the situation correctly).   Positronium has an average life-time that can be determined by experiment but there is variation between individual positronium atoms, it's a bit like other radioactive decay, a seemingly random process as far as we know.

But you still got it wrong, and I edited your post, putting my name on the words that were mine.

   Sorry and thanks.

I didn't say comoving distance or comoving coordinates, I said proper distance. No light that we see today has ever been a greater proper distance from here than 6BLY.

   OK, I think I see what you've done but I'm still not sure how you got the figure to be as low as 6 BLY.   It's certainly under 13.8 BLY.  Anyway, this is what I get:

Take co-moving co-ordinates where t = 0 is the singularity where the scale factor a(0) = 0.  Assume a photon was emitted by a distant star at some co-moving time t_E  with  t_E > 0.   We don't really care how far away the star is now or ever was.  Our focus is on the distance between us and the photon.  We will assume the photon has reached us now, at co-moving time t_o   with  t_o > t_E.

 Star                    Photon                 Us (Earth)   
   *                        ~~>                        @   
                              <----------------------->    d(t) = Distance from photon to Earth at co-moving time t

So we want of find d(t) = the proper distance between us and the photon at a co-moving time, t      with    t_E ≤ t ≤ t_o.

   The co-moving distance χ(t)   =      =  an evolving (decreasing) separation between the photon and us as time progresses, measured in the co-moving frame.  Note that the lower integral limit is t and not t_E.  We are not interested in the distance covered from the time of emission at the star, only in the co-moving distance from wherever the photon is at our arbitary time t and us here on earth (the location of where it will be observed by us at time t_o).

   So that the proper distance between us and the photon at co-moving time t  is 
d(t) =  a(t) .  χ(t)   =     a(t) .       (I think that formula actually requires k = curvature of the FRW universe to be zero but that is our current belief anyway).

Fix  t   with    t_E ≤ t ≤ t_o      So that the lower integral limit and the quantity a(t) become constants.

   Then  a(t) .    =      

[Equation 1]

   

Without assuming an explicit form for a(t) in [equation 1] we can only make limited progress.
Throughout the integral   a(t) ≤ a(t')    since t' ≥ lower integral limt and we can reasonably assume the scale factor has been a monotonically increasing function since the big bang.

   Hence,     ≤      =  c . [t_o - t]  ≤ c [t_o ]     since we have 0 ≤ t ≤ t_o

   The co-moving time, t_o, when the photon was observed by us cannot be greater than the age of the universe = approx. 13.8 BY.  So the proper distance between the photon and us is never greater than 13.8 BLY.

   We can un-fix t now  -  our result holds for arbitrary t with   t_E ≤ t ≤ t_o.   So we have that the proper distance bewteen us (here on earth) and a photon traveling toward us was never greater than  13.8 BLY.
   I can't see how you get this down to 6 BLY unless you assume a(t) has a particular form and apply this in [equation 1].

Best Wishes.

[Halc]

I can't see how you get this down to 6 BLY unless you assume a(t) has a particular form and apply this in [equation 1].

I followed and agree with all you posted, but it seems a lot of work just to show that it can't be > 13.8.
I did it the easy way and just looked up the graphs drawn for the papers illustrating reasonably (15 years?) recent solutions to FLRW metric that best match observations.

Same picture, the top being proper distance and the bottom being comoving distance.
The red curve is our past light cone, which gets at its widest about 5.8 Glr just before it crosses the Hubble sphere at t=~3.7 Gyr.
Light from the most distant star visible is the dotted line at comoving 31Glr which crosses our light cone at a proper distance of only about ~2.5 Gl, far closer than light emitted more recently.