The first one "220" has a nice discussion, in particular a comment by pfdietz:
> It increases the rate of production of neutral antihydrogen from antiprotons and positrons by a factor of 8. It doesn't increase the efficiency of production of antiprotons, which is the extremely inefficient, energy intensive part.
Can't wait for the day we can trap more antimatter so that governments that right now pretend this is for science weaponize it into a bomb way larger and harder to detect than nukes
As a side note, it's mind boggling that overwhelming majority (more than 98%) of the visible universe's mass are only from two most lightweight of chemical elements namely Hydrogen and Helium.
There is a theory that primordial black holes formed in the very early universe. I'm not sure when this process would happen relative to the formation of atoms. But, if it actually happened, it would have been long before stars started forming.
Yes, it's a little mind boggling because the typical human context is this rocky ball of what is ultimately a very uncommon distribution of heavy elements. It's a strange feeling to know that almost everything is utterly unlike the everyday human experience. If you turn down the uhm acksshuwlly a few notches I think parent post's point is quite obvious.
As I learned it long ago in school, elements up to the mass of iron are formed by stellar fusion. That's the point where fusion is no longer exothermic. Any element on earth that is heavier than iron is the product of a supernova. So we live on a ball of supernova debris.
Most of what we live on, the vast majority, is iron or lighter. So it's more that we're sprinkled with supernova debris. But we are made out of stardust, so that's something.
An interesting fact is that while almost all of the Solar System has started as gas, which has then condensed here into solid bodies that have then aggregated into planets, a small part of the original matter of the Solar System has consisted of solid dust particles that have come as such from the stellar explosions that have propelled them.
So we can identify in meteorites or on the surface of other bodies not affected by weather, like the Moon or asteroids, small mineral grains that are true stardust, i.e. interstellar grains that have remained unchanged since long before the formation of the Earth and of the Solar System.
We can identify such grains by their abnormal isotopic composition, in comparison with the matter of the Solar System. While many such interstellar grains should be just silicates, those are hard to extract from the rocks formed here, which are similar chemically.
Because of that, the interstellar grains that are best known are those which come from stellar systems that chemically are unlike the Solar System. In most stellar systems, there is more oxygen than carbon and those stellar systems are like ours, with planets having iron cores covered by mantles and crusts made of silicates, covered then by a layer of ice.
In the other kind of stellar systems, there is more carbon than oxygen and there the planets would be formed from minerals that are very rare on Earth, i.e. mainly from silicon carbide and various metallic carbides and also with great amounts of graphite and diamonds.
So most of the interstellar grains (i.e. true stardust) that have been identified and studied are grains of silicon carbide, graphite, diamond or titanium carbide, which are easy to extract from the silicates formed in the Solar System.
The elements heavier than iron are not formed by fusion because of the asymmetry in the initial conditions.
The Universe that we can see has started from a mixture of equal amounts of free neutrons and protons (at a temperature of a few tens of MeV, matter has the simplest possible structure, consisting of free neutrons, free protons, free electrons, free positrons, photons and various kinds of neutrinos; upon cooling, nuclei form, then positrons annihilate, then atoms form), which have formed in the beginning hydrogen, helium and some lithium. Then, through fusion, the next elements until iron have been generated.
Iron is not the last element generated, a few elements after it have also been generated by fusion, because while they have lower binding energies than iron, their binding energies are still greater than of the lighter elements that can fuse into them.
However after the peak of the iron, the abundance of the following elements generated by fusion drops very quickly, e.g. down to germanium that is about 8 thousand times less abundant than iron.
The elements heavier than germanium are produced only in negligible amounts by fusion. They are produced mostly by neutron capture and sometimes by proton capture, and such events happen mostly during supernova explosions or neutron star collisions, because only then high concentrations of neutrons with high energies are present.
Neutron capture produces elements with Z until 100, i.e. until fermium (after that, spontaneous fission happens too fast, before beta-decay can raise the Z and enough extra neutrons can be captured to form a nucleus with long enough half-life). However the half-life of the heaviest elements decreases very quickly with Z, so the elements heavier than plutonium usually decay before reaching a stellar system from the explosion that has generated them. At its formation, it is likely that Earth also contained plutonium (244Pu has a half-life of over 80 million years, enough to survive an interstellar journey), but it has completely decayed until now, leaving uranium as the heaviest primordial element on Earth.
You can skip the first 42 minutes that are about how bad is an article titled "how antimatter space craft will work". This part is absolutely boring as hell !
If you can electromagnetically trap enough antimatter to use it as fuel you could as well trap a miniature charged black hole that can be fed regular matter to produce power, which skips the whole inefficient part of making antimatter.
That's the point, evaporation turns matter into energy. You can tune power by chosing mass of the black hole and then feed it regular matter at a steady rate.
It's a pretty fundamental prediction though, and it's been derived in many different ways, all of which give the same prediction.
It's closely related to the Unruh effect, which is a direct consequence of pure QFT. The Unruh effect describes how an accelerated observer sees a different vacuum from an inertial observer - they see radiation that the inertial observer doesn't.
Hawking radiation is essentially this same effect, except that "acceleration" is replaced by "gravity" (Einstein's equivalence principle.) There's a bit more to it, but that's the basic intuition.
For Hawking radiation to be wrong would require some fundamental changes to GR, QFT, or both.
A lot of great science progress followed after some "fundamental prediction" turned out to be wrong :). Wouldnt it be awesome to learn that blackholes, in fact, do not evaporate at all? That would be exciting
It's much easier to make a fission reactor than a fission bomb, and much easier to make a fusion bomb than a fusion reaction. They are not even that similar.
It's way easier to make a fission bomb than a fission reactor. I reactor has to stay in the very narrow window where it's critical but not prompt critical. Even pure fission bombs can be marvels of engineering but the simplest gun-type bomb is easier to build than the simplest nuclear reactor.
You can even dissolve the uranium in the water and use the same substance for both fuel and propellant and so capable of reaching far higher temperatures than those that would cause any engine to melt.
There have been several proposals. This paper proposes a feasable mechanism[1]:
-"a SBH could be artificially created by firing a huge number of gamma rays from a spherically converging laser. The idea is to pack so much energy into such a small space that a BH will form."
The biggest problem is that if you're creating it with lasers, you're only going to get the energy out that you put in. You really want to be able to feed it matter, which would effectively make it an anything-to-gamma-radiation converter, which means you have to feed it quite a lot of matter, against the radiation pressure of all that energy coming out. The paper mentioned assumes a worst case of not being able to feed the black hole at all, but doesn't (in my skim) address the fact that this means you have to put in all the energy you'll be using for the lifetime of the black hole at the creation of it, which seems significantly more outrageously infeasible than the bare necessity of creating a black hole at all.
I admit to invoking the phrase “Where we’re going, we won’t need eyes to see” at least once a year when something feels like it’s going horribly wrong.
Before we get too excited, this current "breakthrough" is making less than 1 antihydrogen atom per second. This corresponds to a delivered annihilation power of less than 1 nanowatt.
Neutrons were first definitively observed in 1932.
First nuclear reactor was 1942, and bomb was 1945.
Once the science is established, we have smart engineers to make a short work out of it.
Fusion energy is really the only counterexample in history, which makes me think we are still missing some crucial physics about how it works, for example in stars. Specifically the particle physics view of how it's reliably triggered with minimal energy.
The antiproton decelerator at CERN has been operational for 25 years, and they have plenty of smart engineers there. Unlike in the 1940s, the underlying physics has been well understood for many decades. I would argue that nuclear fission is the counter example that happens to be surprisingly easy to do.
All experiments at the AD are strongly limited by the low rates. If there was a straightforward way to improve this by many orders of magnitude, they would have done it a long time ago.
> Fusion energy is really the only counterexample in history, which makes me think we are still missing some crucial physics about how it works
This is magical thinking. We know how fusion works in great detail. And “reliably triggered with minimal energy” is essentially not a thing, unless by minimal energy you mean something like 10 million times the energy of an air particle at room temperature, for every particle in a reactor.
What we’re trying to do is recreate the conditions at the core of a star - which is powered by gravity due to hundreds of thousands of Earth masses. And since we don’t have the benefit of gravity anything like that, we actually have to make our plasmas significantly hotter than the core of a star. And then contain that somehow, in a way that can be maintained over time despite how neutron radiation will compromise any material used to house it.
The reality is, we still don’t know if usable fusion power is even possible - there’s no guarantee that it is - let alone how to achieve it. The state of the art is orders of magnitude away from even being able to break even and achieve the same power out as was put into the whole system.
> at the core of a star - which is powered by gravity
That is what I meant, I doubt we really understand what 'powered by gravity' means. You could win a Nobel prize or two by discovering all the details involved here. You would also win a Nobel prize by definitively proving that nothing special happens, you just have high temperatures and high pressures.
The way we are trying to study fusion is like rubbing larger and larger rocks to produce more fire.
The processes involved in solar fusion have been well understood since the 1930s [1,2]. Hans Bethe won a Nobel Prize for this in 1967. The problem is that one cannot produce stellar densities and pressures in any kind of apparatus.
What's the key point regarding how we would get a bajillion times more anti-matter than we can now generate, and without expending all the energy we now expend on getting it?
His point seems to be that we haven't yet seriously tried optimizing for energy efficiency of producing antimatter. It's a call to action. If we actually tried it's plausible that we could get to a level that, while still fantastically inefficient in an absolute sense, would still be worthwhile for spaceflight propulsion, where energy density is vitally important. As far as I know, antimatter is the most energy dense fuel possible in known physics by many orders of magnitude.
Also he proposes a few ways that antimatter could be practically used for propulsion, including as a catalyst for fission which seems interesting.
Can't we argue for the low amount of anti-matter as a type of anthropic principle? The early universe was super dense meaning that areas with imbalance would quickly annihilate and leave only one type of matter. Then, due to rapid expansion, our observable universe is dominated by only one type of matter. If we imagine a universe with a more even mix it would be less welcoming to life, so we are less likely to observe it. Has someone modeled something like this?
The anthropic principle doesn't imply that our entire observable universe has to contain only matter.
Why shouldn't we observe clouds of anti matter and matter annihilating millions or billions of light years away? Why does the annihilation have to have happened so early on that we can't see any evidence anywhere?
I think there does need to be an explanation and it can't be an anthropic principle cop out.
Not only there is no evidence for the existence of antimatter in quantities comparable with matter, but there also is no logical necessity for this.
People who entertain the idea of an initial state with equal amounts of matter and antimatter do this because thus the properties of the matter that are conserved, except the energy, would sum to zero in the initial state.
However, such people forget that not only the particle-antiparticle pairs that can be generated or annihilated through electromagnetic interactions have this property that the conserved quantities except the energy sum to zero.
The particle-antiparticle symmetry is important only for the electromagnetic interactions, while other interactions have more complex symmetries.
All the so-called weak interactions are equivalent with the generation or annihilation of groups of 4 particles, for which all the conserved properties except energy sum to zero. Such a group of 4 particles typically consists of a quark, an antiquark, a charged lepton or anti-lepton and a neutrino or antineutrino.
For instance the beta decay of a neutron into a proton is equivalent with the generation of 4 particles, an u quark, an anti-d quark, an electron and an antineutrino. The electron and the antineutrino fly away, while the anti-d quark annihilates a d quark, so the net effect for the nucleus is a change of a d quark into an u quark, which transforms a neutron into a proton.
The generation and annihilation of groups of 4 particles in the weak interactions are mediated by the W bosons, but this is a detail of the mechanism of the interactions, which is necessary for computations of numeric values, but not for the explanation of the global effect of the weak interactions, for which the transient existence of the W intermediate bosons can be ignored.
So besides the symmetry between a particle and an anti-particle, we have a symmetry that binds certain groups of 4 quarks and leptons.
There is a third symmetry, which binds groups of 8 particles. For instance, there are 3 kinds of u quarks, 3 kinds of d quarks, electrons and neutrinos, a total of 8 particles that belong to the so-called first generation of matter particles (i.e. the lightest such particles).
All the conserved quantities except energy sum to zero for this group of 8 particles. The neutrino is necessary in this group so that the spin will also sum to zero, not only the electric charge and the hadronic charge.
These 8 kinds of particles are exactly those that are supposed to compose in equal quantities the matter of the Universe at the Big Bang.
So all the conserved quantities except energy sum to zero for the Universe at the Big Bang, when it is composed entirely of ordinary matter, without any antimatter.
Therefore there is no need for antimatter in the initial state.
There is no known reason for this symmetry between the 8 particles of a generation of quarks and leptons, except that this allows for the initial state at the Big Bang to have a zero sum for the conserved properties.
It can be speculated that this symmetry might be associated with a supplementary hyper-weak interaction, in the same way as the symmetry between certain groups of 4 quarks and leptons is associated with the weak interaction. Such an interaction would allow the generation and annihilation of ordinary matter, without antimatter, but with an extraordinarily low probability.
I think the same subject was addressed in both of these...
https://news.ycombinator.com/item?id=45979220
https://news.ycombinator.com/item?id=46011889
The first one "220" has a nice discussion, in particular a comment by pfdietz:
> It increases the rate of production of neutral antihydrogen from antiprotons and positrons by a factor of 8. It doesn't increase the efficiency of production of antiprotons, which is the extremely inefficient, energy intensive part.
Can't wait for the day we can trap more antimatter so that governments that right now pretend this is for science weaponize it into a bomb way larger and harder to detect than nukes
https://www.youtube.com/watch?v=-hYNl1hnwUc&t=115s
As a side note, it's mind boggling that overwhelming majority (more than 98%) of the visible universe's mass are only from two most lightweight of chemical elements namely Hydrogen and Helium.
> it's mind boggling that overwhelming majority
is it though? I mean literally everything has to start there and the only way get to heavier elements is via stars and many-many iterations.
it's not like heavier things popped into existence.... or did they...
There is a theory that primordial black holes formed in the very early universe. I'm not sure when this process would happen relative to the formation of atoms. But, if it actually happened, it would have been long before stars started forming.
Yes, it's a little mind boggling because the typical human context is this rocky ball of what is ultimately a very uncommon distribution of heavy elements. It's a strange feeling to know that almost everything is utterly unlike the everyday human experience. If you turn down the uhm acksshuwlly a few notches I think parent post's point is quite obvious.
The average density of matter in the universe is one proton per five cubic meters or so. We're very much the outlier!
https://xkcd.com/2640/
The alt text is on point.
Feels like most resources on the map aren’t mined yet.
And earth contains so much of heavier elements.
As I learned it long ago in school, elements up to the mass of iron are formed by stellar fusion. That's the point where fusion is no longer exothermic. Any element on earth that is heavier than iron is the product of a supernova. So we live on a ball of supernova debris.
Most of what we live on, the vast majority, is iron or lighter. So it's more that we're sprinkled with supernova debris. But we are made out of stardust, so that's something.
An interesting fact is that while almost all of the Solar System has started as gas, which has then condensed here into solid bodies that have then aggregated into planets, a small part of the original matter of the Solar System has consisted of solid dust particles that have come as such from the stellar explosions that have propelled them.
So we can identify in meteorites or on the surface of other bodies not affected by weather, like the Moon or asteroids, small mineral grains that are true stardust, i.e. interstellar grains that have remained unchanged since long before the formation of the Earth and of the Solar System.
We can identify such grains by their abnormal isotopic composition, in comparison with the matter of the Solar System. While many such interstellar grains should be just silicates, those are hard to extract from the rocks formed here, which are similar chemically.
Because of that, the interstellar grains that are best known are those which come from stellar systems that chemically are unlike the Solar System. In most stellar systems, there is more oxygen than carbon and those stellar systems are like ours, with planets having iron cores covered by mantles and crusts made of silicates, covered then by a layer of ice.
In the other kind of stellar systems, there is more carbon than oxygen and there the planets would be formed from minerals that are very rare on Earth, i.e. mainly from silicon carbide and various metallic carbides and also with great amounts of graphite and diamonds.
So most of the interstellar grains (i.e. true stardust) that have been identified and studied are grains of silicon carbide, graphite, diamond or titanium carbide, which are easy to extract from the silicates formed in the Solar System.
> elements up to the mass of iron are formed by stellar fusion
And elements down to the mass of iron can also be formed. But iron is at the bottom of the well.
The elements heavier than iron are not formed by fusion because of the asymmetry in the initial conditions.
The Universe that we can see has started from a mixture of equal amounts of free neutrons and protons (at a temperature of a few tens of MeV, matter has the simplest possible structure, consisting of free neutrons, free protons, free electrons, free positrons, photons and various kinds of neutrinos; upon cooling, nuclei form, then positrons annihilate, then atoms form), which have formed in the beginning hydrogen, helium and some lithium. Then, through fusion, the next elements until iron have been generated.
Iron is not the last element generated, a few elements after it have also been generated by fusion, because while they have lower binding energies than iron, their binding energies are still greater than of the lighter elements that can fuse into them.
However after the peak of the iron, the abundance of the following elements generated by fusion drops very quickly, e.g. down to germanium that is about 8 thousand times less abundant than iron.
The elements heavier than germanium are produced only in negligible amounts by fusion. They are produced mostly by neutron capture and sometimes by proton capture, and such events happen mostly during supernova explosions or neutron star collisions, because only then high concentrations of neutrons with high energies are present.
Neutron capture produces elements with Z until 100, i.e. until fermium (after that, spontaneous fission happens too fast, before beta-decay can raise the Z and enough extra neutrons can be captured to form a nucleus with long enough half-life). However the half-life of the heaviest elements decreases very quickly with Z, so the elements heavier than plutonium usually decay before reaching a stellar system from the explosion that has generated them. At its formation, it is likely that Earth also contained plutonium (244Pu has a half-life of over 80 million years, enough to survive an interstellar journey), but it has completely decayed until now, leaving uranium as the heaviest primordial element on Earth.
This piece argues that antimatter could be feasible for space propulsion and we could start developing it now: https://news.ycombinator.com/item?id=46073414
For those who are time-rich and knowledge-poor:
https://youtube.com/watch?v=i6jMnz6nlkw
(Angela is genuinely a great science communicator and that video is time well spent if you are interested in this topic.)
You can skip the first 42 minutes that are about how bad is an article titled "how antimatter space craft will work". This part is absolutely boring as hell !
Angela is great, albeit her rants can get quite windy.
If you can electromagnetically trap enough antimatter to use it as fuel you could as well trap a miniature charged black hole that can be fed regular matter to produce power, which skips the whole inefficient part of making antimatter.
Miniature black holes would just evaporate. Antimatter wouldn't.
That's the point, evaporation turns matter into energy. You can tune power by chosing mass of the black hole and then feed it regular matter at a steady rate.
Minor nit-pick but Hawking Radiation hasn't been observed and remains a theoretical prediction.
It's a pretty fundamental prediction though, and it's been derived in many different ways, all of which give the same prediction.
It's closely related to the Unruh effect, which is a direct consequence of pure QFT. The Unruh effect describes how an accelerated observer sees a different vacuum from an inertial observer - they see radiation that the inertial observer doesn't.
Hawking radiation is essentially this same effect, except that "acceleration" is replaced by "gravity" (Einstein's equivalence principle.) There's a bit more to it, but that's the basic intuition.
For Hawking radiation to be wrong would require some fundamental changes to GR, QFT, or both.
A lot of great science progress followed after some "fundamental prediction" turned out to be wrong :). Wouldnt it be awesome to learn that blackholes, in fact, do not evaporate at all? That would be exciting
It's pretty widely accepted though. He himself hated the idea so you can expect he did the calculations thoroughly.
I love that major scientists had a intense hatred for the concepts forced upon them by the universe. Einstein and quantum mechanics come to mind
Not before efficiently converting a large amount of mass into usable energy.
But you want that to happen in space and to control the output of energy.
Otherwise you just have a bomb.
The difference between a bomb and a reactor is just clever engineering.
It's much easier to make a fission reactor than a fission bomb, and much easier to make a fusion bomb than a fusion reaction. They are not even that similar.
It's way easier to make a fission bomb than a fission reactor. I reactor has to stay in the very narrow window where it's critical but not prompt critical. Even pure fission bombs can be marvels of engineering but the simplest gun-type bomb is easier to build than the simplest nuclear reactor.
Dual use technology, you say?
In the same way that atomic weapons and radioisotope generators both convert mass into energy. It's just a matter of slightly different timescales.
How could we harness this energy and make it usable?
You use it to boil water.
You can even dissolve the uranium in the water and use the same substance for both fuel and propellant and so capable of reaching far higher temperatures than those that would cause any engine to melt.
https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket
The real question is if we'll get back hole or antimatter powered steam engines before GTA 6
It's almost a meme at this point
If I knew that, I'd probably have more important things to do than comment it here.
You could pen a carefully-worded a letter of demands and send it to some Billionaire? A bit on the risky side, but - hey, you only live once etc.
We know how to make antimatter and have actually done it. We have no realistic way to obtain a black hole of any size.
If you can create enough AM to last a space voyage you certainly know how to build big enough particle accelerators.
Depends. Do we know how to obtain a miniature black hole?
There have been several proposals. This paper proposes a feasable mechanism[1]:
-"a SBH could be artificially created by firing a huge number of gamma rays from a spherically converging laser. The idea is to pack so much energy into such a small space that a BH will form."
1. https://arxiv.org/abs/0908.1803
The biggest problem is that if you're creating it with lasers, you're only going to get the energy out that you put in. You really want to be able to feed it matter, which would effectively make it an anything-to-gamma-radiation converter, which means you have to feed it quite a lot of matter, against the radiation pressure of all that energy coming out. The paper mentioned assumes a worst case of not being able to feed the black hole at all, but doesn't (in my skim) address the fact that this means you have to put in all the energy you'll be using for the lifetime of the black hole at the creation of it, which seems significantly more outrageously infeasible than the bare necessity of creating a black hole at all.
Does anyone address the fact that a black hole will be falling towards the center of the earth at 1g? How do you handle a black hole?
Black hole is the safest energy generator per unit of energy produced.
There’s a recent paper on the formation of such a “kugelblitz”; it’s argued to be unfeasible.
https://arxiv.org/abs/2405.02389
The romulan empire does this.
They made a movie about this. It didn’t end so well for the crew.
I admit to invoking the phrase “Where we’re going, we won’t need eyes to see” at least once a year when something feels like it’s going horribly wrong.
> a miniature charged black hole that can be fed regular matter to produce power,
What form of power and through what principle?
Hawking radiation, I think. Yes, this is at best speculatively feasible.
Probably more like a water wheel - matter spinning around the hole can be accelerated.
A spacecraft carrying a blackhole as propulsion means probably would have poor power to weight ratio.
Not at all. It would have one of the best power to weight ratios possible.
Now as to whether you could use all that power....
Before we get too excited, this current "breakthrough" is making less than 1 antihydrogen atom per second. This corresponds to a delivered annihilation power of less than 1 nanowatt.
Neutrons were first definitively observed in 1932.
First nuclear reactor was 1942, and bomb was 1945.
Once the science is established, we have smart engineers to make a short work out of it.
Fusion energy is really the only counterexample in history, which makes me think we are still missing some crucial physics about how it works, for example in stars. Specifically the particle physics view of how it's reliably triggered with minimal energy.
The antiproton decelerator at CERN has been operational for 25 years, and they have plenty of smart engineers there. Unlike in the 1940s, the underlying physics has been well understood for many decades. I would argue that nuclear fission is the counter example that happens to be surprisingly easy to do.
CERN is trying to do fundamental physics, not trying to weaponize antimatter. If/when it comes to that, the pace will pick up.
Also, 25 years to the breakthrough discussed in the article seems like a reasonably good pace.
All experiments at the AD are strongly limited by the low rates. If there was a straightforward way to improve this by many orders of magnitude, they would have done it a long time ago.
> Fusion energy is really the only counterexample in history, which makes me think we are still missing some crucial physics about how it works
This is magical thinking. We know how fusion works in great detail. And “reliably triggered with minimal energy” is essentially not a thing, unless by minimal energy you mean something like 10 million times the energy of an air particle at room temperature, for every particle in a reactor.
What we’re trying to do is recreate the conditions at the core of a star - which is powered by gravity due to hundreds of thousands of Earth masses. And since we don’t have the benefit of gravity anything like that, we actually have to make our plasmas significantly hotter than the core of a star. And then contain that somehow, in a way that can be maintained over time despite how neutron radiation will compromise any material used to house it.
The reality is, we still don’t know if usable fusion power is even possible - there’s no guarantee that it is - let alone how to achieve it. The state of the art is orders of magnitude away from even being able to break even and achieve the same power out as was put into the whole system.
> at the core of a star - which is powered by gravity
That is what I meant, I doubt we really understand what 'powered by gravity' means. You could win a Nobel prize or two by discovering all the details involved here. You would also win a Nobel prize by definitively proving that nothing special happens, you just have high temperatures and high pressures.
The way we are trying to study fusion is like rubbing larger and larger rocks to produce more fire.
The processes involved in solar fusion have been well understood since the 1930s [1,2]. Hans Bethe won a Nobel Prize for this in 1967. The problem is that one cannot produce stellar densities and pressures in any kind of apparatus.
[1] https://en.wikipedia.org/wiki/CNO_cycle
[2] https://en.wikipedia.org/wiki/Proton%E2%80%93proton_chain
There was also a great episode on antimatter engines recently by PBS Space Time.
https://www.youtube.com/watch?v=eA4X9P98ess
What's the key point regarding how we would get a bajillion times more anti-matter than we can now generate, and without expending all the energy we now expend on getting it?
His point seems to be that we haven't yet seriously tried optimizing for energy efficiency of producing antimatter. It's a call to action. If we actually tried it's plausible that we could get to a level that, while still fantastically inefficient in an absolute sense, would still be worthwhile for spaceflight propulsion, where energy density is vitally important. As far as I know, antimatter is the most energy dense fuel possible in known physics by many orders of magnitude.
Also he proposes a few ways that antimatter could be practically used for propulsion, including as a catalyst for fission which seems interesting.
Can't we argue for the low amount of anti-matter as a type of anthropic principle? The early universe was super dense meaning that areas with imbalance would quickly annihilate and leave only one type of matter. Then, due to rapid expansion, our observable universe is dominated by only one type of matter. If we imagine a universe with a more even mix it would be less welcoming to life, so we are less likely to observe it. Has someone modeled something like this?
The anthropic principle doesn't imply that our entire observable universe has to contain only matter.
Why shouldn't we observe clouds of anti matter and matter annihilating millions or billions of light years away? Why does the annihilation have to have happened so early on that we can't see any evidence anywhere?
I think there does need to be an explanation and it can't be an anthropic principle cop out.
Not only there is no evidence for the existence of antimatter in quantities comparable with matter, but there also is no logical necessity for this.
People who entertain the idea of an initial state with equal amounts of matter and antimatter do this because thus the properties of the matter that are conserved, except the energy, would sum to zero in the initial state.
However, such people forget that not only the particle-antiparticle pairs that can be generated or annihilated through electromagnetic interactions have this property that the conserved quantities except the energy sum to zero.
The particle-antiparticle symmetry is important only for the electromagnetic interactions, while other interactions have more complex symmetries.
All the so-called weak interactions are equivalent with the generation or annihilation of groups of 4 particles, for which all the conserved properties except energy sum to zero. Such a group of 4 particles typically consists of a quark, an antiquark, a charged lepton or anti-lepton and a neutrino or antineutrino.
For instance the beta decay of a neutron into a proton is equivalent with the generation of 4 particles, an u quark, an anti-d quark, an electron and an antineutrino. The electron and the antineutrino fly away, while the anti-d quark annihilates a d quark, so the net effect for the nucleus is a change of a d quark into an u quark, which transforms a neutron into a proton.
The generation and annihilation of groups of 4 particles in the weak interactions are mediated by the W bosons, but this is a detail of the mechanism of the interactions, which is necessary for computations of numeric values, but not for the explanation of the global effect of the weak interactions, for which the transient existence of the W intermediate bosons can be ignored.
So besides the symmetry between a particle and an anti-particle, we have a symmetry that binds certain groups of 4 quarks and leptons.
There is a third symmetry, which binds groups of 8 particles. For instance, there are 3 kinds of u quarks, 3 kinds of d quarks, electrons and neutrinos, a total of 8 particles that belong to the so-called first generation of matter particles (i.e. the lightest such particles).
All the conserved quantities except energy sum to zero for this group of 8 particles. The neutrino is necessary in this group so that the spin will also sum to zero, not only the electric charge and the hadronic charge.
These 8 kinds of particles are exactly those that are supposed to compose in equal quantities the matter of the Universe at the Big Bang.
So all the conserved quantities except energy sum to zero for the Universe at the Big Bang, when it is composed entirely of ordinary matter, without any antimatter.
Therefore there is no need for antimatter in the initial state.
There is no known reason for this symmetry between the 8 particles of a generation of quarks and leptons, except that this allows for the initial state at the Big Bang to have a zero sum for the conserved properties.
It can be speculated that this symmetry might be associated with a supplementary hyper-weak interaction, in the same way as the symmetry between certain groups of 4 quarks and leptons is associated with the weak interaction. Such an interaction would allow the generation and annihilation of ordinary matter, without antimatter, but with an extraordinarily low probability.
How many times does the rate need to be increased 10x before it's a problem?
If I remember correctly, 6.023x10^23 protons (with electrons) is one gram of hydrogen.