One of the possible explanations of the unsolved problem of baryon asymmetry is that there may be regions of the universe where antimatter dominates. No such regions have been spotted so far but if they existed, what would they be like? Would their structures and systems function the same as their counterparts in the regions where matter dominates? Or would they carry (slightly or conspicuously) different properties that would render them antithetic to matter stuff in more respects than the electrical charge of particles?
Really? How would they know? Antimatter looks just like matter, so far as we can tell. There could, theoretically, be vast regions swimming with antimatter and we might never know it.
Apart from the distinctive gamma radiation emitted from between areas of matter and antimatter. Space isn’t empty enough to avoid that.
Except we have loads and loads of unexplained sources of gamma radiation. Do we really know enough to know that some of them aren’t from interfaces between Matter and Anti-Matter bubbles?
As far as we can tell anti-matter behaves like matter in all ways except those tied to the “anti” properties.
The spectrum of anti-hydrogen was measured recently and appeared to be the same as regular hydrogen.
Gamma rays from Matter/Antimatter annihilation have distinctive energy levels. Electron/positron annihilation produces two photons of energy 511KeV.
Best on intuition only, I don’t expect there would any difference but I find symmetry fascinating.
A thought I had long ago was that if there really are extra coiled up spatial dimensions matter then antimatter may rotate, bounce, and otherwise move through those dimensions just differently enough as to be “seeing” each other as slightly asymmetric and to miss hitting each other on occasion when even in overlapping three extended dimension coordinates … meaning that there may be more antimatter bouncing (so to speak) in coiled dimensions than we can easily observe and that minimally interacts with matter. In that case, my thinking went, the main thing that would be seen would be more gravity than would otherwise be expected by observable matter, since gravity is the force that should travel through all the small coiled dimensions …
More informed minds than mine here though have told me nah.
Strictly speaking, C(harge)-symmetry does not hold for the weak force (see C-symmetry - Wikipedia) but I find it hard to believe that any earth-bound observation could detect the difference. But any interactions between matter and anti-matter regions would stick out like a sore thumb. There is a CPT-symmetry, q.g., which means that if you interchanged matter and anti-matter, left and right, and reversed time, the result would be perfectly symmetric.
The situation’s a bit more nuanced than that. It’s true that anti-matter “looks like” conventional matter, in the sense that interaction of anti-electrons and anti-nucleons via electromagnetic forces and gravitational forces are exactly the same as those of conventional matter.
The problem comes in with neutrinos. Neutrinos are uncharged particles that always* travel at or near the speed of light. Neutrinos also have “spin”, which means that they act like little spinning balls (i.e., they have angular momentum.) As you might expect, there are also anti-neutrinos. But it turns out that there’s a fundamental asymmetry between neutrinos & antineutrinos. Neutrinos are always “left-handed”: if you stick out your left thumb in the direction they’re traveling, your left fingers will curl in the direction of your rotation. Anti-neutrinos, on the other hand, are always “right-handed”: the same statement holds for your right hand. We have never observed a right-handed neutrino or a left-handed antineutrino.
For a long time, physicists thought that they could salvage this symmetry between matter and antimatter by saying “the laws of physics are the same if you swap particles for antiparticles and look at everything in a mirror.” Under this transformation, a left-handed neutrino is taken to a right-handed antineutrino, and everything’s hunky-dory. This is what physicists call CP symmetry: “C” for “charge conjugation” (i.e., swapping particles for anti-particles), and “P” for “parity” (i.e., the mirror image.)
CP symmetry turns out to hold almost all of the time, but there is a very slight asymmetry between the situations even when you perform this swap. And it turns out that it’s a good thing, because this asymmetry is what (we think) allowed particles slightly outnumber antiparticles in the early universe; if they really were absolutely symmetric, then the number of each would have been exactly equal in the early Universe, everything would have annihilated with everything else, and there wouldn’t have been enough matter left over to condense into planets that are home to apes with laptops.
*All of this, BTW, is according to the “Standard Model” of particle physics, which assumes that neutrinos are massless particles. We now know that neutrinos do in fact have a very small mass, but I’m not sure how it affects the parity properties of neutrinos. If a more informed person than myself could fill in this gap, I’d be grateful.)
Massive particles cannot inherently have a fixed parity. If you have a left-handed neutrino, and it has mass, then it’s necessarily moving at less than c. That, in turn, means that there’s some other reference frame that’s moving even faster than it. In that reference frame, the particle’s direction of motion is reversed, but its spin is unchanged, and therefore its parity is reversed. The parity of neutrinos is still a thing, but it’s a property of the processes that create (and absorb) neutrinos, not of the neutrinos themselves.
There are two competing theories for how to describe this phenomenon. Under the Dirac theory, neutrinos and antineutrinos still have opposite lepton numbers and so are distinct, and so right-handed neutrinos and left-handed antineutrinos, though quite rare in any useful reference frame, would still exist and be distinguishable (though nearly impossible to detect). Under the Majorana theory, there isn’t actually any such thing as lepton numbers, neutrinos are actually their own antiparticle, and the only thing that distinguishes between what we’ve historically called neutrinos and antineutrinos is their spin state. In this interpretation, then, if you did change the parity of a neutrino, it would still be detectable, just as what experiments would call an antineutrino.
Last I saw, everyone agreed that the evidence was 100% clear in favor of one of those theories. That is to say, the proponents of the Majorana theory argued that the evidence was 100% in favor of Majorana, and the proponents of the Dirac theory argued that the evidence was 100% in favor of Dirac. Historically, the Dirac interpretation has been more popular, but I think the tide might be turning towards Majorana. The last time I did any work on neutrinos, we just dodged the issue by presenting an analysis for each case, and told the reader to decide which one to use.
I have a question. I read about this method somewhere, but googling around doesn’t reveal a whole lot of support for this idea.
I read that you can build an immensely powerful pulse laser, where the laser pulses are coordinated between multiple beams and all converge on a single point. It is possible to make the laser so powerful, and the point of convergence so tiny, that the energy density per unit of space at that point is the same as for matter, and thus matter will spontaneously form.
E = mc^2 in reverse. You get a pair of antimatter and matter, though there may be a slight asymmetry in favor of matter.
If you had even greater resources, you could use free electron lasers and have a continuous beam, and basically produce either antiprotons or antielectrons on a continuous basis. Since the free electron lasers are using a lot of superconducting wiring, they have efficiencies in the 90s. You’d probably need to build this thing in solar orbit, but you could actually reasonably efficiently produce antihydrogen doing this, at something like 40% net efficiency, where 1 laser is making antiprotons (and protons) and a much smaller one is making the antielectrons.
The next step, in order to make practical starship fuel, is you need to force the antiprotons to fuse together. Antihydrogen is far too difficult to transport. This fusion step is immensely difficult but perhaps a civilization that can produce apparatus this massive could find a way to reach the needed pressures and temperatures for a reasonable reaction rate. You’d need to keep fusing and fusing the antimatter, without touching it with anything but electromagnetic fields and or laser light, until you reach something heavy and stable enough to transport.
All that fusing would cost a lot of energy, but for the specific case of a starship, reducing the mass of your fuel greatly helps since the rocket equation requires you burn fuel to carry your fuel, and it would also greatly increase your maximum starship performance.
You could use immensely powerful (and immensely high-frequency-- It’s not just a matter of density) light sources to produce matter, but you’ll get ordinary matter and antimatter in equal amounts. And it’s a heck of a lot easier to produce electrons/positrons in this way than protons/antiprotons.
If it’s just high-density energy that you want, the magnetic fields in some neutron stars can have an energy eight orders of magnitude greater than that of water. Note that I said that that’s just the magnetic fields themselves: The actual matter of the neutron star itself is much denser yet.
So the wavelength needs to be extremely high - meaning you need something like a free electron laser to generate the beam, which needs to be gamma rays. How does a neutron star help you?
I mean, the goal of the exercise isn’t to just screw around. You want to reach another star and slow down when you get there. One method might be if you can fuse your way to an element, or alloy (even trickier to forge an alloy when you can’t touch it but probably doable in low gravity with lasers as tweezers), that is a Type 1 superconductor.
Then it will just reject magnetic fields and thus your antimatter container need be nothing more than a tank with permanent magnets in the walls. In low gravity and in deep space (assuming you cool the tank walls sufficiently), it’s passively stable. More laser tweezers and various fields would be needed to pull bits off the fuel masses and then feed it into the engine. The engine is a chamber, open to space on as many sides as possible (so the gamma rays escape into space), and you manipulate any charged pions so they funneled out the back, giving you thrust at 30% efficiency. Which is incredible, it would mean your isp is (C * 0.3) /9.8.
You need this so you can slow down at another star. Light sails or beams or pellets or something could get your ship up to speed (and the pellet beam method would probably give you a lot more thrust) but it doesn’t do you any good several lightyears later when you are far out of range of the pellet launcher or laser.
I understand that we are nowhere close to being able to build something like this, it’s just a napkin sketch, but I understand it is the best solution to the problem of interstellar travel that we know fundamental physics will allow. (whether or not the engineering problems are solvable is a different story, but there’s no missing pieces. Everything I said is something that’s been done at least once on a tiny scale)
The pellet beam is just tiny iron micro (nano) beads, fired out of a gigantic superconducting quench gun at 99% of the speed of light. The starship receives the pellets with some type of magnetic decelerator. Some designs it catches them, but you can also let them just fly right on through, transferring some of the momentum of the pellet to the ship.
Since superconducting magnets are immensely strong for their mass and have a high attraction to iron, you can probably get accelerations at launch of multiple gravities, so long as the pellet stream keeps up. Using superconductors, there’s very little heat produced by the receiver on the ship. Once the ship has left the range of the pellet gun (which is probably has a barrel of sparse magnets stretched across multiple AU) it would tear down the receiver with robots and use the same mass to form the antimatter engine.
The huge advantage of this is that it (1) evades the rocket equation. You don’t carry the pellets on the ship, so it doesn’t need thrust (or propellant) to accelerate the propellant. (2) all the waste heat associated with accelerating the pellets is paid by the pellet gun, which doesn’t go anywhere. (obviously, each gun element must be tethered to an asteroid or have station keeping engines). So the ship need not be loaded down with heat radiators. It only needs small ones, and it builds the big ones needed for the antimatter engine from the same matter used in the pellet gun.
You need, other than the ability to make things across solar system scales, obviously drexlerian nanotechnology and some form of machine sentience. The starship crew are solid state modules that are constantly being replaced as the years of the journey wear on, and the reason you can rebuild the ship constantly, as if it were alive, is you can tear down any component to the atoms it was made of and rebuild it, using nanoscale factory lines.
To be fair, when people in the 19th century imagined what life would be like now, they were totally wrong, because their predictions left out technology they didn’t know was even possible. No doubt my low rent napkin sketch of a working starship - which I don’t feel leaves anything fundamental out - is wrong in that there’s probably an easier way using tricks not yet known. But it seems so relatively close. “All” we have to do is solve the problem of machine sentience, either by copying a brain directly or getting real close with synthetic algorithms. Once we solve that, we’ll have enough smarts to work out (over decades, I don’t expect the Singularity to happen overnight but decades is very fast) how to build nanoscale factory equipment. Once we have that, we can build exponentially growing industry since there are no more bottlenecks. No longer will factories need workers to staff them at all. No longer need they occupy many square miles, a factory that can copy itself is the size of a refrigerator or maybe a car.
Today we have to spend about half a person’s working lifespan on educating them and the process is incomplete and error prone. And once they get really good at what they do, they can only do it a few decades before their body just fails them, since our bodies were never meant for any of this. AIs would be both technically immortal and able to share knowledge and skills with each other perfectly and in seconds.
And with exponentially growing industry - that can produce more processing cores to support more artificial intelligences who can then work out ways to make the growth even faster and more efficient - solving problems like the above are straightforward. With all these AIs we’d have a population equivalent of usable thinking equivalent to a world with trillions of humans or more.
The ultimate limits are the fact that nearly all the mass in the solar system is in the star, we only actually have tiny flecks of matter leftover in orbit around it. That’s the ultimate limit - once we turn all the rocks here into more robots, factories, and so forth, that’s all we can do in this solar system. But there’s probably plenty to build enough starships to reach the nearby ones.
Of course, it doesn’t have to work out this way. I can’t imagine what can go wrong, but it doesn’t mean it can’t go horribly wrong. In Charle’s Stross’s books, all these computers get infested with parasitic, sentient malware and essentially our solar system ends up as a collection of computers around the star that are occupied running the malware 99.9% of the time. Cooperative projects like a starship just don’t happen.
Nitpick: You need short wavelengths, not long. And I don’t think that even a free-electron laser would be enough: When you’ve got enough energy to produce electrons for scratch, it’s quite likely that you’ve got approximations that are going to break down in any equipment involving lasers.
And I only mentioned neutron-star magnetic fields as an example of energy density not being enough. Those things have energy density far greater than ordinary matter, but still aren’t matter in any usual sense.
Anyway, there’s nothing in the laws of physics that would prohibit making antimatter for use as rocket fuel, but it would still be a truly monumental undertaking. I suspect that the infrastructure ends up being so large that even without putting it on the vehicle (and subjecting yourself to the pain of the Rocket Equation), it’s still too big to be practical in any meaningful sense.
Not really. The score or so extra dimensions are there in order to make complex models like string theory to work. A “dimension” is an abstract construct that may not have a physical/spatial manifestation that something could “move through” in any meaningful sense. They are proposed to be coiled up because we have not been able to otherwise observe them.
Not getting how what you wrote after the “not really” follows from it but that’s no matter … my response is to the bit of “… because we have not been able to otherwise observe them.” Now I seem to recall there being other math reasons why they need to be coiled up but the “we’d be seeing them bit” is off. IF, hypothetically, I was made of stuff that could in some dimension other than X, Y, and Z only move and interact with things from coordinates 35 to 42, how would I “see” other things that were shaped like exact mirror images of me that could in that same dimension only move within the range of coordinates 43 to 50? If, in that hypothetical that I not proposing is the universe we live in, I could move within that from 35 to 45 and those other things could move between 40 and 50 with some of me staying in the 35 to 40 and some of mirror me staying in 45 to 50, how would we appear to each other?
If what I am made of cannot move along some coiled dimension but is stuck in one range of it, how can I see the rest of it? Why would I expect to be able to? I could only see that which intersects with the portions that dimension that I can move with and only the portions of those things that intersect there. Remember Flatland? Remember the Flatlander’s view of the sphere moving through its plane? As a point becoming two points spreading apart and then coming back together again to a single point and then disappearing.
Antmatter is not some mysterious thing. We’ve made it, and done tests on it. Heck, it’s not even always unambiguous, given a pair of particles, to say which one is the matter particle and which is the antimatter. And out of all of the tests we’ve done, it’s behaved exactly like matter in almost all of them, and almost exactly like matter in all of them. If antimatter had any property which made it harder to detect, we’d know.