-anti-matter is incredibly difficult to produce (the total produced so far in labs is about enough to boil 1 litre of water)
-anti-matter is incredibly expensive to produce (currently around 1 trillion USD per milligram)
-anti-matter is very difficult to store and manage
-a lot of the energy will be given off as high energy gamma rays, which will be difficult to convert into thrust
-the high energy gamma rays will be very destructive to the spacecraft and crew (biological or robotic) without lots of shielding
-over a ton of anti-matter would be required to get a ton of payload to a nearby star at ~40% the speed of light
So don't start saving up for a ticket on an anti-matter spaceship. Or maybe do start saving, so your ancestors can take advantage of compound interest to buy a ticket in a few millenia, if we ever manage to develop the technology.
It all sounds awful until you get to this line. What are the equivalent numbers for fusion? All else being equal, 250 tons of fuel per 1 ton payload? And that payload has to include the reactor and storage and shielding and everything.
https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...
I've found direct fusion drive as a concept a company is working on: https://en.wikipedia.org/wiki/Direct_Fusion_Drive
If you need to lift all the fuel from Earth, sure, antimatter all the way. Although, they already suggest making it in space for some reasons.
Shielding is also going to be a huge issue for anti-matter drives (even without a human crew). Anyone know how the energy of EM radiation emitted from a fusion reaction compares to that emitted from an matter:anti-matter reaction?
My apologies if this is off-topic, but I have recently read and heard a couple cases of "ancestors" being used instead of "descendants", which has sparked my curiosity. Is this a common brain-lapse result?
In Count to a Trillion, a small group found a way to siphon antimatter off a natural source and effectively ruled the world forever due to the near limitless energy and fast interstellar travel.
It’s just that governments and research institutions don’t have that incentive. Look how fast reusable rockets, and fairing reuse happened once money was on the line. (To use some space examples)
I guess there’s just no getting around fact that an antimatter reaction that creates 9x10^10 MJ/kg requires more robust electrical generation capabilities than we currently have (I.e fusion)
The fact that something is ludicrously expensive today when manufactured in ultra-small quantities says nothing about the cost if humanity goes all in on mass producing it.
The only really conceivable shortcut for antimatter would be the discovery of large quantities of it somewhere close enough for us to essentially mine it. We've looked for the gamma signature of matter-antimatter annihilation in space, and we don't see it anywhere.
So it isn't IMPOSSIBLE for antimatter to become affordable, but it's extraordinarily unlikely. If we want some sort of superior rocket, it's probably going to have to involve either some unforeseen tech, or our ability to spend planetary energy budgets on rocket fuel.
And assuming the laws of energy conservation holds up, you need to find a better way to generate the electricity that is being used for antimatter production.
The second issue is less obvious - the Milky Way is some 13 billion years old, has hundreds of millions of stars, but is only 100,000 light years across. Fact - the technological limitations of "today" (this century, this mya), are with empirical certainty not relevant.
Interstellar voyage is not amenable to the time preferences and capabilities of the great ape. The only stuff that could transport life across the galaxy does so slowly and without propulsion.
It says that a Rand study says they might be able to bring down the cost to "$6.4x10^10 per gram" with a dedicated factory. That is still so many orders of magnitude from anything financially feasible.
IANAP (I Am Not A Physicist) but it seems like the only "realistic" way to create lightspeed-propulsion quantities of antimatter might be to convert entire planetoids and planets to iron.
I have no idea what that means. Care to clarify?
But there have been proposals for other ways of traveling through space, that involve manipulating space itself, so the distance between point A and point B are smaller. Think of wormholes, or similar. One such proposal is the Alcubierre drive, which warps space around it to cause the total distance traveled to get from point A to point B to be much smaller.
But such exotic drives require exotic forms of matter that we have no evidence of
I've been baffled, though, by the insistence on this kind of antimatter production. Antimatter production is inherently expensive because we are fighting conservation of baryon number, conservation of lepton number, et al.
Physics does have one theoretical method around this, which never seems to be addressed for these proposals, and that's the baffling issue of black holes having no hair. You throw whatever past the event horizon, all that is conserved is mass, angular momentum, and charge. Baryon number, lepton number, strangeness ... all lost. Re-emission as Hawking radiation, it is thought, would simply be this sort of thing, redistributed without regard to what went in, so long as mass, angular momentum, and charge are accounted for. You would conceivably get out as much antimatter as matter.
Now, that's worth looking at. And not from a "we JUST need to capture an itty black hole" (the word "just" does an enormous amount of lifting here) perspective. Rather, passage through the event horizon somehow strips off (we think, some think it might be preserved in some fashion) all of the variables which account for matter versus antimatter. And yet the event horizon isn't a hunk of matter, it's a ... membrane, a boundary, generated by matter at some distance (as a function of our usual three variables). Somehow, this warpage of spacetime operates on matter, shaving it so it has no hair.
That's what fascinates me, in the sense of the theoretical having some extremely juicy practical results.
Even for classical black holes I don't think that's the consensus among physicists. The general assumption seems to be that quantum numbers are still conserved when you throw stuff into black holes, just like electrical charge.
Now, as for Hawking radiation, it's worth pointing out that Hawking's calculation only works for large black holes (low curvature near the event horizon). Once a large BH has shrunken down to a tiny BH (-> high curvature at the event horizon), the calculation no longer works and it is unclear what will happen. Some people say the BH will evaporate completely, some say there will be a remnant – who knows. (Both solutions have their issues.)
All in all, I'm skeptical of the "shaving" you describe.
"Suppose two black holes have the same masses, electrical charges, and angular momenta, but the first black hole was made by collapsing ordinary matter whereas the second was made out of antimatter; nevertheless, then the conjecture states they will be completely indistinguishable to an observer outside the event horizon. None of the special particle physics pseudo-charges (i.e., the global charges baryonic number, leptonic number, etc., all of which would be different for the originating masses of matter that created the black holes) are conserved in the black hole, or if they are conserved somehow then their values would be unobservable from the outside"
Now, this is all still hypothetical, as we haven't a black hole on hand, but this has been the mainstream view on black holes for decades.
That was discussed but AFAIR nobody really believed this was actually going to happen.
> What are the logistics on capturing one of those, keeping it fed, and making some definitive observations?
Generally speaking, the logistics are that you hope that will never create a black hole. You can't really "capture" a BH: It will fall down (towards Earth) like everything else and then start eating its way to the core…
Unless… you manage to create an electrically charged black hole. Then you might be able to confine it e.g. between two condensator plates, by calibrating the voltage in such a way that electric and gravitational force on the BH cancel out.
Now, one problem would be that we expect small black holes to radiate heavily (Hawking radiation), meaning we would have to keep feeding the BH to prevent it from evaporating. (Unless we believe in black hole remnants, see my other comment.) This whole endeavor would make stabilizing the BH between the condensator plates much more challenging. And we'd also still have to worry about what all that radiation will do to our equipment…
Long story short: Don't do it!
It's still a bit of a mystery why there was more matter than antimatter when the universe began.
"Chemical combustion: ~ a few MJ/kg • Solid Propellants: ~ 5 MJ/kg • Liquid Propellants: ~ 1 MJ/kg"
Which doesn't look right. My son (a rocketry enthusiast) tells me:
"Liquid engines have energy density between 12 and 20MJ/kg. Apparently they just left off the 0 and likely meant to put 10MJ/kg"
"Your son is correct, that is a typo - it should be 10's of MJ/kg for liquid rocket propellants - thanks for the catch! Thanks too for the heads up re the discussion on antimatter, it is a fascinating topic and great to see there is interest in the community!"
I agree, but for scale I want to add that a glass of water has like 1000000000000000000 millions of protons in it.
1. antimatter is incredibly energy dense, so would be an excellent fuel.
2. there's essentially none of it sitting around, so we need to make it, which takes lots of energy, but that's fine - it's an energy transport system, not an energy production system.
3. just set up square kilometers of solar panels in orbit around the sun or Mercury, use them to power particle accelerators which produce antimatter.
the sun emits enormous amounts of energy - we don't need some magic new energy source, just magic new ways to temporarily bundle up energy in a dense form.
edit: someone did the maths[0]:
> Where will we get the energy to run these magic matter factories? Some of the prototype factories will be built on Earth, but for large scale production we certainly don’t want to power these machines by burning fossil fuels on Earth. There is plenty of energy in space. At the distance of the Earth from the Sun, the Sun delivers over a kilowatt of energy for each square meter of collector, or a gigawatt (1,000,000,000 watts) per square kilometer. A collector array of one hundred kilometers on a side would provide a power input of ten terawatts (10,000,000,000,000), enough to run a number of antimatter factories at full power, producing a gram of antimatter a day.
from TFA:
> Preliminary mission analysis:
> - 10 kg instrument payload could be sent to 250 AU in 10 years using 30 mg of anti-H
> - A similar probe could be sent to Alpha Centauri in 40 years using grams of anti-H
[0]: https://worldbuilding.stackexchange.com/questions/240278/how...
Please don't retire for another 10 years (or more).
