Anti-matter for space-travel?

[This is a transcript with links to references.]

I got some questions the other week about an article in business insider  that says it’s possible to power rockets with anti-matter and the only thing that’s preventing us from doing it is: cost. Really? The business insider writes about anti-matter. Should you buy stocks? Let’s have a look.

 Antimatter isn’t science fiction, it’s stuff we know actually exist. Each particle has an anti-particle, that’s its identical twin, except it has the opposite electric charge. The electron  for example has an antiparticle called the positron. Particles that have no charges can be their own antiparticles.

When matter meets anti-matter  they annihilate, and that process releases a lot of energy in the form of very fast particles. Most of them are photons,  that are the quanta of light, but depending on what type of matter and anti-matter you brought together the reaction might also release charged particles.

And just so there are no misunderstandings.  That’s really unhealthy. You generally don’t want highly energetic particles blasting through your body, be that photons or anything else.

 Where is the antimatter? Well, any anti-matter that was present in the early universe would have annihilated with normal matter, leaving behind mostly photons. We are what’s left over, basically. You know like cold turkey the day after thanksgiving.

 But anti-particles can and have been created in particle colliders,  they have also been found in cosmic radiation reaching us from outer space,  and they are naturally produced in some types of nuclear decay.

So much about what we know. What makes anti-matter interesting from the point of view of propulsion  is that its energy density is enormously high. In fact, the energy density contained in a certain mass of anti-matter is the highest we know, period.

 The energy density of nuclear fission fuel, like Uranium 235 is already enormously high.  1 kilo gram gives you about 24 Giga Watt hours.  For nuclear fusion the energy you can derive from 1 kilogram of fuel, say hydrogen, is about 10 times higher.

And the energy you can derive from antimatter is another factor 100 higher. So, one kilogram of antimatter plus the same amount of matter would produce 25 Tera Watt hours.

A ton of it could deliver electric power for the entire world for a year.

If the stuff exists, and we can create energy with it,  why aren’t we using anti-matter to power the world? That’s because producing it is extremely inefficient and takes up like a million times more energy than you can make from it.

CERN has an experiment in which they produce anti-hydrogen, that’s an anti-proton with a positron around it. The create it by diverting part of the collider’s proton beam and slamming it into a metal target. This anti-hydrogen is the biggest anti-matter thing that’s ever been produced.

But running a particle collider that size isn’t cheap.  CERN has estimated they produced about 1 billionth of a gram of anti-hydrogen,  at a cost of half a billion Euro or so, that’d be a quintillion Euro per gram. And if we’re talking about numbers of which you aren’t sure just how many zeros they have, it’s generally safe to assume that it’s a lot of money.

It is somewhat cheaper to use antimatter that’s produced naturally, most notably that’s positrons  which are emitted in some kind of nuclear decay. And this brings me back to the Business Insider article  because it’s really about a company called positron dynamics  which says that’s what they want to do: Collect positrons and power rockets with that.

But the problem with antimatter isn’t just the production. You also need to store it without it touching anything.  Usually you do that by some clever combination of electric and magnetic fields. It’s possible, but it’s very tricky, even for small amounts. And when I say small, I mean small.  Like CERN usually deals with like a dozen or so antihydrogen atoms. So how would you store a gram of the stuff? I have no idea. I certainly wouldn’t want it to be stored anywhere near me.

With the positrons  you have the additional problem that when they’re created they’re usually quite fast and if you want to trap them you first have to slow them down.

But Okay, business insider  is right that the stuff contains a lot of energy and it’s very expensive. Can you use it for propulsion? Yes, but leaving aside the question of producing and storing the stuff it’s not as simple as it sounds.

You see just releasing energy doesn’t necessarily propel you anywhere.  Hydrogen bombs release a lot of energy, but they’re mainly good for propelling people into afterlife.  The way you propel a rocket is basically by throwing stuff out one end. You don’t need energy, you need momentum, that’s essentially directed energy.

So what you have to do with the matter-antimatter reaction is that you must find a way to direct it somehow.  If you’d be using hydrogen and anti-hydrogen then part of the particles that would be created would be charged. You could then use electric and magnetic fields  redirect the charged particles into one direction. Then whatever is generating the fields will be pushed into the other direction.

Problem is that the matter-antimatter reaction doesn’t just produce charged particles but also a lot of photons  and those will wreck your rocket very quickly.

The company positron dynamics says they’ll use these highly energetic photons to generate charged ions and then direct these ions.  Their CEO gave a TED talk  a few years back with this figure  to illustrate the idea. And that’s all well and fine, except you know some of those the gamma rays will also hit your rocket  and well, I wouldn’t want to sit inside, but maybe that’s just me.

 What would it get us? Well, it could accelerate rockets much more than current propulsion devices. The guy from Positron Dynamics estimates that the positron driven rocket could bring people to Proxima Centauri  within 5 years. Proxima Centauri is about 4 point 2 light years away. It seems a bit overoptimistic to me, but yeah, in principle there’s little doubt that it could dramatically improve space travel.

So why aren’t we doing it? It’s because the technological gap is just too large.  You see the way that I think about scientific and technological developments is that one thing builds on another. You can’t built the 6th floor before you’ve begun the 2ndbasically. Anti-matter propulsion can work,  and I quite agree with Elon Musk that it’s the future of space flight.

But there’s just too much tech missing at the moment. You need to produce  the stuff you need to catch it and store it and then you need to figure out how to build the propulsion system itself. It’s not going to happen in the next 300 years or so.

The obvious next thing for propulsion is nuclear fusion, if we can ever get that to work.

Why is Sabine talking about space travel propulsion now,  has she totally lost it? You know it might sound weird coming from a middle-aged mother of two but it’s stuff like this that got me interested in physics in the first place. Antimatter, warp drives, alien civilizations. I strongly believe that the future of humanity is in space. And I am totally supportive of people like the guy from positron dynamics. But I still wouldn’t buy the stocks.

Hello

Hi Elon

No, I don’t want to sign a petition against Zuckerberg, what are you talking about.

That’s anti-matter not anti-Meta.

Ah, don’t worry about zuckerberg, his fam will want to have him back in Proxima Centauri soon enough.

Love you too.