Why Antimatter Engines Could Launch In Your Lifetime

0:00 Thank you to Displate for Supporting

0:02 PBS Antimatter drives sound like science fiction,

0:04 but they may not be as far as you think.

0:08 There’s a version that could, just maybe, launch within your lifetime.

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0:40 Next, If you believe in humanity’s propensity for interstellar travel,

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1:00 Link in the description.

1:02 Now, on to the episode.

1:04 Warp speed, hyperdrive, jumpdrives, space folding, subspace, wormholes,

1:08 infinite improbability drives… We’re pretty good at coming

1:10 up with ways to travel faster than light, at least in fiction.

1:14 Sadly, Einstein’s relativity theory tells us it’s

1:17 almost certainly impossible to travel faster than light,

1:20 at least for macroscopic objects over meaningful distances.

1:24 Probably our future exploration of the galaxy will have to take the slow road.

1:29 That said, we can certainly do a lot

1:31 better than the current generation of chemical rockets,

1:34 which are still burning stuff for energy, you know, like a steam train.

1:39 In terms of energy per kilogram the antimatter drive is near as efficient

1:45 as you can get within the bounds of the laws of physics,

1:49 if you care about those.

1:51 As such, its also a staple of science fiction,

1:54 and it also feels as far of as the warp engine.

1:59 But there have been lots of advances in recent years

2:02 that warrant this update on the timeline of antimatter-powered space travel.

2:06 And there are early versions that you might even see launch.

2:10 Let’s start with the requisite antimatter review.

2:13 We’ve talked about antimatter before about how, in 1928,

2:17 Paul Dirac discovered it in his equations as he tried to bring

2:21 quantum mechanics into agreement with special

2:23 relativity and found negative energy electron.

2:26 And how it in 1932 it was

2:30 discovered in reality when Carl Anderson noticed cosmic

2:33 ray electrons curving the wrong way in a magnetic

2:37 field they proved to be Dirac's anti-electrons—positrons.

2:41 So what is antimatter really?

2:43 It’s pretty fair to describe it as the mirror image of regular matter,

2:47 and every particle of matter has an antimatter counterpart.

2:51 It's the Bizarro to Superman, the Wario to Mario, the Dirac to Feynman.

2:56 The same but oh so different.

2:58 Antimatter is inevitable in a universe with the symmetries of ours.

3:02 If we think of particles as swirls and vibrations in the quantum fields,

3:06 well, those fields can swirl and vibrate backwards?

3:10 The laws of physics are almost completely symmetric if we,

3:15 say, flip all quantum charges to be their opposites,

3:18 or reflect to their mirror image, or if we reverse the flow of time.

3:25 Antimatter is this—charge-reversed and mirror reflected matter,

3:28 which can equivalently be thought of as time-reversed matter.

3:32 The matter-antimatter symmetry means it’s possible

3:35 to produce particles matter out of a pure vacuum as long as you also produce

3:41 the corresponding particles of antimatter to balance.

3:44 And also as long as you have the energy to account for the new mass,

3:49 according to Einstein’s equation E=mc^2.

3:51 This process is called pair production.

3:54 But if the laws of physics are symmetric in time this process works backwards.

4:00 A matter-antimatter pair can also be un-created—annihilated—to produce energy.

4:06 And again, E=mc^2, so take the mass,

4:09 multiply it by the speed of light, which is a big number,

4:13 then multiply it by the speed of light again,

4:15 and you get a very, very big number.

4:17 And that’s why antimatter is the ultimate energy-dense spaceship fuel.

4:22 For example annihilate an espresso with an anti-espresso and you'll

4:28 get an explosion with the power of the modern h-bomb.

4:31 There are of course complications to using anti-matter or we'd be

4:35 flying around with it and blowing each other up with it already.

4:40 It's very difficult to make the stuff

4:42 in quantity and even more difficult to store.

4:46 And it's also complicated to harness it in rocket propulsion.

4:50 But before we can talk about solving those we need to dispel a misconception.

4:56 There’s a commonly repeated statement

4:58 that matter and antimatter annihilation releases

5:01 ‘pure energy.’ There’s not really any such thing as pure energy.

5:06 Energy is a property possessed by systems.

5:09 It can take many forms—mass energy, kinetic, potential, etc.

5:13 The most charitable interpretation of “pure energy”

5:16 is energy that’s easily accessible and usable.

5:19 Mass-energy is the least pure or “free” in that sense,

5:22 and that's the price of it being the most compact form of energy.

5:28 And that mass energy isn’t necessarily all liberated in annihilation.

5:33 That process will often produce other massive particles,

5:37 so some energy remains locked away.

5:41 The idea of this process producing

5:44 “pure energy” is probably from electron-positron annihilation.

5:47 In that case, the ingoing particles have such low masses

5:50 that the energy they produce typically isn’t enough to produce other particles.

5:55 Instead they annihilate into photons, and the rare neutrino.

5:59 The entire energy content of those photons can be captured pretty efficiently.

6:05 But to accelerate our spaceship we really need momentum, not energy.

6:12 Basically, we need to throw stuff out the back of our ship

6:15 as fast as possible so that momentum conservation accelerates us forwards.

6:19 Although our annihilation photons have plenty of energy,

6:24 they are massless and so carry relatively little momentum.

6:28 Also, very high energy photons are hard to direct efficiently.

6:33 Ideally we want to harness whatever energy we

6:36 produce to blast massive particles out behind us.

6:40 It’s certainly possible to do that with the energy

6:43 of these electron/positron annihilation photons in an indirect manner,

6:47 and I’ll come back to options.

6:50 But there’s another issue with this “high efficiency” type

6:54 of annihilation and that has to do with storage, which I'll also come back to.

6:58 But all of this is nudging us towards using

7:02 more massive versions of anti-matter to power our spaceship.

7:06 Massive anti-particles are much harder to create,

7:09 but let’s see where we’re at with this.

7:12 So we had position in 1932.

7:15 And the next anti-thing discovered was 2000 times heavier,

7:20 the anti-proton in 1955.

7:23 Followed quickly by the antineutron And then we

7:27 started to produce antinuclei with anti-protons and anti-neutrons together.

7:31 First anti-deuterium then anti-tritium then antihelium-4.

7:35 More exotic anti-nuclei followed.

7:38 with the current record being anti-hyperhydrogen-4.

7:42 All of these heavier particles are identified

7:45 in the debris of high-energy particle collisions,

7:48 which means they’re both rare and difficult to capture.

7:52 There are two steps to this: first slow the particle down then trap it.

8:00 These collision products start out moving fast—often at a good

8:04 fraction of light speed--and they’re moving in a random direction.

8:08 So they need to be channeled and then slowed.

8:12 The world leaders in this antimatter

8:14 trapping are at CERN’s Antiproton Decelerator.

8:19 It’s an anti-accelerator for capturing anti-protons.

8:24 Once slowed, the antiparticle needs to be trapped.

8:26 This is arguably the harder part.

8:29 If antimatter touches matter it annihilates,

8:31 which means there’s no such thing as an antimatter-resistant

8:35 material container that isn’t itself made of antimatter.

8:39 The solution is a non-material container, which means a force field,

8:44 and the electromagnetic field offers good

8:47 options for both charged and non-charged antimatter.

8:50 The most famous containment device for charged antimatter is the Penning trap,

8:56 in which electrodes at either end of the trap create an electric

8:59 field that keep charged particles in the middle along the axis,

9:03 while a magnetic field along the axis prevents

9:06 any from drifting away in the radial direction.

9:10 This is the technology used by the BASE collaboration,

9:13 who have managed to store a hundred antiprotons

9:16 for a full year in their larger Penning trap.

9:20 Electromagnetic containment like this relies

9:22 on the anti-particle having a net charge.

9:25 But that introduces a new problem.

9:28 Like charges repel, and so end up as a big diffuse

9:34 cloud—which defeats the point of using

9:37 antimatter as a compact energy-dense fuel.

9:39 Now, if you want to keep the particles close together you need a colossal

9:44 EM field with a similarly colossal

9:46 anti-matter containment device again defeating the point.

9:50 This why we can't just us positrons or just anti-protons as fuel.

9:55 Happily there’s a work around.

9:57 If you trap both positrons and antiprotons in, for example,

10:02 a Penning trap, and then cool them, they’ll combine into anti-hydrogen atoms.

10:08 Of course, now you have the problem that the resulting

10:12 anti-atom is electrically neutral and so immediately escapes the Penning trap,

10:16 which is presumably a bad thing.

10:19 A more sophisticated trap is needed.

10:21 The current trick is to use the fact that the anti-hydrogen

10:25 atom has a magnetic moment—like a little bar magnet.

10:28 It’ll try to align and move in the direction of a magnetic gradient.

10:34 So if you have a magnetic field with a minimum value in all 3 dimensions,

10:39 any anti-hydrogen in that field will move

10:42 towards the minimum and get stuck there.

10:44 This is called a magnetic minimum trap.

10:47 Because the new anti-hydrogen will immediately

10:49 escape its Penning trap after formed,

10:52 the magnetic minimum trap has to be superimposed directly over the Penning trap.

10:59 Now, the magnetic minimum trap is far weaker than the Penning trap.

11:04 And so anti-hydrogen needs to be really cold,

11:07 colder than around 1 Kelvin to have a chance of remaining trapped.

11:12 Various novel methods like laser cooling

11:15 are used to further chill this antihydrogen.

11:18 But despite best efforts, this whole process is still very inefficient,

11:22 with only a tiny fraction of produced

11:26 anti-protons and positrons being converted to captured anti-hydrogen.

11:30 It’s also difficult to keep the much of this stuff trapped for long.

11:34 The current record is by the ALPHA collaboration.

11:37 The team trapped 112 antiatoms for times ranging from one-fifth of a second

11:43 to up to 1,000 seconds.Not exactly long enough for an interstellar journey.

11:48 But now that we have actual anti-hydrogen, a new possibility opens up.

11:54 Anti-hydrogen is expected to behave essentially identically to regular hydrogen.

11:59 And that means that if we have enough of it

12:02 it can undergo phase transitions into more useful forms.

12:06 Hydrogen freezes at 14 Kelvin at atmospheric pressure,

12:10 and so the hope is that enough antihydrogen would solidify at that point.

12:16 That’s the dream actually— antihydrogen ice

12:20 fuel pellets suspended in magnetic fields.

12:24 OK, so we have our antimatter fuel.

12:26 Let’s look at how we can use it.

12:29 Bring a hydrogen and antihydrogen together and first

12:33 the electron and positron annihilate producing high-energy gamma rays.

12:37 The following proton-antiproton annihilation is way more complicated.

12:41 That’s because we’re really annihilating three quarks with three antiquarks.

12:45 When the hydrogen and antihydrogen come together,

12:46 the individual quarks don’t all annihilate.

12:48 That only happens to the first quark and antiquark to make contact.

12:53 They’re likely to produce particles like a gluon or W boson,

12:57 which in turn creates more quarks,

13:00 fragmenting the two baryons into a shrapnel of particles like pions.

13:05 Sure enough, this is exactly how antiprotons were discovered

13:09 in collision experiments by Segre and Chamberlain in 1955.

13:15 These pions give us another approach at antimatter propulsion.

13:19 They can be used directly as our “working mass”,

13:23 which means the stuff you throw out the back for momentum exchange, your thrust.

13:29 This design is called a ‘pion rocket’.

13:31 It’s possible because the charged pions can be channeled by a magnetic field.

13:37 Unfortunately, the energy released in hydrogen-antihydrogen

13:40 annihilation also ends up as neutral pions,

13:43 gamma rays, and neutrinos that ignore our magnetic field.

13:46 If we really don’t want to waste the energy

13:50 from these, there other ways to capture it.

13:52 One approach is to capture the photons

13:55 and kinetic energy of the particles to generate electricity.

13:58 That electricity can then be used

14:00 to accelerate other particles to provide thrust,

14:03 as in an ion drive, which we already employ for very steady,

14:08 if slow acceleration in space craft.

14:11 That can be done whether or not

14:13 the annihilation products are also used for propulsion.

14:16 And that makes it also an option for electron-positron annihilation.

14:21 OK, so how long before we get our first antimatter craft?

14:25 Well, quite a while for our first crewed interstellar craft,

14:29 but maybe not so long for our first unmanned probe,

14:32 especially for our own solar system.

14:34 That might be possible if we use antimatter

14:37 in a sort of hybrid mode with nuclear fission or fusion.

14:42 Now nuclear power is proposed in a couple of space travel scenarios.

14:48 The more prosaic it to use a fission

14:50 or fusion reactor generating electricity to power an ion drive.

14:54 A more radical, but actually perfectly within our technological

14:58 grasp is nuclear pulse propulsion like the Orion project.

15:03 In these, a series of fission or fusion bombs are

15:07 detonated behind the ship or behind a sail to propel it.

15:12 Both direct nuclear reactor and nuclear pulse propulsion suffer

15:16 from the same challenge—the size of the device needed.

15:22 Fusion reactors need to be huge to sustain a reaction with net-positive output,

15:29 and this is why we don’t yet have a commercial fusion reactor.

15:35 Nukes need to be big in order to initiate their chain reaction.

15:40 But if a small amount of antimatter can be employed to kick off the reaction,

15:46 then these devices can be made much smaller.

15:48 Let’s take the H-bomb example.

15:50 A key component of the hydrogen bomb is

15:53 some sort of fission core—typically a ball of plutonium.

15:57 This is surrounded by a layer of heavy hydrogen.

16:00 The plutonium is detonated as a fission bomb,

16:03 providing the heat and neutrons needed to ignite fusion in the hydrogen.

16:07 Because of the need for a critical mass of plutonium,

16:10 there’s a minimum size for a hydrogen bomb,

16:13 which means a minimum explosive output.

16:15 That in turn means that any Orion-type spaceship needs

16:19 to be pretty huge to capture and withstand that energy.

16:23 But what if we replace some or all

16:26 of the plutonium core with a tiny grain of antimatter?

16:29 Then we can build a much smaller device of the same style,

16:33 with the annihilation of the antimatter providing the energy

16:35 and particle bombardment needed to ignite a smaller fission core,

16:40 or even directly ignite fusion.

16:43 Either way, this enables a more manageable thermonuclear explosion,

16:48 which would work on a smaller craft.

16:51 This approach is called antimatter-catalyzed

16:54 nuclear pulse propulsion, and more generally,

16:58 antimatter-catalyzed fusion or fission may be

17:00 useful in powering spacecraft in various ways.

17:04 Really, the main advantage of doing this that we need far less

17:08 antimatter because most of the energy is from the “classical” nuclear fuel.

17:13 The amount of antimatter needed is as low as micrograms by some estimates.

17:19 It’ll take us a few decades to produce that much antimatter at current rates,

17:24 but then we can build an antimatter-catalyzed fission

17:28 craft that can reach the Oort cloud—further than

17:32 anything we've ever set out there— and that would

17:35 be in a mere 10 years travel time.

17:38 Or so at least one proposal has calculated.

17:42 Other proposals for non-crewed craft are also within distant,

17:47 but not sci-fi-future level reach.

17:50 It’s remotely possible that the first

17:53 antimatter-enabled launch will be in our lifetimes!

17:56 For some of us, anyway.

17:58 And not of all of this is just theoretical.

18:02 Early experiments have shown that antiprotons cause extreme

18:06 amplification of fission in non-critical heavy metal samples.

18:10 So we started out wanting a not-too-disappointing

18:14 alternative to the impossible FTL drives.

18:17 What’s it really going to take to build a proper sci-fi antimatter drive?

18:22 The main holdup is the rate at which we can produce and store antimatter.

18:27 Once we can make enough to solidify, it gets a bit easier with the storage.

18:33 At current rates this is going to take centuries to millennia.

18:38 So we can build more and bigger

18:41 colliders that are devoted just to antimatter harvesting.

18:45 But there’s also the possibility of harvesting it in space.

18:50 Space is full of radiation and high energy cosmic rays,

18:54 with high energy collisions making antiprotons all the time.

18:57 The PAMELA satellite discovered that Earth’s magnetic field

19:01 confines antiprotons and positrons produced by those collisions.

19:06 So maybe we can harvest this antimatter in the Van

19:10 Allen belts to fuel up for an interstellar journey.

19:14 A side advantage to harvesting in space is safety.

19:17 I’m not sure I’d want to blasting off from Earth’s

19:20 surface on top of a stash of delicately suspended antimatter.

19:24 So, that’s where we stand with antimatter drives.

19:27 They’re not quite around the corner,

19:30 but also there are plausible paths to the first versions.

19:34 And at least for those versions the technical challenges seem pretty solvable.

19:41 Our first crewed, interstellar antimatter-powered

19:44 spaceship is not in our lifetime, but if we want it to happen it will.

19:50 One day we can cross the galaxy by annihilating

19:54 the reflections in the quantum symmetries of spacetime.

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