[whatshisface]

The resonance width is inversely proportional to the lifetime, and if the resonance width is about 400keV, the particle would live for about 10^-21 seconds. For comparison, neutrons decay via the weak force in about 800 seconds, and delta baryons, a randomly chosen strong force decay, live for 10^-24 seconds. That makes this tetraquark long-lived for a strong decay, but that's way, way faster than a weak decay.

https://en.wikipedia.org/wiki/Resonance_(particle_physics)

[im3w1l]

Despite free neutrons decaying in 800s, there are many stable elements containing neutrons. Would it be possible to imagine a tetraquark as an ingredient of a stable particle?

[dukwon]

No. The presence of a valence antiquark guarantees it will decay at some point.

[evanb]

That’s not the full explanation; the antiproton is stable (or, is as stable as the proton).

It’s that the net baryon number is 0 and that the flavor ‘quantum numbers’ aren’t conserved by the weak force. If you could turn off the weak force, this cc ubar dbar tetraquark would be absolutely stable.

[BurningFrog]

Then I wonder if this "particle" is just some components that takes 10^-21 seconds to bounce away from each other after colliding?

[ShinyRice]

They don't just "bounce away", some of its quarks turn into different ones through interactions, and that is enough to class that process as a decay.

Also, quarks can't bounce away from each other. Lone quarks do not and cannot exist in nature, as far as we know.

[phkahler]

Are these decays equivalent to drops to a lower energy state where that energy is mass?

[whatshisface]

The term "lower energy state" is a funny one, because isn't energy conserved? What's happening is a drop to a more spread out state, where you have several particles making great time flying away from each other instead of one high-energy-density locus in the center.

Edit: Just to clarify, the time-variant system exception does not apply in this case. It really is an entropy thing, moreso than an energy thing (which is constant in every particle decay that happens on Earth.)

[MengerSponge]

Fun fact: in the most general case, energy is not a conserved quantity.

https://www.preposterousuniverse.com/blog/2010/02/22/energy-...

[whatshisface]

The idea that spacetime itself has energy, which balances out the apparent lack of energy conservation in matter fields, is a far stronger interpretation than the article suggests. Spacetime energy bends spacetime, which is why gravitational waves exist. It's even called the stress-energy tensor.

[tux3]

This is lovely. Wikipedia sent me from energy not being conserved, to time translation symmetry breaking, to time crystals as a source of perpetual motion.

Just need to find a tie-in to 5G and I can write my own time-cube parody website.

(The article is also very interesting, of course!)

[teknopaul]

my friend that thinks if he plays the breaks properly on his EV he generates energy, will be happy very hear this.

[renewiltord]

All he has to do is push his Tesla to the top of the hill!

[debrice]

Isn’t information a better unit?

[thechao]

MengerSponge's article is raising an extremely subtle point about how we translate modern physical theories into English: we do it poorly.

Conservation laws (Noether's theorem) are dependent on the way the physics is voiced, mathematically. Saying "energy is conserved" is the moral equivalent of looking at Newton's laws and just ignoring GR. GR tells us new, precise, and amazing things about conservation laws. It's just that, unfortunately, they're a little hard to translate into English.

[whatshisface]

>Saying "energy is conserved" is the moral equivalent of looking at Newton's laws and just ignoring GR.

If you're not expecting the spacetime background to be changing rapidly during your experiment, it's pretty moral to say energy is conserved.

[Gravityloss]

So not usable as spaceship fuel

[whatshisface]

If you're allowed to use something produced in an accelerator as your fuel you can't beat antimatter, which is as stable as normal matter until Kirk orders warp one.

[adrianN]

I like black holes because once you have one you don't need an accelerator for refueling. https://arxiv.org/abs/0908.1803

[hnuser123456]

Huh, "black holes move when you push them" is interesting. I suppose you could feed it with a beam, but focusing a beam down to attometer-size beamwidth seems like the hardest part, ignoring making a subatomic black hole in the first place. But sure, I suppose capturing the radiation and redirecting most of it back into the black hole to push it and maintain its size, and just enough spare to push the ship itself at the same speed is feasible. Feels like a "free energy" invention but I don't see where it fails, especially if you could capture the majority of the radiation and feed it back into the black hole directionally, minus whatever used to accelerate the rest of the ship.

I see, after more thinking, redirecting the radiation into the black hole would push the ship backwards with equal energy, so half the energy needs to be reflected back into the black hole at the correct direction, and the other half needs to shoot out the back as exhaust, and you'd need additional mass to prevent the black hole from shrinking and getting hotter.

They seem to conceptualize a ~100-year black hole which balances semi-feasible mass, power output, and lifespan, which is radius 2.7 attometers, 1.8 million tons, and 17 petawatts (!) of power. Looks like the saturn V was about 50 GW of power, so having ~500,000x the power, with only less than 1000x the mass (2900 tons vs 1.8m), means this thing would propel at hundreds of G's of acceleration, unless the ship itself was another 500 million tons? It looks like the WTC towers were "only" about 500,000 tons, so if you wanted to drop the acceleration to something survivable by humans, you would either need a much larger, colder black hole, or a ship of proportions of 1,000 WTCs.

The 10-attometer black hole, with "only" 1 petawatt of power and mass 6.7 million tons and lifetime 5,000 years, seems more reasonable, you'd want a ship with mass 58 million tons to have Saturn V levels of acceleration, only 100 WTCs and the black hole is still only about 10% the mass of the ship. Still, this is only about 6x the width of a proton where we're trying to beam on the order of a petawatt. We would probably need a lot of lasers packed densely together near the back of the ship to focus together on this point to avoid the beam itself being near capable of creating black holes, all coming from the same direction where we need to exhaust equally (or more) as much power to get the ship to keep up with the black hole.

Next step would be to figure out how big of a net we'd need to collect enough mass to maintain the black hole but I've spent enough time on this already.

Alcubierre drives almost seem more reasonable than this, almost.

Oh, and the temperature of this thing would be around a trillion degrees, pretty sure most of that radiation would be gamma rays. Need to figure out how to reflect gamma rays with efficiency. This is apparently around the temperature of a SMBH's accretion disk, the temperature of a new neutron star, and the temperature where matter doubles in mass due to relativistic effects.

All this being said, if we can balance the mass of the black hole with that of the ship, with a black hole with lifetime 5000 years, and we achieve 1g constant acceleration, we can cross the galaxy in 24 years and park it for up to a few thousand years before needing to feed it to prevent it from getting too small/hot. https://en.wikipedia.org/wiki/Space_travel_using_constant_ac...

Imagine if you could see the other side of the galaxy and make it back to Earth before you turn 50 (though Earth will have experienced 200,000 years), or, since once you're already at such relativistic speed, see Andromeda and come back before you're 60. Apparently we could round trip to the edge of the (Earth's?) visible universe in right around 100 years. Of course, by time you made it back, Earth would be 26 billion years older, the sun will have exploded, etc. Of course, if these are drone ships, we don't need to worry about human-survivable acceleration, and we could retrieve data much faster, but then no biological lifeform would have been there.

[Gravityloss]

Power scales to the square of exhaust velocity while thrust linearly, so if you have a very good power to weight power source like a black hole, you can use a very high exhaust velocity and thus can get by with very little reaction mass. Which is good.

Also, you can make a spaceship out of an asteroid (or from asteroid materials) so multi WTC mass is not a problem.

[hnuser123456]

I see. I was assuming we were thrusting with nothing but gamma radiation, which is the speed of light, unbeatable, but I see with such small black holes, we may end up with some matter evaporating from it. I suppose that's one more benefit to larger, colder black holes: more likely to have lightspeed radiation than massive particles.

We could go for something closer to the temperature of the sun / wavelength of green light, about 10^19kg, easy to reflect, but that's about 0.1% the mass of the moon and would only output about 3.5 microwatts. Could try to balance between an amazingly reflective mirror to EUV or shorter wavelengths, which is more physically realistic than reflecting gamma rays, and usable power output, but that's simply out of reach at the moment. I wonder if we had an atomically smooth mirror, the smallest wavelength we could reflect with efficiency, and thus the highest power black hole we could use. Maybe it makes more sense for the black hole to have much higher mass than the spaceship, but 1g acceleration will be difficult.

Looks like most atoms are about 0.1nm, so 10^18kg black hole, for 350 microwatts. Like turning on a laser pointer attached to the moon and expecting usable thrust. Still far too large and not powerful enough to be a useful power source like a a 10^8kg black hole for around a petawatt. Either we figure out how to reflect deep gamma rays or it won't happen.

[zinglersen]

This was such a cool read, thank you!

What is hardest for me to comprehend is probably the last part about time being relative. All this stuff makes me think about how everything is made of the same matter and then, what am "I"?

Btw this part from the wiki link cracked me up "Constant acceleration is notable for several reasons: It is a fast form of travel."

[Gravityloss]

There's always many considerations. Energy density, stability, what kind of energy can you convert it to, can it be directed easily, how hard is it to store etc...

[whatshisface]

Antimatter's reputation for being incredibly difficult to store comes from the fact that it's produced as individual particles. A superconducting antimatter hockey puck would be much easier to store than a cloud of antiprotons of the same mass.

And yeah, you'll need a way to build gamma ray mirrors before antimatter reactions will push you in any direction (the energy comes flying out isotropically and we can't presently do anything to stop or direct it), but we can cross that parsec when we come to it. :-)

[jjk166]

Presumably antimatter would be your energy source, but not your propellant. The gamma rays from a small number of annihilations would heat up a much larger amount of normal matter.

At least for the first generation, you likely also wouldn't be using antimatter as the main source of energy, but rather as a method of initiating some other reaction. For example where in a conventional fission reaction you get a relatively clean split of a nucleus into two halves plus a few extra neutrons to drive a chain reaction, an antiproton will blast apart such a nucleus like a billiard break, allowing fission reactions with much less than a conventional critical mass. A quick burst of positrons hitting the surface of some lithium deuteride would be able to replace a fission primary and make a pure-fusion explosion. Either of these options could be used as either incredibly low-mass nukes for an orion drive or as a light weight reactor for a more conventional nuclear propulsion method. While about 600 times less energy dense than pure antimatter, you're still talking 10 million times better energy density than our best current rocket fuels, while using several orders of magnitude less antimatter.

[human]

I would be so nervous to be in the middle of space with an hockey puck of antimatter.

[yccs27]

I guess I'd be just as nervous in space with thousands of tonnes of explosive propellant. Spaceflight always operates on the very edge of what's possible, not of what's safe.

[ashtonkem]

Gamma ray mirrors sound like they’d be extremely useful for nuclear power too.

[8note]

Superconductors pretty notoriously need to be kept cold, which adds another difficulty of cooling the antimatter without touching it

[whatshisface]

If it was surrounded by a cold mass that it could radiate photons to across a vacuum, its equilibrium temperature would be that of its container.

[kadoban]

For now. By the time anyone could even possibly create enough antimatter to matter (heh), critical temperatures should be much higher. The record is broken fairly commonly.

[coldacid]

I think that's what the dilithium crystals are for. ;D

[BurningFrog]

Needs to be a very short trip.