In this week's "Ask a Physicist" we're going to consider an incredibly ill-conceived mission: a trip to a neutron star to extract the gooey neutrons inside. What happens next? You die. We'll find out how.
You folks send me lots of questions about topics in physics and astronomy, mostly quite sane, but every now and again I get a suggestion for a mission that is so awesomely foolhardy that I have no choice but to follow it to its natural conclusion. Reader Mark Skelton asks:
I know neutron star material is some of the most dense matter in the universe, with a teaspoon weighing as much as a mountain. What would happen if we could in fact extract this material from a neutron star?
Let me start with this: DO NOT TRY THIS EXPERIMENT.
Though the name "neutron star" seems innocuous enough, beware. They are tiny, but deadly remnants of massive stars. Though they are two to three times the mass of the sun, they are compact enough to fit (relatively comfortably, actually) inside the borders of Philadelphia.
Because of this insane density, the gravity is incredibly high. You might well expect the entire thing to collapse into a black hole, and you'd be very nearly right. This is precisely the reason that neutron stars can't be more than a few times the mass of the sun. Otherwise, they would become black holes.
Neutron stars are held up by something known as "degeneracy pressure," which is really a fancy way of saying that the neutrons are packed together asses to elbows, and can't be pushed any tighter. This is a consequence of the famous "Pauli Exclusion Principle," which says, in essence, that two neutrons (or electrons, for that matter) can't be in precisely the same place and state as one another.
In case you're looking for a talking point for your next cocktail party, you might also be interested in knowing that degeneracy pressure (of electrons, rather than neutrons) are also what hold up white dwarves. We talk a lot about neutron stars and white dwarves in Chapter 5 of my book.
The degeneracy pressure is so rigid that if you have a supergiant star with a neutron star core, and the material from the envelope starts to collapse, the ricochet of material off the neutron star will ultimately produce an explosion that can outshine an entire galaxy. You might know this as a supernova. Have you ever seen the Crab Nebula (see picture above)? That's what happens when you mess with a neutron star.
So what would happen if you were fool enough to approach one of these things?
You and your ship will be ripped apart by gravity and magnetic fields
Landing is going to be incredibly tough. Neutron stars can spin at thousands of times per second and many of them have magnetic fields over ten million times stronger than the Earth's. This is going to adversely affect you in a few ways. First, magnetic fields at those levels are almost certainly going to destroy anything with ferromagnetic materials (a fancy word for things like iron that you can make magnets out of) as well your computer systems.
Also, the combination of spinning and strong magnetic fields mean that neutron stars essentially have their own defense system. You may know them as "pulsars" and they basically consist of a high-energy radiation beam sweeping through the sky every fraction of a second. Finally, have you ever tried to land on a planet where the surface is rotating at thousands of kilometers a second? It isn't easy.
But supposing you could land on the surface of the neutron star. Sure, it's something like a million degrees Kelvin, but compared to the other problems you're likely to encounter, that's child's play. The gravity is something like two hundred billion times that on the surface of the earth. If that doesn't concern you, consider that the difference in gravity between your head and your feet is approximately sixty million g's. Lest you think that everything in the universe would kill you in exactly the same way, consider that the surface of our sun is only about 6000K, and has a gravity of about 27 g's. Comparatively speaking, that's nothing.
Because I like you, I'll allow you to survive for a bit longer. I'll assume you have access to the Enterprise and transporter technology and whatnot, and landing on the surface isn't necessary. Let's suppose you transport up a teaspoon of neutron star from the core directly into the cargo bay. I say "the core" because the outer crust is kind of boring — it's mostly heavy elements like iron. To get to the pure product, you need to dig deeper.
What happens next? Here's where the fun really starts.
You first need to realize that we're talking about densities of about 10^18 kg/m^3, which means that the total mass in a teaspoon is somewhere on the order of 10 billion tons. I've crunched the numbers and indeed, Mark is right. That's about right for a reasonable sized mountain.
Inside a neutron star, there's a delicate balance between the tremendous gravity of the star, and the degeneracy pressure of the neutrons. Once we extract the neutrons, all bets are off. We no longer have the gravitational pressure to compress our neutrons together, and remember, these neutrons are at temperatures of a millions degrees. The gas pressure is huge. Even if you could use a transporter to teleport your neutron star into the hold of your ship, the sudden decrease in pressure will cause the gas to explosively expand. Assuming a decent-sized cargo hold, they'll end up producing a pressure of something like a quadrillion times atmospheric normal, and a density on the order of ten million times that of solid rock.
Do not stand in your cargo bay when you beam up your neutron star material. I cannot stress this enough.
A Neutron Bomb
Supposing the expansion of neutrons didn't destroy your ship outright, the worst is yet to come. Inside a neutron star, the degeneracy pressure also stops neutrons from doing what they'd normally want to do: decay. Neutrons can hang out for a very long time if they're inside atomic nuclei, but on their own, they're not terribly long-lived, at least on normal human timescales. Compared to many subatomic particles that only last a billionth of a second or even less, the 10 minute lifetime of a neutron is incredibly long. After that ten minutes (on average) a neutron decays into a proton, an electron, and normally undetectable anti-neutrino.
Not a big deal, right? Wrong. We finally get to invoke the most famous equation in all of physics: E=mc^2. This tells us how much energy is going to be released by every single decay.
Take the mass of the neutron and subtract the masses of the proton, the electron, and the negligible mass of the antineutrino, and that's the mass lost. Multiply that by the speed of light squared, and you have the energy released. In the case of neutron decay, about 0.08% of the mass gets converted to energy in the process, which doesn't sound like too much, but multiply it over your teaspoon of neutron star, and it ends up producing an equivalent energy of about 10^27 J — or roughly the energy put out by the sun in 2 or 3 seconds.
Maybe you don't have a great intuitive feel for exactly how much energy that is, so another way of putting it is that the neutron decays release the equivalent energy of a trillion megaton nuclear device. To put things in perspective, this about 50 trillion times the energy of the first nuclear bombs, and would easily be enough to destroy all life on earth.
Bear in mind that the half-life of neutrons are about 10 minutes, which means everything would be dead and done quite quickly. Congratulations, you've teleported a live nuclear device on to your ship.
Best of luck to you.
Dave Goldberg is the author, with Jeff Blomquist, of "A User's Guide to the Universe: Surviving the Perils of Black Holes, Time Paradoxes, and Quantum Uncertainty." (follow us on twitter, facebook, twitter or our blog.) He is an Associate Professor of Physics at Drexel University. Feel free to send email to email@example.com with any questions about the universe.
Top image of the Crab Nebula via NASA, ESA and Allison Loll/Jeff Hester (Arizona State University). Acknowledgement: Davide De Martin (ESA/Hubble). Illustration of a spaceship with a neutron star by Rick Sternbach.