‘Nuclear pasta’ might be the strongest stuff in the known universe
Neutron star innards are not your mom’s lasagna.
This composite image shows RCW 103, the remains of a supernova explosion located about 9,000 light years from Earth, and its central source, in three bands of X-ray light detected by NASA’s Chandra X-ray Observatory. Image credit: NASA
Researchers may have identified the strongest stuff in all of space—and despite what its name suggests, it’s not a carbohydrate.
The results of simulations published last September in the journal Physical Review Letters suggest that in neutron stars, a layer below the outer crust, called “nuclear pasta,” is harder to break than anything in the known universe. It’s stronger than steel—by about a million trillion times.
Neutron stars, remnants of some massive, exploded stars, are incredibly dense. Like Earth, they generally have a liquid core surrounded by an outer crust. They are so dense that at about half a mile deep in the crust, atoms get pushed together so tightly that their nuclei touch. This causes the nuclei to rearrange into long chains and plates, which researchers in 1983 likened to spaghetti and lasagna.
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Just like that, “nuclear pasta” became part of the astrophysics vernacular. (And in case you’re wondering which of the two types of nuclear noodle is stronger, study author and McGill University postdoctoral research fellow Matthew Caplan says lasagna is stronger than spaghetti because of its higher density. Lasagna, in this case, can be thought of as the result of several spaghettis being smushed together side by side, forming a sort of sheet.)
Nuclear pasta’s density, says Carl-Johan Haster, a postdoctoral associate for MIT’s LIGO Lab, results from the way neutron stars are formed. When a star runs out of energy—if it’s big enough—it will become so dense that it begins to compress down on itself. This turns the protons in the nuclei of the star’s atoms to neutrons.
Neutrons “can’t be compressed infinitely at these masses, so then, at some point, they start effectively bunching up on each other,” Haster says. “The thing that is left over, the sphere of bunched up neutrons…then becomes a neutron star.”
This process makes a neutron star—and the pasta huddled beneath its crust—remarkably dense. The pasta clocks in at around a billion tons per teaspoon.
Researchers have been using computer models to study nuclear pasta for years. In the new study’s simulation, the team stretched and squeezed pasta to determine its strength. Because of the sheer number of protons and neutrons involved—3 million in one simulation—this requires 2 million processor hours.
“It would take more than two centuries on a single laptop processor to run that simulation,” Caplan says. “So in order to actually run that simulation in the lifetime of a human, or in the year or two that we did, we used a supercomputer, and we ran on hundreds of processors simultaneously for about a year.”
The immense strength of the pasta suggested by the simulation came as a bit of a surprise to William Newton, an astrophysicist at Texas A&M University-Commerce who was not involved with the study.
“It actually predicts the opposite of what we had generally assumed was going to be the case,” Newton says. “I think most people [familiar with nuclear pasta] would have expected these pasta phases to actually be a lot weaker than the rest of the crust.”
Knowing more about the internal structure of neutron stars could prove useful for the team at the Laser Interferometer Gravitational-Wave Observatory (LIGO), which has famously detected the ripples that travel through the fabric of spacetime, known as gravitational waves. Gravitational waves strong enough for LIGO to detect from across the cosmos can occur as a result of two black holes or two neutron stars colliding and merging. Objects spinning on their own, such as a single neutron star, theoretically also send gravitational waves rippling out. But the waves from a single spinning neutron star are much weaker, and hence harder to detect, than those from a big collision.
But if there were a lump of pasta just a few inches long buried just under the surface of a neutron star—something the authors of the new study call a “mountain”—the gravitational wave signals coming from the star would be much stronger.
“If you have a neutron star on its own, with a little mountain on it…it’s not perfectly spherical,” Haster says. “And anything that’s not perfectly spherical is then, in practice, emitting gravitational waves.”
The mountains, relative to the neutron stars themselves, are tiny. Neutron stars have a diameter of around 15 and a half miles and the mountains on them could only rise up tens of centimeters high—a matter of inches or feet—the paper’s authors suggest. “The strength of the material tells you how big of a pile you can make with it,” Caplan said in an email.
If a neutron star with a large enough mountain is spinning within the range of detection for LIGO, the observatory detectors may be able to do something unprecedented: pick up gravitational wave signals from a lone spinning neutron star without having to rely on a collision of two objects.
Whether LIGO will detect a lumpy neutron star spinning in space remains to be seen. For now, at least, we know there’s one type of noodle, an atomic al dente, that no human could possibly sink their teeth into.
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