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I've read on NASA's page on neutron star that one teaspoonful of that star would weigh over 20 billion tonnes on Earth. If it was somehow possible to bring it to earth would it:

  1. Burn and disappear during Earth atmosphere entry?

  2. Assuming we have 20 billion tonnes of mass occupying the volume of a teaspoon here on Earth, would it fall through the ground under its own weight?

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2 Answers 2

up vote 8 down vote accepted

The reason that the density is so high is because the pressures are so immense. If we somehow teleported a teaspoonful of neutron star material to earth, it would very rapidly inflate because the pressures aren't high enough to crush it into its dense form. This would effectively be an enormous explosion.

It is difficult to describe what it would inflate out into - the neutron star material can be imagined as an incredibly dense soup of neutrons with some protons and leptons in small numbers. The protons and leptons would make neutron-rich elements like deuterium, but most of the matter would consist of free neutrons. These free neutrons would undergo beta decay to produce neutrinos, protons, and electrons, which would likely recombine to make very large amounts of hydrogen, some helium, and a few heavier atoms. In all of these cases, the atoms would be neutron-rich isotopes, though.

The behavior would look most like a very rapidly expanding gas. It would explode with such force that it wouldn't even need to "fall through the ground" - it would obliterate the floor entirely.

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just want to highlight that we don't know this for sure: its possible that after reaching nuclear densities, matter is essentially stable entirely due to nuclear strong force. It is believed that islands of nuclear stability become more strong as $Z$ and $N$ grow above 150. However finding such super-dense pieces of matter will be almost impossible in the surface of the earth, since this will likely sink inmediately until reaching the center of the earth –  lurscher May 19 '11 at 5:50
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@lurscher: If it's all neutrons, it cannot be stable after an appropriate amount of beta decay, which happens enormously quickly as the mass of a bound proton in a proton free environment is much less than a neutron squeezed by other neutrons through the exclusion principle. Once it decays into proton-neutron material, it will fall apart like any charged droplet into nuclei of various sizes. The only way nuclear stuff can be stabilized is if it somehow becomes net-neutral strange matter of some kind, and then it would slowly gobble up normal matter. Such strange matter probably doesn't exist. –  Ron Maimon Nov 5 '12 at 17:43

If we take neutron star material at say a density of $\sim 10^{17}$ kg/m$^{3}$ the neutrons have an internal kinetic energy density of $3 \times 10^{32}$ J/m$^{3}$. So even in a teaspoonful (say 5ml), there is $1.5\times10^{27}$ J of kinetic energy (more than the Sun emits in a second, or a billion or so atom bombs) and this will be released instantaneously. Yes, I know that is just inconceivable, but that's physics for you.

The energy is in the form of around $10^{38}$ neutrons travelling at around 0.1-0.2$c$. So roughly speaking it is like half the neutrons (about 250 million tonnes) travelling at 0.1$c$ ploughing into the Earth. If I have done my Maths right, that is roughly equivalent to a 40km radius near-earth asteroid hitting the Earth at 30 km/s.

So, falling through the Earth is not the issue - vapourising a significant chunk of it is.

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