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Comment by mapt | Hacker News Reader

Notably, the possibilities are quasi-finite because of the solar neutrino noise floor. When we reach that, we stop needing to scale to 10x larger detectors for a 90% reduction in reaction cross section, and they start to need to be 100x larger, in my understanding.

Even in a world with WIMPs, we aren't necessarily privileged to have them show up above the solar neutrino noise floor.

It's going to be funny if the reason we get a mission to Neptune's moon Triton a century from now is to house a dark matter search experiment, once the cost for continuing to scale Earth's search becomes dramatically worse.

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Is it the solar neutrino noise floor, or distant supernova neutrino noise floor?

Maybe the justification for interstellar colonization will be so we can run a DM experiment on the near-relativistic ship during the trip. Boost those dark matter particles to some good fraction of c for easier detection.

The "floor" is due to solar neutrinos - the sun is a continuous source of neutrinos and is located much closer to us than virtually every past supernova was. It is also a soft floor, as there are analysis tricks that can be played to distinguish between interactions from neutrinos and WIMPs at the statistical level. For this reason, the current fashion is to refer to a "neutrino fog," which can be entered into to some depth, as opposed to a "neutrino floor."

It's actually due to both (for different DM particle masses).

https://physics.aps.org/assets/0ce5ba95-0367-46ce-890b-de3a4...

(which is from https://physics.aps.org/articles/v14/96)

This is absolutely correct, as there are no solar neutrinos with energy above ~20 MeV (but below that, the solars dominate). Thanks for the clarification.

The interesting thing is the physical process creating the floor is a fairly novel one, coherent scattering of neutrinos off nuclei. This is where the neutrino scatters off all the nucleons together, with the contributions adding coherently, rather than incoherent scattering off individual nucleons.

This process was first observed only fairly recently, at the Spallation Neutron Source using neutrinos from stopped pions. IIRC, they realized a service corridor under the target room would be unintentionally suitable for the experiment.

https://coherent.ornl.gov/

Considering time dilation, a detector inside a relativistic spacecraft will be hit by a lot of neutrinos in the apparent short duration of the mission.

I wonder what is the speed at which interstellar neutrino bombardment becomes lethal to a crew considering it’s essentially impossible to shield against them. You can put a mountain in front of the ship, but neutrinos will just pass through it like is a cloud.

OTOH if the ship is so fast neutrinos are a problem to the crew, that mountain will be suffering some intense ablation from everything including neutrinos that’ll hit it.

Neutrinos already move at relativistic speeds, so they aren't much of a problem.

The real possible problem would be massive dark matter particles. These would be traveling now at around 300 km/s, so a relativistic spacecraft would boost their speeds by three orders of magnitude and their energies by six orders of magnitude or higher. And, of course, the number passing through a given area in the vehicle per unit of time would also be orders of magnitude higher. If the interaction cross section is high enough (and not so high they could be shielded) then travel at that speed becomes impossible.

I wasn’t thinking of the speed of the neutrinos, but the increase in encounter frequency. From a passenger’s perspective, the number of interactions per second will be much higher, not just because the ship is covering large distances and hitting more particles (although I’m not sure about that - it might cancel out) but hitting them more frequently from the crew’s perspective because of time dilation.

No large amount of time dilation is likely to be practical, even with laser propelled light sails or antimatter. And the heat load on the vehicle from running into interstellar matter would be very high.

The more massive DM particle issue I discussed might be problematic even with very little time dilation.

Whatever tech is used, it'll need to be something really fancy (as in new, fancy laws of physics being bent in fancy ways) to make near light-speed travel feasible.

Even Alcubierre-White warp drives would create interesting problems because compressing spacetime in front of the ship would heat up whatever matter is there.

Unless something miraculous happens, I don't think we'll live to see these problems in real life, but, if we do, it'll be fun to work in solving them.

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