Okay, so I have here a nice chunk of unobtanium, don’t ask how I got it.
It has the property that it absorbs all the neutrinos that hit it, not allowing any to pass.
How much energy is it absorbing? Does it have a noticeable force pushing on it in any particular direction?
Is this a question that we actually can have an answer to, or would it require better knowledge of the actual mass of neutrinos first?
I don’t have the time to run the math at the moment, but there would definitely be a directionality - there’s a big source of neutrinos in our neighborhood, so your chunk of unobtanium would feel a net force away from the Sun.
Which means your magic material could be used to build a solar sail that works even on the other side of a planet!
I suppose. I was more thinking that it could be used to generate heat, the force on it was actually more of an afterthought.
I would assume that you would also have a force that is against the Earth’s movement against the overall movement of the galaxy, or even of the CMB.
I’m just wondering if it is a useful amount of heat or force, or at levels that we would only be detecting in laboratory conditions.
Not really worth patententing my unobtanium recipe otherwise.
Don’t neutrinos pass through things without any interaction?
Yes, ordinary matter they pass through with very, very little interaction. I think the usual reference is that they can pass through a light year of lead and only have about a 50/50 chance of interacting.
But not my unobtanium, it absorbs them all. (Note, I do not actually have any of this substance, this is meant as a thought experiment.)
I think that higher energy neutrinos interact more with ordinary matter btw. It’s just that the Sun generally puts out neutrinos in an energy range too low for significant interaction.
From wikipedia
The highest flux of solar neutrinos come directly from the proton-proton interaction, and have a low energy, up to 400 keV. There are also several other significant production mechanisms, with energies up to 18 MeV. From the Earth, the amount of neutrino flux at Earth is around 7·10^10particles·cm−2·s −1The number of neutrinos can be predicted with great confidence by the Standard Solar Model. However, the number of electron neutrinos detected on Earth was only 1/3 of the predicted number, and this was known as the “solar neutrino problem”.
So if we assume 7x10^10 particles/sec at 400keV that gives around 3x10^16 ev or 0.048 Watts per square centimeter. So a 1mx1m unobtanium solar panel will pick up about 480 Watts worth of neutrinos, so a similar order of magnitude to a standard solar panel. The momentum on the other hand is going to be pretty negligible. Given that neutrinos are near massless and travel at nearly the speed of light, the relationship between Neturino energy and momentum with be similar to that of a photon so momentum=energy/c. Resulting in around 1.6x10^-5 kg m/(sec^2) for each square meter of unobtanium.
Whereas, for comparison, the measured Total Solar Irradiance (TSI) for the Sun’s electromagnetic output (full-spectrum, measured above the atmosphere) is 1,665 Watts/m2.
Similar to a photon implies a neutrino sail should still work though, right? Or are there way fewer neutrinos than photos so the momentum affect fails for that reason?
I think this answers my question in the next post actually- it implies the neutrinos would offer about 1/4th of the momentum of photons?
Photons bounce off a solar sail, but neutrinos would be absorbed, so another factor of two decrease in effectiveness.
Ah, good point - you’d want unobtonuium that reflects neutrinos rather than absorbing them for a sail.
Yep. Fortunately, Ultraunobtainium ™ is reflective and only a bit more expensive.
Thanks, that’s exactly the information I was looking for!
But I’m not sure on the math, now that I think about it. That would mean that nearly a third of fusion energy is lost to neutrinos.
Checking on it, I think you misplaced a decimal, where 3x10^16 ev should be 0.0048 Watts.
Or did I miss something there?
Hmmm, according to this: Neutrinos from the Sun ,
I suppose if I’d gone that route in my looking for this, I would have found the answer, still the journey has been very informative all around.
So, my unobtanium is junk, only generating 48 watts per square meter, I suppose I’ll just toss it then. Anyone want it as a doorstop?
Although… In looking deeper into things, now that I have found a better angle on looking, I find that the cosmic neutrino background has 9 or so orders of magnitude more neutrinos in it than solar, but they are at much, much lower energy level.
Since these neutrinos are so cold, colder than the CMB, would that mean that they would actually be sapping heat out of my unobtanium sample, or would the motion of the Earth against the Cosmic Neutrino Background produce any significant heat?
So you can use it for cooling beer. Neat!
The discrepancy might be that I assumed that each neutrino had 400 kev, when the article I got it from said “up to 400kev” so the numbers I presented may have been overly optimistic.
The CNB won’t be sapping energy out of your unobtainium; it’d just be adding only a very small amount of energy.
But your unobtainium will still be cooling down.
The catch is that it’ll be emitting neutrino blackbody radiation, and nothing else near it is going to be impeding the loss of that radiation in any meaningful way. This isn’t a significant source of cooling for ordinary matter, the kind that is utterly transparent to neutrinos, but as you absorb, so shall you emit: Even unobtainium must follow the laws of thermodynamics, and so it’ll emit neutrinos just as it absorbs them. It’ll come to an equilibrium temperature significantly below that of its surroundings. Which might in fact be quite practical.
Somebody should write a book with those!
Chronos already covered this slightly, but … The neutrinos are not cold, neither is the CMB, they are both “energetic”. The CMB is “cold” in the sense that it has the spectrum of radiation from a very cold blackbody, so any object that is warmer will be losing a lot more energy by its own blackbody radiation than it receives from the universe.