Stupid question, posed but apparently not answered in TFA: What if a small black hole approached a person, or even came into contact? Would we see it as, say, someone's arm suddenly falling off for no apparent reason? Or would the size of the black hole be so infinitesimal that for practical purposes it wouldn't matter?
FTA: "'I think someone asked me what would happen if a primordial black hole passed through a human body,' recalls Tung, who did a quick pencil-and-paper calculation to find that if such a black hole zinged within 1 meter of a person, the force of the black hole would push the person 6 meters, or about 20 feet away in a single second. Tung also found that the odds were astronomically unlikely that a primordial black hole would pass anywhere near a person on Earth."
Wouldn’t a black hole that small evaporate pretty quickly?
Iirc a black hole with the mass of an asteroid (whose event horizon would be very small indeed) would have a hawking temperature smaller than the cosmic microwave radiation so there wouldn't be any net evaporation.
Hmm what mass of black hole is needed to rip your arm off? Probably more than an asteroid I’m guessing?
Probably a fairly significant multiple of earth's mass. What multiple of your arm's weight would be required to pull it off? (and tidal forces are generally less than the absolute gravitational pull)
I asked o1-preview a couple times, it gave me:
*Answer:*
Approximately 27 trillion kilograms—the black hole would need about 2.7 × 10¹³ kg of mass to exert tidal forces strong enough at 1 meter to tear off an arm. - This radius is much smaller than an atomic nucleus, so at a distance of 1 meter, the person is far outside the event horizon.
No, the black hole with a mass of approximately \( 2.7 \times 10^{13} \) kilograms would not evaporate quickly due to Hawking radiation. Even though this mass is relatively small compared to stellar black holes, such a black hole would have an extremely long evaporation time—*on the order of \( 5 \times 10^{16} \) years, which is about 50 quadrillion years*. This duration is vastly longer than the current age of the universe, which is approximately \( 13.8 \times 10^{9} \) years (13.8 billion years).
The heavier the black hole, the lower the hawking temperature.
Only the tiniest black holes would have hawking temperature high enough to evaporate given the presence of the CMB
Here's another answer to the Fermi Paradox, that while life is normal and abundant, there are lots of random possibilities for destruction such as a primordial black hole coming by.
The apparent rarity of intelligent life might be a statistical matter of there being lots of opportunities for such life to arise, but also lots of hazards of random destruction.
... on reflection, while this might be intellectually satisfying, it's not particularly nice to contemplate.
That sounds like a variation of the Great Filter[0]
Would a primordial black hole have a small accretion disk? If so, it seems like that disk would give off radiation.
Probably not. Accretion disks are temporary, they only exist for a few thousands/millions of years after a capture event. Something has to get really close, and the smaller your object is, the more unlikely it is for something to come close enough to be consumed.
Plus the rate at which a black hole can consume mass is proportional to mass/radius. If it's too small, it can only consume matter very slowly, which means the accretion disk would be cold and quiet.
so wouldn't small black holes since the dawn of the universe be very fast by now? 13 billion years of bouncing past particles must add up.
"Dark matter" sounds a lot like "we've run out of ideas so we're going back in the direction of 'the ether' and chariots pulling the sun across the sky and the like".
Dark matter is an observed discrepancy between measurements. There appears to be a lot of unaccounted-for mass, based on many experimental results. There are several theories as to what that observed discrepancy could be caused by. This is a well-covered topic, anywhere you could hope to learn about astrophysics. What it may sound like to you is not what it is.
I'd recommend watching a video from PBS Spacetime if you want to learn about it. They have several.
The original dark matter turned out to be Neptune. The second dark matter was planet Vulcan (which turned out to not exist)
Dark matter has a roughly 50/50 track record.
Sure. Just like ether. They will sort it out, eventually.
https://www.earth.com/news/dark-matter-does-not-exist-univer...
Also no WIMPs were found so far.
https://www.newscientist.com/article/2445058-another-blow-fo...
Now we're doing Mars shots to prove it.
Discussion of your first link:
https://news.ycombinator.com/item?id=39726388
I'm not qualified to judge the merit of that theory, really... but my impression is that it doesn't have the explanatory power to be a great candidate.
I guess we shall see. There's also MOND.
Yeah I just feel like they're talking about a problem they've been unable to solve, rather than a solution.
Einstein and Planck just solved stuff, with a fraction of the talk and resources.
Now Physics seems to be so stagnant it's mostly an exercise in making descriptions for the things it's failing at, as though a problem can somehow match the value of a solution if you give it a cool name and talk about it enough.
> Einstein and Planck just solved stuff,
Their ideas, solutions, and problems didn't come about from magic.
> Yeah I just feel like they're talking about a problem they've been unable to solve, rather than a solution.
That's right. There is not yet a definitive answer on what dark matter is, just several theories. WIMPs, MOND, primordial black holes...
> Now Physics seems to be so stagnant
Crapping on modern physics for a lack of progress seems a popular thing amongst us lay people, but despite all the credit you give them (and that they deserve,) I doubt Einstein or Planck would have just intuited the right solution to dark matter.
Shitting on all of science ever is easy. Actually doing it is hard.
No not all of science, just science since 1990-ish
There's been longer periods of worse stagnation in science.
Galaxies don't behave how they should with our understanding of gravitational mechanics. The only thing that fixes the model is the addition of a lot of mass that we can't see.
That would be easy enough to explain away as an incomplete model, except we have observed galaxies which do fit our models. The only way to explain that is if those galaxies are missing this invisible mass. There's no other way to reconcile these observations.
How would you square these observations? Under our models of gravity, most galaxies behave as though they contain much more mass than what we observe. However, some behave exactly as Newton would predict. The misbehaving galaxies can be made to fit the model by adding mass in very particular locations, mainly as a halo or shell around the outside.
We can show how much mass and where it must be to make galaxies behave as expected. These calculations are the same for most galaxies with dark matter, and galaxies without dark matter require no correction. At this point we have two explanations: either gravity behaves radically differently in different places in the universe, or there's a new type of massive particle that we haven't discovered.
We know already that the standard model of particle physics is incomplete, and there are more particles to be discovered. We are also very confident that the laws of physics are the same throughout space and time, and we are very confident that our models of gravity are correct on large scales.
The simplest explanation that fits all available data is dark matter. Positing that there's a new type of massive particle that we haven't discovered isn't really a shocking revelation.
With our calculations. Which we know use wrong distance, don’t take spin direction into account, and use newton instead of Einstein.
And use things like pi=3=1 and a whole bunch of other simplifications.
It’s a miracle it predicts anything at all.
Uh, no. None of that explains the fact that the large structure of the galaxies we observe is not possible without extra mass. This has nothing to do with observational error or rounded calculations. The shape of the galaxies and their internal structures are clearly wrong.
Besides, this also ignores the fact that we observe galaxies both with and without dark matter. These galaixes have very different structures that cannot arise from the same physics unless we consider dark matter.
I'm sure armies of physicists, astronomers, and generally smart people have not considered the most basic simple possible explanations for why observations of some galaxies do not match our models.
They have. Distance calculations are off, they know it, and trying to make it more accurate. They can’t use Einstein because it’s too complicated so they use approximations, and they know it.
If the basic assumptions of your calculations are wrong it’s not surprising if your conclusions are as well.
Here a link for one of the simple explanations they didn’t take into account leading to wrong redshift measurements https://www.mdpi.com/2571-712X/7/3/41
There are a bunch of good reasons to believe there really is matter there that we can't detect aside from its gravity. Here's a talk: <https://www.youtube.com/watch?v=h8KV9Ht5o_M>.
Yeah. What if space/time is just noisy?!?
That "noise" would be called dark energy until we understand its source.
Must be from the Gods on Olympos QTing around.