[HN Gopher] Physicists observationally confirm Hawking's black h...
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Physicists observationally confirm Hawking's black hole theorem for
first time
Author : _Microft
Score : 204 points
Date : 2021-07-01 06:23 UTC (16 hours ago)
(HTM) web link (news.mit.edu)
(TXT) w3m dump (news.mit.edu)
| ordu wrote:
| _> There are certain rules that even the most extreme objects in
| the universe must obey._
|
| A strange way to put it. The more object is extreme the harder it
| would be for it to disobey laws. If we try to imagine what forces
| are involved, all we'll find is that our imagination has it's
| limits. I'd be less surprised if some quark disobeyed laws,
| because it small, forces are minuscule and... who is to notice?
| Maybe they disobey laws all the time, just scientists fail to
| catch them red handed.
| justinboogaard wrote:
| I don't think the quote is saying objects have to obey laws
| because they are extreme (and therefore easy to notice) I think
| it's saying all objects have to follow the established rules of
| physics, regardless of how unusual they are. It's an
| interesting idea though! Known, "extreme" objects may be less
| fruitful places to study unknown forces because they have
| qualities that overwhelming drown out smaller less known
| forces.
| lennoff wrote:
| So black holes can't evaporate? How does Hawking radiation works
| if the back hole are has to stay the same?
| SiempreViernes wrote:
| I think it's more a case of the press release people wanting to
| call it a unqualified "law", as far as we know black holes do
| evaporate slowly.
|
| But there remains is a statement about how the final area
| relates to the area of the two merging black holes.
| jfengel wrote:
| Black holes can't evaporate now because the cosmic background
| radiation is too hot. The black holes are colder the CMBR, so
| they absorb heat and grow (albeit very, very slightly).
|
| Eventually the CMBR will cool down and the holes will be able
| to evaporate, but not for an insanely long time.
| __MatrixMan__ wrote:
| I imagine this holds only for black holes that are massive
| enough to be stable?
|
| All those itsy bitsy ones created by the LHC, they've
| evaporated, yes?
| wruza wrote:
| I believe that LHC has no chance ever to harvest enough
| energy to create a sensible-sized black hole. It's still
| mc-squared (give or take an order of magnitude and my
| layman mistakes), so 1g BH takes about 1e14 joules or 5
| minutes of average EU electricity output. Also, there is no
| sea nearby to cool it off afterwards. Also, a planck-sized
| BH weighs 1e-5 g.
|
| _A mass similar to Mount Everest[13][note 1] has a
| Schwarzschild radius much smaller than a nanometre.[note 2]
| Its average density at that size would be so high that no
| known mechanism could form such extremely compact objects_
|
| It seems that at energies available to us they are
| basically either virtual or non-existent. This contradicts
| the common notion that cosmic rays create microbhs
| occasionally, but I guess we have to wait for a physicist
| to clarify this.
| jfengel wrote:
| In theory, it might have made a black hole. It would have
| lasted a ridiculously small period of time, but be quite
| recognizable by the energy it gave off. Instead of the
| usual decay patterns, it would give off a spray just of
| photons, like a black body at a recognizable (and very
| high) temperature.
|
| We didn't see that, and in fact theory predicted that it
| was insanely unlikely that we would. But there's nothing
| wrong with the possibility of a black hole much, much,
| much smaller than a gram, with a radius smaller than the
| Planck length.
|
| If we had seen it, it would have been insanely
| informative. But it wasn't ever gonna happen.
| morei wrote:
| Do you know what theory that might be?
|
| The difficulty in producing a black hole is getting the
| energy density high enough. We have no known mechanism to
| get an energy density that's even close the right order
| of magnitude.
|
| Maybe you meant that in theory quantum fluctuations might
| do it? Unfortunately, this is really a non-answer. The
| probability is so ridiculously low that it's not
| practicably distinguishable from zero. (It's _vastly_
| more likely that every measurement ever taken and that
| _will_ be ever taken is wrong, than that the event
| actually happened).
| jfengel wrote:
| Correct. It's not so much that the small ones are unstable,
| but just that there's a continuous curve of lifetimes
| that's a function of mass.
|
| For the LHC, the lifetime of a black hole it could
| conceivably create would be 10^-86 seconds. It didn't even
| do that, but if it had, it would have evaporated before it
| moved the diameter of an electron. There's no functional
| difference between that black hole and a vastly bigger one
| besides the mass... but it's a difference of many, many,
| many orders of magnitude.
| edem wrote:
| They _do_ evaporate, but they also absorb CMB, and right now
| CMB > evaporation. Later, when CMB fully dissipated they can
| evaporate in practice.
| kstrauser wrote:
| > Later, when CMB fully dissipated
|
| That's one of the most understated uses of "later" I've
| heard.
| eximius wrote:
| Alright, I'm confused. How does this square with Hawking
| radiation? How can a black hole shrink without shrinking?
|
| The article mentions both in the context that they are reconciled
| but not how they are reconciled.
| edem wrote:
| If you drain your bathwater while also running the bath you can
| have a bathtub that's slowly filling up.
| morebortplates wrote:
| Yes, black holes shrink because of Hawking radiation, but in
| reality this doesn't really happen because black holes are much
| colder than their surrounding space. Actually they are the
| coldest objects in nature. Stellar black holes have a
| temperature of a few Nanokelvins and the average temperature of
| space is 2.7 K so there's a net gain of energy/mass from
| absorbed photons from CMB radiation vs emitted photons via
| Hawking radiation. In order to have a higher temperature than
| the CMB a black hole would have to be really small with a mass
| about half of that of the moon.
| phkahler wrote:
| From the outside they must look cold since they can't radiate
| heat any more than light. That doesnt imply anything about
| the inside.
| ttul wrote:
| The inside is empty space.
| zaarn wrote:
| The inside is empty time, space waved you a goodbye at
| the event horizon.
| morebortplates wrote:
| What's "inside" of a black hole, meaning behind the event
| horizon is forever causally cut off from our universe. The
| Hawking radiation comes from the space around the event
| horizon. In theory black holes can become very hot if their
| mass is small. This happens at the end of their life which
| is in the order of 10^80 years for stellar black holes.
|
| Here is a calculator to play with some values.
| https://www.vttoth.com/CMS/physics-notes/311-hawking-
| radiati...
| jfengel wrote:
| Note that this evaporation time assumes a universe at
| absolute zero. That 10^80 years can't even begin until
| the CMBR cools enough, perhaps 10^40 years.
|
| Admittedly that's an eyeblink compared to the evaporation
| time scale, but it does mean that we won't observe any
| evaporation until many orders of magnitude longer than
| the universe has existed.
| db48x wrote:
| Or unless you manufacture or discover a low-mass black
| hole
| jfengel wrote:
| That's right. If we could create one in a supercollider,
| it would would be so small that it would be hot enough to
| evaporate instantly.
|
| There might also be a range of primordial black holes
| formed directly out of pre-CMBR energy. They'd have to be
| small enough to be hotter than the CMBR, but not so hot
| that they'd already have evaporated in the last 14
| billion years. That's a relatively narrow range, all
| things considered, but if primordial black holes exist at
| all then they could exist at any range.
| edem wrote:
| It _will_ happen, but first dark energy needs to be strong
| enough to expand the universe fast enough (faster than light)
| so that CMB wouldn't be able to reach anything.
| morebortplates wrote:
| Well there are already portions of space that are expanding
| faster than the speed of light relative to our position.
| (see cosmic horizon). The CMB is not just a glowing heat
| somewhere far away, it's everywhere in the universe in
| every volume of space. The moment when every point in space
| (on a Planck-lenght-scale i guess) will be expanding faster
| than the speed of light relative to one another, than
| space-time itself will rip apart and that's the end of our
| universe - at least that is what the Big-Rip theory
| proposes.
| eru wrote:
| > In order to have a higher temperature than the CMB a black
| hole would have to be really small with a mass about half of
| that of the moon.
|
| Either that, or you just wait a couple eternities for the CMB
| to cool down enough.
| morebortplates wrote:
| Or primordial black holes with such a small mass actually
| do exist, who knows...
| sandworm101 wrote:
| Or you make a smaller one and watch it shrink. It is
| theoretically possible to construct a smaller black hole by
| cramming the necessary mass/energy into a small enough
| space. (Plug a death star into the LHC's big brother.) Such
| a hole would be very hot and short-lived.
| Yajirobe wrote:
| how can the temperature of a black hole make sense? Is it the
| temperature of the singularity point? Is it the temperature
| of the space inside the event horizon (but it can't be, as
| that space is empty)?
| lagadu wrote:
| My understanding is it's the temperature of the event
| horizon: because it doesn't emit any blackbody radiation,
| other than the tiny amount from Hawking radiation, any heat
| measurement from it would be approximately 0k.
| wyager wrote:
| Temperature has several definitions that produce the same
| number under normal circumstances but may or may not be
| applicable in extreme circumstances. In this case, I
| imagine they could be using the thermodynamic definition
| (dE/dS, the marginal change in energy per marginal change
| in entropy - I'm not sure if a BH has well-defined entropy)
| or something to do with the emission curve of space around
| the black hole. I vaguely recall something about empty
| space behaving like a blackbody under a gravitational
| gradient, so maybe they can use that. You could also use
| amount of Hawking radiation per surface area. It's possible
| that some of those produce the same number.
| colechristensen wrote:
| it is a phenomenon at the event horizon at with radiation
| is emitted in a way comparable to the radiation produced by
| every object according to temperature
| Filligree wrote:
| "Temperature" in this case refers to the amount of energy
| they emit due to hawking radiation.
|
| By comparing to a black-body curve, you can define a
| temperature for the hole. Obviously it's not a real object
| with a real temperature -- if it were, I believe the
| temperature might be infinite -- but this still works for
| the purposes of deciding whether it'll grow or shrink.
| streamofdigits wrote:
| This might help
| https://physics.stackexchange.com/questions/169886/black-hol...
|
| A black hole merger of this size is unlikely to have any
| significant quantum aspect
| jleahy wrote:
| They're not reconciled, it's just garbage reporting.
|
| The 'area theorem' they are referring to was by Bekenstein and
| others, not Hawking. It's basically the equivalent of the
| second law of thermodynamics for black holes (dA/dt>=0 instead
| of dS/dt>=0). Hawking's insight was that this formula was wrong
| and the area could decrease due to radiation.
| untoxicness wrote:
| While I agree that the article should have mentioned black
| hole evaporation, I would like to point out that "dA/dt > 0"
| is commonly referred to as "Hawking's area theorem" as a
| quick online search can verify and Stephen Hawking certainly
| did publish on this topic.
| aliasEli wrote:
| It is a pity about the reporting.
|
| But it is a fascinating area of research. I never expected
| that we could measure gravitational waves in our life time.
| Cthulhu_ wrote:
| I'm amazed - to the point of skepticism - that with such
| minute forces they can extrapolate so much information and
| prove theories.
|
| I mean I'm no astrophysicist, I like the "pop sci" bits,
| but when I look closer I'm seeing a lot of small numbers
| and statistics that imply something - e.g. exoplanets based
| on minute wobbles and brightness variations, water on said
| exoplanets based on spectrography. It's theories based on
| tiny but statistically significant data.
|
| The pop sci then comes in and makes statements like "second
| Earth found!11", which is like, whoa hold on, when you look
| closer all they found is a wobble or dimness variation that
| kind of implies there might be a planet at a certain
| distance from its host star.
|
| Anyway I don't dispute the findings or that there are
| exoplanets or whatever, I'm just impressed that they are
| able to make confident claims on what little information we
| can receive from here.
| phkahler wrote:
| Sometimes I think the claims are overstating things, but
| since nobody can use the claim to actually do anything it
| gets a pass.
| brabel wrote:
| > e.g. exoplanets based on minute wobbles and brightness
| variations, water on said exoplanets based on
| spectrography.
|
| The minute wobbles may be tiny, but they can plot a curve
| of those wobbles and see very clearly that it changes in
| a certain way that can only be caused by a planet (or
| something spherical and with a certain mass). If there
| was a competing theory of how you can get this exact
| curve in some other way, I am sure we would consider them
| as alternative possibilities and not be able to tell them
| apart, but as far as I know, there isn't any competing
| theories at all... so we can have very high confidence
| there's a planet there.
|
| Regarding water detection: yeah, spectrography is just
| mind blowing, but again, given what we know, there's just
| nothing that could justify believing that when you detect
| radiation that fit exactly what you would expect from
| water molecules, that it could be something else
| instead... unless you come up with a convincing
| "something else", your only option is to conclude that
| the detection is accurate, otherwise you would need to
| stay open to the possibility of absolutely everything
| possibly having alternative explanations we haven't
| thought of yet (though every now and then, that indeed
| can happen and we need to adjust all our theories that
| are based on the changed body of knowledge), and progress
| would not be possible in any area (you need to accept
| something before you can build on top of it).
| lanerobertlane wrote:
| The problem is the 'pop sci' reporting. "Earth-like" or
| "second earth" could easily be swapped for "Venus-like"
| or "second venus" in 99% of cases where pop-sci uses
| "Earth like" and still be factually correct.
|
| However, Wobbles and Transit photometry are done over
| time and plot trends which definitively show that
| something with a certain mass is orbiting with a certain
| period around the star. There isn't really anything else
| it could be except an exoplanet, unless our understanding
| of how physics works was way off, which we know it isn't.
|
| as for Spectrography brabel sums it up in their comment
| very well.
| nine_k wrote:
| I suppose Venus has a different enough thermal spectrum
| due to surface temperatures being 3x Earth's, roughly
| 900K vs 300K.
|
| Also, Venus must have a big enough sulfur line in the
| atmosphere, which is absent in Earth atmosphere.
|
| Probably Mars and Earth could be considered close, save
| for the oxygen line, but not Venus and Earth,
| eru wrote:
| It's fascinating what's possible.
|
| For example, we know more about the chemical composition
| of other galaxies than we know about the centre of the
| earth. Just because we can infer so much from their light
| spectrum.
|
| (For empirical information about the centre of the earth,
| we are basically limited to seismic data and perhaps the
| magnetic field and bumps in gravity?)
| DSingularity wrote:
| Why not launch a probe and point its sensors at earth?
| nine_k wrote:
| We hardly resolve any individual _stars_ in other
| galaxies (maybe some giant stars inside Andromeda?), so
| our idea of chemical composition of other galaxies is
| very... averaged.
|
| We barely can resolve largest and closest exoplanets into
| a few pixels.
| floxy wrote:
| >We barely can resolve largest and closest exoplanets
| into a few pixels
|
| For now...
|
| https://arxiv.org/abs/1802.08421
|
| https://www.youtube.com/watch?v=NQFqDKRAROI
| db48x wrote:
| The Earth is opaque. I don't have a source to cite for
| this, but I think you can check it easily enough.
| josu wrote:
| It is not just gravitational waves, they look at light,
| with telescopes (visible, infra and ultra),
| spectrographs, interferometers, radio signals, gamma
| rays...
|
| Then they combine all that info to come up with cohesive
| theories. LIGO just by itself would be almost useless.
| meowface wrote:
| >water on said exoplanets based on spectrography
|
| Even though from a theoretical perspective it should be
| _way_ easier and more reasonable to detect particular
| molecules on distant planets via spectroscopy compared
| than to detect things on the mind-blowingly minuscule
| scale of gravitational waves, I think distant
| spectroscopy might actually be more prone to error, or at
| least more prone to false positives.
|
| Just speculating since I have zero expertise in this
| area, but part of it may be because light from all sorts
| of sources is reaching us all the time, while
| gravitational waves significant enough to be feasibly
| detected pretty much only come from the top percentile of
| the most energetic events in the universe.
|
| I think if you can discern a gravitational wave-induced
| spacetime wobble at least once and infer the motion that
| could've caused it (e.g. black holes/neutron stars
| merging) and see it matches theoretical expectations, you
| may continue to have a lot of false negatives, but you
| probably aren't at high risk of future false positives.
|
| Whereas with spectroscopy, there seem to be a lot of
| things that can cause both false positives and false
| negatives even if you do have many prior detections that
| you believe are accurate. For spectroscopy, both error
| rates should go down over time as technology and
| techniques improve, but it seems like it may potentially
| be an inherently more "murky" observation technique, even
| if it's far simpler and far less expensive than
| gravitational wave detection.
|
| (Someone please correct me if I'm wrong about any of
| this, because there's a pretty good chance I am.)
| Florin_Andrei wrote:
| I think what they really meant in the article is that it can
| never decrease through processes other than the radiation.
| E.g., BH mergers and so on, the area always increases.
| rakkhi wrote:
| Exactly what I came here to ask. Is Hawking's theory that the
| area never decreases or that it does not decrease in a black
| hole merger as was tested here. "central law for black holes
| predicts that the area of their event horizons -- the boundary
| beyond which nothing can ever escape -- should never shrink"
| this seems to imply the former to me.
|
| Does that mean with Hawking radiation the black hole
| effectively evaporates by loosing mass (?) from the inside
| without the boundary area never shrinking?
| realYitzi wrote:
| Anyone can think of any practical effect this might have or is
| this something that at the moment seems to have only scientific
| relevance.
| eru wrote:
| Well, if the result had turned out the other way, we might have
| seen some practical effects, because it would overturn some
| well-established theories that we also use for predicting more
| practical things.
| tibbetts wrote:
| I'm waiting for a Greg Egan short story to explain this
| better.
| wyager wrote:
| Looks like he already has one on BH boundaries:
| https://www.gregegan.net/PLANCK/Planck.html
| edem wrote:
| Note that by the time black hole evaporation would be significant
| the CMB will be long gone because of dark energy (universe
| expanding). All this means that evaporation is delayed till there
| is nothing else left apart from black holes (because of the
| faster than light recession of space itself).
| notorandit wrote:
| If you have BH evaporation you cannot keep the horizon surface
| constant.
| Ankaios wrote:
| Here's the paper:
|
| https://journals.aps.org/prl/accepted/36074Y8aM291c462a4e264...
|
| https://arxiv.org/abs/2012.04486
| louloulou wrote:
| Thank you! It drives me crazy that news articles about a paper
| never link to the actual damn paper.
| 867-5309 wrote:
| so they're 95% sure.. how do they even come up with a figure like
| that? they didn't bother saying. might be equivalent to 'give or
| take a few trillion tonnes'
| Cthulhu_ wrote:
| https://www.zmescience.com/science/what-5-sigma-means-042342...
| 95% means 2-sigma, when it comes things like the Higgs boson,
| they announced the results with 5-sigma certainty, which is a
| very good indication of statistical significance and
| confidence.
| wyager wrote:
| It's more that p-value is a _bad_ indication of anything
| (since it's vulnerable to p-hacking and all kinds of other
| issues), so physics just picks a really extreme publication
| threshold to avoid getting inundated with spurious
| developments.
| mhh__ wrote:
| It will say in the paper how they went about the error
| analysis.
|
| These estimates are however, subjective. There is a good paper
| on this called "Bayesian methods in particle physics"
| (something like that).
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