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on Gopher (inofficial)
URI Visit Hacker News on the Web
COMMENT PAGE FOR:
URI Dead Stars Donât Radiate
deepsun wrote 1 hour 9 min ago:
Should it be a big embarrassment for Phys. Rev. Lett., a big dip in
their reputation?
The whole point of respectable journals is that they filter out bad
quality papers.
qnleigh wrote 1 hour 16 min ago:
Is there a simple way to understand why massive objects don't radiate
gravitationally? Accelerating observers see a bath of thermal radiation
via something called the Unruh effect. If you're standing on a planet,
you're accelerating under gravity, and therefore don't you see Unruh
radiation? Does this have any connection to Hawking radiation?
dataflow wrote 50 min ago:
> If you're standing on a planet, you're accelerating under gravity,
and therefore don't you see Unruh radiation?
Layman here, but if you're standing, you're not actually
accelerating, right? You'd only be accelerating if there was nothing
under you holding you up, meaning if you were falling down.
nazgul17 wrote 28 min ago:
Also a layman. But as long as your temperature is not absolute
zero, particles inside you are moving, and if they have mass, they
would indeed radiate gravitationally - until they slow down to a
stop, that being absolute zero.
My understanding from pop science videos is that they can indeed
evaporate, but only through decay mediated by the weak force.
qnleigh wrote 28 min ago:
Ah yeah there are multiple definitions of 'acceleration' here.
Unruh radiation occurs when you're not 'in an inertial reference
frame,' loosely meaning that you feel acceleration. So in a rocket
in space or (presumably) standing on Earth's surface.
What you say makes intuitive sense, but it was actually the
opposite logic that lead Einstein to his general theory of
relativity. Here's a slightly dorky but very good Veritasium video
that explains this issue and general relativity
URI [1]: https://youtu.be/XRr1kaXKBsU?si=1iudoAx5kWgWHHt-
m3kw9 wrote 1 hour 44 min ago:
Anyone that predicts an event that far out in the future let alone 100
years out I would bet against any day of the week. This is couple
trillion of trillions years using physics no way of proving
JohnMakin wrote 1 hour 48 min ago:
Good news for boltzmann brains
m3kw9 wrote 1 hour 43 min ago:
One can argue there are 8billion Boltzmann brains on earth already
globnomulous wrote 3 hours 16 min ago:
This is partly why I roll my eyes when people who don't do research in
a field start telling me about the "studies [they] found" while
researching a topic. Unless you know the field and the research methods
and have actually practiced them, reading studies is pointless, because
you're too ignorant to evaluate them.
NKosmatos wrote 4 hours 27 min ago:
Ah yes, our favorite HN âentertainmentâ. Scientists, quantum
physicists in our case, having a beef about Hawking radiation :-)
Besides some high level ideas, which even us normal people can
understand, there are so many details linked in the original post that
you need an MSc/PhD to fully understand them.
For the time being, letâs just keep that the universe has a few extra
trillion years, and isnât expected to decay in 10â·â¸ years ;-)
w10-1 wrote 5 hours 14 min ago:
Ok, we all understand the ancient problem and its current
manifestation.
But what can be done? Science is not supposed to be the realm of
disinformation, but it seems to have no real defenses. People are
being paid to lie, no one is being paid to say they are liars, and from
the outside scientific dispute looks a lot like politics, so scientists
lose credibility by association.
That's a real problem.
khanan wrote 7 hours 27 min ago:
Let's see what Neil deGrasse Tyson says about this.
A_D_E_P_T wrote 7 hours 34 min ago:
lol, I wrote a very similar comment here a few days ago: [1] It's true,
that paper is nonsense. There's not really much else to say. Preprint
servers sometimes publish the sort of stuff that wouldn't pass peer
review. (Remember that S.Korean "superconductor" from about two years
ago!?) The press should be cautious when writing about it.
URI [1]: https://news.ycombinator.com/item?id=43964524
disentanglement wrote 7 hours 0 min ago:
Although that paper even made it to PRL. I guess I should have
written up some similar nonsense and sent it to PRL, might have
improved my career chances.
layer8 wrote 7 hours 40 min ago:
This detail caught my eye:
> [in their 1975 paper] Ashtekar and Magnon also assume that spacetime
is globally hyperbolic
Isnât the modern assumption that spacetime is globally flat?
senderista wrote 6 hours 8 min ago:
The term refers to causal structure:
URI [1]: https://en.wikipedia.org/wiki/Globally_hyperbolic_manifold
thayne wrote 7 hours 50 min ago:
The title is... odd.
White dwarfs and neutron stars are generally considered "dead stars",
since they no longer have active fusion processes. But they do radiate
from energy left over from the star's "death". (Mostly thermal energy
for a white dwarf, for neutron stars there is also a lot in angular
momentum and the spinning magnetic field.) In theory, they will
eventually radiate all of their energy away and become black dwarfs or
cold neutron stars, but IIRC, that would take longer than the current
lifetime of the universe.
GuB-42 wrote 6 hours 47 min ago:
I second that. A more accurate title would be "Only black holes emit
Hawking radiation".
AFAIK everything above above absolute zero radiates, which
effectively means that everything radiates. Black holes would be an
exception if it wasn't for Hawking radiation.
In addition, (stellar) black holes are dead stars. Or at least,
that's one way to see them.
mlhpdx wrote 8 hours 7 min ago:
> As Mark Twain said, âA lie can travel around the world and back
again while the truth is lacing up its boots.â Actually he probably
didnât say thatâbut everyone keeps saying he did, illustrating the
point perfectly.
Well played.
deepsun wrote 1 hour 13 min ago:
"As Mark Twain famously never said" (c)
BlueTemplar wrote 8 hours 21 min ago:
> As Mark Twain said, âA lie can travel around the world and back
again while the truth is lacing up its boots.â Actually he probably
didnât say thatâbut everyone keeps saying he did, illustrating the
point perfectly.
It was Gandalf who said that of course. And before you try to
contradict me, let me point out that Gandalf is a wizard that has no
need to bother with silly things like spacetime continuity.
P.S.: [1] > In conclusion, there exists a family of expressions
contrasting the dissemination of lies and truths, and these adages have
been evolving for more than 300 years. Jonathan Swift can properly be
credited with the statement he wrote in 1710 [(that does not mention
footwear yet)].
URI [1]: https://quoteinvestigator.com/2014/07/13/truth/
cubefox wrote 8 hours 57 min ago:
HN discussion at the time:
Universe expected to decay in 10â·â¸ years, much sooner than
previously thought (phys.org) [1] 223 points, 5 days ago, 323 comments
URI [1]: https://news.ycombinator.com/item?id=43961226
quantadev wrote 9 hours 8 min ago:
In black holes we have essentially a "loss of a dimension" (it's a much
bigger story to explain what that even means, that I won't attempt
here), so it might be the case that the three-quark arrangement known
as 'baryons' only forms according to number of space dimensions (3D ==
3 Quarks), making baryons only happen in 3D, so that when stuff reaches
an event horizon, the quarks rip apart and rearrange into something
where there's simply no such thing as a baryon (i.e. in 2D space). I'm
someone who thinks the 'surface' of an event horizon is where the laws
are preserved, and that the singularity or even perhaps the entire
interior inside black holes may simply not exist at all.
Much of where Relativity "breaks" spacetime (i.e. problems with
infinities and divide-by-zero) can be solved by looking at things as a
loss of a dimension. For example, length contraction is compressing out
a dimension (at light speed), and also time dilation (at event
horizons, or light speed) is a removal of a dimension as well. Yes,
this is similar to Holographic Principle, if you're noticing that. In
my view even Lorentz equation itself is an expression of how you can
smoothly transform an N-Dimensional space down to an (N-1)-Dimensional
space, which happens on an exponential-like curve where the asymptote
is reached right when the dimension is "lost". I think "time" always
seems like a special dimension, no matter what dimensionality you're
in, because it's the 'next one up' or 'next one down' in this hierarchy
of dimensionality in spaces. This is the exact reason 'time' in the
Minkowski Space distance formula must be assigned the opposite sign
(+/-) from the other dimensions, and holds true regardless of whether
you assume time to be positive v.s. negative (i.e. called Metric
Signature). This of course implies our entire 4D universe is itself a
space embedded in a larger space, and technically it's also an "event
horizon" from the perspective of higher dimensions.
nabla9 wrote 8 hours 19 min ago:
> I'm someone who thinks the 'surface' of an event horizon is where
the laws are preserved,
I don't think this is a good way to think it. If black hole is big
enough, there is nothing strange happening in the event horizon, no
significant length contraction, nothing.
quantadev wrote 8 hours 16 min ago:
Some "infinities" of singularity are at the center sure, but all
the maximal Relativistic effects are at the EH surface. It's even
proven that the entropy (informational content roughly) is equal to
the EH area divided by the number of planc-length square areas, as
the amount of quantum arrangements of information that are allowed
"inside". That is a HUGE hint everything's remaining on the
surface.
For example, when you see a clock fall into a BH you see it stop
ticking at the EH, not at the center. It's a common misconception
that everything about them is at the center, but everything
interesting is at the surface.
BlueTemplar wrote 8 hours 28 min ago:
> I'm someone who thinks the 'surface' of an event horizon is where
the laws are preserved, and that the singularity or even perhaps the
entire interior inside black holes may simply not exist at all.
Sounds tempting, but then what happens at the transition : when a
sphere of matter gets just a little bit too dense ?
quantadev wrote 8 hours 19 min ago:
It's just like the Lorentz Tranform or any other of the laws of
Relativity. Things can get very massive and/or time can slow way
down, but ultimately there's not a "problem" (i.e. mathematical
failure requiring the theory to be extended) until the speed of
light is reached, as an asymptotic limit.
But you're raising a good point that maybe Lorentz is pointing to
'non-integer dimensionality' where even enough mass crammed into a
small enough space causes the "new maths" to begin to noticeably
take hold. Like I said I see Lorentz as a way to transform
dimensionality from N-D to (N +/- 1)D, but in a continuous and
'differentiable' way.
In super simplistic terms Lorentz is a "compression" function where
one dimension of space is compressed perfectly flat, which is the
mathematical equivalent of removing that dimension from the
'degrees of freedom' of the system.
zabzonk wrote 9 hours 15 min ago:
But they do fade away? (Blondie)
dudeinjapan wrote 9 hours 4 min ago:
It's better to burn out than to fade away.
cvoss wrote 9 hours 19 min ago:
> It would also mean that quantum field theory in curved spacetime can
only be consistent if baryon number fails to be conserved! This would
be utterly shocking.
Is it really shocking (today)? I mean, isn't this a logical consequence
of Hawking radiation for black holes? I thought we were shocked by this
a long time ago, but now we're ok with it. The authors of the paper in
question may very well be wrong in their calculations (I can't say),
but this blog post doesn't smell good to me because of doubtful
statements like these, passed off as so obviously true that you must be
an idiot not to agree. That kind of emotional writing does not become
someone whose profession should focus on scientific persuasion.
From Wikipedia [0], itself citing Daniel Harlow, a quantum gravity
physicist at MIT:
> The conservation of baryon number is not consistent with the physics
of black hole evaporation via Hawking radiation.
[0]
URI [1]: https://en.m.wikipedia.org/wiki/Baryon_number
pfdietz wrote 2 hours 37 min ago:
Also, the Standard Model does allow nonconservation of baryon number,
nonperturbatively.
gus_massa wrote 4 hours 39 min ago:
>> if baryon number fails to be conserved! This would be utterly
shocking.
> Is it really shocking (today)?
Moreover, there are a few experiments that try to measure the proton
decay (that would break the baryon number conservation.) They are run
on Earth, far away form any black hole. For now, all of them failed
to find a decay, and the conclusion is that the half life of protons
is at least 2.4E34 years. [1] I found an old article by
quantamagazine explaining one of the experiment. It's a huge pool of
very pure water and a lot of detectors. No black hole required. [2]
(HN discussion [3] )
URI [1]: https://en.wikipedia.org/wiki/Proton_decay#Experimental_evid...
URI [2]: https://www.quantamagazine.org/no-proton-decay-means-grand-u...
URI [3]: https://news.ycombinator.com/item?id=13201065
molticrystal wrote 7 hours 14 min ago:
>That kind of emotional writing does not become someone whose
profession should focus on scientific persuasion.
What you'd probably prefer reading is one of the sources John Carlos
Baez cites [0]:
Comment on âGravitational Pair Production and Black Hole
Evaporationâ
Antonio Ferreiro1, José Navarro-Salas, and Silvia Pla
Where they take the equation used in the paper, and outline how there
is a better way than using that equation
"... is obtained to the lowest order in a perturbative expansion,
while the standard way to obtain the non-perturbative Schwinger
effect using the weak field approximation is to perform a resummation
of all terms"
and how the one in the paper being critiqued can't handle situations
arising from electromagnetic cases, much less the gravitational one
properly. These are the statements Baez makes but the cited paper
gives in a much more professional tone and method.
URI [1]: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.13...
AlecBG wrote 8 hours 52 min ago:
I'm not sure what more you want from him, there are many papers and
even a textbook linked?
It's bloody John Baez, the man knows his stuff.
On you actual point, it is shocking because its claimed that baryon
number is not conserved without black holes getting involved
throwawaymaths wrote 2 hours 16 min ago:
> shocking because its claimed that baryon number is not conserved
without black holes getting involved
Isn't it also speculated that there's hawking radiation caused by
the event horizon at the edge of the visible universe in an
accelerating frame?
lupire wrote 8 hours 4 min ago:
Are you saying that when Baez referred to "curved spacetime" he was
excluding black holes (because the paper was claiming that
non--black-holes have Hawking radiation?) or are you saying
something else?
AlecBG wrote 7 hours 24 min ago:
well he certainly mentions a result where if there is an
everywhere timelike Killing vector field (+ some other
assumptions) you can prove that Hawking radiation doesn't occur
and that does not include for example the Schwarzschild solution
because the Killing vector field partial/partial t becomes
non-timelike on the horizon.
So for example if you take a dead star in a vacuum with nothing
else in the universe (and make certain technical assumptions)
then you can prove that the star does not emit Hawking radiation.
That's quite a strong result, and certainly does make the result
seem shocking.
jiggawatts wrote 8 hours 58 min ago:
> The conservation of baryon number is not consistent with the
physics of black hole evaporation via Hawking radiation.
There are other black hole models that can conserve these quantum
numbers!
Speaking of things that are so obviously true that you must be an
idiot not to agree, there are statements so obviously false that you
have to be an idiot to agree: People keep repeating the nonsense put
out by Penrose, which require non-physical timelike infinities to
work.
The current "pop science" (nearly science fiction) statement is that
it is possible to fall into a black hole and there is "nothing
special" about the event horizon.
Quite often, just one paragraph over, the statement is then made that
an external observer will never observe the victim falling in.
The two observers can't disagree on such matters!
To say otherwise means that you'd have to believe that the Universe
splits (when!?) such that there are two observers so that they can
disagree. Or stop believing in logic, consistency, observers, and
everything we hold dear as physicists.
This is all patent nonsense by the same person that keeps insisting
that brains are "quantum" despite being 309K and organic.
If the external observer doesn't observe the victim falling in, then
the victim never falls in, full stop. That's the objective reality.
Penrose diagrams say otherwise because they include the time at
infinity, which is non-physical.
Even if the time at infinity was "reachable", which isn't even
mathematically sound, let alone physically, Hawking radiation is a
thing, so it doesn't matter anyway: Black holes have finite
lifetimes!
There is only one logically consistent and physically sound
interpretation of black holes: nothing can ever fall in. Inbound
victims slow down relative to the outside, which means that from
their perspective as they approach the black hole they see its flow
of time "speed up". Hence, they also see its Hawking evaporation
speed up. To maintain consistency with outside observers, this
evaporation must occur fast enough that the victim can never reach
any surface. Instead, the black hole recedes from them, evaporating
faster and faster.
This model (and similar ones), can preserve all quantum numbers,
because there is no firewall, no boundary, nothing to "reset" quantum
fields. Everything is continuous, consistent, and quantum numbers are
preserved. Outside observers see exactly what we currently expect,
black holes look and work the same, they evaporate, etc...
machina_ex_deus wrote 6 hours 10 min ago:
First of all, kruskal coordinates show beyond doubt that the event
horizon is just a regular null hypersurface that the observer
wouldn't notice crossing locally. (Of course if you look around, at
the moment of crossing into the event horizon you see everything
else that was falling into it unfreeze and continue crossing).
If you want to take into account the evaporation of the black hole,
then you should look at something like the vaidya metric. The mass
function is a function of the ingoing Eddington coordinate v, which
takes on a specific value when you cross the event horizon, and so
you observe the black hole at a specific mass as you cross the
event horizon. Contradicting your layman understanding of time
dilation for the observer relative to the black hole.
Once you cross the horizon, the r coordinate becomes timelike, and
so you are forced to move to decreasing r value just like a regular
observer is forced to move to increasing t value. Your entire
future, all your future light cone is within the black hole and it
all terminates at the singularity. Minewhile, the t coordinate is
space like which is what gives you space like separation from the
mess that had happened in the original gravitational collapse. You
wouldn't be blasted by a frozen supernova like you have said.
You can kind of say the universe splits at the event horizon, the
time like coordinate changes from t to r and the future of the
black hole branch of the universe is permanently cut off from the
rest of the universe.
In rotating and charged black holes it is different, and you
observe the evaporation of the black hole once you cross the Cauchy
horizon. If the black hole is eternal (because someone kept feeding
radiation to the black hole, maybe by reflecting the hawking
radiation inwards), then you would in fact see timelike infinity as
you reach the Cauchy horizon, so this time like infinity is quite
physical. You would need to avoid being vaporized by blue shifted
incoming radiation.
jiggawatts wrote 10 min ago:
Take a closer look at a picture of Kruskal coordinates, e.g.: [1]
Those closer-and-closer line spacings are hiding a mathematical
infinity, which isn't physical for finite-lifetime black holes.
Conversely, look at: [2] The ordinary Schwarzschild metric
diagram in that article makes it crystal clear that in-falling
observers asymptotically approach the horizon, but never cross
it.
Read the next section as well, which uses the "Tortoise
coordinate"... which again uses the mathematical infinity to
allow the horizon to be crossed.
I really don't understand why people keep arguing about this!
If you find yourself writing an infinity symbol, you've failed at
physics. Stop, go back, rethink your mathematics.
URI [1]: https://upload.wikimedia.org/wikipedia/commons/1/1c/Krus...
URI [2]: https://en.wikipedia.org/wiki/Eddington%E2%80%93Finkelst...
nextaccountic wrote 7 hours 43 min ago:
> The current "pop science" (nearly science fiction) statement is
that it is possible to fall into a black hole and there is "nothing
special" about the event horizon.
How is this not true? From the point of view of whoever is falling,
and supposing the black hole is very large
bencyoung wrote 6 hours 2 min ago:
Consider that every "surface" inside the event horizon is like a
stronger event horizon so passing through you'd certainly notice
things like not being able to see your feet any more as the light
wouldn't be able to travel out to your eyes! There would be a lot
of other stuff happening too so you may not notice exactly, but
the event horizon is definitely noticeable!
ldunn wrote 42 min ago:
Why wouldn't you be able to see your feet? Your head is also
falling through the horizon (hopefully - otherwise you are
going to be very unhappy), so the light from your feet doesn't
need to escape the horizon for you to see it.
quantadev wrote 7 hours 16 min ago:
Nobody knows what happens at the event horizon, but we do know
from the perspective of an outside observer things about physics
'break'. It makes sense that there's a flip-side to that
'breakage' (on the inside of the surface, or even "only at" the
surface) that isn't just normal space as if nothing happened.
For example there's no mathematics at all that mankind has ever
known where an asymptotic approach towards some limit doesn't
have a mirror version (usually inverted) on the other side of the
asymptote. If we see time stop, at the EH it seems wrong to
assume there's nothing "stopped" similarly from the other side
too. So this means the surface has to be very special. You don't
just pass by it and not notice as you fall in, imo.
amluto wrote 5 hours 1 min ago:
> For example there's no mathematics at all that mankind has
ever known where an asymptotic approach towards some limit
doesn't have a mirror version (usually inverted) on the other
side of the asymptote.
Thatâs a strong statement. 1/sqrt(x), over the reals,
doesnât have an inverted world for x<0. Maybe you could argue
that it does exist, weirdly rotated, outside the reals?
In any event, the Schwarzchild metric itself is an actual
example of this. From the perspective of a doomed spaceship at
the event horizon, the Schwarzchild metric is quite civilized.
The stuff after the horizon is a different story, but thatâs
not immediately after crossing the event horizon â it might
be whole nanoseconds later :)
Go take a GR class. Itâs fun and mind-bending.
quantadev wrote 3 hours 23 min ago:
What I meant to say was "asymptotically approaches infinity"
for 'f(x)' at some limiting value 'x' and thus a left/right
mirroring of the function. I shouldn't assume people know I
mean vertical just because I say asymptotic, so thanks for
catching that imprecision in my wording.
As you probably know, horizontal asymptotes are never what we
think of as the 'problematic' parts of Relativity, because
when something approaches a constant that's never something
that breaks the math.
The Schwarzchild metric, being a relationship of 6 different
variables I think, has some relationships that go to infinity
asymptotically at the EH radius and some things that approach
a constant at that radius, so it's an example of the kind of
asymptotic I was talking about _and_ one like your
"horizontal" example.
feoren wrote 8 hours 27 min ago:
This claim is different from the overwhelmingly accepted scientific
consensus, so it's on you to provide evidence. You say the two
observers can't disagree on whether the victim falls in in finite
time; tens of thousands of Ph.D. physicists say they can disagree.
Where is literally any citation, any evidence at all of what you're
claiming?
jiggawatts wrote 6 hours 47 min ago:
> overwhelmingly accepted scientific consensus
There is no consensus, quite the opposite: it was very well known
that neither classical GR nor quantum mechanics are able to model
a black hole!
People like to argue this as if it is settled science, right
after saying two contradictory things about it, both from
simplified, incomplete models.
amluto wrote 8 hours 39 min ago:
> The two observers can't disagree on such matters!
Why not?
If a spaceship fell toward a black hole and, as it approached the
event horizon, one observer saw it turn into a horse and the other
saw it turn into a cat, that would be very strange indeed, and one
would suspect at least one of the observers of being wrong.
But if one observer sees it fall through the event horizon and the
other observer waits⦠and waits⦠and gets bored and starts
doing some math and determines that they could spend literally
forever and never actually observe the spacecraft falling through
the event horizon, then whatâs the inconsistency? You might say
âwell, the first observer could fire up their communication laser
and tell the second observer that âyes, the spaceship fell in at
such-and-such timeâ, and the second observer would now have an
inconsistent view of the state of the universeâ, but this isnât
actually correct: the first observerâs message will never reach
the second observer!
jiggawatts wrote 6 hours 35 min ago:
> Why not?
Because that's not how relativity works! Two observers can
disagree only on the order and relative timing of events, not
what the events are or the total number of events. There are far
more restrictions than that, but those are sufficient for my
point.
The whole quantum information loss problem is just this, but
dressed up in fancy terminology. It's the problem with black
holes that the "number of things" (particles, events, whatever)
is "lost" when matter falls into them.
The modern -- accepted -- resolution to this problem is that this
information is not lost, preserving quantum numbers, etc...
How exactly this occurs is still being debated, but my point is
that if you believe any variant of QM information preservation,
then the only logically consistent view is that nothing can fall
past an event horizon from any perspective, including the
perspective of the infalling observers.
If you disagree and believe the out-dated GR model that an
astronaut can't even tell[1] that they've crossed the event
horizon, ask yourself this simple question: When does the
astronaut experience this "non-event"[1]? Don't start with the
mathematics! Instead, start with this simple thought experiment:
The non-victim partner far away from the black hole holds up a
light that blinks on an off once a second. The victim is looking
outward and is watching the blinking speed up. How many blinks do
they count at the time they cross the horizon?
Now think through the scenario again, but this time assume the
spaceship turns the light off when they observe that the black
hole has finished evaporating. When does the in-falling astronaut
observe the blinking stop? Keep in mind that every "toy model"
makes the simplification here that the blinking rate goes to
infinity as the astronaut falls in! (I.e.: "They see the entire
history of the universe play out." is a common quote)
[1] Isn't that a strong enough hint for everybody that there is
no horizon!?
amluto wrote 5 hours 46 min ago:
> Because that's not how relativity works! Two observers can
disagree only on the order and relative timing of events, not
what the events are or the total number of events.
No, and this has nothing to do with quantum mechanics or the
no-hair theorem or anything particularly fancy.
As a toy example, suppose you have a frame with a (co-moving,
but it doesnât really matter) time coordinate t. A series of
events happen at the origin (x=y=z=0 in this frame) at various
times t.
Thereâs another observer in a frame with a time coordinate
t'. The frames are related by t' = t - 1/t for t<0. The t=-10
event happens at t'=-9.9. The t=-4 event happens at
tâ=-3.25. The t=-1 event happens at tâ=0. t=-1/100 happens
at t'=99.99. t=0 gets closer and closer to happening but never
actually happens. t=1 doesnât even come close.
Critically, the t' observer does not observe t=0 or t=1 in some
inconsistent manner. There is no disagreement between the
observers as to what happens at t>=0. To the contrary, those
events are simply not present in the t' observerâs coordinate
system!
Note that the transformation above isnât about when light
from an event gets to the t' observer â itâs the actual
relativistic transformation between two frames.
The Schwarzchild metric has a nastier transformation than this.
If you toss a rock into an isolated black hole from far away,
you will see the rock get progressively closer to the event
horizon, and you will never see it fall in. But the rock is in
trouble: its co-movimg coordinate system ends not long after it
crosses the horizon. That latter phenomenon is called the
âsingularityâ, itâs solidly inside the event horizon, and
itâs not avoidable by coordinate system trickery. While
general relativity does not explain what happens when one
encounters the singularity, one might imagine that itâs fatal
to the rock the reaches it.
edit: FWIW, you also say:
> The current "pop science" (nearly science fiction) statement
is that it is possible to fall into a black hole and there is
"nothing special" about the event horizon.
Iâm not sure what youâre talking about. In a pure GR model
of an isolated black hole, as you fall in, you will observe
tidal forces. In a smaller black hole, the tidal forces will
squash you long before you reach the event horizon. In a large
enough black hole, they will not! Your view of the sky would
certainly look very, very distorted and delightfully and
possibly dangerously blue-shifted, but weâre talking about an
isolated black hole. Nothing to see in the sky, and you may
well survive your visit to the event horizon. Then,
dramatically less than one second later for any credibly sized
black hole, you will meet the singularity, and IIRC you should
probably expect to be squashed by tidal forces before that.
Source: I took the class and did the math. I assume this is
what the âpop scienceâ youâre talking about is saying,
and itâs not wrong.
P.S. Iâve never tried to calculate how lethal the
blue-shifted sky would be. Naively considering just the time
transformation, it should be infinitely lethal at the event
horizon. But trying to apply intuition based on only part of a
relativistic transformation is a great way to reach incorrect
conclusions.
jodrellblank wrote 8 hours 41 min ago:
> "To maintain consistency with outside observers, this evaporation
must occur fast enough that the victim can never reach any surface.
Instead, the black hole recedes from them, evaporating faster and
faster."
If this is radiating a star's mass worth Hawking radiation
particles, is it like the Solar Wind, and if it's happening ever
faster is there a point where it would start pushing the victim
away from the black hole again? (the 'victim' can be a solar sail
if that helps)
pixl97 wrote 8 hours 24 min ago:
I don't think the hawking radiation occurs at the edge of the
event horizon itself.
Arvin Ash just did a video on this [1] It appears to occur
outside the event horizon in a large area.
URI [1]: https://www.youtube.com/watch?v=UxVssUb0MsA
jiggawatts wrote 8 hours 30 min ago:
Yes, infalling victims will have a rather unpleasant time as they
discover that black holes are secretly supernovas frozen in time.
Outside observers see the victim's own black body radiation
become extremely redshifted, asymptotically matching the black
hole's black body radiation.
If you mathematically "undo" this distortion for both, then what
you are really observing from the outside is a star's worth of
matter getting converted to pure energy and the infalling victims
getting blasted in the face by that.
The victims can't make it back out "whole and intact" in the same
sense that you're not going to keep your atomic integrity if
you're up close and personal to a supernova.
Your quantum numbers however... those can be preserved nicely.
Ygg2 wrote 4 hours 10 min ago:
> black holes are secretly supernovas frozen in time.
I don't think that's true. What kills you isn't radiation of
the singularity, but cosmic microwave background (and other
infalling radiation) turned to visible light, then x-rays, then
gamma rays.
quantadev wrote 7 hours 12 min ago:
How are they getting blasted in the face when such a blast
would necessarily have to be moving faster than light?
hparadiz wrote 4 hours 28 min ago:
Because time slows down relative the outside observed as you
get closer to the event horizon any matter falling inwards
starts to compress blocking the matter above it until
eventually that matter is compressed to an extreme against
the event horizon. The photons that get fired off from that
interaction away from the black hole are able to escape and
that's why we can see some black holes as being extremely
bright. However matter that is spiraling inwards will be
blasted by hundreds of years worth of photons from every
direction while inside this matter and energy goop and all
sorts of other particles in a matter of moments relative to
how it is experiencing time.
quantadev wrote 3 hours 13 min ago:
Ah, I gotcha, thanks for explaining. Yeah the 'accretion
disk' are what this is normally called. Lots of matter is
getting smashed right outside the EH, creating heat energy,
and like you said it's able to blast out photons.
detourdog wrote 9 hours 43 min ago:
I couldn't really make heads or tails of this but if they aren't are
emitting are they absorbing instead?
I feel like the only way not to emit is to absorb.
kurthr wrote 9 hours 30 min ago:
Naw, this is Hawking Radiation a "quantum phenomena" that in the
original paper doesn't conserve mass/baryons. It's weird that it was
originally published (fantastic claims require fantastic evidence). I
don't really like the headline of TFA either since it seems conflate
all sorts of radiation.
The original paper is 2023 (Phys Review Letters). There was a
rebuttal in PRL in 2024. I don't know why this is still a big deal
now in 2025 other than Science Alert decided to write (another?)
hyperbolic article based on crap. Still boring.
gruturo wrote 9 hours 45 min ago:
Without a gravity well whose escape velocity exceeds c, how are they
supposing hawking radiation happens in this scenario?
Both virtual particles-antiparticles survive (and promptly disappear
because one didn't just cross an event horizon).
EA-3167 wrote 9 hours 38 min ago:
You have to remember the "one particle in the pair fails to escape
the event horizon" explanation is a simplification of the alleged
reality, which is the scattering of particles (or fields) in the
presence of an event horizon. As far as I know there is no intuitive,
non-mathematical way to describe this accurately, so science
communicators of all stripes tend to approximate it in ways that can
mislead the audience.
The man himself (Hawking) said: "One might picture this negative
energy flux in the following way. Just outside the event horizon
there will be virtual pairs of particles, one with negative energy
and one with positive energy. It should be emphasized that these
pictures of the mechanism responsible for the thermal emission and
area decrease are heuristic only and should not be taken too
literally."
gruturo wrote 9 hours 7 min ago:
Thanks! I just learned something!
pixl97 wrote 8 hours 21 min ago:
Arvin Ash just did an episode on exactly this effect. The modern
way we understand it is much to simplified.
URI [1]: https://www.youtube.com/watch?v=UxVssUb0MsA
coolcase wrote 7 hours 38 min ago:
Now I am confused as what he says at the end seems to agree
with the paper
"Hawking radiation doesn't just come from black holes but from
any collapsed star"
Sharlin wrote 9 hours 39 min ago:
That one's a big white lie of how Hawking radiation works. It's not
even an approximation, just a far-fetched metaphor that Hawking made
up, presumably to satisfy science journalists.
nimish wrote 9 hours 45 min ago:
There's an issue this highlights and it's not that the original authors
were stupid so much as there's clearly a lot of knowledge held in
silos.
That's not a good thing if your goal is to advance everyone's
knowledge. Whatever is going on in academia is failing relatively
closely related fields which is not good.
grues-dinner wrote 2 hours 0 min ago:
> clearly a lot of knowledge held in silos.
I don't think it's quite that, since the eventual goal is to publish,
not only publicly, but as publicly as possible. More like it seems
like everyone tends to hold their cards quite close to their chest
until the moment of pre-print publication. Which means you can be
working on something that someone could have told you months or years
ago you have a problem.
The scientific equivalent of polishing a branch before making a pull
request, only to be told "this has a huge memory leak and moreover
what you want already works if you use this other API".
I'm not really sure there's a human-scale solution: the research
landscape is so vast that you can't connect everyone to everyone else
and have everyone in need of valuable input get it, and have everyone
able to give it not be inundated with half-baked rubbish. Even if you
assume everyone from the top to bottom has pure motivations and
incentives for doing the research in the first place (in the pull
request analogy CVE spammers, for example).
Perhaps not having the universities themselves so keen for PR that
they'll slap a press release together about anything that looks
clickable without due diligence would at least prevent making a
public spectacle outside of the academic circle now and then, but it
wouldn't solve the fundamental issues.
boznz wrote 7 hours 13 min ago:
A Lot of these physics papers are interesting but ultimately just
noise. An untested Theory is NOT fact, it's just someone (with or
without a PhD) pulling something out of their arse that might explain
things. Most of cosmology and physics is still theory (even the big
bang, and string theory) and even if 90% of theory fits facts, they
could still be wrong. I am seeing more and more of these un-testable
theories, built on other un-testable theories, citing other
un-testable theories, this is why theoretical physics is in a crisis
IMHO.
MY mother and father also have an untested theory that explains all
this too it's called "God", most Sci-Fi authors have plenty, and I am
sure AI's will soon add to this pile.
Kudos to those scientists that create testable papers or
experimentally prove stuff.
kmm wrote 8 hours 19 min ago:
Is it really that siloed? The condition mentioned in the article
(there being a global timelike Killing field) is discussed in all
introductory texts on quantum field theory in curved spaces, it's
even present in the first few paragraphs of the relevant Wikipedia
article[1]. Even if it doesn't apply here, the authors ought to have
mentioned why not.
I don't think they were stupid per se, nor malicious, but perhaps
cavalier in pushing a result with such unexpected consequences
without getting a consult.
1:
URI [1]: https://en.wikipedia.org/wiki/Quantum_field_theory_in_curved...
moefh wrote 9 hours 8 min ago:
> There's an issue this highlights [...] there's clearly a lot of
knowledge held in silos.
I think the real issue this highlights -- which is something everyone
knows and still everyone does -- is that people love to spread and
discuss sensational stories, and no one likes to hear naysayers
ruining the fun.
Look the discussion of the original story here in HN[1]. There's a
comment by A_D_E_P_T way down in the discussion explaining why the
paper is nonsense and pointing to one of the replies objecting to it
mentioned in the article from this post. That comment was downvoted
by HN readers. I know because it was greyed out when I upvoted it
days ago.
So there's no knowledge silo -- us simple folk just want to discuss
the newest breakthrough without looking too hard, because that spoils
the fun.
URI [1]: https://news.ycombinator.com/item?id=43961226
gus_massa wrote 4 hours 20 min ago:
It's a good comment, but too technical. It's difficult to know if
it makes sense. I think it's good, but I'm used to read weird stuff
in papers. Anyway, my level of general relativity is too low to
understand all the details.
I skip that whole thread because I was expecting an overhyped
result and I have to sleep from time to time [1] . I'd have upvoted
that comment, especially if it was gray.
The comment is like ELI35[1], but for HN it's better to write a
ELI25[2] version. Or perhaps a ELI25 introduction and a second
ELI35 part with even more technical details. (I never liked
ELI5[3].) [1] I just finished my postdoc in General Relativity.
[2] I just finished my major in Geology. I know atoms and calculus,
but I have no idea what covariant is. Moreover, whatever gauge
means is not the type of gauges I know.
[3] I just want a lollipop.
URI [1]: https://xkcd.com/386/
jhalstead wrote 8 hours 4 min ago:
Direct link to A_D_E_P_T's comment:
URI [1]: https://news.ycombinator.com/item?id=43964524
ryandrake wrote 8 hours 43 min ago:
I don't think there's a lack of skepticism on HN of all places.
Every article that gets posted that discusses even a mild
scientific result brings at least one HN commenter out of the
woodwork to dunk on it. You can bank on it--there is always That
Guy who has to argue against it, whether he's right or not.
Also, the comment you reference was probably downvoted because of
the tone, not because of some HN bias against naysayers. Starting
out your comment with "It's nonsense." is about as conducive to a
productive conversation as starting it out with "You're wrong."
andrewflnr wrote 36 min ago:
The point of a statement like "it's nonsense" is to prevent a
conservation that should not happen, because it will be dumb.
It's the right thing to say iff it's correct.
gnramires wrote 9 hours 2 min ago:
I also think this kind of idea can be fun speculation, but I think
there are better things to have fun that aren't promoting wrong
ideas (like literal Science Fiction speculation!). When we can
build fun on top, the physics of our reality doesn't need to be
(academically) fun by itself :)
ajross wrote 9 hours 17 min ago:
It certainly wasn't in "silos", it's all on arxiv!
But yes: the world is complicated and it's easy to make mistakes
outside your core field. The point of the scientific process is to
get things in front of eyeballs who can spot the mistakes, c.f. the
linked blog post. Then everyone fights about it or points and laughs
or whatever, and the world moves on. The system worked.
What the process is not good at is filtering new ideas before people
turn them into news headlines. And sure, that sucks. But it's not a
problem with "academia failing", at all. The eyeballs worked!
tekla wrote 9 hours 23 min ago:
99.999999999% of people do not have enough knowledge to even dream of
beginning to understand a majority of research. Adults can barely
read, much less be able to pass Calc 1.
coolcase wrote 7 hours 56 min ago:
That percentage of the human population is everyone.
lupire wrote 8 hours 11 min ago:
Wher are you hiding 92 Billion people?
kurthr wrote 9 hours 27 min ago:
Well, there's another aspect which is that the original authors and
pop-sci journalists don't seem to be able to understand where they
went wrong or how outrageous their claims are, precisely because
their jobs depend on not understanding. The could have corrected it.
We could not still be circling this drain 2 years later, but we are.
Kinda classic. Kinda boring.
EA-3167 wrote 9 hours 25 min ago:
It helps that this is a genuinely difficult process to understand
and requires an enormous fluency with QFT. Most people who fit that
bill have better things to do with their time than write popular
science articles or correct them.
fishsticks89 wrote 9 hours 54 min ago:
URI [1]: https://archive.is/mG1KS
Wowfunhappy wrote 9 hours 51 min ago:
As best I can tell there's no paywall on TFA, so I really don't think
there's any reason to go through archive.is, which adds its own
advertisements (if you don't block them).
Out_of_Characte wrote 9 hours 8 min ago:
archiving isn't just to circumvent a paywall. There's also the HN
hug of death, possible geoblocks or an actual interest in archiving
the article as it was written at the time these comments.
DIR <- back to front page