How does gravity escape a black hole?
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- Опубликовано: 5 июн 2024
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If nothing gets out of a black hole, how does gravity do it? Something with virtual gravitons? Is this really necessary? It's tricky question, but this is what I can say without resorting to equations.
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I love Sabine’s videos. However in recent times they seem to always end the moment they get interesting. I would love to have longer videos again.
This video is particularly disappointing. It ended with "it doesn't have to". Chasing the algorithm unfortunately has it's limit.
@@MrElvis1971 I agree.
Yeah this was a terrible video. "Its a math thing". Really??? Thats all shes got?
Same. I miss the ones I had to try really hard to keep up. As for this one I could've learned everything from this video with a good google search or even chat gpt
Terrible is a bit of a stretch (of a spaghettification?). It comes down to the fact that gravity as far as we can tell is a literal deformation of space, and gravitons are, as far as we know currently, a mathematical trick we use to do certain calculations. The point is that “how does gravity get out of a black hole” isn’t a sensible question.
I wonder if the reason I remember my wife's birthday after 40 plus years is because I do the taxes.
I wonder how you might test that theorem. Possibly, have someone else do your taxes that year, and then see if you remember her birthday.
Heh. The reason I recall my sister's birthday is it's Tax Day. 😂
🤣🤣🤣
LOL That's great.
It's the same with our wedding date. We wed after being together for 45 years, only because it makes potential inheritance and estate issues easier for our children. The only time I need the wedding date is when I complete our tax returns for the 10% Marriage transfer allowance (in the UK) . So after 10 years of doing that it's now enshrined in my brain.
Every time i see a video on gravity I come away with the same feeling; no one really knows what gravity is and how it really works.
Like governments.
Bingo. Their explanations are not scientific as they cannot actually use the scientific method to test them. In the end, all they have is claims stacked upon claims with little to no epistemology and evidence to support them. Their arguments become "Einstein must be right because general relativity is the best theory we have," which is a circularly fallacious argument; especially considering the 1,900% correction to the mass of the universe in the form of dark matter and dark energy they have to add to the theory ad hoc in order to make it comport with observations.
I mean you could say no one really knows anything. As far as history goes there’s always been a more accurate theory to discover so it’s likely everything we currently accept as true will be replaced someday even if it takes centuries
@@johnmckown1267 women?
@@deananderson7714 You could. Some raise an eyebrow when I opine that our 'knowledge' is tentative/provisional/a working approximation of reality.
What baffles me is the following - Why is it always postulated that an outside observer, say sitting on earth seeing someone falling into a black hole becoming frozen at the event horizon, but when a another perhaps smaller black hole or neutron star falls into and combines with a black hole and it therefore behaves as all masses following an ever decreasing faster and faster orbit before joining with the first black hole. Why do we measure through LIGO a rapid chirp not a long drawn out seemingly frozen endless time period of a series of ultra long and endlessly slow gravity wave pulses? It seems to me that gravity does not play by the same rules somehow!
I asked the same question ;)
Yeah good question, why can two event horizons touch each other when a falling astronaut can never reach an event horizon?
We do, that is where the "chirp" end. That is the beginning on the slow occillations. They are just so slow that they can't be observed.
@@Harkmagic That makes sense. So the 2 event horizons never touch either. Even the surface of the original star that collapsed never reaches the event horizon. It's just too red-shifted to see anymore.
Nothing freezes at the horizon, everything reddens and vanishes in short order. The difference between the GW merger signals and the observation of a traveler falling across the horizon is that the GW are emitted much further out.
An old adage about chocolate. Coffee makes getting up possible but chocolate makes it worthwhile.
Vanilla always plays the supporting role.
During a difficult and busy time in my life, I ate dark chocolate before leaving bed every day. It was worthwhile
One with earliest recorded mentions in the 15th century and the other in the last 5000 years.
or a boiler if it's winter!
Both coffee and chocolate are awful. 🤮
I and some others have asked a similar question on the hypothetical charged black hole: how does the electric field get out of the horizon?
Ah, yes, that question is not so easy to answer. Only thing I can say about this is that the electric field of a black hole is not a propagating field, it doesn't travel, it just sits there. The part you feel outside isn't inside the black hole in the first place. You could ask what happens if you could manage to shift around charges inside the black hole and you wouldn't be able to measure this outside the black hole.
@@SabineHossenfelder Thank you Sabine. So its as if the entire horizon sphere acts like a big sphere of charge? But its weird though shouldn't this mean a discontinuity of electric field at the horizon boundary?
And its especially weird since the horizon is "not special" and can also shift when the black hole grows/shrinks... if that surface is being treated as a surface charge
It's the same as mass - outside observers never see anything actually enter the BH. From their perspective, it all sits at the surface.
Interesting
I remember the lecture about spinning blackholes. Just before the lunch break, the prof said that a spinning blackhole has a singularity that looks like a donut. I don't remember anything after that.
Donuts do exactly the same to me.
It's a ring, i.e. that donut is thin as a line. Kerr solution describes it mathematically.
@@thedeemon It follows that a singularity must be of zero size in at least one dimension. I was making a joke.
You donut remember anything after that?
I think I was at that lecture; my mind was the hole in the donut.
I asked Brian Greene this very question many years ago. He said it was due to “virtual gravity”, but it was too complicated to explain without at least a master’s degree understanding of physics, so he doesn’t know either.
I think you mean to say "virtual gravitons", which would communicate the curvature in exactly the same way virtual photons communicate the field of a static charge configuration.
@@kylelochlann5053 he really used the term “virtual gravity”, but the term may have morphed in 25 years.
This was back when he was the leading proponent of String Theory and had just published The Elegant Universe.
Brian Greene is not a scientist I would ever trust. He's a believer. He lacks skepticism.
@@Jesus.the.Christ - Brian is good enough to have accepted that his favorite pet theory, String Theory, has been effectively defeated by facts. He was a bit depressed at first but then moved on, with some help of the science of psychedelics. I like the guy, he's a great science communicator.
Pseudo science...
Around the 4th minute I suddenly remembered Sabine is married. Now I worry for Mr. Hosenfelder...
Would have to take him a really long way from earth to find a black hole, so I think he's safe for now...
Consider him simultaneously both alive and dead. That's how quantum mechanics works in the macroscopic world.
If something is pulling a body,then there must be some force doing it,like gravity.Otherwise,the body is just falling.
Sabine's marriage put an end to the annual "Win a date with Sabine Hossenfelder Contest !!!"
HoSen? Hahaha 😂😂
I recently started to doubt the existence of gravitons. In General Relativity, gravity is not even a force. So why would it be a force in particle physics? So if graviton exist, it is most probably in the same sense as phonons (quanta of sound): a useful mathematical device but not a real particle
Well, you are in good company. I know many people who question the existence of gravitons. That said, they are compatible with gravity not being a force for the following reason. As you probably know, you can define a force for gravity once you have fixed a background and reference system. This is how you recover Newtonian gravity from GR. If you want to define gravitons, you also have to define them relative to a background. Loosely speaking, they are waves atop something, but you first have to define what that something is. What this means is that the definition of gravitons requires the same additional assumptions as what you need to assign a force to gravity. It's more of a linguistic problem than a technical one. (Though that doesn't mean that gravitons exist...)
@@SabineHossenfelder Yes, that is what I geberalky meant but not in such eloquent way. Thanks for the insight.
@@SabineHossenfelder if Gravity would be a Quantum field then we would have exactly the same problem we have today to explain the Measurement Problem. Everything should be in a state of superposition but it's not. The Wave Function decays into a state but this phenomenon cannot be explained by adding yet another quantum field. However if gravity was not quantifiable then adding it to the mix would not result in the same issue. I like the idea that Sir Roger Penrose has that precisely it's the fact that Gravity is not a quantum field that eventually triggers the collapse of the wave function.
Yeah, my old question is how do you quantise changes in gravitational attraction between two dancing ants at opposite "sides" of the universe (which would still be "huge" compared to those changes at an atomic level.)
@@zemm9003 Yes, exactly!
I've never been convinced about "gravitons". I think they are as practical as String Theory. Crunch the numbers all you want, it does not exist.
Frankly, I've never understood what the point of a graviton is--but I viewed that as my problem, lol.
If GR is the best theory of gravity we have, how do you get from there to gravitons? QM *has* particles already. I'm not even getting to practicality here. I just don't understand What the point is in even talking about them.
@@bsadewitz well its sort of the progress of science right? You find a force, you find the things that interact with that force, you find the things that transmit that force, then you can manipulate that force.
We find light, see it interacts with matter, discover the EM spectrum, figure out photons. Along the way you get tech, starting at candles, then lightbulbs, then plasma lamps, then LEDs in many particular spectrums.
Having electricity was cool, but being able to directly manipulate electron beams is how we got TVs. Imagine the applications of being able to manipulate gravity.
@@keatoncampbell820 Well, yeah, except where are the gravitons? There is no quantized gravity that works. And man, have they ever been looking. We don't have a particle. We have the geometry of spacetime. Our best theory of gravity doesn't treat it as a particle at all.
@@keatoncampbell820 And candles? People were using candles when the leading theory of vision in some places was that our eyes illuminate the world--and before that lol.
@@bsadewitz yeah I don't disagree. Just explaining WHY we want gravitons to be real. And I don't really understand your dig at candles?
It was literally my example of the first step of understanding...
I dont think gravitons are a thing. I don't think magnetons are a thing either. But if they did exist it would be useful.
Please breathe my guy
Whatever happened to the paper that suggested that a black holes vacuum energy causes the expansion of space around it?
On another note, It's so fascinating that Einstein's theory of general relativity basically implies that black holes are one-way portals to the end of the universe
Or, black holes could be one way portals to the beginning of another universe!
Yup, an implication, not necessarily a fact.
Given the disks around a lot of these things, in many cases you'd still just burn on your way in!
Because the disk just sits there while you get wiped across it?
@@deltalima6703more because the (accretion) disk is a disk of such high speed and energy particles that it would be similar to sticking your head in a particle accelerator akin to the LHC times 1,000,000...
@@Sanquinity You would burn from the thermal energy of the plasma, not because you got blasted by a particle radiation as you would be travelling zero speed relative if you fell with the accretion disc. If you went in the opposite of traffic or crossed the disc, how much particle radiation would depend on where you intersected and your own speed.
You also can take paths that don't cross the disc, but I wouldn't take the polar routes unless you enjoy showering with relativistic jets, aka, particle radiation.
how does one enter a blackhole safely then?
@@mdsmatheus One doesn't, as far as we know.
I'm surprised that Sabine made such a basic error of the strength of gravity at the event horizon. It doesn't get weaker with a larger BH, it's actually the same strength for any size BH, that's why light can't escape at that point. What changes are the tidal forces which get smaller the bigger the BH. So, a supermassive BH won't spaghettify you outside the horizon, but a small one will.
I thought this as well at first (though I thought it a deliberate simplification, but one I thought shouldn’t be done) but, try calculating it?
r_s = 2GM/c^2
Far enough away, we have that F=GMm/r^2
(Maybe this holds throughout? Like even inside the horizon? I’m not sure, I don’t really know GR),
If you take r = (1 + epsilon) r_s ,
then, F=GMm/(((1+epsilon) 2GM/c^2)^2) = m/((4GM)•(1+epsilon)^2)
which, is inversely proportional to the mass of the BH.
Also, the Schwartzchild radius happens to also be the location where the escape velocity, if one computes it in a Newtonian way neglecting relativity, reaches c,
But, “the location where the escape velocity is a certain value” isn’t the same thing as the force/acceleration being a certain value!
It isn’t like there’s a particular amount of force you can apply to light which makes it not go in some direction?
@@drdca8263isn't the whole point of black holes "not being a vacuum" that it really isn't sucking things in, but rather that space curves inwards so much that you can't take any path that leads outwards anymore?
That description doesn't mention anything about how much force is applied. Only that space curves inwards.
@@Sanquinity so, rather than “the force applied”, you might want to consider it instead, “the amount of force (in the opposite direction) that would *need* to be applied, in order to produce the right amount of ‘proper acceleration’ in order to stay at the same position relative to the black hole”
Yeah she meant to say the rate of change of gravity. As in it's the least steep there - you'll have a low jerk (derivative of acceleration).
I love Sabine. "No amount of chocolate with get you out of it"
Great timing and perfect deadman. AND she's a great scientist
I absolutely agree, I just hope, her husband hadn´t to face this experience.
Thanks! That clarifies pretty much all the questions I had about how gravity works, near black holes and otherwise.
Thank you so much for your guidance, we now will have even more Black Hole tourism I can't wait!
Nothing to see there
@@Iohannis42 Read "Lost Horizon"
The statement of the anniversary of your wife is really your terrain, Sabine: exact science.
What’s traveling when we measure gravitational waves with LIGAR ? Gravity seems to send ripples across space. I’d like to ride a gravitational wave on a cosmic surf board.
SPOUSE: I don't want anything special for my (n)0th birthday
ME: Okie dokie
Love how quirky and educational your videos are. Reminds me of the cool teachers from school that genuinely loved teaching others about their favorite subject. Followed and will be back again
Off topic, I'm studying for a toefl exam and the fact that your videos have a quiz to know how much did we get from the video is truly helpful!! thanks for the content, big fan!
Thanks for a black hole video :)
1:36 That is an interesting concept of less gravity at the event horizon of a larger black hole. I always thought the event horizon was the demarcation of where light could no longer escape the gravity (a point of balance like angular velocity).
Opps, I fixed a period omission. Sorry Sabine and thank you lol
Actually this leaves me with numerous questions for me to look more deeply at:
- What part does the size and spin of a black hole play upon gravity. Is it just like free fall due to angular velocity at a closer distance, and the gravity is still there, or is gravity actually less?
-Would this allow the accretion disk (and photons with enough angular velocity) to settle closer the the event horizon?
- Could a black hole theoretically become massive enough that that gravity becomes low enough for particle to escape (aka the event horizon disappears)? "If" that were possible would all the mass inside suddenly inflate?
@@axle.studentrotating black holes do allow for a closer ISCO in the same plane as their angular velocity, yes.
@@Mernom Thank you, I have seen some interesting videos depicting photon orbits, but they are a little generic (lacking in information granularity).
@@axle.student Vererasium has a video about what is it that you actually see when you look at a black hole. It explorers the structure.
2:15 what an animated treat! Thank u video team 😅❤
I've been thinking about this problem for 2 decades since I first read Hawking's A Brief Histroy of Time. Guess I will have to continue thinking about it because this vid only sparked more questions than answers for me. Sabine, please do a more in depth follow up...or a whole series! Thanks
What do you think about this? en.wikipedia.org/wiki/Ant_on_a_rubber_rope Do you think this holds true for gravitons exiting a black hole?
@@DanjoBanks Then wouldn't it work for photons to also exit a black hole? Interesting problem though that I had not heard of before. Thanks for sharing it with me.
@@HyperHowie56 Good point. I hadn't considered that. I'm not sure
Thanks so much for creating and sharing this informative video. Great job. Keep it up.
❤A question I never thought about. Expands the intellectual horizon.😊
Gravitational waves are not caused by mathematical equations, but mathematical equations are used to describe the observed effects of gravitational interaction between mass. If gravitational waves are observed to travel through free space at the same speed as electromagnetic waves, then their interaction with free space must somehow be related to the permeability, permittivity, and impedance of free space. Or maybe free space has a different set of gravitational properties derivable from permeability and permittivity, whose combination gives the same speed of gravitational waves through free space.
Gravitational waves do not travel through free space or interact with free space. The waves ARE fluctuations of free space, the substrate. They are the same entity.
The higgs field is it's own thing.
@@viralsheddingzombie5324: We _don't_ really know that.
@@absalomdraconis _that_ is their definition. we already set it as that, thats the basis we are already building on since einstein, as he said gravity ==== curvature of spacetime
Yea but that's just the linguistic technicality. I think the original problem stays there. Gravitational waves do have a finite propagation speed and the question is why is it so close to c.
In that sense they travel through space. To me you are saying that wave doesn't travel through water because it's water. Yes, but the disturbance still appears as if it was travelling through the medium. But it's true that it's different from a photon.
What I like is when two black holes merge, say 30 and 16 solar mass, several solar mass of the black holes is converted to gravitational waves escape the black hole.
I've been more curious about a similar related question "Draw a straight line between three points. Point A is the Observer. Point B is a Black Hole. Point C is a Gravitational Wave source such as the merger of two black hole or of two neutron stars. Point C create Gravitational waves and the waves are heading in the observers direction, as the gravitational waves arrive at the black hole, what do the gravitational waves do? Do they:
1: Pass into the event horizon never to leave
2: Pass into the even horizon and out the otherside
3: Simply is bent around the black hole
If the gravitational wave pass through the event horizon and out the other side could it theoretically carry information about the state of the black hole to the observer at point A? sort of like gravitational wave spectrometry.
They go around. Just like light, we see how it gets warped around massive objects (gravitational lensing). Gravitational waves propagate at light speed.
At the center of every black hole is an AI creating paper clips at an uncontrolled speed.
Semi-organized quark soup.
Maths checks out
Math checks out
Sabine: I signed up for Brilliant using your link (last night) and LOVE it!!!! I started with algebra because I’ve always been math-phobic. So far I’m acing all the lessons & hated turning it off! Don’t ask my age; soon I’ll be measuring it in light years!!!!
Many thanks for your videos!!
Light years measure distance.
@@gbormann71 Yes, I know that, but it seemed appropriate for a bit of hyperbole!
At 3:15, it is stated that we measure the gravitational effect of an object as just before it fell in.
But this is in contradiction with the hairless theorem.
At 3:40 it is stated that if ourself is falling into the black hole, it is not an infinite time.
But this doesn’t take Hawking Radiation into account. The black hole under me will evaporate before I fall into it.
As I understand gravity is the curvature of space and matter (particles) do not have gravitational interaction with each other just with space. Also as I understood quantum mechanics describes the interaction between particles(and in my opinion, it(quantum mechanics) uses probabilities because space has "noises" aka quantum foam, etc., and interactions happening in space). Why physicists are trying to use quantum mechanics to study/describe gravity? Has it worked on particle - space interactions before?
Quantum mechanics requires a uniform spacetime to work properly.
Without a quantum theory that can handle gravity, explaining an object like the singularity which is the intersection of both theories is impossible.
@@Mernom It should be quantum because if not theory will not work(will not describe everything). Am I understood correctly?
@@SlimGreen because the singularity exists at the scale where quantum effects become relevant.
GR only deals with mass and spacetime. It does not deal with matter structure or composition. You need other theories for that, but since quantum doesn't play well with curved spacetime, we can't ask it how would stuff work at that scale under that gravitational effect.
@@Mernom Quantum effects are relevant not only on the singularity level. Nobody knows what is happening on singularity or if singularity exists(it's just a mathematical concept). We just know that the speed of light is not enough to get from strongly curved space(like a black hole). It is the end of humanity's knowledge for now.
@@Mernom, For example, Plank length and Plank time state that the time and space are discrete. But I highly doubt that it is so. For a time, I am sure that in this case should be some frequency in processes in particles that prove it. speed of light over Plank length does not tell that it is the smallest amount of time, because we know that the light can be trapped in the black hole.
1:16 - your blood won't be able to traverse your body in the direction towards outside of the BH, electric impulses in your brain won't be able to go the neurons closer to BH's center to the neurons further from it. This wouldn't come unnoticed. Even presuming that your capability of noticing something will remain intact, which is unlikely due to above. I wouldn't call this "nothing"
Maybe but how long does that last when you are travelling near the speed of light? And once you are inside - what is the direction of gravity?
That applies only if you could somehow sit at the horizon, moving outwards at light speed,half-in,half-out of the hole. In reality you are falling into it.Blood inside the hole can't leave,but your body is moving into the hole at light speed and will meet that blood in short order. Your objection is like saying you can't drive a car because your heart wouldn't be able to pump blood fast enough to keep up with it. Relative motion matters.
@@garethdean6382 Relative motion matters, but if I'm not mistaken the acceleration here would grow too fast and rip you open.
Your heart in the car has a way of "catching up" and getting up to speed with your body because we're not talking about extreme differences in acceleration, but as you fall into a black hole the acceleration would grow faster and faster as you get closer.
There will be a point in which the difference in acceleration between your head and your legs will be significant enough to literally rip you open: all of your body is still falling towards it, but the relative speed of your limbs is only destined to grow as you move towards the center. And *that* rips you open.
So yea, it's still all relative motion lol
@@Random-ly1kgWhy would your acceleration accelerate near a black hole?
@@gbormann71 Force of gravity goes by the squared inverse of the distance to the center.
The closer you are to the center, the bigger the acceleration
On Earth it's pretty much constant because we're approximately on a sphere (so your distance to the center of the Earth is constant), but in some cases we too need to account for the vertical movement
[ hope I got your question, not sure ]
So what do you think: do gravitational waves bend around a black hole due to the gravitational field?
GW follow the same geodesic paths as e/m waves.
The way I always understood it was like the event horizon was like a sheer cliff and space itself was falling off the edge of the cliff, dragging more space with it. Plus most black holes start from stars, and don't just pop into existence. So the gravity well is already leaning toward the black hole before it even forms.
I think of it as a slope. At the event horizon the slope is vertical. 1/0+. Infinity.
Outside observer will not see you stuck on the horizon, smushed infinitesmally thin, either. Your mass adds to BH. Horizons gets a smidge bigger, and gone!
You cant just coast into a BH anyways. BH spins, space rotates, you are forced to orbit it. Faster and faster. At the horizon you orbit so fast that space is contracted via relativity to the point where you are smeared across the whole thing as if it was a single point.
Its not survivable and its causally disconnected, so it actually does not even make a difference if there is a beyond the event horizon or what is there.
It's just the curvature of space-time, which is extremely hard to visualize (the funnel analogy removes one spatial dimension and doesn't represent time even, the Penrose diagram removes two spatial dimensions instead in order to represent time in Einsteinian coordinates). I also thought like you in the past but it's not the case. Space may still be dragged by the rotation of the black hole (real BH's singularities are probably rings and not dots because they have a momentum or spin that already existed when the black hole was created and is inherited from the parent star, or the combined momenta of the various aggregating BHs), this space-time dragging effect is even more notorious when BHs merge, causing the gravitational waves (which are not quantum waves but classical ones in space-time as medium).
Since I am neither a physicist nor a geek, let me fuse these two virtually contradicting observations: 1) A distant observer would never see an object crossing the event horizon; 2) Here on Earth, we can observe objects merging with a black hole within seconds, via gravitational waves. Point 1a) A small object (like a human) would not feel any temporal change in its journey through the event horizon of a supermassive (not spaghettifying), non-rotating, non-magnetic black hole, because even if time dilatation applies to him too, his physiology, biochemistry, heart beating, nerve impulse propagation and thus his perception will also be equally slower, so he won't notice that time slowed down around him. 1b) From the perspective of an outside distant observer, the human falling into the black hole would appear slowing and slowing, hence light emitted from him should be more and more scarce due to time dilatation, so finally, visually it will be impossible to see the victim crossing the event horizon. The victim would become a non-emitter, a completely black object. Like this, we can also say that light does not cross the event horizon because time is not "ticking" at all at the event horizon, and thus no physical process, even the crossing of light, can happen in finite time.
1c) The disappearance of a large object such as a star, when merging with a black hole, would be visible by an outside observer, because most part of the star is far enough from the event horizon, thus time dilatation is not sufficient to prevent light leaving from it. Only the part in physical proximity with the event horizon is subject to "invisibility". So the star would be seen as smaller and smaller and finally, no light emitted. 2) Gravitational waves are a result of the sudden merger of two smaller curvatures of spacetime into one larger (I hope I phrased everything sufficiently well). Their propagation is not subject to time dilatation.
Also check my comment, pls. Thanks.
As we have measured gravitational waves of 2 black holes colliding, didn't we then observe one black hole fall into another black hole?
Sorry if this is perhaps a stupid question, I am not a physicist.
We observed two smaller black holes merge into a larger black hole.
@@kylelochlann5053 I know, but how can, from our perspective, one black hole ever merge with the other? Things never appear to cross the event horizon, that would also count for another black hole wouldn't it?
@@Richard-bq3ni We don't see anything crossing the horizon during the merger - LIGO is receiving signals created outside the horizons of the black holes.
In the merger process the horizons expand outward to reach the other black hole and merge, oscillate wildly, and then settle down into a larger black hole.
@@kylelochlann5053 All right. Give me some time to process this, I try to visualise. Don't wait for it as my brain red shifts into eternity 😉
Not being able to see something fall into a black hole needs special conditions. One of these is that the hole doesn't change in mass, another is that the falling object is tiny compared to the hole. (A third is that you don't go and check on the object.)
In the case of mergers the holes lose a significant amount of energy, even before the 'ringdown' of the merger. And since a hole's radius depends on its mass, you can fit two holes of mass x inside one hole of mass 2x. The merging holes actually warp and grow as they combine. The final hole is always bigger in volume than the ones that formed it.
I just thought of a possible strange consequence of time dilation as one approaches a black hole. I'm excited to read what you all think...
As you approach a black hole, your clock shows down (relative to a distant observer). How slow does it get a you approach the singularity? Does it depend on the mass of the black hole? Other factors?
As you approach the singularity, this point of infinite density and infinite acceleration, would your clock approach infinite slowness?
My thought is that if your clock goes slowly enough, the black hole could evaporate through Hawking radiation before you ever reach the singularity. There is no need to worry about a point of infinite density or infinite acceleration because everything's clocks are running so unbelievably slowly that the object itself will have ceased to exist by the time you "get there".
Thoughts?
I haven't even watched the video yet, I just clicked on it so I could comment that the question is so brilliant, I wonder why I never thought of it before...
How long does it take to fall into the singularity?
My hypothesis is that it takes a very long time, and by the time you have reached the singularity, the black hole has evaporated via Hawking radiation. So, yes, there is a singularity, but you can never reach it because the black hole evaporates while you are falling.
The gravitational stuff propagates at c, you fall at less than c.
I suspect that, if there's such a thing as a singularity (which I am not convinced of), that it's a particle made out of space-time which innately has no interior volume: sott of the idea of a holographic universe, but turned inside-out so that the holographic surfaces are surrounded by what they create instead of surrounding it.
The singularity is reached very very quickly. For a free-fall across the horizon the maximum amount of time is πm and the least amount of time is 4m/3.
I was wondering that, too, something doesn't seem to add up for me.
If it takes forever to fall into the black hole, it is evaporated before you fall any further, so what is, you fall or you evaporate?
@@kylelochlann5053but with time delation this may extend to a long time, so wouldnt you evaporate before you reach the it?
@@kylelochlann5053 In which reference frame?
If you shine a light toward the singularity, don't the photons get 'stretched' by the gravitational field in the process of spaghettification?
Here on the outside, we can never see anything cross the event horizon. The idea that something falling in will pass right through the horizon is really just a guess. A very good guess, based on solid mathematics, but still a guess, because it can never, in principle, be confirmed experimentally.
What's the difference between something that cannot, in principle, be detected, and something that does not exist? I contend that there is no difference. Therefore, since it can't be detected, the inside of a black hole does not exist. Black holes are literally holes in spacetime. There is no there there.
I think you may be on to something with black holes, but consider this counter argument about observability: the size of the observable universe is the distance light can travel to earth since the big bang. Every day it increases. Would you argue that the stars on the edge we can see today didn't exist yesterday?
Black holes can be detected thru their gravitational influence on other objects, even if they cant be observed directly
@@reij1 Since the universe is expanding, it works the other way: things we can see today may disappear tomorrow. But to answer the question as amended, It is not really a well-defined question, because there is no universal now due to the relativity of simultaneity. However, adjusting or that, a mass outside our observable universe could be inferred from the effects it has on things inside our observable universe.
@@whisperwalkful Sure, but you can't see _inside_ the event horizon, and that is my point. The mass, and charge, of a black hole appear as if spread out over the surface of the horizon.
I thought that the whole point of a black hole was that it bends spacetime, and it's the relationship between objects within this that is called gravity, so how could gravity 'escape' anything?
She posed and answered that exact question
A grammatical error: You say (approximately) "we see the observer and its gravitation just before it fell in", the observer hasn't fallen in yet, that's why we see him/her outside the horizon. This is a similar mistake as people saying "things appear to shrink when they move according to Einstein's relativity theory". No, things DO shrink, it's real, it's not just how things appear to you. The person falling into the black hole is still at the horizon and not inside yet.
In short: the reality of the object itself is not more real than how others perceive that object (at least according to Einstein's theory).
I hope I live to see what gravity is and what causes it. I often think this whole quantum/graviton approach is an attempt to put round gravity into the square hole of quantum mechanics. My sense is that the understanding of what gravity is will take a leap of inspiration akin to Einstein's relativity.. It may well be that a new mathematics will have to be invented to explain it.
Good shout - I’m not even convinced by the photon exchange model of the EM force let alone an equivalent particle exchange interpretation for gravity. I’m just not buying it.
@@alwayscurious413 ... I've had doubts for a long time about dark matter. It's invisible stuff, they say, with far more mass and gravity than ordinary matter, yet no one has a chunk of it and no one can point to any object made of it. What else has gravity that doesnt come together as a solid?
A half century on there have been so many expensive experiments looking for it, and so many Nobel Prize hopefuls seeking it. That nothing has been found suggests the theory has been wrong.
Clearly, we have a long way to go on gravity - on many fronts.
Gravity is the curvature of the gravitational field (which is spacetime) and it's caused by the stress-energy of matter. Hope you're still alive to read this.
@@kylelochlann5053 ... Your description is one part tautology and the other part effect. It's not helpful in understanding the issue. Understanding what gravity is at its most fundamental level will open the door to manipulating it, and possibly to efficient space flight.
@@frankhoffman3566 We measure the gravitational field be space and time itself, how could this be more fundamental? What we don't understand well is matter.
I have another beginner question which even watching countless videos about black holes could not answer:
According to the equivalence principle, you can fall into a black hole and should not be able to notice when you cross the even horizon. But how do molecular bonds hold molecules together when one atom is inside the black hole and the other atom is outside? How does the atom inside of the black hole tell the atom outside the black hole about its existence? How does any complex structure composed of different parts survive when crossing the event horizon?
A really big black hole and you would not notice. A small one and much bad times.
@@fehmeh6292 Both small and large black holes have event horizons. The problem mentioned above - if it is indeed a problem - applies to small and large black holes alike.
Where you're going wrong is imagining a particle at rest at the horizon, which is impossible (horizons are null hypersurfaces). Let's say you fall into a black hole feet first and when you're feet cross the horizon they send a flash up towards your eyes. The flash at the horizon cannot cross the horizon but you're passing across at the speed of light towards the light and the light arrives at your eyes in a time frame as it normal would outside the horizon.
You have to think about this relativistically. From the perspective of someone falling into a black hole there is no event horizon. You experience stronger and stronger pull towards the center until no matter what you do, you can't get away from it. But as you speed up, the lengths contract and time slows down so from your perspective everything is business as usual.
Now this is where the size of the black hole comes into play, because a small one will tear you apart very quickly, while for a large black hole, the pull at the event horizon should be weak enough for you to preserve your body in one piece for somewhat longer amount of time.
I have had this same question stuck in my mind for a long time. I’v heard everyone mention about going inside blackholes and not noticing anything when crossing the horizon however, as you mentioned, a complex system such as a human body composed of nerve fibres along which impulses travel would seem to malfunction once it has, or part of it has, crossed the event horizon. Imagine you have crossed the event horizon feet first, how would the blood travel from the lower part of your body upwards to provide circulation when movement is only allowed downwards towards the centre of the blackhole. It seems your blood would have to travel upwards, away from the blackhole’s centre, to get to your brain to keep your body functioning. I feel like I am missing something or that this is a problem because we have tried to make a visual representation of the precise equations describing a blackhole then again I am unsure.
Excellent explanation as always 👍👍
Thank you Dr. Hossenfelder for such explanations.
Schwarzschild black holes probably don't exist. Why? Because everything in the universe rotates. So when black holes form they're Kerr black holes, and Kerr black holes have radically different causal structure. Their singularity can be avoided! Isn't this what the recent paper by Kerr is about?
The Kerr black hole you're imagining is of the unphysical analytic continuation in an eternal spacetime. If you introduce some perturbation the causal structure changes (mass inflation singularity at the Cauchy horizon, BKL (or maybe null) central singularity, etc).
Kerr's recent paper is much ado about nothing. What he found is a pair of principle null curves that asymptote to the inner horizon with finite affine parameter such that curvature scalars do not diverge. I haven't come across a simple way to say this that would work on my grandmother. Anyway, it's something we've always known and doesn't exclude the existence of singularities but rather suggests that geodesic completeness is not a guarantee of a singularity.
They could eventually slow down over a long enough period of time to lose all rotational energy
@@Iohannis42 Yes, but they could as well spin up due to accretion.
@@zdzislawmeglicki2262 Yes, but without new inputs to the system, it will eventually slow down again.
From external perspective time of black hole completely stops on event horizon. Singularity does not exists.
Gravity begins at point A and travels to point B. We see this in the progress of gravitational waves. If the Sun were to disappear, it's gravity would take over eight minutes to stop reaching the Earth.
When a black hole forms, it's event horizon forms, but the gravity traveling out is not stopped.
I want to say that nothing but gravity can get out of the event horizon.
Maybe it already got out and once it's there, it's there? I guess it's the changes to curvature that have to travel, not the curvature itself?
@@DobesVandermeer Black holes are not static - they are moving in spacetime. If some black hole is closing to another mass object, spacetime at this object has to change its curvature (higher gravity). So there has to be some information (gravitational waves, gravitons or whatsever) traveling from BH in each direction form it.
@@vitlibovicky6741 maybe the information in question is coded into the fabric of space-time itself, not transported by some mediator particle.
@@furrball No, it could not. Then it will be static, but it is not. Every moving mass changes curvature of spacetime (gravity) in all space. This changes has to be transported somehow - at speed of light.
In the "One more thing to worry about" category...
Question about black hole mass and gravity: from the outside perspective the gravitational effect of matter falling into a black hole never leaves the event horizon, doesn't that mean the observed gravitational effect of the entire black hole is independent of the actual mass and position of matter inside?
I know it's pure theory at this point, but theoretically if the matter that falls into a black hole ends up somewhere else either in our universe or some other destination, wouldn't that mean that it's mass is curving space time both at the event horizon and wherever it ends up? Also, does that mean that the entire observed mass of a black hole from the outside is spread out on the event horizon?
Yes. This is why w can't drop things into a hole and measure their gravity to figure out what's inside. As something approaches a hole it slows, with closer parts slowing more, 'flattening' it. At the same time its outward light warps and bends until some rays can wrap right around the hole. This makes it look like the object spreads out. It seems to form a shell around the hole, perfectly matching it but a little bigger.
There cannot be any matter in a black hole.
I understand that space curves around a black hole so nothing has to "get out", but to me the question is- how does space "know" there is such a large mass there in the first place if it can't interact with it?
Sabine: Given that - as both a participant and as an observer - you are 6’ tall and your feet are at the very “surface” of the Event Horizon of a very small Black Hole while your head is some 6’ further distant from (above) said “surface”. Then, if you were watching your own feet fall through said “surface”, would your own feet appear (to your eyes and mind some 6’ further distant) to become redder and redder and take forever to fall through, or would your feet (followed by the rest of you) simply appear (to you) to fall through near instantaneously? I realize that you personally are not6’ tall, but work with me on this.
Sorry, but I don't understand what scenario you are thinking of. Is it someone who falls through the horizon (if so, how fast), or someone who hovers at the horizon (if so, how)?
@@SabineHossenfelder
I interpret it as falling through the horizon.
The gradient of spatial curvature is very large near a small black hole, and it's the large gradient that spaghettifies or rips apart an object. S/he wouldn't remain 6 feet tall as her feet fall through the event horizon; s/he would be ripped apart.
@@SabineHossenfelder I believe what Eric is asking is this: As you fall feet-first into a blackhole, do you observe your own feet redshifting into oblivion as they reach the event horizon, and/or appearing to freeze in time, since your head is still some distance from the event horizon? Assuming a large enough blackhole that you're not spaghettified, when you're halfway into the event horizon, can your head outside the horizon no longer see your feet inside the horizon?
It's closer to appearing to fall through near instantaneously. This is because your head is going to fall through in what appears to you to be finite time; the process only appears to take infinite time to outside observers, not to observers falling through the event horizon.
My physics Prof showed me a proposition, that can do without using quantum gravity: Since a black hole must update its surrounding spacetime curvature upon feeding a new snack (e.g. a star), there is no simple answer to this question. From a point of view of relativity, spacetime-curvature of every piece of mass, that crosses the event horizon leaves its curvature frozen on the horizon. Therefore, we can weigh the black hole, although it can't communicate back its actual total spacetime curvature.
Though, isn't spacetime itself able to exceed the speed of light? Actually idk
@@theeyeofomnipotent Spacetime itself can propagate FTL (faster than light, see Rindler horizon) indeed, however it can't carry information within an existing volume of spacetime with FTL veliocity. Such information- and energy transfers can be found in so called gravitational waves (Details see Wikipedia). The only known mean to achieve apparent (!) FTL-travel is based on the Alcubierre metrics (Details see Wikipedia). With currently known technology, about 8…12c are possible.
@@debrainwasher Spacetime doesn't propagate, not even sure that's a coherent thought.
@@kylelochlann5053 Yes Sir. Spacetime can and does propagate. When the universe expands, spacetime expands with it. Even faster than light. Alcubierre metric describes spacetime kinetics. Further, wie have gravitational waves, that can carry information and energy within a quadropole-oscillation train. Since spacetime curvature is the cause - not the reason - of gravity, we get a moving wave of spacetime.
@@debrainwasher Most of what you have there isn''t right. You can have a perturbation of the metric that propagates, but that is spacetime itself propagating. There is no physics of space expanding, rather, it's an interpretation of the expansion of the spatial coordinates in the FLRW metric and the Alcubierre metric describes a "warp bubble" but that is not spacetime propagating either.
Gravitational waves are only quadrupole to leading order.
Gravity is the condition upon the gravitation field such that the Riemann curvature is non-trivial on one or more components, R^α_{βγδ}≠0. The gravitational field is spacetime and the curvature of spacetime is the curvature of the gravitational field, i.e. gravity.
I appreciate your diligence.
There seems to be a connected paradox to which I haven't found yet any solutions. Event horizons deform when black holes merge, and essentially, when any body with nonzero mass falls into a black hole. It settles into new shape after ringdown, and the dynamics of this deformation reflects trajectory of the infalling body.
Now imagine that the infalling body is a spaceship consisting of two halves and having a Schroedinger cat (=any quantum system in superposition) onboard. Autopilot determines when ship crosses the EH, by observing it's apparent size and the distortion of the sky around it, and opens the box. If the cat is alive, the halves separate and fall further on (slightly) different trajectories; otherwise, the ship continues falling as a whole.
In the first case, deformation of EH, observable from afar, will be different from the second case. This difference of course is extremely minuscule but it is principially distinct from zero. It can be made oomags more than Planck length in a not-so-unrealistic setting. Thus, watching the EH deformation, a distant observer can obtain information which _did_not_exist_ before the ship crossed EH.
Is there some delicate solution, or it really does make for superluminal information propagation?
More, I haven't found any descriptions of this paradox anywhere yet, it was invented in a chat over a couple of beers. But I can't believe that noone have thought about it =]
PS to me, it means that if lightspeed is an absolute limit, then black holes must be something like described in arxiv.org/pdf/1311.4538, with all mass located immediately above EH and nothing below it. (although details here are way beyond my background)
I've always heard, from your perspective you fall into the black hole, from Earth's perspective, you appear to be stuck on the event horizon forever. If this is true, doesn't than mean we never see any accretion whatsoever from our perspective? Anything falling into the black hole we never see get there..
Wouldn´t all these stopped pictures wrap the event horizon after some time for an external observer like an advertising column? I know thats crap, but we could see, how the aliens look like who have already fallen in.
Yes, that's true. Though most of the time when astrophysicists talk about accretion, they are concerned with the accretion disk which is outside of the black hole horizon.
@@Thomas-gk42 The slowed down images don't orbit around the black hole, they just seem to leave it very slowly. There are photons which wrap, but they usually come from behind the black hole. This is what you see in the famous Interstellar rendering.
Hmm, in the simulation renderings of the accretion disk, it shows the sections of the disk closer to the black hole spinning faster and getting hotter (as you would expect). But then, as the matter in the disk gets closer to the horizon, the disk should get redder, start slowing down, and then apparently freeze. Is that correct?
@@SabineHossenfelder Thank you professor Bee.
01:18
_... whereas if you cross a black hole horizon is nothing._
Not exactly nothing. crossing the EH means you're doomed to be shredded eventually.
Unless you're a star, then you are shredded way before entering, for some reason.
edit: I meant it the other way around. A star is shredded before it reaches the horizon , and so would a person, i think.
@@heisag
Depends on the size of the BH, I guess.
Weird stuff happens to the singularity of spinning and charged blackholes. They end up like rings. I think this may work like a blender.
Unless you could place yourself in a bubble wherein all atoms in the system are affected as one unit
we all will be shredded eventually.. by entropy 🥲
Each time you hit Einstein, my hairdresser asks "did he have dandruff?"
I never thought of this, great question.
This is the kind of content I prefer! Much better than bashing on about flat earthers.
User name checks out
They think Mars is flat too.
Excellent channel. Love the comments section.
Politicians are so repulsive but how do we use them for transport to where we really want to go?
The particle that carries the political force is the moron.
I hope this helps
Get an anarcho democratic party on the right curvature and the future of politicians will be dark indeed.
The sleaziest ones have the most caloric energy when burned in a court room.
You don’t , they’ll use you to get to where they want to go
@@Jackiee_Chann Perfect reply.
I had read a theory about being able to possibly make a black hole blow up like a bomb. I wondered how that would be possible if every model says going past the even horizon guarantees with absolute inevitability that you will meet the center. So much so that you could reverse space and time in the equations because the "where end point" was as good as assumed as time outside of a black hole is sure to have a "later."
since its gravity is so strong, would we see the object falling getting redder and redder until it gets invisible? since gravity would be pulling light and making it longer?
Yes
Noooo - Sabine you fell into a black hole - phew you’re back again
I would have expected the astronaut falling in your backhole animation to vanish into the red until it disappeared. Not just get stopped with the same color. I know that seems small, but people with little knowledge will get confused believing that is the actual person and not the photons.
Sabine! I have been wondering about a question for a while. I understand that for an outside observer the freefalling observer’s time seems to stop as the spacecraft approaches the event horizon. but does it mean that for the observer inside the freefalling spacecraft the time of the outside observer (and thus the time of the whole external universe) speeds up infinitely so the rest lifetime of the universe (let’s assume it is finite) gets degenerated to a moment? or does the mathematics of general relativity say otherwise?
Remember, the object isn't sitting at the horizon, it's accelerating towards the hole. At the horizon it's moving away from the outside universe at light speed. As it falls towards the hole, it will see any far away observer redshift. The hole's gravity will help counteract this, since it will blueshift light but this isn't enough to allow infinite compression. The same 'spaghettification' that affects objects falling into a hole will also stretch signals, their view of the outside universe.
@@garethdean6382 1, there is no acceleration. freefalling is an inertial motion. there is no force, thus there is no acceleration. this is the most important lesson of relativity. Einstein simply extended the concepts of "straight line" (using non-Euclidian geometry) [see: geodetics] and inertia (constant motion along straight lines) [thus in general relativity every gravitational motion is an inertial motion along geodesics].
2, it's not true that we move with the speed of light at the event horizon. you confuse our speed with the escape velocity. and our speed cannot reach the speed of light, anyway. at the event horizon the escape velocity is the speed of light in vacuum not because there is so strong force (since there is no force at all, as I have already mentioned), but because there is no such world line (4D spacetime trajectory) that goes out from the black hole. even reaching the speed of light you can stay at that horizon at most.
3, spaghettification is not universal. it depends on the size of the black hole and its geometry. or to be more precise, it depends on the mass and the angular momentum of the black hole. if the black hole is large enough, the tidal effects are not lethal. and if the black hole is spinning, it makes it even more complicated (and sometimes less dangerous).
but I still didn't get answer to my question: what does Einstein's theory say? would we see the whole (rest) history of the universe when we cross the event horizon or not?
To me this means that either A) the phenomena of gravity is an effect of spacetime reacting to the presence of energy while energy itself isn't doing anything particular to broadcast its presence in terms of gravity, or B) general relativity is an approximation of what's really going on and black holes aren't actually completely black (aside from hawking radiation).
More on B: I've been building analytical simulators as a hobby for quite a while and its actually quite amazing how well things like a maximum speed of travel through space, spacetime relativity and quantum uncertainty lend themselves as justifications for computational optimizations IF your goal is to have an interesting universe. The natural consequence of which is that in my simulations, black holes and their internals do in fact experience time, its just that the interaction frequency of anything inside the event horizon is what I normally afford to a single elementary particle, divided among all the particles inside the black hole.
If reality works anything like that, it would mean that there is an actual limit on time dialation, as well as gravitational pull which cannot exceed the speed of light. From our perspective, we can't really tell the difference between whether photons that cross the event horizon stay there potentially forever or for a google ^ google years, either way, the black hole will evaporate well before it becomes relevant.
I assumed gravity doesn't escape a black hole because a black hole IS gravity. Just like water doesn't escape a whirlpool.
How can a thing (black hole) be the effect it creates (gravity.) That is like claiming any mass is gravity because it causes gravity.
@@wesbaumguardner8829 A black hole is just an area of mass condensed to a very small area. Mass warps space-time. This warping is gravity or what Einstein called it. We do not know what space-time is made of or how mass interacts with it. Is it graviton or are they just something we made up to make the math that describes it work?
Is spacetime is emergent? Meaning its not fundamental it arises from something else? We might be barking up the wrong tree. This is a huge blindspot in our understanding. We do not know all the rules of this game yet.
Not knowing all the rules makes it difficult to engineer things like future space travel that is not based on archaic rockets, light sails etc etc. I have a strong feeling we are still cavemen in this area.
We have stagnated for too long on this maybe we will never get there. just like monkeys have yet to build a chip factory or even a basic airplane. Are we just chimps at a slightly higher level?
@@wesbaumguardner8829 well technically it's not exactly gravity but its quintessential gravity, it's pretty much the only thing it is no?
This is an amazing question. Maybe gravity is a true DC quantity. It can vary and have waves but waves may not be necessary.
Gravity gets out of a black hole because you already can't see it so there's no rule it can't just leave whenever it wants to.
So does this mean a black hole's mass is on its event horizon? And if this is so, and you fall through the event horizon, what is it that continues to pull you towards the singularity? Also, if you take forever to cross the horizon to the outside observer, when do you actually get pulled apart from tidal forces from the outside observer's perspective - at the end of time? And if time ends for the outside observer, how can there be enough time left for you to be pulled apart within the horizon?
Okay, I love this channel and I love Sabine. But call it a coincidence or something but as a five-year-old, I had a babysitter with the exact same voice as Sabine's. "You don't behave, I take away your toys and make you sit in the corner facing the wall."
I hope that one day someone evolves the general relativity theory to not call singularity to a black hole just because light cannot escape. It is just a warp of space higher than the velocity of light in space, not an infinite point. Just my opinion. :)
We currently do not have any theories that can give us any concrete answers about what form 'the singularity' actually takes.
Would you like a beer or a cookie, good man?
No one calls a singularity a black hole. The black hole is the whole space(time) inside the event horizon, not only the singularity.
@@starventure me? :) I think beer 😂
@@bjornfeuerbacher5514 I think you get my point, densities become infinite
Black holes are like Space Opera TV series. If fall in, there is no escape.
I just had a thought about the expanding universe.
Could it be that space, like a stretchy fabric, once stretched, does not return completely to its original state?
So gravity stretches it, and once that object dissapears,... space wants to become uniform again, so that one place pushes on the space outside (not just because the gravity is gone, but also because there is now an "extra" space pressure), and the wave travels out,... to stretch the universe a tiny, tiny bit to in order to become uniform again?
Maybe we cannot find the reason for the expansion, because it is perfectly hidden by gravity, and the wave. And we also cannot measure that new extra space.
Or space could also be under tiny, constant tension, from all the outside gravity, and that could contribute too to the stretch.
Just a thought.
Gravity's relationship with black holes is like a never-ending love affair. It's stuck in the ultimate cosmic romance novel, where every page turn leads to another 'Can't Leave You, Black Hole' chapter.
From what I understood, gravity should be more seem like a slippy floor that you slide down towards a heavy object that bends the surface of the slippy floor. So space is essentially a multiplanar slideshow at which the heaviest objects gets most of matter slipped towards it.
We move always straight forward in space time! But if the space time itself is "curved" for us it seems that there is a force.
Was what I was expecting for your take Sabina. I’m certainly on the “panel” for gravitons being purely ‘virtual’. But I’m also on the “panel” that space-time is purely ‘virtual’ as well 😊
Gravitons most likely are like the Cheshire cat’s smile … a fleeting afterimage of a very peculiar exchange.
But, If objects falling into a black hole appear to take forever to do so from our perspective, then from our perspective nothing has ever entered a black hole from the outside, yet. That seems like a logical tautology. So everything we measure about a black hole is measuring what it was like before anything from the outside ever crossed its event horizon. For example, that means we have never observed a black hole that has eaten a star, only ones that are on the verge of eating stars, right?
And, from our perspective, it takes infinite time to actually cross the event horison and get inside a black hole. So, any person or object who falls into a black hole will get there only after our Universe dies. And if black holes are really evaporating, this object will never reach event horizon because that horizon will be shrinking as the black hole evaporates.
Love the channel and watch every episode but I still have a question - by what mechanism does a massive object actually warp spacetime? How is this accomplished? I understand that it does warp but what actually is the underlying process? Maybe I am missing something simple but I never seem to see this explained - just that it happens. Thanks for all the great videos I've learned tons from you!!!
What if empty spacetime exerts positive pressure. Everything from moons, planets, stars and black holes absorbs this positive pressure, which causes a net negative pressure in the direction of the black hole (or whatever massive object). Gravity doesn't have to escape the black hole because gravity is a negated (absorbed) positive pressure caused by empty spacetime (the expansion of the universe).
Great idea to have a quiz! But.. i failed to find the one for this video... please, provide a proper link in a description! Thanks!)
Thanks Sabine!
Something I can't figure out...
How is it that, from the outside world we never see something cross the horizon of a black hole, but still we detect black hole collisions? Shouldn't the collision "never occur" from our point of view? How could even black holes evolve in time (mass increase for ex), when observed from any point not falling into them? Thanks 🙏❤️
Sabine, one of your animations had what looked like space flowing into the black hole. In fact, space is constantly flowing into every massive/energetic object, not just a black hole, and this is why things fall toward heavy objects. Could you do a video on this? Or maybe you already did, forgive me, I don't know all your videos!
It is the spatial coordinates of the Gullstrand-Painleve metric that flow into mass.
Dr SH word choice is important. Whoa - by definition the acceleration at the event horizon is the speed of light - always. The stretch you’d experience is proportional to the gradient of the gravity - how fast gravity increases, which is smaller for large black holes, as you tried to say. Best.
Sabine, don't forget that gravity is not a force!
theoretical question, if you were traveling at close to the speed of light, and crossed the horizon in an orbit of a remote black hole, could you just wait it out? Like, having the tech to suspend yourself indefinitely or whatever, could you wait out the evaporation of the black hole while maintaining orbit as close to the horizon as possible, and eventually end up outside of the horizon again as it evaporates? or would your orbit just slowly decay regardless? is there a minimal mass that possibly could 'escape' through this method, no matter how infinitesimal? and lastly, if technically possible, would you be able to discern any information while just beyond the horizon, or would it be more like all the information at once with no way to differentiate what's what?
once you „crossed” you are done, you got only seconds to live
What would be necessary to capture the EM produced by the merger of charged blackholes? Would it "sound" the same as the gravitational waves or would it come out distorted by them?
1:37
Can you explain that? I thought the definition of the event horizon was the exact distance from the center of the black hole that light couldn't escape. Would that not require the exact same amount of gravitational pull regardless of black hole radius? If the gravitational pull became less, light could then escape? What am I missing here?
From my understanding it's possible to send a very high-speed probe that follows a tight curve that just dips into the event horizon of a large black hole but retains enough momentum to exit.
Nope. Because, no matter what acceleration you manage to give it, the final speed of any massive object will always be below c, and you'd need a speed above c (impossible) to exit the event horizon. In fact the probe's de facto "event horizon" will always be outside the true event horizon, it would be trapped even before reaching the real event horizon.
Even if you managed to create a probe with a dipping arm long enough that the core of the probe would be free from falling in (make the arm in the shape of a dipper or spoon for joke effect), the arm would be literally cut from the probe, because the EM interaction that holds it together can't also escape the black hole once inside the event horizon.
How would a field-based theory of gravity (involving gravitons) account for phenomena like frame dragging, the geodetic effect, gravitational time dilation and redshifting, or the thought-experiment of a kugelblitz?
Black holes are creepier and ultimately more destructive than stars because any time you increase the gravitational pull you speed up the velocity of everything around it and increase the chances of collisions (physical and/or gravitational) between those objects at every increasing velocities. Anything attempting to orbit just outside the event horizon (even for a super massive black hole with an event horizon that spans the diameter of our solar system), is forced to attempt to move at a significant fraction of the speed of light, and it's interacting directly with all the other objects also moving at similar relativistic speeds, but not necessarily the same speed or direction. The net results is not that you just appear to freeze in time from an outside observer's perspective as you approach and cross the event horizon, but instead more than likely will smash into and are turned into dust and radiation against everything else orbiting or falling into the black hole, including most likely the "outside" observer. This is why black holes aren't really black. They're a hole in physics surrounded by near certain death.
Or put more personally. Yes if you were to fall directly into a super massive black hole the tidal forces might not be so high that get spaghettified before or after the event horizon, but you've already been accelerating a long long time such that what remains of your ship/body/etc has already been shredded by collisions with other relativistic speed objects and intense radiation.
Question, when two black holes merge we have seen gravity waves. Given that the collision would have to be within both black holes event horizons, the waving of space time would seem to be initiated at the point of collision within the horizons but spreads beyond them into the universe at large. Does this mean that gravity waves can indeed exit the black holes? or is the warping of space only coming from out side the event horizons??
As time dilates when you get closer to the black hole, wouldn't your perspective of how long the black hole takes to evaporate get dilated as well? If you fall in, which happens first, reaching the singularity or it evaporating away? If it evaporates before you can get there and you could somehow survive the intense blueshifting, would you then find yourself suddenly trillions of years in the future?
May I have some questions?
1) How does reaching the speed of light threshold compare to crossing the event horizon in terms of time dilatation and redshift?
2) Can the environment appear for a viewer inside the event horizon as an expanding space? By this I don't mean an expansion that is caused by the expanding universe outside. What I mean is that could what we observe from the outside as an infinitely dense zero volume object, be observed an expanding space from another perspective? Why? Because that volume is infinitely small and I would guess the fabric of space there must get more and more dense too as we get nearer to that center of zero volume. Hence if you'd go through an imaginary circle in your journey towards the center, your local space would shrink (due to the increasing density of space), making the circle you leave behind look bigger, as if it would expand. So the observer becoming smaller would actually create the illusion of expansion.
3) If we cross the event horizon, from our point of view, could we ever reach the singularity? I mean if space keeps contacting in front of us making us ever smaller and smaller, then the distance we travel would become shorter and shorter, then we may never reach the center, if the total distance we can travel with our contracting rate is shorter than the distance is to the center. Say the center is 2 meter away, but you can only travel .5 meter today, then .25 next day, .125, .0625 and so on... and even with infinite time it would add up only to 1 meter.
+ a bonus question: with the above said conditions, i.e. falling into a black hole having crossed the event horizon, what would we see around us? An expanding space with a background radiation coming from the center of expansion with an infinitely small wavelength in the past and redshifted around us in the present?
You may have questions. I may or may not have the answers.
1. They're the same. Both are accelerating to the speed of light, slowing their clocks relative to the outside universe or observers. The method of acceleration is not important.
2. I'm not sure what you mean but a black hole does not shrink space or objects in it.
3. That's like Zeno's paradox. Again black holes do not shrink space but I can answer this, no. There is an ultimate shortest distance, the Planck length, *correction: which is the shortest measurable distance and that distance cannot be subdivided for very complicated reasons that would require a novel to explain. The same issue would occur and this is why I mention Zeno's paradox, just walking to and crossing a line on the ground. First you have to cover half the distance, then half that distance (a quarter) and then half that and half that and on and on ad infinitum. You don't need a black hole for that and we know from experience that you can walk to and cross a line.
@@Nefville
I think your answer 2) about black holes must be wrong providing 1) is right. Why do black holes shrink space? Time dilatation goes hand in hand with length contraction. Moreover, the volume of the black hole converges to zero nearing the center, hence length contraction must happen in all 3 dimensions.
3) Is not entirely Zenos paradox, as your reference frame is changing with space becoming more dense. In Zenos paradox the reference frame is the same. The whole idea is very similar to approaching the speed of light limit, where you can always add to your speed until the end of time, but never reach the speed of light, because the amount you can increase your speed with becomes ever smaller. Zenos paradox applies to an euclidean coordinate system, but not a hyperbolic.
As to the Planck length, good point, but what if the Planck length is relative to your reference frame as well? As you contract the Planck length contracts too? You wouldn't notice anything I think, other than space expanding around you.
It is because a geometric chronon field exists also where it is geodesic; not only where there is "matter" where the geometric chronon field is not geodesic. Spacetime itself becomes the source of gravity. That is why it does not need to escape the BH.
Excellent question!
sabine am i correct to say that infinity towards the singularity is just a mathematical construct BECAUSE if mass was squeezed out of existence it wouldnt be causally connected to the observable increase of the mass and size of the black hole, therefor i suspect matter to be converted to quantum energy