How does gravity escape a black hole?

[@Krmpfpks]

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.

[@MrElvis1971]

This video is particularly disappointing. It ended with "it doesn't have to". Chasing the algorithm unfortunately has it's limit.

[@Krmpfpks]

@@MrElvis1971 I agree.

[@luddite31]

Yeah this was a terrible video. "Its a math thing". Really??? Thats all shes got?

[@eduardomachado3740]

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

[@williamyoungblood4221]

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.

[@curtisblake261]

An old adage about chocolate. Coffee makes getting up possible but chocolate makes it worthwhile.

[@meandyouagainstthealgorith5787]

Vanilla always plays the supporting role.

[@mikemhz]

During a difficult and busy time in my life, I ate dark chocolate before leaving bed every day. It was worthwhile

[@VeteranVandal]

One with earliest recorded mentions in the 15th century and the other in the last 5000 years.

[@yetti423]

or a boiler if it's winter!

[@Dolemite23554]

Both coffee and chocolate are awful. 🤮

[@retiefgregorovich810]

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.

[@johnmckown1267]

Like governments.

[@wesbaumguardner8829]

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.

[@deananderson7714]

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

[@fred_2021]

@@johnmckown1267 women?

[@fred_2021]

@@deananderson7714 You could. Some raise an eyebrow when I opine that our 'knowledge' is tentative/provisional/a working approximation of reality.

[@johnfranks9271]

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!

[@rouhihossein]

I asked the same question ;)

[@JCAtkeson3]

Yeah good question, why can two event horizons touch each other when a falling astronaut can never reach an event horizon?

[@Harkmagic]

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.

[@JCAtkeson3]

@@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.

[@kylelochlann5053]

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.

[@kensmith5694]

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.

[@kubhlaikhan2015]

Donuts do exactly the same to me.

[@thedeemon]

It's a ring, i.e. that donut is thin as a line. Kerr solution describes it mathematically.

[@kensmith5694]

@@thedeemon It follows that a singularity must be of zero size in at least one dimension. I was making a joke.

[@JCAtkeson3]

You donut remember anything after that?

[@Guy-z6o]

I think I was at that lecture; my mind was the hole in the donut.

[@curtisblake261]

I wonder if the reason I remember my wife's birthday after 40 plus years is because I do the taxes.

[@heypauly2002]

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.

[@geodkyt]

Heh. The reason I recall my sister's birthday is it's Tax Day. 😂

[@jab-gn3sw]

🤣🤣🤣

[@Absaalookemensch]

LOL That's great.

[@robertrjm8115]

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.

[@GeoffryGifari]

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?

[@SabineHossenfelder]

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.

[@GeoffryGifari]

@@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?

[@GeoffryGifari]

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

[@Mernom]

It's the same as mass - outside observers never see anything actually enter the BH. From their perspective, it all sits at the surface.

[@poppy3879]

Interesting

[@arctic_haze]

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

[@SabineHossenfelder]

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...)

[@arctic_haze]

@@SabineHossenfelder Yes, that is what I geberalky meant but not in such eloquent way. Thanks for the insight.

[@zemm9003]

​@@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.

[@dw620]

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.)

[@SabineHossenfelder]

@@zemm9003 Yes, exactly!

[@Lucas.boninsegna]

I love the fact that Sabine doesnt seem to get excited about physics because its beautiful, but instead because it's mind blowing.

[@Musix4me-Clarinet]

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.

[@bsadewitz]

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.

[@keatoncampbell820]

​@@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.

[@bsadewitz]

@@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.

[@bsadewitz]

@@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.

[@keatoncampbell820]

@@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

[@andrewsuryali8540]

Around the 4th minute I suddenly remembered Sabine is married. Now I worry for Mr. Hosenfelder...

[@seriousmaran9414]

Would have to take him a really long way from earth to find a black hole, so I think he's safe for now...

[@dasistdiewahrheit9585]

Consider him simultaneously both alive and dead. That's how quantum mechanics works in the macroscopic world.

[@rjrich2322]

If something is pulling a body,then there must be some force doing it,like gravity.Otherwise,the body is just falling.

[@BigZebraCom]

Sabine's marriage put an end to the annual "Win a date with Sabine Hossenfelder Contest !!!"

[@nolanr1400]

HoSen? Hahaha 😂😂

[@UplandJones1]

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.

[@kylelochlann5053]

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.

[@UplandJones1]

@@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.

[@Jesus.the.Christ]

Brian Greene is not a scientist I would ever trust. He's a believer. He lacks skepticism.

[@LuisAldamiz]

@@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.

[@CreepsCompilation]

Pseudo science...

[@Shin_mid]

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

[@bbbl67]

Or, black holes could be one way portals to the beginning of another universe!

[@Guy-z6o]

Yup, an implication, not necessarily a fact.

[@seanmcghee2373]

I love Sabine. "No amount of chocolate with get you out of it" Great timing and perfect deadman. AND she's a great scientist

[@Thomas-gk42]

I absolutely agree, I just hope, her husband hadn´t to face this experience.

[@JonDoe-zi3mh]

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.

[@drdca8263]

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.

[@drdca8263]

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?

[@Sanquinity]

​@@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.

[@drdca8263]

@@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”

[@George70220]

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).

[@TheMelnTeam]

Given the disks around a lot of these things, in many cases you'd still just burn on your way in!

[@deltalima6703]

Because the disk just sits there while you get wiped across it?

[@Sanquinity]

​​@@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...

[@oohhboy-funhouse]

@@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.

[@mdsmatheus]

how does one enter a blackhole safely then?

[@Sanquinity]

@@mdsmatheus One doesn't, as far as we know.

[@graemep.1316]

2:15 what an animated treat! Thank u video team 😅❤

[@annecarter5181]

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!!

[@gbormann71]

Light years measure distance.

[@annecarter5181]

@@gbormann71 Yes, I know that, but it seemed appropriate for a bit of hyperbole!

[@HyperHowie56]

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

[@HyperHowie56]

@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.

[@greganderson1681]

Thanks! That clarifies pretty much all the questions I had about how gravity works, near black holes and otherwise.

[@Thomas-gk42]

❤A question I never thought about. Expands the intellectual horizon.😊

[@frytura]

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?

[@martf1061]

Yes

[@axle.student]

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).

[@axle.student]

Opps, I fixed a period omission. Sorry Sabine and thank you lol

[@axle.student]

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?

[@Mernom]

​@@axle.studentrotating black holes do allow for a closer ISCO in the same plane as their angular velocity, yes.

[@axle.student]

@@Mernom Thank you, I have seen some interesting videos depicting photon orbits, but they are a little generic (lacking in information granularity).

[@Mernom]

@@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.

[@samedwards6683]

Thanks so much for creating and sharing this informative video. Great job. Keep it up.

[@ClodODirt]

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...

[@rudyberkvens-be]

The statement of the anniversary of your wife is really your terrain, Sabine: exact science.

[@kurtiserikson7334]

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.

[@kylebeatty7643]

SPOUSE: I don't want anything special for my (n)0th birthday ME: Okie dokie

[@elmer6123]

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.

[@viralsheddingzombie5324]

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.

[@Mernom]

The higgs field is it's own thing.

[@absalomdraconis]

​@@viralsheddingzombie5324: We _don't_ really know that.

[@eyeofthasky]

@@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

[@OmateYayami]

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.

[@n-steam]

For that poor person falling into the black hole, if it takes us an infinite time to see him cross the event horizon, then from the travellers perspective time outside of the black hole "speeds up" to such an extent the end of the universe (including the lifespan of the blackhole) arrives before that happens, so does he ever cross the event horizon?

[@andrewclimo5709]

What an excellent physics essay question!!!

[@martf1061]

1:45 Ok, now i get it.. The UNIVERSE IS FLAT.. but the EARTH IS NOT ...

[@pullupterraine199]

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.

[@alinochis1514]

Also check my comment, pls. Thanks.

[@michaelsommers2356]

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.

[@reij1]

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?

[@whisperwalkful]

Black holes can be detected thru their gravitational influence on other objects, even if they cant be observed directly

[@michaelsommers2356]

@@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.

[@michaelsommers2356]

@@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.

[@frankhoffman3566]

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.

[@alwayscurious413]

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.

[@frankhoffman3566]

@@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.

[@kylelochlann5053]

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.

[@frankhoffman3566]

@@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.

[@kylelochlann5053]

@@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.

[@Blackwingk]

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."

[@juzoli]

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.

[@debrainwasher]

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.

[@theeyeofomnipotent]

Though, isn't spacetime itself able to exceed the speed of light? Actually idk

[@debrainwasher]

@@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.

[@kylelochlann5053]

@@debrainwasher Spacetime doesn't propagate, not even sure that's a coherent thought.

[@debrainwasher]

@@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.

[@kylelochlann5053]

@@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.

[@ISK_VAGR]

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.

[@davidwhitlock7137]

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.

[@absalomdraconis]

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.

[@kylelochlann5053]

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.

[@MrX-nv8kp]

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?

[@MrX-nv8kp]

​@@kylelochlann5053but with time delation this may extend to a long time, so wouldnt you evaporate before you reach the it?

[@davidwhitlock7137]

@@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?

[@lcgmilllz3514]

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

[@juzoli]

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.

[@DeeNeuro]

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..

[@Thomas-gk42]

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.

[@SabineHossenfelder]

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.

[@SabineHossenfelder]

@@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.

[@Walter-Montalvo]

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?

[@Thomas-gk42]

@@SabineHossenfelder Thank you professor Bee.

[@Richard-bq3ni]

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.

[@kylelochlann5053]

We observed two smaller black holes merge into a larger black hole.

[@Richard-bq3ni]

@@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?

[@kylelochlann5053]

@@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.

[@Richard-bq3ni]

@@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 😉

[@garethdean6382]

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.

[@carlbrenninkmeijer8925]

Thank you so much for your guidance, we now will have even more Black Hole tourism I can't wait!

[@Iohannis42]

Nothing to see there

[@carlbrenninkmeijer8925]

@@Iohannis42 Read "Lost Horizon"

[@amirpatel1934]

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.

[@thedeemon]

They go around. Just like light, we see how it gets warped around massive objects (gravitational lensing). Gravitational waves propagate at light speed.

[@sabinrawr]

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?

[@ericberman4193]

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.

[@SabineHossenfelder]

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)?

[@jensphiliphohmann1876]

​@@SabineHossenfelder I interpret it as falling through the horizon.

[@brothermine2292]

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.

[@stevenjones8575]

@@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?

[@psychohist]

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.

[@zdzislawmeglicki2262]

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?

[@kylelochlann5053]

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.

[@Iohannis42]

They could eventually slow down over a long enough period of time to lose all rotational energy

[@zdzislawmeglicki2262]

@@Iohannis42 Yes, but they could as well spin up due to accretion.

[@Iohannis42]

@@zdzislawmeglicki2262 Yes, but without new inputs to the system, it will eventually slow down again.

[@DefleMask]

From external perspective time of black hole completely stops on event horizon. Singularity does not exists.

[@sojourn6207]

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?

[@KCUFyoufordoxingme]

A really big black hole and you would not notice. A small one and much bad times.

[@sojourn6207]

@@KCUFyoufordoxingme 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.

[@kylelochlann5053]

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.

[@JanVerny]

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.

[@thepuzzlingparadox]

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.

[@eeman1335]

What is causing the space-time around the black hole to bend if nothing gets out.? They say information beyond the singularity is cut off from the rest of the universe, but something is still connected less the space-time around it would not be influenced or aka "bent". So, information can't be completely cut off, and something indeed has to be getting out.

[@HuygensOptics]

So what do you think: do gravitational waves bend around a black hole due to the gravitational field?

[@kylelochlann5053]

GW follow the same geodesic paths as e/m waves.

[@TheKevinGHutton]

I assumed gravity doesn't escape a black hole because a black hole IS gravity. Just like water doesn't escape a whirlpool.

[@wesbaumguardner8829]

How can a thing (black hole) be the effect it creates (gravity.) That is like claiming any mass is gravity because it causes gravity.

[@Physics072]

@@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?

[@The_Art_Style]

​@@wesbaumguardner8829 well technically it's not exactly gravity but its quintessential gravity, it's pretty much the only thing it is no?

[@tinfoilhomer909]

This is the kind of content I prefer! Much better than bashing on about flat earthers.

[@nas8318]

User name checks out

[@rickdworsky6457]

They think Mars is flat too.

[@RegHoldsworth-ri7hh]

Excellent channel. Love the comments section.

[@Vondoodle]

Noooo - Sabine you fell into a black hole - phew you’re back again

[@allhitstaken6200]

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?

[@Boritis]

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.

[@henrythegreatamerican8136]

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.

[@Campfire_Bandit]

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?

[@garethdean6382]

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.

[@kylelochlann5053]

There cannot be any matter in a black hole.

[@floretionguru2977]

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?

[@davidbarbina7923]

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!

[@Plystire]

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?

[@pavoladam4457]

And I am surprised that after a month, You are the only one complaining. But I am with You: it seems like Sabine made a mistake in this regard. (Unfortunately, it's not the first mistake I encountered in her videos :(.)

[@deandeann1541]

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?

[@mob1235]

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.

[@daemn42]

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.

[@rodneyleonard8714]

That's a really good question.

[@randygilmour]

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.

[@NorthstriderGaming]

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.

[@ZedOhZed]

Sabine, at 1:35 what you said is wrong. The strength of gravity at the schwarzschild radius is always the same, by definition. I think you meant to say the gradient of gravity is less extreme for larger black holes.

[@kylelochlann5053]

No, the black hole surface gravity is weaker at the horizons of larger black holes, along with other measures of the gravitational field. But you're right, she said "gravitational pull" which is exactly zero at the event horizon of every black hole and everywhere else in the universe, and it is the tidal forces (geodesic deviation) that weaker at the horizon of larger black holes.

[@ZedOhZed]

@@kylelochlann5053 No, black holes don't have "surface gravity". No known surface exists. The event horizon is not a surface. If gravity was weaker at the event horizon, then by definition it wouldn't be an event horizon. Light would escape from inside if gravity was weaker. And yes you are right, the gradient is exactly what I said is weaker. You just repeated me.

[@SkyDarmos]

I briefly discuss this in my book. In my view one can't believe in gravitons and event horizons at the same time. Anyway, it is interesting to see how Sabine addresses these things. 1. She explains the fact that we can feel the gravity of things that have falling into a black hole by the fact that from our perspective they can never fall through the horizon, because that would take an infinite amount of time. 2. She explains the fact that things inside the black hole exert gravity on things outside of the black hole by the fact that gravitons are virtual particles and thus do not have to obey standard laws of physics, such as the speed of light limit. Her two explanations represent a double standard in my view, because if nothing can fall in from our perspective, then it must also be true that the star that formed the black hole has also not collapsed below its Schwarzschild radius, and that would mean that there is no event horizon from our perspective. That means we arrive back at my explanation of black holes, namely they still shine and exert gravity, because they are just collapsing stars frozen in time.

[@perlindholm4129]

Idea - You might need a cross-connections type gravity to restore matter after a black hole has been around. In machine learning you end up with cross connection which a dense keras layer resolves. The universe might be persuaded to materialize a cross connect aware gravity equation to fix the mess black holes have made.

[@carlbrenninkmeijer8925]

Each time you hit Einstein, my hairdresser asks "did he have dandruff?"

[@vanosaur]

We would never see anything fall into a black hole in a finite length of time. But you know what does take a finite time? For the black hole to evaporate, thanks to Hawking radiation. So, from our point of view, wouldn't the black hole disappear before anything has had a chance to cross the event horizon?

[@DanielScholtus]

Well Gravity does travel, and it travels at the speed of light, supposedly... that's why we have gravity waves, no?

[@HcVRGbyOB9CHK0chBKaX]

Excellent explanation as always 👍👍

[@RadoslavFicko]

Assuming that the relation (m.c^2/√(1-v^2/c^2) -m.c^2 = G.M.m/R , where the square of the velocity is v^2 = c^2.[1 - R^2.c^4/(G.M+c^2. R)^2] and substituting in the Schwarzschild metric ds^2 =-c^2.(R^2.c^4/(GM+c^2.R)^2)dt^2+........we get that the Schwarzschild radius R must be zero.

[@Mattias_the_unimpressive]

My internal explanation was just "gravity is space falling into massive objects, therefore it doesn't need to get out"

[@johnpayne7873]

Gravitons most likely are like the Cheshire cat’s smile … a fleeting afterimage of a very peculiar exchange.

[@John_O_Maoilearca]

I'm confused now: I thought gravity is not a force. But if it has a 'pull', that does sound like a force. Also, if gravity is simply curvature of space, how can a graviton be a particle of this curved space?

[@franciscook5819]

Mass does leave black holes. When black holes merge, the resultant black hole has a mass less than the sum of the two original black holes. The "energy" difference is in the form of gravitational waves (averages about 5% of the original mass). P.S. I'd love to see this explained in terms of the fundamental "mass" particle the Higgs Boson.

[@kylelochlann5053]

GW and the Higgs boson have nothing in common.

[@franciscook5819]

@@kylelochlann5053 Mass is reputedly an expression of the Higgs boson. The mass of the resultant blackhole is less than that of the constituent black holes. How does Higgs fit into this?

[@kylelochlann5053]

@@franciscook5819 The Higgs field couples to some particles, but most of the mass in the universe has nothing to do with the Higgs mechanism.

[@glencmac]

Gravity doesn't get out. Gravity isn't a particle. It isn't a force. Gravity is the warping of space/time.

[@davidg4288]

Yet gravitational waves still propagate at the speed of light. This may be obvious to those that can do the relativity math, possibly the space/time warp *has* to propagate disturbances at the speed of light?

[@dexter8705]

Gravity is not getting out... It is gravity. And a black hole is a vacuum cleaner but it's not sucking in matter.. it pulls in space and matter gets dragged along with that space.

[@maxwellsimon4538]

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.

[@deltalima6703]

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.

[@LuisAldamiz]

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).

[@MrPoornakumar]

A valid question. The fact that gravity escaped the event "horizon" of a Black Hole, shows that gravity isn't just a wave with momentum. Otherwise (but for its gravity), we wouldn't have noticed even the existence of any BH.

[@jonwhite9633]

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!!!

[@duran9664]

Spacetime is everywhere including the inside black hole 🤏

[@ibrahimsaidali3140]

A: If something is right I will remember it. B: Something in between. C: And if something wrong I will ignore it.

[@patrickmuller3248]

Ok, BUT: Here is another QUESTION: We deteced gravitational waves from black hole (BH) mergers. The press release went something like this: "BH "A" with 8 solar masses merged with a BH "B" with 10 solar masses. They became BH "C" with 15(!) solar masses, which means 3 solar masses went into the gravitational waves" So, how does the mass get out of the black holes and converted into the energy to create the waves?

[@Mycochef]

Sabine, don't forget that gravity is not a force!

[@aSpyIntheHaus]

"It's like forgetting your wife's birthday" = such much trouble for Mr Sabine

[@ZeroOskul]

The ideas presented at the start of the video are specific to the ridiculous ideas in MOND and Q-Grav and have nothing to do with General Relativity. There is no pull of gravity in General Relativity, there is just motion and massive bodies falling relative to each other in space. The more massive a body is, the slower time goes for it and the less massive a body is, the faster time goes for it, also the farther a small body is relative to a large body, the faster the small body's time goes. In General Relativity, time is relative to the observer, so the speeding up and slowing down of time relative to massive bodies is responsible for the illusion of a pull of gravity. Massive bodies distend and warp SpaceTime. In space, if no bodies are around, forever takes no time... but if there are massive bodies around, forever takes forever.

[@johnnynephrite6147]

So Sabine's reading not only my comments on her channel, but my comments on other channels. Cool!

[@0x0404]

Awhile back I was thinking gravity was the sum of unaligned magnetic pull of everything nearby.

[@DhruvRed]

Gravity isn’t a force but it’s due to bending of space time around the massive objects

[@xhelloselm]

The thing with the whole gravity is just spacetime curvature is: Why does its effect have a finite speed then? If it‘s not some force mediated by a particle but just the geometry of spacetime, why does it have to obey the finite speed of causality for stuff *inside* it? Especially since space itself is expanding faster. If a block hole disappeared, would the change in gravitational pull be instantaneous? As I understand, it wouldn’t. So that is a transfer of information (mass disappearing) with finite speed C, and it would cross the event horizon unhindered. Why?

[@romansasik9087]

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!

[@kylelochlann5053]

It is the spatial coordinates of the Gullstrand-Painleve metric that flow into mass.

[@wulphstein]

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).

[@prabagaranm7767]

We commonly imagine falling through a black hole's event horizon to be harmless. However, wouldn't an observer be subjected to intense, high-energy electromagnetic radiation (blueshifted) from the surrounding universe due to the BH extreme gravity?

[@paulmcdonald9592]

I appreciate your diligence.

[@kingstonchi]

Sabine, we kept hearing that gravity is just the curvature of space, not a force. So when Voyager 1 and 2 got 'gravity assisted' by Jupiter and/or Saturn to turn and accelerate to go to where they were going, did the Voyagers exert force onto the warped space that made them curve and turn ? similar to the Roulette ball exerting force onto the track it spins on ? If affirmative, does this mean that force can act on space ? What happens when space is "forced upon" ?

[@kylelochlann5053]

There is no force of gravity. A "gravity assist" is just taking advantage of a particular choice of geodesic paths.

[@kingstonchi]

@@kylelochlann5053 then, like the roller coaster cars hitting the curved rails hard when turning abruptly, did the Voyagers also hit the curving space hard as their paths were changed turning by the curved space ? Also, it was reported that they sped up by those 'assists', how did gravity accelerate them if there were no force ?

[@kylelochlann5053]

@@kingstonchi No, the exact opposite actually. Geodesic curves are path of zero force and zero acceleration. Accelerometers attached to the crafts would read zero, everywhere. They only appear to accelerate to us because we cannot see all of spacetime (we see our own particular 3D sub-manifold).

[@olasek7972]

it is really much simpler to just stick with Newtonian physics to understand Voyager’s path, gravity assist is pure classical orbital mechanics. Gravity assist in this case actually is a tiny transfer of energy from the moving planet to the spacecraft.

[@kingstonchi]

@@olasek7972 Exactly my point ! But they must put on this mantra of gravity is no force [although walks and quacks like a ..] Everything just to stick up for 'no force' .. only warped space .. But warped space can compel Voyagers to change course of motion .. which is why I asked whether Voyagers exert force upon the warped space, as roller coaster carts exert force to the rails they run on .. How do you exert force on space ? You said they increased energy from this assistance, I am not sure that they would agree with that .. again, no force .. no energy transfer .. He was trying to say that there's really no speed change either .. He seems to indicate that the APPEARANCE of increased speed observed by us on Earth is not the real speed increase that the objects are experiencing [again, relativity ..] I wish they would take time to fully explain what they are talking about .. instead of simply making statements .. no force .. just geodesic pathway ..

[@Bananniel]

Isn't the gravitytional pull the same at the eventhorizon of every black hole, no mater its size? When it's the point at which not even light can escape the black hole, doesn't that mean imply that every eventhorizon has the same gravitational pull?

[@Spherical_Cow]

Yes; Sabine misspoke in her effort to simplify the description. What she was really talking about, was not the strength of gravity at the event horizon, but the magnitude of gravitational gradient - i.e. how rapidly gravity increases as you continue falling toward the center of the black hole. For gigantic, supermassive black holes - which can get big enough to contain our entire solar system - the strength of gravity at the event horizon is almost the same as it is 100 km inside the event horizon. That's another way of saying that supermassive black holes have a low gravitational gradient at the event horizon, and infalling objects won't feel much stretching or pinching force as they cross the event horizon. By contrast, for a stellar-mass black hole, the entire event horizon fits inside a 10 km diameter sphere, so upon crossing the event horizon, infalling objects begin to immediately experience very large tidal forces as the gravitational force increases very steeply toward the much-closer singularity. So, stellar-mass black holes have a very high gravitational gradient at their event horizon.

[@Bananniel]

@@Spherical_Cow Ah I see. It does make a lot more sense, if she meant the gravitational gradient instead of the strength of the gravitational force. Thank you for your elaboration.

[@kylelochlann5053]

The gravitational pull is zero, everywhere in the universe. The surface gravity of a black hole decreases with increasing size (mass parameter).

[@TroyIII]

What do you need gravitons for. If two objects exchange particles, they will cause them to move away from each other. Having mass objects exchange gravitons will not explain the gravitation. Maybe it is time to conclude for a fact that nobody managed to prove the existence of gravitons, because they do not exist.

[@noelwass4738]

Thank you, Sabine, you have been saying what I suspected about black holes. This is about the "experiences" or history of an object falling into a black hole extending beyond what the outside observer would or could observe. The outside observer sees events only up to a certain point of the object's history. Time would be slowing down for the outside observer and the outside observer would see the falling object forever stuck on the outside of the event horizon and the images getting redshifted as well. The falling object would have "experiences" that extend beyond this. These "experiences" could include finding out what the inside of a black hole is like but not being able to communicate the experiences. If the falling object had an inbuilt clock, the clock would appear to stop at a certain point according to the outside observer but would in fact continue to function as normal according to the falling object's time.

[@Chazulu2]

The redshifting of any light emitted by any object falling into the black hole would be redshifted beyond the ability to detect at infinity/Earth well before their clocks appear to stop. An object just inside of the event horizon with a hypothetical super blue plank wavelength laser pointer aimed strait at Earth/infinity would not be able to use that laser to communicate any messages to an observer at Earth/infinity because even that maximally energetic light would be redshifted to the level of background noise by the time it reached Earth/infinity, not because the light isn't fast enough. The speed of light is the same for all observers. However, if the laser was pointed at something just outside of the event horizon that had a red light detector and another blue light emitter, then the blue light emitter from inside the event horizon should be able to send binary/digital (but not analog or Quantum) signals to the middle object just outside of the event horizon. That middle object could then take the detected red (shifted from blue) signals from the inner object and use that information to send a blue signal to Earth which would again be redshifted (because the middle object is still in a fairly deep gravity well) but not so much to be undetectable. This game of telephone should (in theory) allow digital information (albeit with a loss of resolution) to be transmitted out of the event horizon of the black hole. That is if the objects near and inside the event horizon weren't being completely obliterated by light coming from the background experiencing the opposite effect and turning to blue ionizing radiation and particle antiparticle pair annihilating radiation. Anihilizing radiation, if you will.

[@noelwass4738]

@@Chazulu2 This is a good theory and I think it would be true if time was not involved and only space involved in the distortion. There is a distortion of both time and space near a black hole horizon from the viewpoint of the outside observer. There would in theory be a trail of falling objects each separated from the other by a reasonably small distance in time and space for which communication between these objects would be possible. That is the theory. The problem I think is that all these objects would still be on the outside of the event horizon (and of course red shifted) according to the stationary outside observer. There are paradoxes here.

[@Chazulu2]

@@noelwass4738 Thank you! So, my guess would be that the light from the cosmic microwave background radiation, or minimally energetic quantum EM fluctuations from flat spacetime/"infinity" become blue shifted enough to cause pair production to occur at the event horizon after having traveled all the way down the gravitational well caused by the black hole. Since pair production has both a matter and antimatter component, it's plausible that as time "stops" for the object falling into the horizon according to a distant observer, that nearby and locally any object crossing the event horizon would be annihilated by the portion of the pair production opposite of it causing half of the photons released from the annihilation to go into the black hole with possibly another round of pair production deeper in, and the other half to be the last escaping source determinable information. If the photon from pair production that goes inwards is itself of a minimum energy level, then a set of concentric horizons could be described, and it's possible that as the size of the black hole becomes smaller, that the distance between concentric horizons gets smaller until they all overlap at the mass of an election or positron, making them naked charged singularities with a minimum fundemental rest mass and charge. The portion of the pair production that is the same as the material falling in (matter vs antimatter) would replace it with a slightly different angle, preventing its momentum from carrying it out the other side. This would effectively cause a beam of electrons entering a black hole to move strictly inwards and/or around the singularity, but never away from it. If Protons and Neutrons are made out of 2 positrons and 1 electron each in distinct tightly bound stable arrangements, then all mater except unionized hydrogen 1 would leave a black hole with a net positive charge due to having more positrons than electrons. The opposite would be true for antimatter.

[@noelwass4738]

@@Chazulu2 You are going into all sorts of details I had not been considering about pair production of particles and so on. Basically, I believe that when an object falls into a black hole there is both a cut off distance as experienced by the outside stationary observer, namely the Schwarzschild radius but also a cut off time as experienced by the outside observer when it somehow observes the slowing down of any clocks for the falling object. The onboard clock would appear to have stopped. The falling object will not experience this cut off distance and also will not experience this cut off time. It will therefore continue to have "experiences" beyond this point in its history in both distance and time. This gives no explanation I know of what happens when two black holes merge.