## How Would an Astronaut Falling Into a Black Hole Die? 412 412

ananyo writes

*"According to the accepted account, an astronaut falling into a black hole would be ripped apart, and his remnants crushed as they plunged into the black hole's infinitely dense core. Calculations by Joseph Polchinski, a string theorist at the Kavli Institute for Theoretical Physics in Santa Barbara, California, though, point to a different end: quantum effects turn the event horizon into a seething maelstrom of particles and anyone who fell in would hit a wall of fire and be burned to a crisp in an instant. There's one problem with the firewall theory. If Polchinski is right, then either general relativity or quantum mechanics is wrong and his work has triggered a mini-crisis in theoretical physics."*
## Re:Gravitational tides will kill you (Score:5, Informative)

Everybody repeat after me: "Black holes ain't yer friend. Don't try to hug them, you will die."

## Re:We must find out for sure! (Score:5, Informative)

I suspect that you're confusing gravity with density - as the black hole's event horizon gets bigger, the density gets lower. I can't remember exactly, but if the event horizon is somewhere around the radius of our solar system, you get an average density around the same as our atmosphere. The gravity's still a heck of a lot higher than 1g at the event horizon though.

## Re:Gravitational time dilation (Score:5, Informative)

## Re:Black hole argument (Score:3, Informative)

I thought that black holes were still theoretical. Or have they been scientifically proved, and I'm just an asshat?

They've been observed - just because there's theoretical work being done about something doesn't mean it hasn't been shown to exist.

## Re:Black hole argument (Score:5, Informative)

Have we been to a black hole? No.

Have we taken photos of an actual black hole? No.

Have we seen gravitational effects that look exactly like what a black hole should have? Yes.

Do those gravitational effects calculate out as something of several to millions of solar masses in a tiny volume that can't exist in any non-black hole way that we are aware of? Yes.

Have we seen the radiation from an accretion disk falling into and being destroyed by a black hole as predicted? Yes.

Is a black hole what astrophysicists think it is? Probably.

Is a black hole what non-scientists (hollywood, general public, dentists, etc) think it is? Probably not.

Do you really exist? This is about black holes, but your existence is only a bit less theoretical than that of a black holes, though some of the specifics of either may not be what is generally thought about them.

And no, a black hole is not god dividing by zero. It's more likely an alien mad scientist multiplying by the square root of negative zero.

## Re:We must find out for sure! (Score:3, Informative)

No, light can most certainly escape from 1 G. Gravity IS the curvature of space. The Event Horizon is the point at which escape velocity = c.

## Re:Misleading (Score:5, Informative)

The whole "what would happen to an astronaut"

...is part of a collection of classic thought experiments by real scientists which predate the internet. Your concerns are misplaced.

## Re:Well.... (Score:4, Informative)

"astronaut" - Someone on who has gone into space.

"fall" - 'to descend under the force of gravity"

"Into" - "to the inside of"; Also "toward or in the direction of:"

"black hole" - "an object in space so dense that its escape velocity exceeds the speed of light"

"die" - "to cease to live; undergo the complete and permanent cessation of all vital functions; become dead."

Or possible in this case: "to cease to exist" literally.

"Ass" - You.

## Re:We must find out for sure! (Score:5, Informative)

No matter the size of a black hole, gravitational acceleration at the event horizon is c per Planck time. That's not "infinite", but it is maximal. Anything at the event horizon of a black hole will have its velocity increased by c toward the singularity as quickly as theoretically possible. You cannot build a rocket to put out that kind of acceleration in the other direction to keep you in place, because you cannot build a rocket to get you up to c at all, in any amount of time. Your talk about time axes sounds like it's an echo of a description of why you can't build a rocket to get up to c. It has nothing to do with black holes specifically, other than that you would need to get that fast to escape the event horizon of one.

If you were made of light, however, you would be moving at c, and you could orbit at the event horizon. If you're just above the event horizon, you could in theory get moving fast enough to orbit just outside of it. And in orbit, you don't feel any acceleration from gravity; you are free-falling around the black hole, and continually missing it. You could also have some flight path requiring 1g of constant acceleration to keep you from falling in. The size of the black hole doesn't matter for any of that; if you are anywhere outside the event horizon, you can find a flight path that will make you feel any amount of acceleration you want, for as long as you have fuel to maintain that kind of acceleration. (For an orbit, you feel zero acceleration, and so need no fuel and can maintain it indefinitely).

Where the size of a black hole does matter, and what I think you were thinking of in your earlier post about black holes of 100 solar masses or such, is tidal forces. These are the forces which pinch and stretch your body in uneven ways. Imagine you had a tetherball pole in the middle of a schoolyard. You stand far off to the east of it, facing it, with your arms outstretched. The lines from both of your hands, and your elbows, and your nose, toward the tetherball pole, are all roughly westward, so if you were to be pulled toward it, your whole body would be pulled more or less evenly. But if you stand right next to it, with your arms stretched out, your nose is pulled west, but your right hand is now north of it, and your left hand is south of it, so they get pulled south and north respectively, and the pole pulls your hands toward each other. If, like gravity, it also pulls harder the closer to it you are, it will pull your face toward it much harder than it will your hands, and make you smack your nose into it and then hit yourself as your hands fall in behind your head; while from a long ways away, all your body parts are pulled with about the same force.

Likewise with black holes. The closer you are to one, the more the different parts of your body (and spaceship, etc) are pulled in different directions and with different magnitudes. The farther you are from it, the more evenly everything is pulled. A very massive black hole has a very large event horizon, so at the event horizon, you are very far from the center of the black hole, and even though you are still experiencing the same acceleration you would feel at the event horizon of any black hole, it's all pulling you in more or less the same direction, so you could orbit there and suffer no ill effects. Around a small black hole though, even if you were orbiting just above its black hole and feeling no acceleration overall, the parts of you closer to it would need to orbit faster to maintain that effect and so would feel pulled and pinchedcompared to the parts of you further away from it, which would have more speed than they need to orbit and so tend to drift away from it. All in all you would feel pulled in every different direction and your body would be ripped apart. Around a larger black hole, even moving at the same speed to maintain orbit the same distance from the event horizon,

allof you would feel roughly the same effects, so you wouldn't even notice them.## Re:Gravitational tides will kill you (Score:4, Informative)

The idea is that the two particles form, and one is closer to the black hole than the other. One of them

barelyfalls in, while the otherbarelymakes it out. No difference in how gravity effects them, just a difference in initial positions.## Re:Gravitational tides will kill you (Score:5, Informative)

Then why would the particle be affected differently than the antiparticle? Why wouldn't *both* fall into the black hole equally?

Both the particle and the antiparticle

areaffected equally by gravity, but gravity is the weakest force in nature. Think about it: a simplechair, held together by the electromagnetic force, supports you above the ground by counteracting the gravitational attraction ofthe entire Earthpulling you down.Since virtual particle pairs start from vacuum, they are always created with equal but opposite momentum. This momentum can't be very big because the attraction between the pair (usually electromagnetic) has to be strong enough to quickly counteract that initial momentum (and bring the particles back together fast enough for them to still count as "virtual"). But just because the momentum can't be very big

doesn'tmean it can't be big enough for one particle to escape a black hole, if the particles happen to pop into existence with one of them pointing in just the right direction to escape. Hawking predicts that the odds are 50/50 on whether it's the matter particle or the antimatter particle that does the escaping; it has nothing to do with the particles responding differently to gravity.(Keep in mind that the escaping particle doesn't have to rocket out in a straight line at escape velocity. Instead, it can take a few swings around the black hole in a rapidly decaying orbit, until it slingshots out on a hyperbolic path. The smaller the black hole gets, the more definite the position is for every matter/antimatter particle pair, and by Heisenberg's uncertainty principle applied to position-momentum, this makes it easier for one of the two particles to escape. A smaller black hole also has the bonus that, looking out from just above the event horizon, more directions point away from the black hole, giving more chances to escape.)

You

couldactually make a black hole that radiates away Hawking radiation with a bias toward antimatter over matter, or vice versa. It's easy: black holes can have an electric charge, so just electrically charge the black hole! Like charges repel, so if the black hole is positively charged, it will preferentially eject positrons instead of electrons. However, the absorbed electrons neutralize the black hole's electric charge, bringing it back to neutral and making the Hawking radiation return to a 50/50 ratio between matter and antimatter.(We suspect that the universe has a small preference for matter over antimatter, and this is why the universe is made of matter. But this mostly happens for some heavy uncharged mesons, not for lightweight simple particles like electrons. Here, "heavy" means "high energy" means "unlikely to appear in Hawking radiation". So the radiation may not strictly be 50/50, but it should be

veryclose.)## Re:Gravitational tides will kill you (Score:5, Informative)

Assuming you're not trolling, that's a nice story, but that's not how science works.

The trouble is that "what we know about black holes" is all theoretical and mathematical.

Usually, the first step in science is to observe something. In the case of black holes, our knowledge of their existence can be traced back to a few experiments [wikipedia.org], which provided pretty solid evidence

againstthe prevailing theories of aether. The observation that doesn't match the expectation means that the theories aren't right, and must be changed.In fact, many of today's experiments are simply re-running old trials, but with more precise technology. Rather than dropping rocks off a tower, we can measure how fast individual atoms fall, giving us a more exact understanding of gravity. Usually the results are a perfect match [wikipedia.org] for what's expected, but sometimes they aren't.

Black holes were invented to explain present-day theories about the motion of stars and galaxies.

Next comes the theory. Starting from the results of those experiments, Einstein hypothesized his theories of relativity, which are really little more than a collection of relationships derived from the assumption that the speed of light in a vacuum is constant. His theories explained the results of previous experiments, and importantly, provided a set of formulas that can be used to make predictions for future experiments.

Mathematics are very useful in describing measured experiments and observations in the physical universe. As soon as mathematics and computer simulations go beyond what is actually observed and measured, it no longer describes the real world were living in.

The relationships in the physical world are described with mathematics. Sometimes, when math is insufficient to easily describe a particular relationship, new mathematical forms [wikipedia.org] are invented to accommodate the real world [wikipedia.org]. Ultimately, though, every physicist knows that the mathematical models do not

prescribereality, butdescribeour understanding of it. Again, we use those models to predict the outcome of future experiments.At the center of these hypothetical, theoretical black holes is this mathematical entity that has been called a "singularity". This is another mathematical fiction that can't exist in the known universe.

That depends on the rules of the known universe. in 1915, Karl Schwarzchild [wikipedia.org] transformed Einstein's theories of relativity into a form that would require black holes. This means that Einstein's formulas can only be correct

ifthe universe allows black holes. If the universe does not allow black holes, then Einsteins formulas must be wrong - though less wrong than the aether theory they replaced.Perhaps it is time to examine some of these widely held theories that require these mathematical fictions.

That's what experiments [arxiv.org] are [harvard.edu] for [harvard.edu].

No one has ever directly observed a black hole and thereby shown that these things even exist in the real world.

Black holes have been observed many times [harvard.edu].

In 1929 an astronomer named Edwin Hubble discovered that "red shift" of distant galaxies. Then he made the assumption (belief, faith) about the cau