Would crossing the event horizon of a black hole be instantly fatal?

[Hypercube]

I recently thought of something. I've heard several times that if a person were to fall into a supermassive black hole, that they would survive quite comfortably for a while even after they've crossed the event horizon due to the lack of significant tidal forces. But there's one thing I don't understand. From what I've read about black holes, the event horizon is the absolute point of no return; it's the point at which outward movement becomes impossible. In other words, once you cross the event horizon, the only possible direction you can move is inward towards the singularity.

 

Now this got me thinking though; if that is true, and past the event horizon nothing can move in a direction away from the singularity, then crossing the event horizon should be almost instantly fatal regardless of the size/mass of the black hole. Because that would mean that your blood would no longer be able to flow, your brain would no longer be able to send nerve impulses, heck, your heart wouldn't even be able to beat because it would by definition involve some kind of movement AWAY from the black hole. So why is it that it is often said a person could survive within the event horizon of a supermassive black hole for quite some time before hitting the singularity?

 

Have I missed something?

[Airbrush]

So why is it that it is often said a person could survive within the event horizon of a supermassive black hole for quite some time before hitting the singularity?

 

How long does it take to be crushed to the size of an atom after penetrating the event horizon for a supermassive black hole? It will not take "quite some time before hitting the singularity. Gettting accelerated to near light speed in a tiny fraction of a second will also cause you to disintegrate.

[dimreepr]

I recently thought of something. I've heard several times that if a person were to fall into a supermassive black hole, that they would survive quite comfortably for a while even after they've crossed the event horizon due to the lack of significant tidal forces. But there's one thing I don't understand. From what I've read about black holes, the event horizon is the absolute point of no return; it's the point at which outward movement becomes impossible. In other words, once you cross the event horizon, the only possible direction you can move is inward towards the singularity.

 

Now this got me thinking though; if that is true, and past the event horizon nothing can move in a direction away from the singularity, then crossing the event horizon should be almost instantly fatal regardless of the size/mass of the black hole. Because that would mean that your blood would no longer be able to flow, your brain would no longer be able to send nerve impulses, heck, your heart wouldn't even be able to beat because it would by definition involve some kind of movement AWAY from the black hole. So why is it that it is often said a person could survive within the event horizon of a supermassive black hole for quite some time before hitting the singularity?

 

Have I missed something?

 

If we take the hyper gravity out of the equation and assume you wouldn’t be mashed or mangled. The event horizon would be unnoticeable, like when approaching a waterfall in a boat that has a top speed of say 10 mph you don’t notice when the speed of the water surpasses 10 mph you’re just going to go over.

 

 

[MigL]

The event horizon is the radial distance where escape velocity is equal to the speed of light. At any point inside, the escape velocity exceeds the speed of light, no light can ever be emitted and the event horizon is black.

 

At this moment you are standing on the surface of the Earth, and without a Saturn 5 booster rocket, you cannot escape from the Earth's gravitatioonal field either ( escape velocity about 18000 mi/hr I think ), yet it doesn't "mean that your blood would no longer be able to flow, your brain would no longer be able to send nerve impulses, heck, your heart wouldn't even be able to beat" does it ??

 

A black hole is no different from any other gravitational source. It has no magical properties.

Edited April 10, 2012 by MigL

[Hypercube]

The event horizon is the radial distance where escape velocity is equal to the speed of light. At any point inside, the escape velocity exceeds the speed of light, no light can ever be emitted and the event horizon is black.

 

At this moment you are standing on the surface of the Earth, and without a Saturn 5 booster rocket, you cannot escape from the Earth's gravitatioonal field either ( escape velocity about 18000 mi/hr I think ), yet it doesn't "mean that your blood would no longer be able to flow, your brain would no longer be able to send nerve impulses, heck, your heart wouldn't even be able to beat" does it ??

 

A black hole is no different from any other gravitational source. It has no magical properties.

 

But it was my understanding that the event horizon is the absolute point of no return; the distance from the singularity at which the pull of gravity towards the black hole becomes too powerful for any force or object to resist it. I've read that in over a dozen textbooks. The fact that here on Earth we are, for example, able to briefly defy gravity by jumping into the air is because the upward force generated by our legs is temporarily greater than the force of gravity pulling us down. And also, correct me if I'm wrong, a person only feels gravity when they try and resist it; in free fall, you feel weightless.

 

Now I admit I'm no PhD astrophysicist, but wouldn't 'falling' into a supermassive black hole essentially be analogous to being in free fall? That is, in the absence of the extreme tidal forces present in less massive black holes, wouldn't you theoretically only be crushed by the black hole's gravity when you 'hit' the singularity? Now in my mind, and mind you I'm not ignorant enough to use this as anything other than a useful analogy, hitting the singularity would be equivalent to a person falling to Earth from a plane; they would not be 'crushed' by the downward gravitational acceleration of the Earth until they hit the ground.

 

Now, assuming I haven't missed something, the only difference between 'free falling' into a supermassive black hole and free falling to the surface of the Earth is that unlike in the latter, the former would require a (although personally I think this will one day be found to be a flaw in the theory) infinite amount of energy to move in opposition to the inward pull of the black hole's gravity, as like you said, the escape velocity of the black hole is greater than the speed of light once you're within the event horizon.

 

Now that you know my reasons for thinking this, I'll rephrase my question: how would it be possible for your essential bodily functions such as blood flow and nerve impulses to continue past the event horizon when the energy required to 'defy' the gravitational pull of the black hole is infinite (or technically greater than infinite according to E=MC^2)? Last time I checked a red blood cell does not have the >infinite amount of energy that would be required to overcome the black hole's gravity and flow in the opposite direction of the singularity.

[MigL]

You seem to grasp the meaning of the event horizon. It is the distance at which the ESCAPE velocity becomes greater than the speed of light. That is the only constraint, the fact that c cannot be equalled or surpassed and so there can be no escape from the black hole.. The gravitational field is by no means infinite, it is governed by distance and mass just like any other.

 

The effects you would feel near a small black hole or near the centre of a large one are due to differential gravity. The gradient is extremely steep such that your feet ( if falling feet first ) would feel much stronger pull due to gravity than your head would, This is spaghettification due to tidal forces. It tends to stretch in the axial direction and compress in the tangential. It can commonly be seen on the sea level of the earth, higher along the axial direction to the moon ( close side and far side ), and lower in the tangential direction.

Edited April 10, 2012 by MigL

[Airbrush]

"Would crossing the event horizon of a black hole be instantly fatal?"

 

It will be instantly fatal far outside the event horizon. The intense acceleration will kill you, about a Million G's well outside the event horizon. The event horizon only applies to light. Humans are more fragile and travel much slower. Long before you reach the event horizon you will be crushed by extreme G forces from instantaneous acceleration towards the singularity.

Edited April 10, 2012 by Airbrush

[Sato]

From the above paragraphed responses, the simple answer is that you (most likely)wouldn't be alive within the event horizon.

 

edit-Unrelated @OP: Tesseract?

Edited April 10, 2012 by Sato

[Airbrush]

On the same topic, but different conditions, suppose we discovered a mini black hole with the mass of the Earth. That would be the size of a walnut. How could we safely approach it? How close could we get to it without getting destroyed?

 

The faster your speed in orbit around it, the closer you could get to it.

[Hypercube]

On the same topic, but different conditions, suppose we discovered a mini black hole with the mass of the Earth. That would be the size of a walnut. How could we safely approach it? How close could we get to it without getting destroyed?

 

The faster your speed in orbit around it, the closer you could get to it.

 

A mini black hole with the mass of the Earth would have exactly the same net amount of gravity as the Earth itself does from what I remember in university physics, albeit condensed into a much, much smaller volume of space. And if its event horizon was only the size of a walnut, I would think it would be very easy to avoid if you knew it was there.

 

But anyway, this lets me ask another question which has always confounded me. What the heck is meant by escape velocity and why does escaping the gravitational field of an astronomical body such as a black hole or planet have a minimum speed/velocity at all? To put the mindset of my confusion to bear, here's what I would intuitively think. If I jump off the ground, I am temporarily overcoming the Earth's gravity. Now, logic would dictate that if I could build some sort of device capable of providing a constant upward thrust of say 4 kph (I.E. it's capable of lifting me off the ground at a constant speed of 4 kilometres per hour, similar to an elevator). By the very fact that it is able to continually overcome the Earth's gravity, a logical layperson would probably say that eventually they would leave the Earth far behind. But the physics textbooks all say that to break orbit, spacecraft must be travelling extremely high speed, 11 km/sec or something.

 

Please explain why if you are able to get off the ground at 4 km/hour that it is physically impossible to leave the Earth, even though technically speaking the gravity get weaker the higher off the ground you are (albeit by a miniscule amount).

[zapatos]

But anyway, this lets me ask another question which has always confounded me. What the heck is meant by escape velocity and why does escaping the gravitational field of an astronomical body such as a black hole or planet have a minimum speed/velocity at all? To put the mindset of my confusion to bear, here's what I would intuitively think. If I jump off the ground, I am temporarily overcoming the Earth's gravity. Now, logic would dictate that if I could build some sort of device capable of providing a constant upward thrust of say 4 kph (I.E. it's capable of lifting me off the ground at a constant speed of 4 kilometres per hour, similar to an elevator). By the very fact that it is able to continually overcome the Earth's gravity, a logical layperson would probably say that eventually they would leave the Earth far behind. But the physics textbooks all say that to break orbit, spacecraft must be travelling extremely high speed, 11 km/sec or something.

 

Please explain why if you are able to get off the ground at 4 km/hour that it is physically impossible to leave the Earth, even though technically speaking the gravity get weaker the higher off the ground you are (albeit by a miniscule amount).

The escape velocity is the velocity you need to achieve to break free from the gravity of a body without any further propulsion. By 'break free' I mean that once the escape velocity is achieved, the gravity of that body will never be able to pull you back to it. The closer you are to the center of mass of the body, the higher the escape velocity. The further you are from the center of mass, the lower the escape velocity.

 

If you maintain a continual upward thrust of 4 kph you will eventually break free from the gravity of earth. The distance from the earth where you break free is the distance at which the escape velocity is 4 kph, which would be somewhere far out in space.

 

It is much more efficient to break free from the gravity of the earth at high speeds rather than a continuous slower speed, because at the slower speed you have to lift the rocket just as far, but you have to provide fuel to fight gravity for a longer time.

 

Because it is more efficient to 'sling' the craft into space, that is the way we do it, and that is why the text books speak in terms of escape velocity.

[Hypercube]

The escape velocity is the velocity you need to achieve to break free from the gravity of a body without any further propulsion. By 'break free' I mean that once the escape velocity is achieved, the gravity of that body will never be able to pull you back to it. The closer you are to the center of mass of the body, the higher the escape velocity. The further you are from the center of mass, the lower the escape velocity.

 

If you maintain a continual upward thrust of 4 kph you will eventually break free from the gravity of earth. The distance from the earth where you break free is the distance at which the escape velocity is 4 kph, which would be somewhere far out in space.

 

It is much more efficient to break free from the gravity of the earth at high speeds rather than a continuous slower speed, because at the slower speed you have to lift the rocket just as far, but you have to provide fuel to fight gravity for a longer time.

 

Because it is more efficient to 'sling' the craft into space, that is the way we do it, and that is why the text books speak in terms of escape velocity.

 

And yet the same can't be said for a black hole?

[zapatos]

And yet the same can't be said for a black hole?

Same is true for a black hole. But remember that the escape velocity increases as you get closer to the center of mass. Since a black hole is so small you can get relatively close to the center of mass. Eventually you reach the critical point at which the amount of mass is great enough, and the distance from the center of mass is close enough, that no amount of energy will allow you to reach escape velocity.

 

We are circling a black hole right now that resides at the center of the galaxy, but we are far enough away to be able to escape its gravitational pull.

Edited April 11, 2012 by zapatos

[Hypercube]

Same is true for a black hole. But remember that the escape velocity increases as you get closer to the center of mass. Since a black hole is so small you can get relatively close to the center of mass. Eventually you reach the critical point at which the amount of mass is great enough, and the distance from the center of mass is close enough, that no amount of energy will allow you to reach escape velocity.

 

We are circling a black hole right now that resides at the center of the galaxy, but we are far enough away to be able to escape its gravitational pull.

 

And the Earth's gravitational field is so weak that even the energy contained in a human's biceps, albeit maintained constantly without weakening, is theoretically enough to break free of the Earth's gravity? Wow, gravity really is the weakest force, isn't it. lol.

[MigL]

As has been mentioned, the mini black hole with the mass of the earth would have the same gravity as the earth itself, so why do you think Airbrush, that " the intense acceleration of about a million Gs" would instantly kill you well outside the event horizon ???

At one earth radius from the centre of the black hole your acceleration would be exactly 1G, not exactly killing acceleration is it ??

 

If you jump into the air at 4 km/hr you are not escaping but merely in an orbit, ie you will come back down !!

 

The escape velocity for a black hole is defined as being greater than the speed of light and that is the reason we call them black holes, not even light can escape them. The 'point of no return' as you call it Airbrush, is fixed by the escape velocity. Since you can never equal or exceed the speed of light you can never return from inside the event horizon.

 

I really hope I don't have to explain this again.

[Mosheh Thezion]

There is no actual evidence for BLACK HOLES... the evidence which exists... is of... massive super dense objects... which are dark.....

the evidence... does not actually suggest holes... or inifinite mass density... or anything of the kind.

big black super dense things... yes... but that is all.

 

In fact... black holes orginates from the fact.. that stars leave behind core masses.. of neutron mass density or so... with variations.... super dense objects....

some... who are obsessed with fusion as the only fuel for stars... say... the cores are made by the super nova...

but..

those cores.. could of been there when the star formed... instead of made by the star.

as such...

the entire idea of black holes.. is (could be) based on a false premise... and so.. MAY NOT EXIST AT ALL.

 

-Mosheh Thezion

[MigL]

And the above doesn't even deserve a reply since there is actual observational evidence for black holes...

[Airbrush]

As has been mentioned, the mini black hole with the mass of the earth would have the same gravity as the earth itself, so why do you think Airbrush, that " the intense acceleration of about a million Gs" would instantly kill you well outside the event horizon ???

 

......At one earth radius from the centre of the black hole your acceleration would be exactly 1G, not exactly killing acceleration is it ??

 

Yes, at one earth radius (about 4,000 miles) the acceleration is 1G and you are safe. The event horizon is the size of a walnut. To remain in orbit around the walnut you will need to travel as fast as a low-orbit satellite.

 

To orbit the black hole at a distance of only 2,000 miles, for example, you will have to travel at a much higher speed to remain in orbit. The closer you get to the walnut, the more Gs are pulling on you. How fast do you need to be traveling to remain in an orbit of only 10 miles? It seems like some extreme Gs will be experienced when you are only a few miles from the walnut, and that will be enough to kill any human. At some point your technology will be inadequate to keep you in orbit, and you fall into it.

 

How close can you safely get to such a mini black hole if the top speed of your spaceship is 20 miles per second?

[imatfaal]

Yes, at one earth radius (about 4,000 miles) the acceleration is 1G and you are safe. The event horizon is the size of a walnut. To remain in orbit around the walnut you will need to travel as fast as a low-orbit satellite.

 

To orbit the black hole at a distance of only 2,000 miles, for example, you will have to travel at a much higher speed to remain in orbit. The closer you get to the walnut, the more Gs are pulling on you. How fast do you need to be traveling to remain in an orbit of only 10 miles? It seems like some extreme Gs will be experienced when you are only a few miles from the walnut, and that will be enough to kill any human. At some point your technology will be inadequate to keep you in orbit, and you fall into it.

 

How close can you safely get to such a mini black hole if the top speed of your spaceship is 20 miles per second?

 

Surely bodies in freefall - ie only force is gravity - will not have a problem with g, until tidal forces become a problem. g forces are a problem to the human body when there is an opposing force - otherwise you just go faster and that's not a problem. In open space (ignoring collisions which we must) why is increased speed a problem? - galiliean relativity would say that all your experience inside the constant velocity ship would be the same as stationary

 

Orbits in free spaces (ie not dragging through an attenuated atmosphere) are fairly stable - the energy needs to be dissipated for the orbit to be unstable (the movement of the earth, sea etc by the moon will do it. As would the tiny amount of atmosphere in low earth orbit, or I think gravitational waves. But all a pretty slow for this example).

 

DocRock gave a great worked example of the magnitude of tidal forces a few months back - I will dig out the link

[Mosheh Thezion]

And the above doesn't even deserve a reply since there is actual observational evidence for black holes...

 

 

 

eh no... there is evidence.. for super massive... super dense things.. which are dark.

 

THAT.. DOES NOT PROVE THEY ARE HOLES IN SPACE... it does not... not prove black holes exist as proposed.

 

For you to say it does, is nothing but an assumption YOU HAVE BEEN TAUGHT.

IF YOU BELIEVE THINGS... just because others tell you its true... YOU GET MISLED.

 

Sorry.

[hypervalent_iodine]

!

Moderator Note

Mosheh Thezion, leave the speculation out of main Science forum and stop hijacking the thread, please.

Do not respond to this.

[ACG52]

IF YOU BELIEVE THINGS... just because others tell you its true...

 

If you don't accept things, in spite of overwhelming evidence, you're a crank.

[questionposter]

eh no... there is evidence.. for super massive... super dense things.. which are dark.

 

THAT.. DOES NOT PROVE THEY ARE HOLES IN SPACE... it does not... not prove black holes exist as proposed.

 

For you to say it does, is nothing but an assumption YOU HAVE BEEN TAUGHT.

IF YOU BELIEVE THINGS... just because others tell you its true... YOU GET MISLED.

 

Sorry.

 

There's plenty of evidence, but there's no direct proof. What else do you think throws stars around it at 40 kilometers per second? Neutron stars can't do that, the physics predicts that when something is denser than a neutron star, it collapses into something which by proportion to size and mass creates an escape velocity greater than light.

[Mosheh Thezion]

There's plenty of evidence, but there's no direct proof. What else do you think throws stars around it at 40 kilometers per second? Neutron stars can't do that, the physics predicts that when something is denser than a neutron star, it collapses into something which by proportion to size and mass creates an escape velocity greater than light.

Thank you, an open mind, there is no proof, but there are very large things, with density greater than neutron stars, that much we know for sure.

I would not dispute that they are black, as they are.

As to the topic, crossing the event horizon would be the point of no return, where the gravity is so great, I would think, yes it would be fatal, probably instantly, as our bodies are smashed against the ship floor as it is pulled in. and as we get closer, the density of our ship would go up, until we match the density of the object we impact.

Personally, I am interested in what happens to the atoms themselves, as they would also squash, and likely the electron clouds would fail, and nucleons would likely start ripping threw those electron clouds as atoms collapse and join the matter in what ever form it exists in the 'proposed' black hole object.

At what gravity level do atoms stop being atoms??? anyone know?

-Mosheh Thezion

 

 

 

[MigL]

If you are in free fall through the event horizon of a massive black hole like the ones found in the centres of galaxies, your body and space ship would undergo the same acceleration until very close to the possible singularity, well inside the event horizon. So I really don't see how your body would get smashed against the ship's floor. Do you know what freefall is or are you making this up as you go along ?

Typical galactic centre black holes have masses of millions of suns and are therefore huge. Even travelling at the speed of light ( something you still cannot do inside the event horizon ) it would take some time to reach the possible singularity. Why don't you take this opportunity to try and figure out the radius of such a black hole and the time it would take to get close to the centre at realistic ( not relativistic ) speeds. The fact that you "personally, are interested in what happens to the atoms themselves, as they would also squash, and likely the electron clouds would fail, and nucleons would likely start ripping threw those electron clouds as atoms collapse and join the matter in what ever form it exists" at the possible singularity, is of no concern because it happens at close distances to the singularity. The other 90% of your subjective time spent inside the event horizon, you wouldn't even know it. Do some reading on black holes, I recommend 'Black Holes and Warped Space Time, Einstein's outrgeous legacy' by Kip Thorne, or would you rather keep making stuff up as you go along ?

 

Again I have always stated large black holes, not walnut sized ones which would have appreciable tidal forces, but then again so would neutron stars at very close range