# Hangin' out near a black hole.

#### hedons

What would happen if you could travel at such a speed (faster than the escape velocity) as to have a stable orbit of a black hole just beyond the event horizon and you were to stick your hand inside the boundary?

Would you be sucked in?

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The gravitational gradient would 'spagettify' your arm drawing it out into a long thin string. Similarly the rest of your body would be strongly deformed - closer parts would orbit at a greater rate than the bits further away.......

Would you be 'sucked in'? Hmm - don't think so. It depends if you lost energy (if so you would spiral in)... Probably not I'd say, but I'm no expert...

#### pervect

Staff Emeritus
hedons said:
What would happen if you could travel at such a speed (faster than the escape velocity) as to have a stable orbit of a black hole just beyond the event horizon and you were to stick your hand inside the boundary?

Would you be sucked in?

There are no orbits around the black hole (stable or otherwise) inside the photon sphere, at a schwarzschild radius of r=3M (geometric units). This is the radius at which the orbital speed around the black hole becomes equal to 'c'. Nothing can move faster than the speed of light, therefore nothing can orbit the black hole inside the photon sphere.

The event horizon itself is at a schwarzschild coordinate of r=2M (inside the photon sphere).

You need to accelerate (with a rocket engine) in order to maintain station at a constant 'r' value in the Schwarzschild coordinate system whenever you are inside the photon sphere r<3M.

The required acceleration goes to infinity as you approach the event horizon -so at some finite point short of the event horizon, you will be crushed, and your hand (if you allow it to dangle unsupported) won't be able to support it's own weight and it will be ripped off.

A fun java applet about orbits around a black hole with some of the supporting equations can be found at

http://www.fourmilab.ch/gravitation/orbits/

Thanks Pervect!!

-Hedons

#### Chronos

Gold Member
You would not have to get within an arms length of the event horizon to be torn apart by tidal forces.

#### Dr.Brain

When you put your hand in the event horizon, your upper body would be normal and the part of your hand inside the 'eh' would feel enormous force leading to 'tidal force' on your body which arises due to difference in pulls on your body , something similar to the tides on earth , You would be torn apart.

Infact as a fact, at about 10^5 km away ...you would start feeling the tidal forces.If any person happens to try to go near a black hole....he would start feeling the tidal forces way back before he reaches the black hole and he would die long before he could have the privilege to put his hand into the event horizon.

#### Labguy

Dr.Brain said:
When you put your hand in the event horizon, your upper body would be normal and the part of your hand inside the 'eh' would feel enormous force leading to 'tidal force' on your body which arises due to difference in pulls on your body , something similar to the tides on earth , You would be torn apart.

Infact as a fact, at about 10^5 km away ...you would start feeling the tidal forces.If any person happens to try to go near a black hole....he would start feeling the tidal forces way back before he reaches the black hole and he would die long before he could have the privilege to put his hand into the event horizon.
How large (massive) is this black hole?

A small one would do as you say, but a giant as the one in the center of M87 at about 2 billion solar masses has such a large EH radius that a person near the EH would not experience large tidal force differences between head to toe, or shoulder and hand. He could fall on through and be torn apart later nearer to the center of mass.

#### Dr.Brain

Labguy said:
How large (massive) is this black hole?

A small one would do as you say, but a giant as the one in the center of M87 at about 2 billion solar masses has such a large EH radius that a person near the EH would not experience large tidal force differences between head to toe, or shoulder and hand. He could fall on through and be torn apart later nearer to the center of mass.
Ofcourse, massive black black holes donot lead to effective tidal forces as the difference in forces along the length of the body is pretty small.

#### pervect

Staff Emeritus
Chronos said:
You would not have to get within an arms length of the event horizon to be torn apart by tidal forces.
That depends on the black hole mass. With a big enough mass, the tidal force at the event horizon is managable. For a popular source see Kip Thorne's book "Black Holes & Time Warps: Einstein's outrageous legacy", or the link quoted below. For a technical source, there is a derivation of the tidal force on an infalling observer in "Gravitation" on page 822 - the tidal forces are on the order of GM/r^3 (I've converted the result to non-geometric units from the original in geometric units). The radial tidal force is the largest at 2GM/r^3 - exactly the same as the Newtonian result, BTW. The observed tidal force is also independent of the radial velocity of the observer.

Google finds an online source in Ted Bunn's black hole FAQ:

http://cosmology.berkeley.edu/Education/BHfaq.html

What would happen to me if I fell into a black hole?
----------------------------------------------------
Let's suppose that you get into your spaceship and point it straight towards the million-solar-mass black hole in the center of our galaxy. (Actually, there's some debate about whether our galaxy contains a central black hole, but let's assume it does for the moment.) Starting from a long way away from the black hole, you just turn off your rockets and coast in. What happens?

At first, you don't feel any gravitational forces at all. Since you're in free fall, every part of your body and your spaceship is being pulled in the same way, and so you feel weightless. (This is exactly the same thing that happens to astronauts in Earth orbit: even though both astronauts and space shuttle are being pulled by the Earth's gravity, they don't feel any gravitational force because everything is being pulled in exactly the same way.) As you get closer and closer to the center of the hole, though, you start to feel "tidal" gravitational forces. Imagine that your feet are closer to the center than your head. The gravitational pull gets stronger as you get closer to the center of the hole, so your feet feel a stronger pull than your head does. As a result you feel "stretched." (This force is called a tidal force because it is exactly like the forces that cause tides on earth.) These tidal forces get more and more intense as you get closer to the center, and eventually they will rip you apart.

For a very large black hole like the one you're falling into, the tidal forces are not really noticeable until you get within about 600,000 kilometers of the center. Note that this is after you've crossed the horizon. If you were falling into a smaller black hole, say one that weighed as much as the Sun, tidal forces would start to make you quite uncomfortable when you were about 6000 kilometers away from the center, and you would have been torn apart by them long before you crossed the horizon. (That's why we decided to let you jump into a big black hole instead of a small one: we wanted you to survive at least until you got inside.)
Note that this remark applies to someone freely falling through the event horizon of a black hole. You can still crush yourself (with a powerful enough rocket) by trying to hover near the event horizon, even with a very large black hole. While the tidal forces there are managable, the proper acceleration required to "hold station" will not be if one gets too close.

I worked out the formula for the proper acceleration for station-keeping in some other thread (this result is also in Wald) - you get a result that goes to infinity as r->2m (i.e. the hovering acceleration increases without bound the closer one gets to the event horizon). I'll look up the actual formula if anyone is interested enough.

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#### gonzo

My GR book mentions an interesting point ... aside from tidal forces, if you are freely falling into a black hole you will be unable to identify the event horizon. Local physics won't change much from point to point.

Also, with spinning holes don't you get weird effects due to the gravitomagnetic forces so that you have sort of "two event horizons" depending on which way a photon orbits the hole ... going around it in the right direction it can get much closer?

#### pervect

Staff Emeritus
gonzo said:
My GR book mentions an interesting point ... aside from tidal forces, if you are freely falling into a black hole you will be unable to identify the event horizon. Local physics won't change much from point to point.

Also, with spinning holes don't you get weird effects due to the gravitomagnetic forces so that you have sort of "two event horizons" depending on which way a photon orbits the hole ... going around it in the right direction it can get much closer?
Kip Thorne has some very good descriptioins of hypothetical journeys into a black hole in his book "Black Holes & Time Warps - Einstein's outrageous legacy". You are right in that the observer falling into a large black hole will not notice anything particularly special about local physics, though if he watches the outside universe he will see it contract to a point when he reaches the horizon. For a very large hole, a hovering observer can get fairly close without crushing themselves, close enough that the black hole will fill more than half the sky.

Spinning black holes are very weird - as I understand it, when you get close enough to them, you are forced to rotate in strict lockstep with the black hole. I haven't looked into this case nearly as much as I have that of the non-spinning black hole, but there is a diagram of this occuring at

http://scholar.uwinnipeg.ca/courses/38/4500.6-001/Cosmology/Rotating_Black_Hole.htm [Broken]

A less technical site with a nice drawing is

http://www.gothosenterprises.com/black_holes/rotating_black_holes.html [Broken]

The inner horizon on these diagrams, though, is currently a matter of some debate and the description of it (especially the one given in this second less technical link) should be taken with a grain of salt.

There is a considerable difference in the inner geometry between the Kerr solution and our best estimates of the geometry of an actual imploding rotating mass. The same can be said for the Schwarzshild solution - while the simple, symmetric solutions of Schwarzschild and Kerr are valid outside the event horizon, they do not reflect the instabilities that would likely break the simple symmetry in the interior region. To put this in another way - in the interior regions, the standard Schwarzschild and Kerr solutions are like a pencil balincing exactly on its point - they are not really stable solutions in the interior regions.

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#### Chronos

Gold Member
And pervect's point is entirely correct. When you approach a super massive black hole, you can cross the event horizon without even noticing it. While you ultimately, you get shredded, it really has nothing to do with the event horizon. My mistake. I learn something new every day and pervect is a good teacher.

#### vincentm

Gold Member
What i wanna know is what happens AFTER you enter one, meaning does anyone know as of now? Are they Wormholes? Surely they have to go somewhere.

#### pervect

Staff Emeritus
vincentm said:
What i wanna know is what happens AFTER you enter one, meaning does anyone know as of now? Are they Wormholes? Surely they have to go somewhere.
I'd suggest reading Kip Thorne's book "Black Holes & Time warps: Einstein's outrageous legacy" for the best available answer to this question.

The short version is, if the black hole is not rotating, you will fall to the singularity at the center, and you will be destroyed by the tidal forces of this singularity before you reach it.

The singularity probably doesn't have the simple structure of the Schwarzschild singularity in the interior region, but (probably) has a rather complicated form. It will still pull you apart. A rather grisly analogy was made to being pulled apart and twisted by a taffy-pulling machine.

When the black hole is rotating, things are a lot less clear. I believe that there is some chance that an inner horizon exists in this case. I believe the math suggests that it may be possible to avoid being torn apart by passing through this inner horizon, but what that means physically is not at all clear. There were some interesting papers I ran across on this topic, but I can't seem to locate them to refresh my memory. Something about numerical simulations of the collapse of a scalar field with angular momentum.

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