# Black holes and Event Horizon: Q's ?

I understand why you would be ripped apart if you enter a black hole, but I don't understand the fact that if the black hole is large enough, You would not be ripped apart if you passed the event horizon.

And, some blackholes you would be ripped apart outside of the event horizon?

Can someone explain this for me?

Chronos
Gold Member
Tidal forces are the culprit. Parts of you nearer the black hole would experience stronger gravity than parts more distant. In a supermassive black hole the difference is hardly even noticeable until you are well within the event horizon. For a 'small' - say 10 solar mass - black hole you would be torn apart long before you reached the event horizon. The force differential is proportionate to the square of the distance between the inboard and outboard parts of an object with respect to the singularity.

Hey, thanks for that. Just what I was looking for!

One more thing. Can you explain to me how time is altered while entering a black hole? Mainly, comparing looking at someone enter a black hole, and actually going into one. I don't understand how time is "altered" ?

Thanks!

Chronos,

Given the statement:

" For a 'small' - say 10 solar mass - black hole you would be torn apart long before you reached the event horizon."

What size would a Black Hole have to be, to cause an object falling into it to appear stationary forever at the horizon?

If one is torn apart prior to a black holes horizon then said object would appear to be a hot soup of particles rotating at the horizon relative to an outside observer.

If the Hole was big enough said object would disappear relative to an outside observer as a whole.

But in between the very small BH and very large BH , the object may hover forever at the horizon as viewed from an outside observer.

What size BH would that be?
Would the mass of the object falling in play a role?

Test,

FtlIsAwesome
Gold Member

Tidal forces are present for objects other than blackholes, too. A neutron star can tear something apart, and it doesn't even have an event horizon. A white dwarf can break stuff apart also.

What size BH would that be?

Any size black hole. The event horizon is the point at which light from an object no longer reaches an external observer. As an object approaches that point you will see it appear to move slower and slower, as it takes the light longer and longer to struggle out of the gravitational field. You will also be observing by radio waves at the end, as the light becomes more and more redshifted.

Would the mass of the object falling in play a role?

Only if it was extremely massive. When two approximately equal mass black holes merge, space time in the area gets put through a blender.

Chronos,

Given the statement:

" For a 'small' - say 10 solar mass - black hole you would be torn apart long before you reached the event horizon."

What size would a Black Hole have to be, to cause an object falling into it to appear stationary forever at the horizon?

If one is torn apart prior to a black holes horizon then said object would appear to be a hot soup of particles rotating at the horizon relative to an outside observer.

If the Hole was big enough said object would disappear relative to an outside observer as a whole.

But in between the very small BH and very large BH , the object may hover forever at the horizon as viewed from an outside observer.

What size BH would that be?
Would the mass of the object falling in play a role?

Test,

we could throw you in one and find out. lol

Hey, thanks for that. Just what I was looking for!

One more thing. Can you explain to me how time is altered while entering a black hole? Mainly, comparing looking at someone enter a black hole, and actually going into one. I don't understand how time is "altered" ?

Thanks!

Time is not altered for the person falling towards the event horizon, but time for him APPEARS to slow to the observer outside the event horizon.

What happens is that the light from the faller (what the observer sees) slows to a trickle as the faller gets closer to the event horizon. This makes it seem that he is slowing down, when in reality we just can't see him finish the fall. It's analogous to a listening to a song on a tape recorder when the batteries are running down.

What happens is that we never see the faller actually fall beyond the event horizon... he is just frozen in time at the event horizon, just like the song is stuck at the spot where the batteries ran out.

Of course, from the faller's perspective, he is going straight towards the black hole at an accelerating velocity and eventually gets torn apart by tidal forces (which has nothing to do with the event horizon).

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DaveC426913
Gold Member

Tidal forces are present for objects other than blackholes, too. A neutron star can tear something apart, and it doesn't even have an event horizon. A white dwarf can break stuff apart also.
So can a planet for all that matter - its Roche Limit. If the Moon came within Earth's Roche Limit the Moon would disintegrate under the same tidal forces.

FtlIsAwesome
Gold Member
So can a planet for all that matter - its Roche Limit. If the Moon came within Earth's Roche Limit the Moon would disintegrate under the same tidal forces.
Yep. It would make a ringlike phenomenon. Saturn's rings are partially within its Roche limit.
The minimum Roche limit for the Moon (assuming it's perfectly rigid) is ~9,500 km, and the maximum Roche limit (assuming it's perfectly fluid) is ~18,300 km. The actual limit will be somewhere between depending on the Moon's rigidity.

Some moonlets in Saturn's rings within the Roche limit are still intact because of their tensile strength, not their self-gravity.

A person is a lot smaller than a moon, so tidal gravity is greatly reduced; planets have negligible effects on him/her. That's why it only shows up in extreme cases such as blackholes, neutron stars, and white dwarfs. Maybe a few other cases such as massive stars or something, but I'm not sure.
Though a person falling into a blackhole has nothing to do with the Roche limit (which is limit where an object's self-gravity is overcome), rather it's the person's "tensile strength".
There's probably some way to calculate when a person will get spaghettified.

EDIT: Found the equation on the Wikipedia Spaghettification page.

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Chronos,

Given the statement:

" For a 'small' - say 10 solar mass - black hole you would be torn apart long before you reached the event horizon."

What size would a Black Hole have to be, to cause an object falling into it to appear stationary forever at the horizon?

If an object can enter a black hole then the size doesn't matter, for a distant observer stationary with the hole any object "appears" to remain at the horizon but slowly red shifts out of existence.

Chronos,
If one is torn apart prior to a black holes horizon then said object would appear to be a hot soup of particles rotating at the horizon relative to an outside observer.

No, orbits around a block hole are tricky, the horizon is at 2M, beyond 6M they are stable timelike orbits, 3 - 6M there are unstable timelike circular orbits, for 2 - 3M there are no timelike circular orbits. Elliptical orbits may enter this region. At 3M there are stable lightlike orbits. There are spacelike orbits everywhere but no object can follow them.

If the Hole was big enough said object would disappear relative to an outside observer as a whole.

But in between the very small BH and very large BH , the object may hover forever at the horizon as viewed from an outside observer.

No. See above.

Yep. It would make a ringlike phenomenon. Saturn's rings are partially within its Roche limit.
The minimum Roche limit for the Moon (assuming it's perfectly rigid) is ~9,500 km, and the maximum Roche limit (assuming it's perfectly fluid) is ~18,300 km. The actual limit will be somewhere between depending on the Moon's rigidity.

Some moonlets in Saturn's rings within the Roche limit are still intact because of their tensile strength, not their self-gravity.

A person is a lot smaller than a moon, so tidal gravity is greatly reduced; planets have negligible effects on him/her. That's why it only shows up in extreme cases such as blackholes, neutron stars, and white dwarfs. Maybe a few other cases such as massive stars or something, but I'm not sure.
Though a person falling into a blackhole has nothing to do with the Roche limit (which is limit where an object's self-gravity is overcome), rather it's the person's "tensile strength".
There's probably some way to calculate when a person will get spaghettified.

EDIT: Found the equation on the Wikipedia Spaghettification page.

Tidal forces are the same (or close to) in Newtonian and GR computations. The tidal acceleration on an extended object near the event horizon is ~c2/Rs2 times the distance from the object's centre of mass, within a factor of two. Thus why small BHs have such extreme tidal forces and big ones are so mild. Rs is ~3,000 metres for the Sun, so a solar mass BH has tidal accelerations of ~1010 m/s2 per metre of object at the event horizon. If the body can be reinforced to take ~1,000 gees then the minimal hole masses ~1,000 solar masses to survive crossing the horizon. Of course the tides get ridiculous near the singularity...

Incidentally the infall time, in the falling reference frame, is $$\pi$$.M/c where M is half Rs. Thus a drop into M87's 8 billion solar mass BH lasts ~35 hours once you cross the event horizon.

so u r saying its takes 35 hours before u r ripped apart by the solar mass black hole?

DaveC426913
Gold Member
so u r saying its takes 35 hours before u r ripped apart by the solar mass black hole?

Not necessarily, 35 hours is the time it takes to fall from the EH to the centre. Where you are spaghettified along the way is a different matter.

And please, do not use textspeak.

In the interest of conveying ideas as clearly as possible, posts are required to show reasonable attention to written English communication standards. This includes the use of proper grammatical structure, punctuation, capitalization, and spelling. SMS messaging shorthand, such as using "u" for "you", is not acceptable.

FtlIsAwesome
Gold Member
so u r saying its takes 35 hours before u r ripped apart by the solar mass black hole?
No, he was saying that's how long it takes to reach the singularity of an 8 billion solar mass blackhole after crossing the event horizon.

No, he was saying that's how long it takes to reach the singularity of an 8 billion solar mass blackhole after crossing the event horizon.
Exactly. The same dive into a solar mass BH takes ~1/64,000th of a second.

Not necessarily, 35 hours is the time it takes to fall from the EH to the centre. Where you are spaghettified along the way is a different matter.

And please, do not use textspeak.