<p>For a planet like the Earth, it is reasonably straightforward to show that the tidal acceleration <em>across the planet</em> is around <span class="math-container">$1g$</span> at the innermost stable circular orbit (ISCO) of a <span class="math-container">$\sim 10^8 M_\odot$</span> black hole (i.e. an orbit of 3 Schwarzschild radii in size).</p>
<p>Since a person is about 1/6400000 of the extent of the planet, then stretching acceleration they experience, which is proportional to the extent of an object, will be just <span class="math-container">$1/6400000$</span> of <span class="math-container">$1g$</span>. This would not be felt.</p>
<p>If you orbit a less massive black hole at its ISCO, then the tidal forces get bigger. But the planet would then be ripped apart, because that's what happens if the tidal acceleration exceeds the surface gravity. There is no configuration where you can stand on an intact planet (of a sensible size) and experience a direct tidal force.</p>
<p>If you are standing on a planet that falls toward a black hole, the planet will disintegrate before you begin to feel any tidal stretching.</p>
<p>In fact it is also a straightforward calculation to show that, for a free falling person, the time from when the stretching becomes noticeable to reaching the singularity is of order a few tenths of a second; about the time it takes sensations to reach and be processed by the brain, irrespective of the black hole mass.</p>

CC BY-SA 4.0
2021-06-30T09:42:22.267Z
What if the planet were incompressable? Almost like a neutron star. If it has a very small radius ( but the same mass as the Earth) will I stay or will I go? Will I find myself on a Lagragian point at a certain distane so I dont feel gravitational effects (except tidal forces)?
Deschele Schilder
CC BY-SA 4.0
2021-06-30T09:53:02.450Z
What if the planet was "like a neutron star"? Then you wouldn't be standing on it you would be smeared over the surface. The compressibility of the planet has nothing to do with it. BTW Neutron stars are quite compressible and have an average equation of state like $P \propto \rho^2$ which is much more compressible than say the Earth.
ProfRob
CC BY-SA 4.0
2021-06-30T10:00:01.857Z
Will there not be a distance where I dont feel the gravity of the neutron star?
Deschele Schilder
CC BY-SA 4.0
2021-06-30T10:05:01.740Z
@DescheleSchilder If you feel the tides (which is not gravity, but gravity gradient!), then you are not on a planet anymore, as it means the planet cannot exist there. There are no incompressible planets.
planetmaker
CC BY-SA 4.0
2021-06-30T11:38:59.463Z
Event horizons are not material. You may pass them without noticing or any adverse effects. It's just the point of no return. Whether tides are dangerous there depends on the mass of the black hole it belongs to.
planetmaker
CC BY-SA 4.0
2021-06-30T08:13:04.093Z
2021-06-30T08:19:36.440Z
2021-06-30T08:19:36.440Z
ProfRob
<p>If you were to swap the sun with a black hole of exactly the same mass, the only thing you’d feel different is cold.
The Earth would go along its slightly elliptical orbits as if nothing happened, as the new black hole and the old sun would have the same gravitational effects.</p>

CC BY-SA 4.0
2021-07-01T09:23:23.857Z
2021-07-01T09:23:23.857Z
Marco Leonardi
<p>Black holes do not &quot;suck&quot; a planet in, rather, think of them as the sun with an insanely multiplied gravitational force, constantly changing your orbit until it isn't even an orbit, rather a suborbit. Once the planet is within extremely close proximity (like  a spacecraft reentering when coming back to Earth), the gravity alters the structural stability of a planet (Sorta like the Kraken if you play KSP) and then rips it apart. While orbiting the tidal forces would only be an extremely small fraction of a g, and you would be disintegrated before your body would be able to perceive the tidal forces. So yeah, in other words you woudn't really experience anything because you would die first...</p>

CC BY-SA 4.0
2021-06-30T23:40:43.170Z
Hi Linus, welcome to Stack Exchange. Please note that we're not like a discussion forum where users post a range of personal opinions; on SE we're looking for *detailed and authoritative answers*. We assess this by voting; the Help Centre says "Voting up a question or answer signals to the rest of the community that a post is interesting, well-researched, and useful, while voting down a post signals the opposite." See the earlier, more detailed answer as an example of what gets upvoted. For further guidance, see [answer] and take our [Tour]. :-)
Chappo Hasn't Forgotten
CC BY-SA 4.0
2021-06-30T13:33:40.513Z
2021-06-30T13:33:40.513Z
Linus Kerman
<p>I can't get my head around it completely. I am standing on an Earth-sized planet circling a black hole. Let's assume the planet to be an incompressible one.</p>
<p>It is observable that matter is sucked in black holes from stars orbiting around them. At a certain distance though this will not happen anymore. The stuff of the star will stay within the star.</p>
<p>What will I feel on distances for which I will not be sucked in the hole? Will I just feel stretched out? Is this stretching gettting stronger up to the point I get sucked in (when the distance grows smaller)?</p>

CC BY-SA 4.0
2021-06-29T20:14:05.687Z
"sucked in" is not how black holes work. They just have gravity.  Ordinary common-or-garden general relativity gravity.  It doesn't suck any more than the Earth is sucking you to the floor.
James K
CC BY-SA 4.0
2021-06-29T20:16:13.083Z
Wont I experience a tidal force when the planet is close enough?
Deschele Schilder
CC BY-SA 4.0
2021-06-29T20:18:32.510Z
"Garden relativity"  Thats a nice one! :)
Deschele Schilder
CC BY-SA 4.0
2021-06-29T20:18:54.003Z
You'll experince tidal forces at all distances, However if you are close enough to be aware of the tidal forces, the tidal forces on the planet will probably have ripped it apart already.  But "tidal forces" doens't equal "sucking"
James K
CC BY-SA 4.0
2021-06-29T20:20:45.847Z
@JamesK Thats why I assumrd the planet to be incompressable. To avoid it being stretched. So I just get stretched upto me hitting the horizon? Will I be sucked in?
Deschele Schilder
CC BY-SA 4.0
2021-06-30T07:07:59.370Z
Tidal forces happen whatever the object a planet is orbiting.
ProfRob
CC BY-SA 4.0
2021-06-30T07:12:37.150Z
@ProfRob But will I notice them? The closer the orbit of the planet the more I will be stretched (or pressed on the other side of the planet). What if my head passes the horizon (when I try to look in)?
Deschele Schilder
CC BY-SA 4.0
2021-06-30T08:05:36.020Z
If you can "feel" the tidal forces then you won't be standing on an intact planet.
ProfRob
CC BY-SA 4.0
2021-06-30T08:09:21.670Z
@ProfRob Cant we assume it to be a very strong one? What if I orbited all alone? (apart from the fact that I would be very lonely...) But thats maybe no astronomy anymore.
Deschele Schilder
CC BY-SA 4.0
2021-06-30T10:45:33.433Z
Stop adding further questions to your original question.
ProfRob
CC BY-SA 4.0
2021-06-30T10:59:10.993Z
@ProfRob There were 2 closing votes  casr asking for more clarity.
Deschele Schilder
CC BY-SA 4.0
2021-07-01T00:09:35.467Z
"incompressible" is a red herring, since it refers to resistance to an *inward* ("pushing") force, whereas you're asking about *tension/shearing* ("pulling") forces. In simplest terms, if ripping forces exceed binding forces, things get ripped. The Earth is already subject to these ripping ("tidal") forces from the Sun, but you can't feel it. If it was possible for the Sun to become a BH of the same mass (it's not), the tidal forces on the Earth would actually *reduce* slightly.
Chappo Hasn't Forgotten
CC BY-SA 4.0
2021-07-01T00:31:49.577Z
Deschele, I'm not sure this question can be rescued from closure, and you've already had [another similar question](https://astronomy.stackexchange.com/questions/44492) closed. Is your main interest whether a planet can orbit a BH? See [this question](https://astronomy.stackexchange.com/questions/500). If you're more interested in whether such a planet could be *habitable*, see [this question](https://astronomy.stackexchange.com/questions/10373). If neither of these address your concern, what are you looking to find out? :-)
Chappo Hasn't Forgotten
CC BY-SA 4.0
2021-07-01T00:33:55.993Z
Does this answer your question? [Orbiting around a black hole](https://astronomy.stackexchange.com/questions/500/orbiting-around-a-black-hole)
Chappo Hasn't Forgotten
<black-hole><gravity><tidal-forces>
What would I experience on a planet orbiting close to a black hole?
Deschele Schilder
# What would I experience on a planet orbiting close to a black hole?

(Asked by: username_0 on 2021-06-29)

I can't get my head around it completely. I am standing on an Earth-sized planet circling a black hole. Let's assume the planet to be an incompressible one.

It is observable that matter is sucked in black holes from stars orbiting around them. At a certain distance though this will not happen anymore. The stuff of the star will stay within the star.

What will I feel on distances for which I will not be sucked in the hole? Will I just feel stretched out? Is this stretching gettting stronger up to the point I get sucked in (when the distance grows smaller)?

**Tags:** black-hole, gravity, tidal-forces

> **Comments:**
> **username_1**: "sucked in" is not how black holes work. They just have gravity.  Ordinary common-or-garden general relativity gravity.  It doesn't suck any more than the Earth is sucking you to the floor.  
> **username_0**: Wont I experience a tidal force when the planet is close enough?  
> **username_0**: "Garden relativity"  Thats a nice one! :)  
> **username_1**: You'll experince tidal forces at all distances, However if you are close enough to be aware of the tidal forces, the tidal forces on the planet will probably have ripped it apart already.  But "tidal forces" doens't equal "sucking"  
> **username_0**: @username_1 Thats why I assumrd the planet to be incompressable. To avoid it being stretched. So I just get stretched upto me hitting the horizon? Will I be sucked in?  
> **username_2**: Tidal forces happen whatever the object a planet is orbiting.  
> **username_0**: @username_2 But will I notice them? The closer the orbit of the planet the more I will be stretched (or pressed on the other side of the planet). What if my head passes the horizon (when I try to look in)?  
> **username_2**: If you can "feel" the tidal forces then you won't be standing on an intact planet.  
> **username_0**: @username_2 Cant we assume it to be a very strong one? What if I orbited all alone? (apart from the fact that I would be very lonely...) But thats maybe no astronomy anymore.  
> **username_2**: Stop adding further questions to your original question.  
> **username_0**: @username_2 There were 2 closing votes  casr asking for more clarity.  
> **username_3**: "incompressible" is a red herring, since it refers to resistance to an *inward* ("pushing") force, whereas you're asking about *tension/shearing* ("pulling") forces. In simplest terms, if ripping forces exceed binding forces, things get ripped. The Earth is already subject to these ripping ("tidal") forces from the Sun, but you can't feel it. If it was possible for the Sun to become a BH of the same mass (it's not), the tidal forces on the Earth would actually *reduce* slightly.  
> **username_3**: username_0, I'm not sure this question can be rescued from closure, and you've already had [another similar question](https://astronomy.stackexchange.com/questions/44492) closed. Is your main interest whether a planet can orbit a BH? See [this question](https://astronomy.stackexchange.com/questions/500). If you're more interested in whether such a planet could be *habitable*, see [this question](https://astronomy.stackexchange.com/questions/10373). If neither of these address your concern, what are you looking to find out? :-)  
> **username_3**: Does this answer your question? [Orbiting around a black hole](https://astronomy.stackexchange.com/questions/500/orbiting-around-a-black-hole)  

---

## 3 answers

### Answer by username_2 on 2021-06-30. Score: 6

For a planet like the Earth, it is reasonably straightforward to show that the tidal acceleration *across the planet* is around $1g$ at the innermost stable circular orbit (ISCO) of a $\sim 10^8 M\_\odot$ black hole (i.e. an orbit of 3 Schwarzschild radii in size).

Since a person is about 1/6400000 of the extent of the planet, then stretching acceleration they experience, which is proportional to the extent of an object, will be just $1/6400000$ of $1g$. This would not be felt.

If you orbit a less massive black hole at its ISCO, then the tidal forces get bigger. But the planet would then be ripped apart, because that's what happens if the tidal acceleration exceeds the surface gravity. There is no configuration where you can stand on an intact planet (of a sensible size) and experience a direct tidal force.

If you are standing on a planet that falls toward a black hole, the planet will disintegrate before you begin to feel any tidal stretching.

In fact it is also a straightforward calculation to show that, for a free falling person, the time from when the stretching becomes noticeable to reaching the singularity is of order a few tenths of a second; about the time it takes sensations to reach and be processed by the brain, irrespective of the black hole mass.

> **Comments:**
> **username_0**: What if the planet were incompressable? Almost like a neutron star. If it has a very small radius ( but the same mass as the Earth) will I stay or will I go? Will I find myself on a Lagragian point at a certain distane so I dont feel gravitational effects (except tidal forces)?  
> **username_2**: What if the planet was "like a neutron star"? Then you wouldn't be standing on it you would be smeared over the surface. The compressibility of the planet has nothing to do with it. BTW Neutron stars are quite compressible and have an average equation of state like $P \propto \rho^2$ which is much more compressible than say the Earth.  
> **username_0**: Will there not be a distance where I dont feel the gravity of the neutron star?  
> **username_4**: @username_0 If you feel the tides (which is not gravity, but gravity gradient!), then you are not on a planet anymore, as it means the planet cannot exist there. There are no incompressible planets.  
> **username_4**: Event horizons are not material. You may pass them without noticing or any adverse effects. It's just the point of no return. Whether tides are dangerous there depends on the mass of the black hole it belongs to.  

---

### Answer by username_5 on 2021-07-01. Score: 1

If you were to swap the sun with a black hole of exactly the same mass, the only thing you’d feel different is cold.
The Earth would go along its slightly elliptical orbits as if nothing happened, as the new black hole and the old sun would have the same gravitational effects.


---

### Answer by username_6 on 2021-06-30. Score: -2

Black holes do not "suck" a planet in, rather, think of them as the sun with an insanely multiplied gravitational force, constantly changing your orbit until it isn't even an orbit, rather a suborbit. Once the planet is within extremely close proximity (like a spacecraft reentering when coming back to Earth), the gravity alters the structural stability of a planet (Sorta like the Kraken if you play KSP) and then rips it apart. While orbiting the tidal forces would only be an extremely small fraction of a g, and you would be disintegrated before your body would be able to perceive the tidal forces. So yeah, in other words you woudn't really experience anything because you would die first...

> **Comments:**
> **username_3**: Hi username_6, welcome to Stack Exchange. Please note that we're not like a discussion forum where users post a range of personal opinions; on SE we're looking for *detailed and authoritative answers*. We assess this by voting; the Help Centre says "Voting up a question or answer signals to the rest of the community that a post is interesting, well-researched, and useful, while voting down a post signals the opposite." See the earlier, more detailed answer as an example of what gets upvoted. For further guidance, see [answer] and take our [Tour]. :-)  

---

