# Possibility of Black Hole research

• B
• Marshall_Mathers
In summary, a black hole of 30 billion suns would be able to hold a spaceship for a few days before it was ripped apart by tidal forces. However, a black hole with the mass of trillions, maybe zillions of stars would be able to hold a spaceship for longer.
Marshall_Mathers
Hey,

If we take a hypothetical black hole with the mass of thirty billion suns which has the schwarzschild radius of roughly the orbit of the venus and let a spaceship fly into it, would it be possible for the spaceship to do some reseach on the physical nature of the singularity and then somehow fly out of it again or is the space around it while it is in the black hole always too contorted so that there is no possibility of escape? Tidal power lessens the farther you are from the singularity (is that right?), so if we throw all possibility overboard: Is there a hypothetical mass limit when a stellar body is able to (by the means of external energy) escape the event horizon again if he crosses it once? A black hole with the mass of trillions, maybe zillions of stars?
Have a lovely day,
Marshall

Once through the event horizon, there is no escape.

Excuse my question, but why? Wait, it's not the tidal spaces that we are battling, it's space itself, right?
Then my followup question: Is there a hipothetical black hole with enough mass that a spaceship could just chill out at the borders of the event horizon without being ripped apart by tidal forces?

Marshall_Mathers said:
Excuse my question, but why?

Well, one might consider it as a definition of an event horizon of a black hole. You can't escape horizon because it's horizon.

Orodruin
weirdoguy said:
Well, one might consider it as a definition of an event horizon of a black hole.
I would go as far as saying that it is the definition of the event horizon.

Marshall_Mathers said:
Is there a hipothetical black hole with enough mass that a spaceship could just chill out at the borders of the event horizon without being ripped apart by tidal forces?
Yes. Super massive black holes can have little tidal gravity.

George Keeling and Chris Miller
Marshall_Mathers said:
it's not the tidal spaces that we are battling, it's space itself, right?

No, it's spacetime. Once you are inside the black hole, moving into the future is the same thing as moving towards the singularity. And there's no way to avoid moving into the future.

enorbet
The Schwarzschild radius of the Sun is 3 kilometers. The Schwarzschild radius of 30 billion suns is 90 billion kilometers, which is 1,000 times the orbital radius of Venus.

https://arxiv.org/abs/0705.1029

90 billion kilometers is 9 * 10^13 meters. Divide that by the speed of light 3 * 10^8 m. It is 300,000 light seconds, or over 3 light days.

The formula given in the reference gives you a maximum time of 3.14 / 2 * 3 days = 5 days to reach the singularity.

Heikki Tuuri said:
The formula given in the reference gives you a maximum time of 3.14 / 2 * 3 days = 5 days to reach the singularity.
15##\mu##s per solar mass is my rule of thumb (derived from the paper you linked). Note that this is a maximum survival time - no path has more proper time from horizon to singularity, and a lot have less.

Heikki Tuuri
Upshot: you can do all the research you want, you just can't publish.

enorbet, SiennaTheGr8, Ibix and 5 others
Marshall_Mathers said:
Hey,

If we take a hypothetical black hole with the mass of thirty billion suns which has the schwarzschild radius of roughly the orbit of the venus and let a spaceship fly into it, would it be possible for the spaceship to do some reseach on the physical nature of the singularity and then somehow fly out of it again or is the space around it while it is in the black hole always too contorted so that there is no possibility of escape? Tidal power lessens the farther you are from the singularity (is that right?), so if we throw all possibility overboard: Is there a hypothetical mass limit when a stellar body is able to (by the means of external energy) escape the event horizon again if he crosses it once? A black hole with the mass of trillions, maybe zillions of stars?
Have a lovely day,
Marshall
The reason your spaceship can't escape once inside the event horizon is because it would need to be moving faster than the speed of light in order to cross back out. So while you're right that the tidal forces are less for a black hole that is massive enough, the spacetime within the event horizon is what prevents you from leaving once you've crossed over. What's more, inside the event horizon, no signal from any other body can ever reach you( a condition known as "spacelike light cones" among physicists) so there's absolutely no possibility of being able to do any research whatsoever anyway!

alantheastronomer said:
inside the event horizon, no signal from any other body can ever reach you( a condition known as "spacelike light cones" among physicists)

I have no idea where you are getting this from, but it's wrong. It is perfectly possible for objects inside the horizon to exchange light signals. There are limitations on it due to the spacetime geometry, but that's true everywhere in any spacetime, and such limitations have nothing whatever to do with "spacelike light cones" (light cones by definition are null surfaces, not spacelike).

What if two black holes are spirally in towards each other? Could you hypothetically fly between them in such a way that the gravity on both sides net to zero? Or dip into into one BH event horizon and then allow the presence of the second BH to allow u to escape? Or possibly waiting right before they collide when a portion of their combined mass is transformed into gravitational wave energy does the event horizon shrink?

Justin Hunt said:
Could you hypothetically fly between them in such a way that the gravity on both sides net to zero?

In the sense that you would stay equidistant between them, yes. But you would still most likely be trapped inside the new hole that the two spiraling in holes merged into.

Justin Hunt said:
Or dip into into one BH event horizon and then allow the presence of the second BH to allow u to escape?

No. Once you're inside the horizon, you can't escape. There actually is only one horizon in spacetime terms; it just looks like a pair of trousers instead of a cylinder, heuristically speaking.

Justin Hunt said:
Or possibly waiting right before they collide when a portion of their combined mass is transformed into gravitational wave energy does the event horizon shrink?

Again, once you're inside the horizon, you can't escape. Also, gravitational wave emission doesn't shrink the horizon; the GWs from the merger get emitted from outside the horizon.

Justin Hunt said:
Could you hypothetically fly between them in such a way that the gravity on both sides net to zero?
Depends what you mean. You can (at least sometimes) find places where your path is not deviating towards one hole or another, simply from symmetry. But you will always be able to see light from distant stars Doppler shifted due to the holes' gravity.
Justin Hunt said:
Or dip into into one BH event horizon and then allow the presence of the second BH to allow u to escape?
No. Once you cross an event horizon, that's it.
Justin Hunt said:
Or possibly waiting right before they collide when a portion of their combined mass is transformed into gravitational wave energy does the event horizon shrink?
The event horizon(s) of a combining black hole are not spherically (or even cylindrically) symmetric, but settle down to it quite quickly. They change shape, rather than shrinking. And the key point is that once you are inside the horizon, you're inside. Whatever changes happen to the horizon, you can't escape it.

What about gravity waves themselves? Is it theoretically possible to create them? And if so encode information in them if u do? Because apparently they are the only thing that could escape from a BH

Justin Hunt said:
What about gravity waves themselves? Is it theoretically possible to create them?

Sure, just smash some neutron stars or black holes into each other.

If you mean, create them with technology we are going to be able to develop any time soon, no. At least not gravitational waves of an intensity anywhere close to what we can detect.

Justin Hunt said:
apparently they are the only thing that could escape from a BH

No. Gravitational waves cannot escape from inside a black hole's event horizon, any more than anything else can. As I've already said, the GWs we observe from black hole mergers come from outside the horizon.

Justin Hunt said:
Is it theoretically possible to create them? And if so encode information in them if u do?
You mean artificially? Sure, you just wave a couple of neutron stars around and modulate the motion. Actually doing that is left as an exercise for the reader.
Justin Hunt said:
Because apparently they are the only thing that could escape from a BH
They don't escape from a black hole. Any gravitational wave you receive must come from within your past lightcone, and this cannot include anything inside an event horizon.

Dale
So my understanding of gravity in relativity is that it is the curvature of spacetime rather than a force in Newtonian physics. My understanding of gravitational waves are a disturbance of the spacetime curvature. Isn’t this fundamentally different from electromagnetic radiation? Does spacetime not exist within the event horizon? I understand why light can’t escape, but please explain why GW can’t as well

Also, I get that for far away objects LIGO can only detect things like BH merges, but u could have a second ship orbiting the BH that could detect weaker gravitational waves and convert the encoded information into another form of communication.

Possibly something like a particle accelerator could make tiny detectable gravity waves.

Justin Hunt said:
Possibly something like a particle accelerator could make tiny detectable gravity waves.
The Earth going round the Sun produces around 100W of gravitational radiation. Nothing less than a couple of stars is going to produce anything detectable.

Also - note that gravity waves are a kind of water wave. We are talking about gravitational waves.
Justin Hunt said:
Isn’t this fundamentally different from electromagnetic radiation? Does spacetime not exist within the event horizon?
Yes, and yes it does.
Justin Hunt said:
I understand why light can’t escape, but please explain why GW can’t as well
Because they propagate at the speed of light. If they could escape, so could light. Or, to put it simply, nothing can escape the event horizon.

Justin Hunt said:
Isn’t this fundamentally different from electromagnetic radiation?

There are some differences, but there are also many similarities.

Justin Hunt said:
please explain why GW can’t as well

Because GWs have to move on the light cones, just like light does. They can't escape from anywhere that light can't escape from.

Justin Hunt said:
Possibly something like a particle accelerator could make tiny detectable gravity waves.

Just accelerating particles isn't enough. You need a nonzero third time derivative of the quadrupole moment; basically that means you need two massive objects orbiting each other, and their masses need to be not exactly the same. If the detector is close to the source, you might not need neutron star masses, but you would still need objects much more massive than anything we can manipulate now or in the foreseeable future.

It seems odd to me that on a BH merge that a significant portion of their total mass can be converted to Gravitational waves that are released outside the BH event horizon. Intuitively it would make sense if they were created from the impact and originated from the impact. Is this related to Hawking radiation in any way?

Dale
Justin Hunt said:
Intuitively it would make sense if they were created from the impact and originated from the impact.

There is no impact; black holes are vacuum. Their "mass" is not because they contain matter; it's a property of the spacetime geometry.

Justin Hunt said:
Is this related to Hawking radiation in any way?

No. Hawking radiation is a quantum process. Gravitational wave production from a black hole merger is a classical process.

Justin Hunt said:
Intuitively
Unfortunately, intuition is a very poor guide to fundamental physics. You can develop and train an intuitive feel for how the maths goes, but that's a rather different thing.

If relativity were intuitive, we would not have all the threads on the twin paradox that we do. And SR is the friendly and straightforward bit of GR.

Put in another way, intuition is something you get from experience. The typical person has very little experience with situations where relativity becomes noticeably different from Newtonian physics and therefore should expect to have their intuition lead them wrong and full of misconceptions from every-day intuition that no longer applies. After working with teaching SR and GR for years, I feel that I finally have some actual intuition that is applicable to those subjects as a result of being familiar with the theories and the math they contain.

Heikki Tuuri and weirdoguy
PeterDonis said:
It is perfectly possible for objects inside the horizon to exchange light signals.
How so?

alantheastronomer said:
How so?
You can see it's possible from a Kruskal diagram. It obeys the same rules as a Minkowski diagram - light travels on 45° lines and timelike paths are above 45°. There's a fairly narrow window for communication between horizon and singularity, but it can happen. And one way communication is always possible, at least in principle.

It's also obvious from the locally Minkowski nature of spacetime. If we are close enough together then we see spacetime between us as flat, so obviously we can communicate. That can't cut off dead outside "local" distances unless something catastrophic (like the singularity) happens to one partner.

Nugatory and PeterDonis
A practical example. Suppose you are on the stern of a somewhat long spaceship, and you're looking at an object in the bow of the spaceship. While you are looking, the front of the space-ship crosses the event horizon of a very large black hole. You snap snapshots just before, at the same time, and just after the front of the spaceship crosses the event horizon of a very large black hole. The black hole is so large that the curvature at the event horizon is negligible.

What do the photos show?

In all three cases, the photos show the object as normal, as if the black hole were not present. The assumptions we made that the black hole is large enough that curvature / tidal effects can be ignored is important for this result to be correct.

For completeness, I should add an explanation of what I mean by "at the same time". Due to the fact that space-time curvature can be neglected, there is essentially a local inertial frame of reference for the space-ship. It is in this local inertial frame that we judge the idea of "at the same time", via the usual convetions we apply to an inertial frame in Special relativity. You might be familiar with the Schwarzschild time coordinate "t". We are NOT using that time coordinate to judge "at the same time", we are using the notion of time associated with the local inertial frame of the ship. Due to the relativity of simultaneity, we need to specify a particular inertial frame to define the particular notion of simultaneity we are using.

To be specific, consider the photograph taken after the bow of the ship falls through the horizon. The r-coordinate of the light is decreasing with time, but slowly. The r-coordiate of the back of the ship is increasing more quickly. At some point, the light ray from the front of the ship hits the back of the ship. This will only happen after the stern of the ship has crossed the event horizon. But it is an example of "objects inside the horizon to exchange light signals."

Technically, we've only analyzed one signal of the two-way exchange. But the analysis is similar for the case of an exchange of signals in the other direction.,

pervect said:
The r-coordiate of the back of the ship is increasing more quickly.
You seem to mean 'is decreasing more quickly', right?

pervect said:
At some point, the light ray from the front of the ship hits the back of the ship. This will only happen after the stern of the ship has crossed the event horizon.
And is it correct that this happens in the moment the stern is crossing the event horizon as this light ray is "frozen" at ##r=2m## because it was emitted in the moment the bow was crossing the event horizon?

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timmdeeg said:
You seem to mean 'is decreasing more quickly', right?

Oops, yes.

And is it correct that this happens in the moment the stern is crossing the event horizon as this light ray is "frozen" at ##r=2m## because it was emitted in the moment the bow was crossing the event horizon?

I had three cases in the original - just before, just at, and just after the crossing. For the middle case, yes, the r-coordinate of the light signal emitted from the bow as it crosses the stern hangs there, and it is actually photographed when the camera reaches the horizon.

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pervect said:
I had three cases in the original - just before, just at, and just after the crossing.
To follow this up I think after the crossing the stern will receive light from the bow for a certain time only depending on the length of the ship. Light emitted later at the bow will no more hit the stern but the singularity.

timmdeeg said:
To follow this up I think after the crossing the stern will receive light from the bow for a certain time only depending on the length of the ship. Light emitted later at the bow will no more hit the stern but the singularity.
Yes, but the stern will continue to “see” the bow until it reaches the singularity. That is, at any point at all in the stern’s world line, there will be a bow emission event just becoming visible.

pervect said:
Technically, we've only analyzed one signal of the two-way exchange. But the analysis is similar for the case of an exchange of signals in the other direction.,
Could you please elaborate a bit on the "other direction". By means of an Eddington-Finkelstein diagram I understand that the stern receives light from the bow but I fail to see the other way round. From the diagram it seems that the bow is in the past of the stern.

PAllen said:
Yes, but the stern will continue to “see” the bow until it reaches the singularity.
Yes, until the stern "reaches the singularity". Just to avoid any misunderstanding.

timmdeeg said:
Could you please elaborate a bit on the "other direction". By means of an Eddington-Finkelstein diagram I understand that the stern receives light from the bow but I fail to see the other way round. From the diagram it seems that the bow is in the past of the stern.
Just think light cones. There are stern world line events in the bow past light cone until the bow reaches the singularity. Thus there is a window of time in which two way signals can be exchanged between the bow and stern, with all relevant emission events occurring inside the horizon. And, of course, the bow therefore sees the stern until it hits the singularity.

It really becomes obvious in the Kruskal chart.

timmdeeg

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