# Entering a black hole theoretically of course

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1. Dec 11, 2014

### LightPiercing

Hey all this is my first post,

Everyone knows you can't enter a black hole because you'll be stretched/turned into noodles due to the immense change in gravity over small lengths. I was reading another thread on the physics forum and happened to come across an equation which would determine if you would get noodled. if there was a change of 10 earth g's over 2 meters you would be in trouble. Is this the main restricting factor when "entering" a black hole? If so would it be possible to find a black hole with less changes in force over a certain length. When you're being accelerated into the abyss, would it be possible to use a counter force(gravity etc) to cross the gravitational lines in a slow and controlled manner? This may be more suited for science fiction but I do have a small understanding of the universe. Any comments appreciated!
PS
(Don't flame too hard if I've violated more than one law of physics).

2. Dec 11, 2014

### Staff: Mentor

I hope not, because it is wrong.
It depends on the size. If the black hole is massive enough, you can cross the event horizon without getting ripped apart by tidal gravity.

You might get issues with intense radiation from the accretion disk, high-energetic particles in general, you are still in space so you need some life support infrastructure. And there could be odd things happening at the event horizon, this is still unclear.

Crossing it slowly is actually more dangerous as then everything else is hitting you with high speed or energy. And it needs some science fiction magic to provide the necessary thrust for your rocket.

3. Dec 11, 2014

### LightPiercing

i agree, everyone and knowing are pretty encompassing words. A better choice would be some people think.. anyways, thanks for the response. Black Holes are really weird. I recently read an article stating that singularities actually have a volume. The article stated it was smaller than any particles we encounter (protons, nuetrons, quarks, etc) but it does have a finite volume, which would clash with the theory that singularities have no volume and are infinitely dense?

do you have any thoughts on this topic?

4. Dec 11, 2014

### Staff: Mentor

A singularity does not have a volume by definition. It is unclear if the center of a black hole has a singularity or maybe something else.

5. Dec 11, 2014

### phinds

Changing it to "some people think" does make the statement less egregious but doesn't change the fundamental wrongness of that belief. People who think that are wrong, period, as mfb explained.

6. Dec 12, 2014

### Tom Arabia

This is actually one of the most if not the most hotly debated topic in fundamental physics and quantum gravity today. Start here: http://en.wikipedia.org/wiki/Firewall_(physics)

7. Dec 12, 2014

### Lamdbaenergy

Maybe you can avoid the radiation by just crossing into the black hole from the direct North or South Pole, where the acretion disk isn't?

8. Dec 12, 2014

### Staff: Mentor

There the full disk is visible to you, that does not make it better. And you still have starlight (or even the cosmic microwave background) that gets blueshifted to intense gamma radiation if you are slow and close enough. Just go for free fall and you avoid that issue.

9. Dec 12, 2014

### DrStupid

Can you estimate the minimum size of the BH to be entered alive?

Last edited: Dec 12, 2014
10. Dec 12, 2014

### Staff: Mentor

You get a reasonable estimate with Newtonian gravity:
$$\frac{da}{dr}=\frac{GM}{r^3}$$
With the Schwarzschild radius for a non-rotating black hole $r=\frac{2MG}{c^2}$ we get
$$\frac{da}{dr}=\frac{c^6}{8(MG)^2}$$ or
$$M^2=\frac{c^6}{8G^2} \frac{dr}{da}$$
With 10g over 2m, this gives about 10,000 solar masses. ~100 milliseconds might be a bit short to watch all the events, so larger black holes probably give a better view.
Why blueshifted? And see the firewall link in post 6.

11. Dec 12, 2014

### DrStupid

Can you show that Newtonian gravity results in a reasonable estimate at the event horizon of a black hole? That's hard to believe.

Doppler effect and less gravitational redshift compared to a distant observer

I'm afraid quantum field theory in curved spacetime exceeds my skills.

12. Dec 12, 2014

### Staff: Mentor

Dimensional analysis. The prefactor is wrong, the order of magnitude should be right.

Ah, relative to a distant observer, okay.

Same here, that's why I just referenced that point.

13. Dec 13, 2014

### DrStupid

How does this work? I would assume that the tidal acceleration need to be estimated on the basis of the relativistic trajectory of two test particles within each others rest frame. I still have doubts that this results in something similar to Newtonian tidal accelerations because the properties of the event horizon fundamentally differ from the Newtonian limit.

14. Dec 13, 2014

### ImperialThinker

wait..wait..wait..wait.. You mean I have to exert an internal force to counter the gravitational force from a black hole's event horizon ?
Gimme a break... am not Goku.

15. Dec 13, 2014

### Staff: Mentor

There are three quantities that can influence the solution (the mass): the gravitational constant, the speed of light and the tidal gravity we want. There is a unique way to combine powers of them to get units of mass: c3 G-1 (da/dx)1/2.
While this analysis does not give the exact solution, the GR solution will have those values in those powers, multiplied by some numerical prefactor - there is no other way the formula could look like. It does not really matter whether that prefactor is .1 or 10. It is not one million.

@ImperialThinker: No, but you can use rockets. They won't help you once you cross the event horizon, but outside they can be used.

16. Dec 13, 2014

### ImperialThinker

@mfb : Turning into super saiyan isn't enough ?

17. Dec 13, 2014

### ellipsis

Apparently not:

18. Dec 13, 2014

### DrStupid

How did you get this result?

19. Dec 14, 2014

### Staff: Mentor

Do you know any example where a dimensional analysis is off by more than a factor of 1000?
Huge numerical prefactors rarely show up in equations.

Do you have a reason to expect a large prefactor here?

20. Dec 14, 2014

### DrStupid

I have no reason not to expect it. Please provide a proper justification for your claim that the prefactor is small and don't ask me for counter-evidence.