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There actually is at least one interesting and potentially important difference between a supermassive black hole and a stellar mass one. Counterintuitively, ev

There actually is at least one interesting and potentially important difference between a supermassive black hole and a stellar mass one. Counterintuitively, even though the total gravitational effect of a huge black hole is larger, the strength of the tidal forces goes down as mass goes up. Tidal forces are the cause of "spaghettification" (yes, that's the real term!), which is the expected cause of death for anyone getting too close: if you fall feet-first toward a black hole, your feet will feel much greater gravitational acceleration than your head and you'll be stretched into spaghetti. My memory is that for a normal stellar-mass black hole this would happen even outside the event horizon (the radius of no return), while for a very large black hole you might be able to safely get close to the event horizon. So any experiments or applications that involved being close to the horizon could be much easier there.

canhascodez 8y ago
Your memory is incorrect. The interesting gravitational effects typically happen much closer to the singularity. The event horizon is an otherwise normal region of space, and more boring the larger the black hole is.
- Steuard 8y ago
  My main point was the same as yours: that the event horizon looks more and more "normal" as a region of space the larger the black hole is, so if you want to get close to a black hole's event horizon to test its properties (e.g. to explore whether quantum effects might cause that "normalness" to break down), visiting a very large black hole is your best bet. I haven't double checked the calculations myself: too much other stuff to do. But glancing around at other people who have looked at the numbers, it seems pretty clear that spaghettification is expected to happen for macroscopic objects well outside the event horizon of a stellar-mass black hole. Wikipedia gives an example of a 10-solar-mass black hole: its event horizon radius is about 30km, but macroscopic objects will be spaghettified at a radius of about 320km. https://en.wikipedia.org/wiki/Spaghettification#Inside_or_outside_the_event_horizon https://en.wikipedia.org/wiki/Spaghettification#Inside_or_ou... (That ratio is roughly reversed for a 10,000-solar-mass black hole.) There are some similar calculations shown in detail on this NASA math worksheet (which is for some reason still using cgs units): https://spacemath.gsfc.nasa.gov/blackh/4Page33.pdf https://spacemath.gsfc.nasa.gov/blackh/4Page33.pdf
  - canhascodez 8y ago
    You're right, I'm wrong.

canhascodez 8y ago

Your memory is incorrect. The interesting gravitational effects typically happen much closer to the singularity. The event horizon is an otherwise normal region of space, and more boring the larger the black hole is.

Steuard 8y ago
My main point was the same as yours: that the event horizon looks more and more "normal" as a region of space the larger the black hole is, so if you want to get close to a black hole's event horizon to test its properties (e.g. to explore whether quantum effects might cause that "normalness" to break down), visiting a very large black hole is your best bet. I haven't double checked the calculations myself: too much other stuff to do. But glancing around at other people who have looked at the numbers, it seems pretty clear that spaghettification is expected to happen for macroscopic objects well outside the event horizon of a stellar-mass black hole. Wikipedia gives an example of a 10-solar-mass black hole: its event horizon radius is about 30km, but macroscopic objects will be spaghettified at a radius of about 320km. https://en.wikipedia.org/wiki/Spaghettification#Inside_or_outside_the_event_horizon https://en.wikipedia.org/wiki/Spaghettification#Inside_or_ou... (That ratio is roughly reversed for a 10,000-solar-mass black hole.) There are some similar calculations shown in detail on this NASA math worksheet (which is for some reason still using cgs units): https://spacemath.gsfc.nasa.gov/blackh/4Page33.pdf https://spacemath.gsfc.nasa.gov/blackh/4Page33.pdf
- canhascodez 8y ago
  You're right, I'm wrong.