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View Full Version : A probe to a black hole.



Buttercup
2015-Sep-29, 06:31 PM
Yes, I know supposedly black holes don't exist (or they might, we're not sure...I'm confused).

Anywho: Supposing black holes exist and we could send a probe to the nearest.

How close could the probe get (and transmit data) to it before it couldn't relay information anymore? I mean in light years or miles (and yes I know about the event horizon...or thought I did).

Noclevername
2015-Sep-29, 06:58 PM
A black hole is a gravity source, so they could go into orbit around it, and unless it has an accretion disk (https://en.wikipedia.org/wiki/Accretion_disc) they could probably get as close as the probe's Roche limit (https://en.wikipedia.org/wiki/Roche_limit) (where tides pull the probe apart).

IsaacKuo
2015-Sep-29, 07:56 PM
It could get closer than the smallest stable orbit. The limit will either be the event horizon or the Roche limit...I think it will typically be the event horizon, for a sufficiently compact probe.

The Schwarzschild radius or the event horizon will be about 3km per solar mass, so a black hole with a mass of 10 suns will be about 30km in radius.

So basically, we're talking miles, not light years.

starcanuck64
2015-Sep-29, 10:12 PM
Yes, I know supposedly black holes don't exist (or they might, we're not sure...I'm confused).

Anywho: Supposing black holes exist and we could send a probe to the nearest.

How close could the probe get (and transmit data) to it before it couldn't relay information anymore? I mean in light years or miles (and yes I know about the event horizon...or thought I did).

They do exist, we're just not sure what's going on at the center where General Relativity indicates a singularity with infinite density, I forget what quantum mechanics have to say about the center....probably that it needs its oil changed.;)

Very massive black holes likely occupy the center of most galaxies and can have masses of millions of solar masses. If a star is large enough, when it undergoes a supernova event at the end of its life-cycle the intense force of the collapse of the star can push the mass at the center beyond a threshold where it becomes a black hole. Essentially what happens is the mass occupies such a limited space that space-time is curved around it beyond the speed of light. Which makes them black.

Another interesting tidbit about black holes, if a star is occluded directly behind the black hole it creates a series of rings around it for the viewer. This is produced by photons that circle the black hole once, twice and so on before continuing on.

John Mendenhall
2015-Sep-29, 10:57 PM
IIRC,. the Roche limit is well inside the evnt horizon.

cjameshuff
2015-Sep-30, 12:14 AM
IIRC,. the Roche limit is well inside the evnt horizon.

That depends on the size of the black hole and on the body falling in, and certainly won't be true in general. I'd expect stellar mass black holes to shred anything material before it reaches the event horizon. The largest supermassive black holes might only be able to pull apart the most fragilely-bound objects.

grant hutchison
2015-Oct-01, 06:10 PM
The Roche limit applies to bodies held together by their own gravity. Monolithic chunks of rock, people and space probes are a lot stronger, so can get a lot closer. It's not a particularly useful parameter for black holes, because it contains a term allowing for the relative densities of the tide-raising body and the tidally deformed body. There are obvious problems with calculating the density of black holes, but the volume you need to plug in to the calculation is just the naive spherical volume calculated from the Schwarzschild radius.

The tidal force at the event horizon varies inversely with the mass of the black hole squared. A 40000-solar-mass black hole will give a human being an easily survivable one-gee gradient from head to foot as she free falls across the event horizon.

Grant Hutchison

publiusr
2015-Oct-02, 07:38 PM
In some respects a magnetar is far more dangerous to spacecraft. Harder to orbit. It's powerful magnetic field would behave much like the popular (mistaken) ideal of how a black hole pulls everything down.
https://en.wikipedia.org/wiki/Magnetar

Jeff Root
2015-Oct-03, 02:47 AM
If you get within 6 Schwarzschild radii of a non-rotating
black hole, your orbit will decay. That distance is often
called "the last stable orbit" and is the closest circular orbit
for any massive object. Light could theoretically orbit at
exactly 3 Schwarzschild radii, "the photon sphere". If it
gets any closer it will spiral in, and if it gets any farther
away it will spiral out and escape. Massive objects cannot
escape that way, just spiral in to their doom. But as long
as you are still outside the event horizon, you have a
chance to fire your rocket engines and escape.

-- Jeff, in Minneapolis