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Q & A: Black Holes vs. Dark Matter

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Most recent answer: 05/14/2012
Q:
If a black hole is entirely comprised of dark matter, and a textbook is thrown in, will we see the textbook get crunched up, or does it disappear?
- Noah (age 14)
United states
A:

Hi Noah,

That's a fantastic question! As you may already know, a black hole is a very massive compact object whose gravitational influence is so strong that it prevents anything within its event horizon from escaping, even light (thus the "black" part). Black holes can form from the collapse of mass of any sort, even gases.

If we were to observe from a safe distance (far, far away) as an object fell into a black hole, we would never see it cross the event horizon- the point at which light cannot escape the hole's gravitational pull. From our frame of reference, the intense gravity would cause time at that point to slow almost to a stop so that the object would appear frozen there, its image growing redder and redder (this because the gravitational force will tug at the light waves forming the image of the textbook as it travels towards our eyes, causing its wavelengths to lengthen and appear more red) until it fades completely. This is exactly what we would see happen to the textbook, though an observer traveling with it would, we think, see nothing special happen to it as they crossed the event horizon.

As for dark matter, I'm guessing your mention of it in the context of black holes was incidental. Dark matter is not  a known type of substance, but rather a placeholder name for something we don't quite understand yet. When we look at things like or the famous , we find that there is less visible matter in each system than its gravitational interactions would seem to indicate. We call the missing matter "dark" because we can't see it but we're fairly certain it's there -- either that, or there is something wrong with our current understanding of gravity. Problems with gravity seem increasingly less likely as we learn more about the Universe, which generally fits well with the standard form of gravity. There have been many candidates for this theorized "dark" matter, but many are already known not to fit the observations. Even black holes have been suggested, but if black holes constituted all of dark matter we would expect to see gravitational lensing (the bending of light as it passes massive objects) when we look through the halo of our own galaxy at stars in other galaxies because we would expect there to be many black holes in that halo. We do not see such lensing, so we conclude that the dark matter we know to be present in galaxy is not black holes. Our galaxy does not seem special in other ways, so it is unlikely that there is black hole dark matter in other galaxies either. Still, we're now fairly certain that dark matter does exist.  Roughly 85% of the mass of all matter in the Universe must  be dark matter, to account for the gravitational forces present.

If the textbook fell into dark matter, the result would be far less interesting than it was for a black hole. Our observations of dark matter indicate that it is weakly interacting, i.e. it doesn't interact with other matter or even with other dark matter. Instead of colliding, the exotic particles would just pass right through the textbook. Pretty cool, huh?

Cheers,
Becca

 

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(published on 05/14/2012)

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