I’m a layman when it comes to astronomy and related topics, so I’d like to clarify a question regarding black holes.
I’ve always understood that blackholes are superdense and possess such enormous gravity that not even light can escape the pull.
However, it seems that in some cases light can escape, as this Hubble space scope story suggests.
Can light escape black holes or not?
I’m hardly an expert, but here’s a quickie explanation of the phenomenon as I understand it:
There is a perimeter around a black hole called the “event horizon”. Nothing which crosses this perimeter comes back out again–including light. Surrounding the black hole outside the event horizon is an accretion disk–accumulated matter that is in the process of being pulled in. As the matter is pulled inward, it is compressed, causing it to give off energy. So a telescope can detect a black hole’s presence by the light given off by its accretion disk.
Balance answered correctly before I finished writing. But here’s my post anyway.
The key paragraph is:
The junk that a black hole eats doesn’t always get pulled straight in. Something traveling at an oblique angle nearby will get trapped in its gravity and orbit just like a planet orbits the sun. The stuff spirals closer and closer, often propelled to high speeds, etc. This crazy plasma surrounds the black hole, and all of said emissions originate from outside of the event horizon–not from inside the black hole itself.
Yup. You guys got it right on. Interestingly, though, there is a way for black holes to evaporate. It is possible to get a particle and its antiparticle to spontaneously appear out of nothing. They cancel each other out, so matter is still conserved. Imagine this happens just on the edge of the event horizon, with the antiparticle closer to the hole than the particle. The ap could fall into the black hole and annihilate its equivalent mass inside, making the black hole less massive. Meanwhile, the particle is far enough away that it flies off. To an observer, it would appear that a particle just escaped from the black hole.
I originally read this in either National Geographic, Scientific American or in one of Steven Hawking’s books.
Out of nothing? Do you mean out of some energy lying around? I don’t get how a particle can just appear from nothingness, though I don’t pretend to understand Quantum Physics…please help me wrap my puny brain around this…
Then you’re normal. As one quantum physicist supposedly said, “Anyone who understands quantum physics, doesn’t understand quantum physics.”
Yep. It is possible for matter to ‘pop in’ and then ‘pop out’ again without changing the mass of the universe. John Wheeler(University of Texas, Austin) postulates…but you can get it directly from him. I tend to disagree. My ex ‘popped in’ from hell and has still not ‘popped out’ again.
Bear with me, but how is a particle (or anything) able to fly off from the vicinity of a black hole? Wouldn’t anything nearby just get sucked-in by the intense gravity?
Look at it this way…when a basketball goes in the basket in what we used to call a ‘toilet swisher’, where it goes around and around and around and around before dropping through, it’s like the light particles that are trapped. Or when a golf ball rims the cup, changes direction, but doesn’t drop. Same kinda thing, on a particle level.
A particle in the vicinity of a black hole will almost but not quite certainly be pulled into the black hole. It is rare for a particle to escape, but not impossible. The smaller the hole the more likely that a particle will escape. This is called Hawking radiation and it is thermal in nature. The larger the black hole, the cooler it is.
Almost, but not quite. Matter - anti-matter annihilatation will not reduce the mass. What happens is that one of the pair (It could be either the particle or the anti-particle) escapes with positive mass, the other with negative mass-energy is absorbed by the hole and reduces its mass. You can increase the mass of a black hole by pouring in matter, anti-matter or just energy in the form of photons. The Hawking radiation is equal amounts of matter and anti-matter.
The no-hair theorem says that the only observable properties of a black hole are the mass, electric charge, and spin. You cannot distinguish a black hole made from matter from one made from anti-matter or from one made from just energy.
That is exactly what virtual particles do. A particle and its anti-particle appear out of the vacuum and (usually) annihilate each other before they can be directly observed.
Just a WAG - but isn’t the “light can’t escape theory” not necessarily an indication of the strength of the gravity of the black hole, but moreso is kind of a way of defining the Event Horizon? Specifically, the Event Horizon is the leading edge of the collapse of space around the BH and that the BH is collapsing the space around it more quickly than the speed of light? Ergo, light doesn’t necessarily get sucked into the BH, it just can’t move fast enough to get away.
Then again, if that were the case, then all of the black holes in the universe would be growing much faster and sucking up more space than we currently observe to be the case, so I’m now getting very confused.
Okay, okay. I recognize that the extent of my astrophysics education is limited to a survey course, but I thought that was the way it was explained to me. Perhaps I am very very wrong?
For those of you wondering about virtual particles popping into and out of existence check this link:
http://focus.aps.org/v2/st28.html
The particles cannot be observed directly since they appear and disappear on the order of Planck Time (smallest slice of time it is conceivable to measure). However, their existence has been proved experimentally as the above link describes.
You’re on the right track here. Light has no mass hence it isn’t affected by ‘gravity’ as we generally think of it (2 masses attracting each other). You can think of gravity as the effect we feel moving through a 4-D (spacelike not timelike) universe.
Think of a 2-D creature on a flat-plane made of some stretchy material. Now imagine you deform the terrain by poking your finger up into the stretchy material making a peak. 2-D creature has NO concept of ‘up’ so it cannot see the peak. Everything looks flat to 2-D guy. Now, 2-D guy starts climbing that peak. 2-D guy thinks he is simply moving forward but now he feels a ‘force’ tugging him backwards. Not sure what it is because he can’t see the peak he’s on, he decides to call this new force gravity and there must be some new virtual particle called a graviton tugging him back. While this explanation is useful for the world he lives in what he really is experiencing is a dimensional effect from a dimension he can’t perceive.
In the case of a Black Hole 4-D space is curved so tightly that no path exists for even light to follow that will take it out of the black hole (I’m not positive but you can think of it as being curved back in on itself like a curling wave about to hit a beach…slide up the wave and eventually you get turned around).
Black holes are limited in size by their mass. They can only grow by acquiring more mass so in general they suck up what’s nearby then just hang there for a REALLY long time before they evaporate. They won’t just keep growing as if it is fundamental part of their nature.
Jeff
Splendid explanation! Thank you.
Way to go Jeff
not meaning to nitpic such a great explanation, but would like to say that a blackhole in your 2d world would be a downward push preventing the 2d ‘man’ from escaping