Black holes have such an intense gravitational field that they slow down light or even suck it in... how!?! wasn't one of the properties of light - that it is pure energy - with no mass!?! If there is no mass how can the blackhole affect the light? if the light has no mass?

Thanks

Andrew

The rule about the speed of light applies more specifically to the speed of light in a vaccum. This is the speed beyond which one may not pass. (Actually, that isn't really true, theoretically. There is nothing in physics to prevent you from traveling faster than the speed of light. That's why tachyons are interesting speculation rather than crackpot theorizing. The problem is that nothing can travel at the speed of light, so you're left with no way to get to speeds beyond it.)

At any rate, energy and mass, according to Einstein, are interchangable. E=mc^2 and all that. Meaning that energy, like mass, is influenced by gravity. Why? Well, take the old example of space being a big rubber sheet, onto which are placed various objects like stars and black holes. These objects deform space (and time), creating the phenomenon we call gravity. Now, something to remember out light, again according to Einstein, is that it will always travel the shortest distance between two points. In other words, light travels in a straight line. Or does it? In a curved universe, the shortest distance is no longer a straight line. So light, when traveling near one of these large depressions in the fabric of space, follows the path of the shortest distance. And so it curves around objects of sufficient mass.

This property of light, and with it much of Einstein's theory, was confirmed by observing the sun during a total eclipse. Stars which we knew should have been behind the sun at the time showed up along the edges, because their light had been bent around the sun to hit our eyes and telescopes.

(Who says that Physics degree was a waste of time?)

So, is it that light is actually being affected by gravity directly like we are to the earth, thus having a mass, or is it that its just some sort of 'field line' that makes the light bend around an object?

so does light (which I thought were just packets of Energy) exist as Energy AND matter - or only matter when it comes into contact with a Gravity field? If so, can we exist as Energy and Matter - and if we weren't near a --- ok big leap here--- if we weren't affected by the gravity of the Earth, Sun, planets and stars - exist as energy and matter? i.e. how can light (if I've guessed correctly) exist as both E and matter?

so DOES 'something' have to have MASS to be affected by gravity?

Light does not slow down as it passes near a black hole, nor does it's path bend. Well, not exactly, anyhow. The truth is stranger than that.

The presence of mass affects the curvature of space. The greater the mass, the greater the curvature. A black hole is not so much a massive object as it is an incredibly dense concentration of mass.

For normal objects, the amount of bending is very slight, but measurable. During Solar eclipses, some stars which should be hidden by the Sun/Moon combination are visible because the Sun's gravity bends the space around it very slightly. This "lens effect" has the effect of "bending" the light's path, although in this case, what is actually happening is that the space itself is bent and the light is actually following a straight path through it.

There is another property which accounts for the apparent slowing of light as it passes near a black hole. As space is warped by an intense gravity field, time slows (time dilation) relative to the less-severely bent space further away. To an outside observer, the light appears to bend and red-shift. For objects travelling very near a black hole, time will seem to slow, as observed by someone much farther away -- to the object near the black hole, it's the rest of the universe that will seem to slow. It's almost as if they were travelling at a high percentage of the speed of light.

There's a neat website about this very subject:

http://casa.colorado.edu/~ajsh/schw.shtml

Go have a look. The universe we live in can be pretty strange.

--Baloo

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"Lassie, her ears pricked up!"

--Atoth the Tamarian [From "Star Trek: Door Repair Guy"]
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Ever see the Stargate SG-1 episode about the black hole?

So is gravity a force or an effect? does the 'warping' idea happen say if you put two oranges infront of each other - put on an infintessimal scale?

As to whether gravity is a force or not...it depends upon your point of view, really. Gravity can be explained as curvature, or in terms of a gravitational field. It's similar, though perhaps subtler, than the way light can be explained as both a wave and a particle. (Though in the case of gravity, its effects are the same regardless of your explanation of what it actual "is".)

I suppose, in theory, one could eventually find a way to explain the universe in such a way as to show that all the forces we see are really just effects of something else. In fact, I think that just such an explanation is part of string theory, but I wouldn't want to bet anything important on my sureness.

Another, somewhat related: Hawking radiation, as I understand it. Supposedly, a particle with positive mass and a particle with equal negative mass can appear for a split second before annhilating each other with no release of energy (since negative mass means negative energy). Hawking radiation is where the negative particle gets seperated from the positive particle by the gravitational attraction of a nearby black hole. The negative particle falls into the black hole, canceling some of its mass/energy, and the positive particle escapes. But tell me: would a gravitational pull have any affect on negative matter? You'd think it'd just push it farther away.