# Does speed of light also mean strength of light?

1. Mar 3, 2009

### ColonelTravis

Physics mega-novice (i.e. moron) here, so I'm probably not thinking correctly about these things. Just curious which item below is close to what's generally understood about light and black holes. Maybe none are nowhere near being close, I would not be surprised.

Concept #1
A gravitational field starts at the center of an object and then goes out farther and farther, eventually dissipating but not technically ending, correct? From what I understand, a black hole isn't like a bucket or vacuum cleaner or something. But does that necessarily mean it's 2D and not 3D? Haven't seen that description before. So I'm wondering if gravity is something of a predator - starts at the center of a black hole, bolts out to whatever distance the event horizon is, grabs the light and then brings it back in. This might be another way of saying gravity has two forces - one that goes out and one that comes back. If Mr. Newton heard this, I'm sure he'd want throw a bunch of rotten apples in my face.

Concept #2
Gravity just sits there like a spider, waiting for some poor slob photon to crash into the web. In this case gravity is (for a lack of a better description) passive, while gravity in the previous example is active.

Concept #3
Gravity warps light around a black hole so much that light can do nothing but orbit what's left of a crushed dead star. Except this act of orbiting is cloaked by gravity and you can't see it.

Concept #4
Gravity sucks equal amounts of light from all directions, except the light does not orbit - everything drains into a single point in the center.

In the first two items above, I'm envisioning light heading into a black hole in the shape of a funnel. But I don't understand how gravity can force light into one spot that way. Wouldn't gravity need to be an equal opportunity sucker, since it came from a collapsed star that (presumably) collapsed evenly like a leaking balloon? That's my thinking with the other two items.

Either way, does fast light equal strong light? Plenty things in the universe are strong yet slow, just like you've got stuff that's slow and weak. I understand light is light. You can't say it's like something else, because there is nothing else like it. But let's say light has been slowed down - like when the Harvard people dropped the speed down to 30-some mph not long ago. Would slow light enter a black hole with less resistance than normal speed light? Or does the speed of gravity, if it really puts its nose to the grindstone, beat light to the event horizon door no matter what?

2. Mar 3, 2009

### redargon

I am no master of all things stringy and black holey and speed of lighty, but I'm pretty sure that the speed of light is constant through a certain medium and can't be changed unless the medium it is travelling through is changed. So in that regard, I'm not sure how the Harvard people dropped the speed of light to ~30mph?

In terms of gravity, I thought it was more along the lines of a force generated by a body due to that body's mass. Two masses attract each other due to the affect of their gravities. So, a black hole which (i think) has a very large mass has a strong gravitational field that attracts other bodies of mass and even photons of light with negligible mass.

I'd be interested to see what some more of our experienced members have to say, but I'm not sure they could abswer your questions directly as they seem very difficult to understand. I don't think that strong is a word you can use with light, no matter how fast it's going. Maybe energy is better word to use.

3. Mar 3, 2009

### confinement

Only concept (1) strikes me as outright wrong, while the interpretations (2), (3), and (4) can all be supported by the details of Einstein's GR. Concept (2) is not exactly correct since, as you said, the gravitational interaction diminishes with distance but is not ever zero; therefore the blackhole has been pulling the photon in all along, rather then just sitting there. Concept (3) is the way that I would describe the process of light entering a blackhole as seen by someone outside the even horizon. Concept (4) is what an observer would see inside the event horizon.

4. Mar 3, 2009

### ColonelTravis

I think my original post is convoluted, sorry. My ultimate question is what maximum gravity does to light, if that light has been slowed down before it gets there. But I wanted to make sure I know what the heck is going on with a black hole in the first place, so that's why I had those 4 concepts.

Scientists have been slowing the speed of light for a while. What I did not know before I posted my question is that the Harvard people have actually stopped it. And they did this 8 years ago, which shows you how in touch I am with all this.
http://www.hno.harvard.edu/gazette/2001/01.24/01-stoplight.html [Broken]

So my one question remains: Is it easier for gravity to capture slow light? But now I've added a second part: Can you capture stopped light?

Let's say light is minding its own business, chugging along toward a black hole at 186,000 mps, then someone (like the Harvard people but a gazillion times more sophisticated with tech stuff) decide to stop that light at the event horizon. Is gravity strong enough to capture it? Or maybe the better question: forget the light, would gravity need to overcome whatever force stopped it? Or maybe the BETTER better question is: if gravity can overpower light, can anything ever overpower gravity?

Last edited by a moderator: May 4, 2017
5. Mar 4, 2009

### v2kkim

A neutrinos has tiny mass but it will still be influenced by gravity. Gravity is just universal. I think gravitational field itself escape BH and propagate fast, but I do not know its speed may slow near BH ?