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Old 8th April 2012, 05:01 PM   #1
C_Felix
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black holes and light speed

Just a thought, If I'm way off base, have a heart....

I remember reading, I think in one of Brian Greene's books, that one cannot travel through space faster than light, but on space faster than light. One can't travel on water faster than the speed of light, but if one were to ride on the water as it were moving along, and it was moving faster than the speed of light, then one can travel faster than light speed that way.


For the space shuttle to lift off (when the program was up and running) it needed to travel upwards with enough speed to overcome the gravity of the Earth.

For something to travel out of a black hole (the poor doomed space shuttle for example) it has to travel faster "up" than the gravity accelerates in down.

Since light can't escape, does that mean gravity is accelerating items, photons or the space shuttle, "down" at faster than the speed of light?


So, if my little hypothesis is correct, then the poor space shuttle and the not so poor photons are traveling faster than light. This should be a "no-no" unless the space shuttle or the photons are riding on the fabric of space and not transversing it.

What about gravitational waves? Einstein had a problem with the following: A star goes supernova and planets cannot be released from its gravitational pull instantly, since it cannot happen instantly, because the information and the lesser gravitational pull cannot be transmitted instantly. So, he theorized the fabric of space and the idea of gravitational waves. (I think he came up with gravitational waves.)

So, if gravitational waves are what cause gravity, are gravitational waves in black holes going faster than light?

Is this possible?


Its late, I'm tired, but hopefully you see where I'm trying to go.
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Old 8th April 2012, 05:42 PM   #2
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I'm not a physicist, but you seem to be neglecting to take into account time dilation.
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Old 8th April 2012, 06:37 PM   #3
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Originally Posted by C_Felix View Post
Since light can't escape, does that mean gravity is accelerating items, photons or the space shuttle, "down" at faster than the speed of light?
No. The problem with such an analysis is that it mixes units (speed and acceleration), and that never makes sense. You can't compare quantities with different units and ask which is bigger, it doesn't make sense.

What's going on is actually far stranger than that, if you're used to thinking of gravity in a Newtonian context. In general relativity, gravity is different from every other force. In fact, strictly speaking it's not treated like a force at all. Rather, gravity curves space, and if you DON'T apply a force (and gravity isn't a force in GR), then you'll travel along what's called a geodesic, which is basically the equivalent of a straight line in curved space (think, for example, of a great circle on the surface of a sphere). Because of the curvature of space, geodesics look curved if you project 4D world lines onto 3D Euclidean space, even though they are "straight", in a sense. So that's what accounts for acceleration with gravity.

Now, black holes do something really strange. The curvature around a black hole becomes so extreme that once you pass the event horizon, space itself becomes reoriented. At that point, it's not even a question of acceleration: you can't escape and you're doomed to hit the singularity because every spatial direction points towards the singularity. The singularity is, quite literally, your future. Now, IF you could move faster than the speed of light, you might be able to escape a black hole. But that's because going faster than light is equivalent to time travel, and you have to go backwards in time in order to escape the event horizon.

Quote:
So, if gravitational waves are what cause gravity, are gravitational waves in black holes going faster than light?
Gravitational waves don't cause gravity, at least not in GR. A quantum theory of gravity might look like that, but we haven't sorted that out yet, and GR itself is explicitly not quantum. In GR, gravity waves are merely how changes in the gravitational field propagate. But outside of the event horizon, the gravitational field of a black hole and the gravitational field of a non-black hole of the same mass are exactly the same. So during collapse, no change needs to propagate outwards. After collapse, the field is already there, so again, no change necessary. Gravity waves won't escape the event horizon, but we don't need them to. In fact, the same thing happens with electric fields from black holes: if the black hole has a net charge, then it will have an electric field. Photons can't escape the black hole, but the field outside the black hole will exist before charges cross the event horizon, so we don't need changes to propagate from inside the black hole.
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Old 8th April 2012, 07:49 PM   #4
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Originally Posted by C_Felix View Post
I remember reading, I think in one of Brian Greene's books, that one cannot travel through space faster than light, but on space faster than light.
Yes, if you could find or create a patch of space which is itself moving (or at least stretching & squeezing). That's why some galaxies are more light-years away from us than the number of years they've had to get there; the space between us and them stretched. But even if we had a way to cause the kinds of changes in space that this calls for as a method of transportation, which we don't, its minimum energy requirements would be outrageous.

Originally Posted by C_Felix View Post
One can't travel on water faster than the speed of light, but if one were to ride on the water as it were moving along, and it was moving faster than the speed of light, then one can travel faster than light speed that way.
That would make sense if the water were space instead of water, but using water in the analogy makes it a pretty bad analogy, because you can't add together the speeds of two material things and get a sum greater than the speed of light.

Originally Posted by C_Felix View Post
For something to travel out of a black hole it has to travel faster "up" than the gravity accelerates in down.
You've just tried to assess which is greater and which is less between a speed and an acceleration rate. That can't work because speed and acceleration are two different things; one speed can be greater than another speed and one acceleration rate can be greater than another acceleration rate, but trying to find such a relationship between a speed and an acceleration rate is like checking a temperature and an air pressure, then asking which is greater.

Gravity works like an acceleration rate, and you need to counter that in order to fly upward against gravity. But that can be done at various speeds. Even a very slow speed will get you out if you keep at it long enough.

There is a thing called "escape velocity", which sounds like it might be the minimum speed to get away, but it isn't. It's the minimum speed you'd need to reach in order to get away without doing any more work. If you're on a rocket, for example, once you're at that speed, you could turn the engines off and coast; you'd start slowing down due to gravity immediately, but not be slowed down enough to fall back down. But you can get away without reaching escape velocity; you would just need to keep the engines going.

Black holes were originally imagined as places with escape velocities greater than the speed of light. That situation is caused by an extreme gravitational acceleration rate. But, although there is a universal speed limit (the speed of light), there is no universal acceleration rate limit, and gravity can be escaped at less than the escape velocity, as long as you can keep using energy to accelerate away from it. The trouble with black holes (at least while you're still outside the event horizon) would be that you'd just need a ridiculously powerful engine and a ridiculous supply of fuel for it to burn.

Originally Posted by C_Felix View Post
Since light can't escape, does that mean gravity is accelerating items, photons or the space shuttle, "down" at faster than the speed of light?
No, acceleration doesn't happen at any velocity. It's a rate of change in velocities. And it doesn't apply to light because light's speed doesn't change. Getting away slowly is possible for a powerful enough rocket with enough fuel, but light has no engines and doesn't burn fuel.

Originally Posted by C_Felix View Post
if gravitational waves are what cause gravity
They aren't. They're just a description of how a change in gravity spreads throughout the gravitational field. For example, if something were to change in the sun, the change would affect Mercury first, then Venus, then Earth, then Mars, and so on. The delay in the timing with distance could be described/depicted as a matter of a wave spreading out from the sun. But the sun's gravity was always there before and after whatever changed to make that wave.

This is a situation where a water analogy isn't too bad. Drop a rock in a pond, and you create waves on the surface, so nearby floating objects bob up and down on those waves sooner than objects that are farther away do. But those waves don't create/cause water. The water was there all along. Your rock just momentarily moved it.

Last edited by Delvo; 8th April 2012 at 07:56 PM.
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Old 9th April 2012, 01:42 AM   #5
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Originally Posted by C_Felix View Post
I remember reading, I think in one of Brian Greene's books, that one cannot travel through space faster than light, but on space faster than light. One can't travel on water faster than the speed of light, but if one were to ride on the water as it were moving along, and it was moving faster than the speed of light, then one can travel faster than light speed that way.
The universe is expanding, and the usual speed-of-light limit doesn't apply, but if you could make use of this you aren't really travelling in the usual sense.

Originally Posted by C_Felix View Post
For the space shuttle to lift off (when the program was up and running) it needed to travel upwards with enough speed to overcome the gravity of the Earth.
Actually that isn't true. If the space shuttle had limitless fuel, it could climb up at a fairly slow rate like it does when it takes off.

Originally Posted by C_Felix View Post
For something to travel out of a black hole (the poor doomed space shuttle for example) it has to travel faster "up" than the gravity accelerates it down.
That isn't true either I'm afraid. Gravity doesn't accelerate a light beam down. Shine a light beam striaght up and it doesn't fall back down like a lofted cannonball.

Originally Posted by C_Felix View Post
Since light can't escape, does that mean gravity is accelerating items, photons or the space shuttle, "down" at faster than the speed of light?
No.

Originally Posted by C_Felix View Post
So, if my little hypothesis is correct, then the poor space shuttle and the not so poor photons are traveling faster than light. This should be a "no-no" unless the space shuttle or the photons are riding on the fabric of space and not transversing it.
It's a no-no.

Originally Posted by C_Felix View Post
What about gravitational waves? Einstein had a problem with the following: A star goes supernova and planets cannot be released from its gravitational pull instantly, since it cannot happen instantly, because the information and the lesser gravitational pull cannot be transmitted instantly. So, he theorized the fabric of space and the idea of gravitational waves. (I think he came up with gravitational waves.)
No problem with gravitational waves.

Originally Posted by C_Felix View Post
So, if gravitational waves are what cause gravity, are gravitational waves in black holes going faster than light?
They aren't I'm afraid. A gravitational wave is a passing variation in the gravitational field. And gravitational waves in black holes aren't going faster than light.

Originally Posted by C_Felix View Post
Is this possible?
Sorry, no.

Originally Posted by C_Felix View Post
Its late, I'm tired, but hopefully you see where I'm trying to go.
If you want to go there, you have to think outside the box.
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Old 9th April 2012, 02:14 AM   #6
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I once read a neat explanation of why you can't add 2 speeds together.

Imagine a space station. One astronaut leaves in one direction at 100 mph and another astronaut leaves in the opposite direction at 100mph. Clearly one astronaut can communicate with the other, say with a beam of light, because your combined speed travelling apart is only 200 mph (in reality a little less ).

If the astronauts now increased their speed away from the space station to half the speed of light, intuition says they can't communicate with each other because their combined speeds are now the speed of light. However, one astronaut can communicate with someone on the space station who can then pass on the message to the other.

Even if the astronauts were travelling at 99% the speed of light away from the space station they can still communicate. Imagine one astronaut sending a message to someone on the space station. When the message arrives the man is asleep - so the message continues on as if he had sent it and the other astronaut can receive it.

Clearly 99% the speed of light PLUS 99% the speed of light is still less than the speed of light.

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Old 9th April 2012, 02:55 AM   #7
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Originally Posted by Delvo View Post
That would make sense if the water were space instead of water, but using water in the analogy makes it a pretty bad analogy, because you can't add together the speeds of two material things and get a sum greater than the speed of light.
Why would you take any analogy overly literally? Doesn't that defeat the purpose of analogies?
What C_Felix references is actually a pretty good analogy: in a frame field of some family of freefalling observers, space as if being sucked towards the black hole at the local escape velocity.

Originally Posted by pilsan View Post
Clearly 99% the speed of light PLUS 99% the speed of light is still less than the speed of light.
It isn't if you don't switch inertial frames. Then it's 198% of the speed of light. The situation forces one to confront the fact the following are two completely different questions:
(1) In the station's frame, at what speed do the ships separate?
(2) In ship A's frame, at what speed does the other ship move?
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Old 9th April 2012, 05:12 AM   #8
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Warning: in no way is a gravitational field anything like "space being sucked inwards".

Vorpal, I'm surprised to hear you repeating this chicken-little nonsense. I thought we'd knocked that on the head in the black holes thread.
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Old 9th April 2012, 07:19 AM   #9
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Originally Posted by Farsight View Post
Warning: in no way is a gravitational field anything like "space being sucked inwards".

Vorpal, I'm surprised to hear you repeating this chicken-little nonsense. I thought we'd knocked that on the head in the black holes thread.
If "knocked on the head" means "Farsight did a runner" when asked to find any mistake or problem with the math, then yes.

There's a nearly exact mathematical equivalence between black hole event horizons and sound propagating in fluids in which the fluid flow velocity increases with position (along pipe or river, say) and at some point exceeds the speed of sound. Sounds emitted in the region past that point cannot escape to the other side, and the equations that govern their propagation are almost identical to those that govern light-speed signals in a black hole geometry.

Last edited by sol invictus; 9th April 2012 at 07:21 AM.
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Old 9th April 2012, 10:19 AM   #10
Vorpal
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Originally Posted by Farsight View Post
Warning: in no way is a gravitational field anything like "space being sucked inwards".
For the case in the OP? It is, for frames attached to observers freefalling from rest an infinity. In general? Of course not.

Originally Posted by Farsight View Post
Vorpal, I'm surprised to hear you repeating this chicken-little nonsense. I thought we'd knocked that on the head in the black holes thread.
Honestly, I spent almost all of that thread ignoring it because I didn't think it would be productive to participate and had other entertainment, so I didn't read much of it. In any case, what's the problem?
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