# Thread: Black holes and light?

1. Newbie
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## Black holes and light?

First off: I'm new new here so I might do somethings in here that may be slightly out of the norm, hope not, but might so if I do please tell me how to improve and tips etc.
I have been told many times about black holes, even with out black holes being my area of reaserch and not my idea of fun space stuff, and I have been told that black holes are "black" because there gravity is so large light can not escape it. Now let's say that the speed I pedal a bike at is not great enough to excit earths gravity, but say I was going straight up in a straight line and could pedal forever at that same speed, I would in theory finally make it out of earths atmosphere and then out of earths gravity.
What is to stop light from continually moving forward and finally breaking free from the black hole?? Is it because the time needed would be greater than the time the black hole has "been" or is it some other theory or law that I have not take into account?

Thanks

2. Originally Posted by Flanimal4114
Now let's say that the speed I pedal a bike at is not great enough to excit earths gravity, but say I was going straight up in a straight line and could pedal forever at that same speed, I would in theory finally make it out of earths atmosphere and then out of earths gravity.
Welcome to the boards, Flanimal.

As much as I've tried, I've never been able to get my bike off the ground to any significant degree, no matter how fast I've pedaled.

I think you're just using a flawed analogy. At one point, light is emitted, but after that, there is no "pedaling." Light doesn't need any additional, continuous "force" to keep traveling.

Think of, say, throwing a baseball straight up. If you could give it enough speed at the point of release, it would escape earth's gravity and keep going. Otherwise, it will fall back to earth. A black hole is where even light doesn't have enough speed at the point of release.

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The analogy is flawed, sorry for that. But let's say a rocket with unlimited fuel could go up wards at a very slow speed, let's say less than even 1m/s. That rocket would however go up for ever and not stop going up little by little, every second of time going one metre.

Now let's say in a black hole light is trying to escape, the light would go up with its speed minus the graver try of the black hole, now even if that were so little, so very little, it would slowly escape 1000 of a metre at a time (this is a fiver of speech and not a fact of the lights speed in a black hole)
So would not it carry on to go up for ever needing no more force and then never having to go back wards?

4. Originally Posted by Flanimal4114
The analogy is flawed, sorry for that.
Actually, all analogies are flawed at some point, including mine. Unlike the baseball, the light isn't "slowed" so that it falls back in the black hole. It is the space that the light is traveling in that is curved back into itself.

5. Unfortunately, it's like trying to cycle up a down elevator.
The problem is that light, at the event horizon, simply doesn't move fast enough to increase its radial distance from the black hole. And within the event horizon, an outward-directed photon is swept radially inwards, towards the singularity.
To a distant observer, the coordinates of an outward-moving photon at the event horizon never change - it just hangs there. A local free-falling observer, crossing the event horizon at the speed of light, sees the photon go past "upwards" at lightspeed. A local stationary observer at the event horizon has to accelerate infinitely hard to stay in place, next to the photon (and therefore can't do it).

The usual analogy for this mathematical result is to say that space moves continuously inwards towards the singularity - slower than lightspeed outside the event horizon, at lightspeed at the event horizon, and faster than lightspeed beneath the event horizon. The photon propagates at lightspeed through this inward-moving space, and therefore cannot make outward headway if it's at or below the event horizon. (What that's actually describing is the behaviour of an inertial coordinate system attached to a freefalling observer.)

Grant Hutchison

That light moves at a specific pace, but it's fighting upstream against space which inside the event horizon is moving inward faster than it.

7. Two posts in a row that use an analogy of "flowing space." I don't get it. Or rather, I do get it, but isn't that implying that space is "something," a non-mainstream idea? I'm confused.

8. Originally Posted by Lucretius
isn't that implying that space is "something," a non-mainstream idea?
Space is something, it contains values and variables, it expands at large distances, space itself is a thing, even the notion of a "vacuum metastability event" is grounded in the idea that space has qualities potentially subject to change.

Ever since relativity, space is a thing. It is mainstream.

9. Originally Posted by Lucretius
Two posts in a row that use an analogy of "flowing space." I don't get it. Or rather, I do get it, but isn't that implying that space is "something," a non-mainstream idea? I'm confused.
It's "something", but it's not the kind of "something" that we're used to considering something. It's the something that other somethings happen in.

10. Originally Posted by Lucretius
Two posts in a row that use an analogy of "flowing space." I don't get it. Or rather, I do get it, but isn't that implying that space is "something," a non-mainstream idea? I'm confused.
As you say (and as I said), an analogy.
I even said what it was an analogy for, which is the mathematics of general relativity - there's a set of coordinates, in which light propagates, which is moving inwards faster than the light moves outwards.

Spacetime is "made of" mathematics - it's a mathematical construct.

Grant Hutchison

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Originally Posted by grant hutchison
As you say (and as I said), an analogy.
I even said what it was an analogy for, which is the mathematics of general relativity - there's a set of coordinates, in which light propagates, which is moving inwards faster than the light moves outwards.

Spacetime is "made of" mathematics - it's a mathematical construct.

Grant Hutchison
Presumably there is a physical aspect, that the mathematics of general relativity describes?

12. Originally Posted by Questing1
Presumably there is a physical aspect, that the mathematics of general relativity describes?
Presumably so, just like there was a physical aspect that the mathematics of Newtonian gravity described. The secret is not to confuse the mathematics with the unknown reality that it (partially) describes, and certainly not to confuse an analogy for part of that mathematics with the whole of the underlying reality.

Grant Hutchison
Last edited by grant hutchison; 2016-Apr-17 at 12:48 PM.

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Originally Posted by Questing1
Presumably there is a physical aspect, that the mathematics of general relativity describes?
But that might be very far from the mathematical forms of GR. Or not. We don't know.

To take an example - we use wave and field equations to describe oceanic and atmospheric effects like tides, gravity waves, flow and so on. So the mathematical forms we are looking at represent continuous functions of various properties we can then evolve and use to predict behaviour. But underneath that the physical aspect that is being described is an ensemble of molecules interacting in very complex ways. We can ignore than on some scales and model our waves quite happily - but it would be tough to extrapolate back to the molecular picture accurately just from the mathematical model we were using.

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Originally Posted by Shaula
But that might be very far from the mathematical forms of GR. Or not. We don't know.

To take an example - we use wave and field equations to describe oceanic and atmospheric effects like tides, gravity waves, flow and so on. So the mathematical forms we are looking at represent continuous functions of various properties we can then evolve and use to predict behaviour. But underneath that the physical aspect that is being described is an ensemble of molecules interacting in very complex ways. We can ignore than on some scales and model our waves quite happily - but it would be tough to extrapolate back to the molecular picture accurately just from the mathematical model we were using.
Great explanation and point. This is why analysis of gravitational waves, gives us limited information about what is waving. But there are different types of waves to discern from, velocity waves travel through a given medium, and dissipation rates being unique. Infer a measure of viscosity or other, which could help identify a liquid or solid type. But applying our understanding of waves to space, and trying to infer what type of physical medium it is, has led to apparent contradictions and confusions. Until somebody uncovers a hidden parameter or two.

15. Order of Kilopi
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Originally Posted by Questing1
This is why analysis of gravitational waves, gives us limited information about what is waving.
That depends on what you mean by "limited", Questing1 . The recent detection of gravitational waves told us that the observations were exactly as if spacetime was waving as predicted by GR, i.e. the apparatus was getting bigger and smaller because of the expansion and contraction of spacetime in waves. It was another confirmed test of general relativity. There is no actual treatment of spacetime as a "type of physical medium".

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Originally Posted by Reality Check
That depends on what you mean by "limited", Questing1 . The recent detection of gravitational waves told us that the observations were exactly as if spacetime was waving as predicted by GR, i.e. the apparatus was getting bigger and smaller because of the expansion and contraction of spacetime in waves. It was another confirmed test of general relativity. There is no actual treatment of spacetime as a "type of physical medium".
I dont want to lead this thread onto thin ice, so I'll save it for ATM. What you say does accurately represent current theory, and it certainly is a most extraordinary kind of circumstance, spacetime distorting the physical universe in this respect. Must narrow down the possibilities for what is taking place. A physical reason for physical effect? And a contender theory for advancement of GR would need to satisfy an awesome number of known constraints.

17. Questing1, you shouldn't even have gone near the ice. Don't even hint at an ATM answer in the Q&A forum again.

18. Order of Kilopi
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Originally Posted by Questing1
What you say does accurately represent current theory, and it certainly is a most extraordinary kind of circumstance, spacetime distorting the physical universe in this respect.
Actually it is very ordinary physics, Questing1. Spacetime is what physical objects are embedded in. Distort spacetime and objects in it are distorted.

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## Blackholes

Originally Posted by Flanimal4114
First off: I'm new new here so I might do somethings in here that may be slightly out of the norm, hope not, but might so if I do please tell me how to improve and tips etc.
I have been told many times about black holes, even with out black holes being my area of reaserch and not my idea of fun space stuff, and I have been told that black holes are "black" because there gravity is so large light can not escape it. Now let's say that the speed I pedal a bike at is not great enough to excit earths gravity, but say I was going straight up in a straight line and could pedal forever at that same speed, I would in theory finally make it out of earths atmosphere and then out of earths gravity.
What is to stop light from continually moving forward and finally breaking free from the black hole?? Is it because the time needed would be greater than the time the black hole has "been" or is it some other theory or law that I have not take into account?

Thanks
Light is not traveling out of blackhole do to concentration of electromagnetic charge inside and gravitational forces that is part of it. Horizon light is visible by reflection of light from inside blackhole on particles that is spinning around on top of blackhole. To support this I have picture showing photons not traveling true concentrated electromagnetic feeld.

20. Originally Posted by zivota11
Light is not traveling out of blackhole do to concentration of electromagnetic charge inside and gravitational forces that is part of it. Horizon light is visible by reflection of light from inside blackhole on particles that is spinning around on top of blackhole. To support this I have picture showing photons not traveling true concentrated electromagnetic feeld.
zivota11,

First, welcome to CQ.

Second, you are advocating a non-mainstream idea. If you wish to do so here, you must do it in our Against The Mainstream (ATM) sub-forum. Do not do so in Q&A; Q&A is only for mainstream answers.

I also very strongly suggest you review our rules (link in my signature).