# Black holes and time dilation

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• FyrenEyce
And if you looked in front of you it would be just blackness. This is what would happen if you were to move faster than the speed of light and enter the event horizon of a black hole.

#### FyrenEyce

According to Einstien's theory of evolution, the closer to the speed of light that an object travels, the slower it appears to move to an outside observer. (see https://www.thenakedscientists.com/forum/index.php?topic=40951.0) To take this to the next step once an object crosses the event horizon of a black hole, by definition it is being pulled faster than the speed of light since light cannot escape. Einstien also postulated that if somehow one could move faster than the speed of light then one could move backwards in time. Could therefore mass that is pulled into a black hole be ejected not in space but in time? I.E. traveling back in time to before the black hole existed?

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Space-time inside a black hole doesn't work the way you think it does. I can't explain it, but others here can.

While I appreciate you reading and replying to my post, telling me I'm wrong without stating reasons why I'm wrong is not helpful to the discussion.

FyrenEyce said:
According to Einstien's theory of evolution, the closer to the speed of light that an object travels, the slower it appears to move to an outside observer. (see https://www.thenakedscientists.com/forum/index.php?topic=40951.0) To take this to the next step once an object crosses the event horizon of a black hole, by definition it is being pulled faster than the speed of light since light cannot escape. Einstien also postulated that if somehow one could move faster than the speed of light then one could move backwards in time. Could therefore mass that is pulled into a black hole be ejected not in space but in time? I.E. traveling back in time to before the black hole existed?
As @phinds indicates, this is not correct. Black holes bend space-time in such a way that, once inside the horizon, the future is toward the singularity. There are no time-like (or null) paths that lead back out to infinity. Once inside the horizon you cannot escape the singularity any more than you can escape next Tuesday.

None of this involves any faster-than-light motion. In general relativity, the notion of relative velocity is less straightforward than you might expect. In order to compare a velocity over here to a velocity over there, one needs to have a coordinate system -- a frame of reference (*). In special relativity one could use an inertial frame of reference to make the comparison. In general relativity with curved space-time, there are no global inertial frames. One can pick out a [non-inertial] frame of reference to use, but the choice you make affects the velocity that you measure. There is no unambiguously right choice. However, one thing remains true. For any object you can always set up a frame of reference centered on that object and have it as close to perfectly inertial as you please, as long as your restrict your attention to a small enough region and interval. This is a so-called "tangent inertial frame". In this frame of reference, light moves at the speed of light and nothing moves faster. Even at or inside an event horizon.

(*) Technically, one can compare velocities without setting up a frame of reference. Instead, one can use parallel transport to take a velocity vector over here, move it over there and compare it with another velocity vector. But the path you use to do the transport matters. So you end up with the same ambiguity. In flat space-time (i.e. special relativity), parallel transport is unambiguous. All paths come out the same. In curved space time (i.e. general relativity) the path matters.

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What I meant by being ejected into time is that if the object were to move faster than light and therefore backward in time, as viewed by an outside observer (if one could observe anything inside an event horizon), is that it could feasibly move backwards to the point that the black hole collapsed into a singlarity. I understand the concept that the speed of light is constant regardless the viewer's own velocity, but the very existence of an event horizon seems to indicate that objects inside of it move faster than light.

Follow me on a thought experiment if you will. Imagine you are dropped into a black hole wearing a magical space suit that allows for you to not be ripped apart and/or crushed. As you approach the event horizon things further from the gravity well appear to speed up and eventually start to blue-shift. Eventually everything behind you is blue-shifted and accelerated to the point where everything is just one big blur from your perspective. And if you look in front of you, it's just blackness. This is the event horizon. As you get sucked further into the black hole, if the speed of light is constant regardless of your own velocity, but can't escape the gravity of the singularity, you would still see the same blue-shifted blur behind you and the same black ahead of you forever, no matter how close to the singularity you get, until you reach the singularity itself. It would functionally be no different than what you would experience if you were to somehow attach a video camera (also magical) to an event horizion of a black hole as it formed and then watch the tape in reverse.

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You are being deceived by coordinate system choices for defining the position of objects inside an event horizon. To preserve sensibility you need to use something like Rindler coordinates. If done correctly, it will quickly become obvious time travel into the past is impossible.

FyrenEyce said:
According to Einstien's theory of evolution, the closer to the speed of light that an object travels, the slower it appears to move to an outside observer. (see https://www.thenakedscientists.com/forum/index.php?topic=40951.0)

The above talks about Special Relativity - the situation where spacetime is flat.

To take this to the next step once an object crosses the event horizon of a black hole, by definition it is being pulled faster than the speed of light since light cannot escape.

Black hole is not a flat spacetime. Now you need to use General Relativity. In GR, speed of light is a local concept: namely, light moves *locally* at the speed of light relative to any object.

Globally, in GR spacetime is curved (not flat), and definition of relative velocities for objects which are spatially separate is coordinate-dependent and there is no way to choose which one is "correct".

At the event horizon, too, light moves locally at the speed of light relative to any object. On the other hand, light which moves outward at the event horizon stays at the same distance from the BH center. This is the basis to informally say that any falling object "falls at the speed of light" there.

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FyrenEyce said:
According to Einstien's theory of evolution

Is that like Darwin's theory of relativity?

FyrenEyce said:
the closer to the speed of light that an object travels, the slower it appears to move to an outside observer.

This is not correct even as a description of what special relativity says. A clock carried by a moving object appears to run slow to the observer who sees it moving relative to him; but the object's own speed is just its speed; there is no relativistic speed that slows its speed to slower than what its speed is (can you see how that doesn't even make sense?).

FyrenEyce said:
once an object crosses the event horizon of a black hole, by definition it is being pulled faster than the speed of light since light cannot escape

This is not correct either. In a curved spacetime (i.e., in the presence of gravity), it is not possible to compare speeds at different locations. You can only compare speeds locally, i.e., a given observer can meaningfully assign speeds only to objects moving past him at the same location. Even inside the horizon of a black hole, no observer will see anything at his location moving faster than light relative to him.

FyrenEyce said:
Einstien also postulated that if somehow one could move faster than the speed of light then one could move backwards in time.

Einstein did not postulate this. Other later physicists speculated about tachyons, hypothetical particles that could move faster than light, but it's not as simple as you think it is. This article might be helpful:

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/tachyons.html

FyrenEyce said:
Imagine you are dropped into a black hole wearing a magical space suit that allows for you to not be ripped apart and/or crushed. As you approach the event horizon things further from the gravity well appear to speed up and eventually start to blue-shift.

This is not correct. An observer who is using rocket power to hover close to the horizon (i.e., not falling in but keeping the same altitude) will see the rest of the universe blue-shifted and sped up; but an observer who is free-falling into the hole will see the rest of the universe as red-shifted and slowed down.

You seem to have a number of misconceptions about black holes; I would advise correcting them before trying to speculate.

Pencilvester
I found this video made by smarter people than me that seems to back my theory.

I have no stake in being correct about this, just trying to share what I thought was an original idea. Anyway, maybe someone will find this interesting.

FyrenEyce said:
I found this video made by smarter people than me that seems to back my theory.

Pop science videos are not valid references. You need to look at textbooks or peer-reviewed papers. If you do, you will see that what the video is describing has nothing whatever to do with your personal speculations. It is just a common (and somewhat misleading) pop science description of how the interior of a black hole looks in particular coordinates.