Time Travel Possibility with Two Black Holes Orbiting Each Other

[slimak]

TL;DR Summary

Hi, I want to ask what happens to someone who passes between two black holes that are orbiting each other so closely that their event horizons are touching. Is it true that such a person would transform into antimatter because he would start to travel backwards in time?

The reason why I think it could happen is that spacetime is being curved really extremely in black holes and when you draw a chart of spacetime near and in black hole , you can see that time axis is being bend towards the center of black hole and that thing is happening from all sides of the black hole, so if you enter black hole from one side , the time axis goes straight to the center of the black hole and theoretically its coming on the other side of the black hole, but in opposite direction.

But of course stuff can't exit the black hole, not even the event horizon, but that is only from the perspective of the viewer outside of black hole and of course all the stuff ends up in the center of black hole because it can't continue its natural trajectory of time axis , because there is center of black hole blocking its path. (same reason why we can't orbit the Earth by simply falling down, there is Earth in a way).

But if there are two orbiting black holes and their event horizons are overlapping or touching, you could travel trough there, but from the point of view of viewer outside, you would just fall in a black hole and at the same time as the antimatter version of you would fall in at the same time as you, but from exactly opposite side of the black holes. But from your point of view you would come on other side (assuming you survive and are not ripped apart by spagetification) and on the other side you would see universe traveling backwards in time, because your time axis would be reversed.

 

[PeterDonis]

I want to ask what happens to someone who passes between two black holes that are orbiting each other so closely that their event horizons are touching.

If the horizons are touching, the holes are merging and it's impossible to pass between them; anyone who tried to pass between them would end up trapped inside the merged hole.

Is it true that such a person would transform into antimatter because he would start to travel backwards in time?

You seem to have two misconceptions here.

First, falling into a black hole or going near the horizon of one does not make you travel backwards in time.

Second, antimatter is not matter that is traveling backwards in time.

when you draw a chart of spacetime near and in black hole , you can see that time axis is being bend towards the center of black hole

No. This and the rest of your post is simply mistaken speculation.

 

[slimak]

No. This and the rest of your post is simply mistaken speculation.

Isn't that how theory of relativity works? That objects with mass bend spacetime around them. Both space and time axis are bend towards the centre of gravity?

First, falling into a black hole or going near the horizon of one does not make you travel backwards in time.

I didn't sugest that it does, I just sugested that behind the event horizon the time axis could become bent so much that will be paraler with the space axis of the spacetime outside of horizon and if that would be the case, the time axis could conect with the time axis coming from the opposite side of black hole right in it's center. Isn't that how relativity works?

 

[Ibix]

Isn't that how theory of relativity works? That objects with mass bend spacetime around them.

Yes, more or less.

Both space and time axis are bend towards the centre of gravity?

No. Curvature is described by the 20 independent components of the Riemann tensor. It doesn't have a single direction. And it certainly doesn't do anything to "the time axis", since there isn't really any such thing, unless you choose to declare some timelike vector field to be "the" time axis. But that would be an arbitrary choice, and the integral curves might approach the black hole or not.

I just sugested that behind the event horizon the time axis could become bent so much that will be paraler with the space axis of the spacetime outside of horizon

This makes no sense, unfortunately. You can't casually compare directions at different points in a curved spacetime, and no sensible process for doing it is going to make a timelike vector "more parallel" to a spacelike vector than a timelike one.

black hole right in it's center

The black hole models I'm familiar with don't have centers (curved spacetime is strange). The singularity is typically closer to a time than a place.

Isn't that how relativity works?

No.

 

[slimak]

Thank you so much for your reply. You taught me a lot today.

 

[Ibix]

And it certainly doesn't do anything to "the time axis", since there isn't really any such thing, unless you choose to declare some timelike vector field to be "the" time axis. But that would be an arbitrary choice, and the integral curves might approach the black hole or not.

Just to add to what I wrote here - all future-directed timelike vectors do point inwards at the event horizon. That's why you can't escape - all your future is inside the hole. But anywhere above the horizon there always exist timelike vectors pointing away from the holes and there's nothing stopping you labelling one of these "the time axis", although the system will fail at the horizon.

You do sometimes see people talking about "light cones tipping towards the horizon", which is closer to what you were talking about. This is a coordinate dependent statement, though. In some coordinate choices the light cones "close up" as you approach the horizon. So, while both statements are true, they come with large "certain point of view" caveats and I wouldn't put too much stock in them.

Thank you so much for your reply. You taught me a lot today.

Curved spacetime is very strange. Unfortunately, that means that you really have to get to grips with the maths if you want to be able to reason about it at all.

 

[slimak]

all future-directed timelike vectors do point inwards at the event horizon. That's why you can't escape - all your future is inside the hole.

Yes, that's what I meant for it to be parallel . Thank you so much for your reply.

I have weird question. When objects follow the timelike vectors inside the event horizon, where would they endup if the singularity would not stop them?

 

[PeterDonis]

You do sometimes see people talking about "light cones tipping towards the horizon", which is closer to what you were talking about. This is a coordinate dependent statement, though.

Actually, there is an invariant sense in which the light cones "tip" towards the horizon: radially outgoing null vectors have an expansion scalar that decreases as the horizon is approached, and is zero at the horizon and negative inside the horizon. But how this shows up in particular coordinate charts does vary a lot from one chart to another.

 

[PeterDonis]

When objects follow the timelike vectors inside the event horizon, where would they endup if the singularity would not stop them?

All timelike worldlines inside the horizon end on the singularity. There is nowhere else to go.

 

[slimak]

Thank you so much for your reply.

 

[PAllen]

Actually, there is an invariant sense in which the light cones "tip" towards the horizon: radially outgoing null vectors have an expansion scalar that decreases as the horizon is approached, and is zero at the horizon and negative inside the horizon. But how this shows up in particular coordinate charts does vary a lot from one chart to another.

While what you state is true, I would say it has nothing to do with light cones. You are discussing the behavior of a congruence of radially outgoing null paths emanating from particular spheres defined by symmetries of the spacetime. This congruence is completely different from the congruence of light paths emitted in all directions from a given event. This latter is what is normally called a light cone. Even in the interior, this has a positive expansion scalar, at least initially. It also has increasing shear, and I don’t know, without calculating it, whether there is a turnaround in the expansion scalar as you have compression in one direction and expansion in another. In no sense is the initial behavior of a light cone different whether the emitting event is outside, on, or inside the horizon. So I go with the view that tipping of light cones is an entirely coordinate dependent statement, and doesn’t exist at all in Kruskal coordinates.

 

[PeterDonis]

You are discussing the behavior of a congruence of radially outgoing null paths emanating from particular spheres defined by symmetries of the spacetime.

Yes, and these null paths are the "outgoing" edges of the light cones of events that lie on the spheres. So the decreasing expansion of those null congruences as the horizon is approached can be thought of as an inward "tipping" of the outgoing edge of the light cones of those events.

I agree, however, that this is only one possible interpretation, and the congruence in question is not the same as the congruence that describes the entire light cone of a particular event.

 

[slimak]

You seem to have two misconceptions here.

First, falling into a black hole or going near the horizon of one does not make you travel backwards in time.

Second, antimatter is not matter that is traveling backwards in time.

I have another weird question. What about antimatter with negative energy, that could be a regular matter traveling backvards in time ?

 

[PeroK]

I have another weird question. What about antimatter with negative energy, that could be a regular matter traveling backvards in time ?

Antimatter has positive energy.

In Quantum Mechanics, the mathematics is usually expressed in terms of matter (e.g. electrons). In this mathematical formalism, the anti-particles (e.g. positrons) manifest themselves as electrons with negative energy or moving backwards in time.

This is not physically significant, as you could express the equations in terms of positrons, where electrons would be the anti-particles (with negative energy or moving backwards in time).

In that respect, there is no fundamental difference between matter and anti-matter. It's purely conventional that the electron is called "matter" and the positron "anti-matter". (Although, of course, there is an over-abundance of matter and anti-matter is rare.) In terms of gravity, both are particles with mass and positive energy and have the same effect on spacetime curvature.

 

[PeterDonis]

What about antimatter with negative energy

As @PeroK has pointed out, antimatter does not have negative energy.

That seems like enough questions based on misconceptions for one thread. Thread closed.