# Tag Info

3

This is too long for a comment so I'll post it as an answer, even though this question is years old. If Alcubierre warp bubbles are physically possible, which is exceedingly unlikely, and if the equivalence principle is correct, you could definitely escape from a black hole in one, because there's nothing locally special about the event horizon. In a large ...

-2

The drive works by warping normal space, creating a bubble that kind of surfs through space time. I don't know what would determine the speed such a ship could achieve so not sure if a natural law would limit the ability to travel beyond visible space. Black holes exist because their extreme mass has warped space beyond to point where light can escape. It ...

0

Maybe a qualitative answer motivated from thermodynamics: If you let your black hole rotate, you reduce the number of symmetries of your system, this will decrease your entropy $S$ which is proportional to the surface area. The surface area however is for sure monotonic increasing with your Schwarzschild radius, therefore, if your break symmetries, $r$ will ...

1

Let me add something to the second part of the question. The evidence for the existence of a black hole starts with the observation that there is a very compact, massive object that is not emitting as much light as a "normal" star of that mass. This in itself does not rule out a neutron star, because it may well be as the OP proposes that the neutron star ...

4

Dark matter as far as gravitational forces go , has the same behavior as normal matter. That is how it was discovered and defined. By balancing gravitational forces in the motion of galaxies etc, it was found that more matter was needed than the matter estimated from the luminosity of the bodies. It was observed that the trajectories would not fit the ...

0

See [1] H. Chung, Dynamics of diffeomorphism degrees of freedom at a horizon, arXiv:1011.0623. [2] H. Chung, Hawking radiation and entropy from horizon degrees of freedom, arXiv:1011.0624. The metric used in these papers is a general metric with a horizon, which applies to a large class of horizons including cosmological horizons. Ref [2] derives entropy ...

5

There is theory that light cone shape does not depend on the reference frame in which it is viewed. So why we draw light cones near black hole differently? In general relativity, frames of reference are local, not global. Each of the light cones in your diagram corresponds to a certain local frame of reference. An observer using that frame of reference ...

1

Light travels along paths with a metric interval of zero. In flat spacetime this would be drawn as a light cone with a 45 degree opening angle in a standard Minkowski space time diagram.Things get a bit weirder in GR when spacetime is curved by mass/energy. In GR, the concept of an invariant speed of light only applies locally in non-accelerating frames of ...

5

This isn't exactly an answer to your question, because as it stands your question can't be answered, but I thought I'd post this because the answer really surprised me. Firstly, the reason your question can't be answered is that you can never get your rope below the event horizon. From the perspective of an observer stationary with respect to the black hole ...

6

What would happen if I were to allow one end of a rope to fall past the event horizon of a black hole while I held the other end? As usual, this is in the context of a Schwarzschild black hole. First, outside the horizon, a object with constant radial coordinate 'feels' a constant proper acceleration, i.e., an accelerometer (think of a weight scale) ...

1

In order to not fall straight in, you would have to be orbiting the black hole very quickly, in fact near the speed of light. By definition event horizon is when not even light can escape as it orbits. (Edit: as John Rennie commented, hovering in a rocket is also an option) So imagine you are whizzing around at nearly the speed of light. You lower Your ...

4

When talking about black holes, you need to take into account time dilation. As you lower a rope into an event horizon, you will see time for the end of the rope slow down. You will not be able to say at some point: "Now the rope has crossed the event horizon", because you would need to wait indefinitely. The rope, on the other hand (or some observer you ...

1

If space-time of a black hole is infinitely curved, how can new volume be created for these particles to occupy? The spacetime of a black hole isn't infinitely curved. Only at the spacetime singularity within a black hole is the curvature infinite. The spacetime near, at, and within the horizon is highly curved but not infinitely so. I recommend ...

2

On paper, a black hole already has infinite density. Two coalescing holes would combine to another object of infinite density. Realistically, we would need quantum gravity to prevent a true singularity from forming,a nd there, we could address, more concretely, what happens when the "masses" in the center of the black holes merge. But until we ...

3

$r=1.5r_s$ for the Schwarzschild solution corresponds to the unstable maximum of the effective potential for a photon, therefore you won't be able to see much in practice, since practically every photon on this orbit will either fall in the black hole or escape to infinity.

8

The distance where light has a circular orbit is actually $1.5r_s$ not the event horizon. This distance is known as the photon sphere. In principle a shell observer hovering at this distance could indeed see their own back. The proper distance is indeed just $2\pi r$, however the object would look bigger than expected because the curvature of spacetime has ...

-4

We do not know that "normal matter can not exist within an event horizon". For all we know, aliens may be sitting around and drinking coffee watching us.* We have theories on what could happen. different theories have different views. Nothing has been tested, scientifically proven as yet. Some calculations (the status thereof) on some theories (Hawking' ...

2

As a non-physicist, this is how I understand it: As a massive object collapses, you get more and more matter packed into less and less space. When you do this, the temperature rises and pressure increases. As the temperature rises, the particles get more energetic. As they gain energy, the equilibrium of various things changes... First, the lower-energy ...

4

Let me attempt a slightly different approach to the other two answers. The geometry of spacetime around a static black hole is described by the Schwarzschild metric. The metric can be written down in various ways, but I think the one that best illustrates what is going on is to use the Gullstrand-Painlevé coordinates, and in particular in the form known as ...

4

We intuitively understand why it is inevitable for a particle to progress forward in time. Relativity tells us, among other things, how objects can affect others, cause other stuff to happen and this is connected with the direction forward in time. We call this structure of "what can cause what" the causal structure. In general relativity, this causal ...

1

Aside from the escape velocity at the surface reaching c, what actually changed? The radial direction becomes time-like with the future direction in the direction of decreasing radius. In other words, the time and radial directions swap character at the horizon. See, for example, these lecture notes where we find: Outside the horizon r is a ...

6

Basically, the reason is that if the matter is to keep a positive mass, the amount of force required to keep the matter distribution stable tends to infinity once the matter is fit within the schwarzschild radius for that given mass. This was proven in a very strong sense, without much in terms of assumptions about the particular form of the metric, by ...

Top 50 recent answers are included