# Tag Info

8

While photons can in principle form a black hole, the black hole will not be massless. The mass of the black hole will be related to the energy of the photons that went into it by Einstein's famous equation $E = mc^2$. The black hole will be a regular black hole, and classically it has an infinite lifetime. Once you include Hawking radiation the black hole ...

6

In 1974 Stephen Hawking published a paper that provides a theoretical basis for the thesis that black holes eventually may radiate away all the mass, light, and other energy they accumulate. Evaporation of black holes has been called Hawking radiation. It takes place so slowly (at least until the black hole shrinks to a small size) that none has been ...

6

There will be spoilers if you keep reading Firstly, he is shown surviving inside black holes. From where did he got oxygen? Perhaps from oxygen bottles. But, in an intense gravitational pull, how he survives? He would have got torn apart! am I right? The popular press says the word black hole and it is a bit vague what they mean because there are some ...

5

The first part of the argumentation is basically right: external objects influence the gravitational field so that the event horizon of the black hole in the middle will no longer be "exactly spherical and isotropic" when additional objects' gravity distorts the field. Well, the reality is that even the spacetime refuses to be spherically symmetric in the ...

4

To someone outside it looks the same as a regular, massive black hole. Classically the lifetime is the lifetime of the universe. It might merge with another black hole. It might last forever. It might meet a singularity in a big crunch if the whole universe contracts to a singularity. If you are worried that it can't decay by Hawking radiation because an ...

4

It is believed, according to our most tested theory of gravity (General Relativity), that objects, such as material from other stars, particles such as electrons and also photons of light, may actually pass through the Event Horizon of a black hole. For a large enough black hole, a person in a spaceship passing through the event horizon may not notice any ...

3

A short answer is that frames themselves are moving towards the black hole and light moves relative to a frame and hence it can be stuck. Nothing passes through a black hole. Things can enter a black hole, they can't can't exit without going faster than light. Where do things go then? The important part of that question is the word "where" you ...

3

The stars in the galaxy don't really orbit the black hole in the center of the galaxy. They all orbit a common center of gravity. Obviously, a lot of the mass is in the black hole, and the center of gravity could very likely lie inside the black hole's event horizon, but it's not required. Look here for a cool animation of Pluto and Charon orbiting a center ...

2

Like most proposals, it is possible of course; in physics we must ultimately test proposals experimentally. In the meantime (i.e. in this case whilst waiting for experimental observation and study of dark matter here on Earth), one must resort to assessing plausibility in the light of what we already know. There are two ways your proposal, if true, could ...

2

I don't think I can explain all the technical stuff, but first things first. Primordial means "at the creation", so if it was created today it wouldn't be primordial. Now "Primordial size" black hole, is probably what you mean and even that is a bit vague as estimates vary on the possible sizes of Primordial black holes, (and there's some uncertainty as ...

2

There is no requirement for a central black hole in a dynamical sense. Many galaxies are not known to have one, or if they do, its mass is relatively small. The gravitational influence of the SMBH can be quite negligible at distances that are only a tiny fraction of the size of a Galaxy. What I mean by this is that say the BH at the centre of the Milky Way ...

2

Yes and no. A vacuum solution could be stationary, could be static, could be neither. For the orbiting black holes you end up with gravitational waves and gravitational radiation. You either have some going out, and the bodies in spiraling or you have some coming in and driving the system or both. And the point is that you have to specify the state of the ...

2

In case your ball is infinitely lighter than the black hole, the answer is infinity. You can never be sure it is not coming back. But in reality your ball has a finite mass which can not be neglected. Its mass is to be added to the black hole's mass M, therefore increasing its size. An outside observer will see that his ball got sucked into the black hole ...

2

The math in that article is based in Cartesian space. Note specifically figure 4, where a portion of a Cartesian plane is pinched in at one side to show the supposed warping due to gravity. Using the shown transformation, she concludes that space is compressed near a black hole rather than stretched. The diagrams after that along with the process ...

2

General relativity is a theory that tells us the geometry of spacetime. However it predicts that in some situations the geometry of spacetime is undefined - this happens when we get a singularity. There is a singularity at the centre of a black hole, and the Big Bang was also a singularity. So we have the odd situation that the theory of general relativity ...

2

Disclaimer for those who know more: I only talk about spherical black holes for simplicity; rotating black holes are more complicated. Indeed, a planet's gravity bends light and allows you to see a little bit farther; this is an observed effect (though not on planets), and is called gravitational lensing. If you've seen the movie Interstellar, you might ...

2

When gravity is strong enough, it bends light towards the source of the gravity. Roughly true So if you were on a small planet and gravity were to gradually increase, would the horizon rise as well, allowing you to see further? Yes! If so, at some point, could you look up at some angle and have the light go all the way around the planet ...

2

Nothing special is happening! Think of a black hole as the accumulation of mass which is exceeding a certain limit. The same laws of gravity are applying before and after exceeding the limit. That means: Mass particles keep on being attracted. They are becoming part of the mass of the black hole. Electromagnetic waves will equally be attracted by the mass ...

2

In $4\times10^9$yr, M31 and MW (Milky Way) will have merged to form an elliptical galaxy. The internal spiral structures of either progenitor and their bars will be destroyed in the process, leaving a smooth ellipsoidal distribution of stars. The supermassive black holes (SMBHs; note that the one in M31 is $\sim100$ times more massive than that in MW) will ...

2

The area of an event horizon is an invariant, so it doesn't matter what coordinate system you use to calculate it. The area is of physical interest because it's proportional to the entropy of the black hole, and in a second law kind of way that means the area cannot decrease.

2

No one knows what happens inside the black hole (I mean inside the event horizon). But inside the event horizon space becomes unidirectional (like time in real world) and therefore whatever enters into it must hit the singularity inside (at least according to classical general relativity). But again no one knows what happens at that singularity. By ...

2

Do black holes have a puff pastry point? No. If a person falls into a certain sized black hole they accelerate very fast, which increases the g forces on them. They don't feel any g forces at all, because there aren't any. These g forces flatten the person out into a pancake. There aren't any, a falling person doesn't flatten into a ...

2

It does seem odd that a star that isn't a black hole can explode, and therefore presumbly lose mass, and still form a black hole. The explanation is that to form a black hole requires a high density not just a high mass. Even a small object such as, well, you or I could form a black hole if compressed enough, though obviously in practice that level of ...

1

(as usual, we assume the static black hole solution for simplicity). If hovering outside the horizon, the gravitational time dilation goes to infinity as the distance from the horizon goes to zero. The elapsed proper time of the twin hovering outside the event horizon is given by \Delta \tau = \Delta t \sqrt{1 - \frac{r_s}{r}} = 10^9 \sqrt{1 - ...

1

The mathematics of general relativity is clear and unambiguous. The trouble comes when you try and describe what is going on it non-mathematical terms, because there is no precise way to do it. Kip Thorne is attempting to talk about black holes in non-mathematical terms, and he is adopting a different perspective from (probably) most of us. That doesn't mean ...

1

Hawking radiation is indeed usually calculated as a semiclassical effect (classical gravitational field and quantum matter), which is invalid at high energy ($\langle T_{\mu\nu}\rangle >> 1$) and high fluctuations ($\langle T_{\mu\nu} T^{\mu\nu} \rangle - \langle T_{\mu\nu} \rangle \langle T^{\mu\nu}\rangle >>1$). Still, it is a result that is ...

1

Due to gravitational time dilation, for an observer of the planet, the frequency of electromagnetic radiation would be slower. Visible light emitted from the planet would appear as infrared or micro-waves. The amplitude of the radiation would not change. Since frequency decreases while amplitude remains constant, the radiometer would receive less ...

1

An object inside a black hole horizon cannot send signals outside of the horizon, but something falling into the black hole can fall through the signal. Take the example of the camera attached to the astronaut's foot. When the astronaut is halfway through the horizon, the camera is about a meter inside the horizon and the transmitter is about a meter ...

1

Your premise is that quantum gravity has effects near the singularity and that the event horizons is a barrier to getting information our from within the horizon. So a simple theoretical investigation is to look at a very very small black hole, one where the outside of the event horizon is still near the singularity and thus the quantum gravity effects are ...

1

If two black holes hit head on at 99% the speed of light, the result would be one black hole sitting stationary in the center of mass frame with roughly twice the mass and some fraction of the energy expended as gravitational radiation.

Only top voted, non community-wiki answers of a minimum length are eligible