# I Does a black hole cut the fabric of spacetime as it changes

1. May 2, 2017

### SD das

Einstein's general theory of relativity describes the stars distort spacetime fabric and it appears a black hole makes a hole in the spacetime fabric. It is evident that the size of a galaxy does not change due to gravity but in large scale the distance between two distant galaxies increases due to the metric expansion of space. Therefore, does a black hole cut the fabric of spacetime as it changes is position as the distance of galaxies increases? It is known, most of the galaxies contain million of stars, many black holes and specifically a supermassive black hole in its centre.

Therefore, I was wondering about the consequence of the curvature of spacetime caused by an entire galaxy! Can you suggest?

2. May 2, 2017

### Staff: Mentor

This is a common misconception. Spacetime is described by a mathematical object known as a metric tensor. This describes the local geometry of spacetime at any point in an analogous way that an electric field describes the force that a charged particle would experience at any point in space. A black hole's singularity causes the metric to develop infinities, indicating a problem. There is no fabric and no hole. It's just a point where the math behaves in a way that makes it impossible to make predictions. Singularities exist in other areas of physics and math and usually indicate that you're doing something incorrectly, don't know the correct laws to use, and sometimes don't represent anything physical at all (cusp singularities of optical caustics).

It does not. There is no cutting of fabric at all.

3. May 2, 2017

### SD das

you say misconception but (1) what is a misconception I don't get, besides (2) are you saying spacetime is described by metric tensor but I see it is described as, "In the mathematical field of differential geometry, a metric tensor is a type of function which takes as input a pair of tangent vectors v and w at a point of a surface (or higher dimensional differentiable manifold) and produces a real number scalar g(v, w) in a way that generalizes many of the familiar properties of" please clarify the issues (1) and (2) above

4. May 2, 2017

### SD das

Why not? ..... In large scale the distance between two distant galaxies increases due to the metric expansion of space... so since a black hole is an entity in a galaxy it will move too.... besides it is known that a black hole put a dent in the spacetime, a dent means, break, crush, cripple, destroy, damage etc. therefore since a black hole moves in the spacetime with its containing galaxy, the dent put by same black hole will drag along the spacetime with its movement in it... however, the dent as dragged by the black hole making a cut might heal as the dent moves that may cause gravitational wave too.. now tell me where my guesswork went wrong, would you?

5. May 2, 2017

### SD das

Note1, Metric Expansion: https://en.wikipedia.org/wiki/Metric_expansion_of_space

6. May 2, 2017

### SD das

Note 2: Black Hole Puts Dent In Space-time...http://www.space.com/1976-black-hole-puts-dent-space-time...

7. May 2, 2017

### SD das

Note 3: What makes a supermassive black hole move through space? https://physics.stackexchange.com/q...-a-supermassive-black-hole-move-through-space

8. May 2, 2017

### phinds

None of the articles contradict in any way anything that Drakkith said. What is your specific point of confusion? If you still think there is a "fabric" then you are adhering to pop-sci nonsense, not science.

And by the way, the statement "It is known, most of the galaxies contain million of stars" is off by a factor of 1,000

9. May 2, 2017

### Staff: Mentor

There is no fabric. The misconception is simply that there is a fabric. Since there is no fabric it doesn't make sense to ask if it can be cut or torn.

It is a common misconception because, unfortunately, many authors of pop-sci media describe it that way. So we completely understand the source of your misconception, and are simply trying to help you learn correct physics instead.

10. May 2, 2017

### PeroK

I took a look at the space.com article:

"Spacetime

Spacetime can be thought of as an elastic sheet that bends under the weight of objects placed upon it. The more massive the object, the more spacetime bends."

The problem with trying to understand this is that you are trying to understand something that is fundamentally wrong. I'm not sure why the elastic sheet analogy is so widespread as it is highly misleading.

Another inaccuracy is that it is not the mass of an object but a combination of its mass and density that curves spacetime. A star may have just as much mass as a black hole, but because of its size we see essentially regular "Newtonian" gravity around it. In fact, even if the Sun were a black hole, the Earth would stay in its current orbit, as it's only close to the black hole that you get extreme curvature.

A galaxy is not very dense, hence the curvature of spacetime outside a galaxy is low.

11. May 2, 2017

### kimbyd

The spacetime surrounding a black hole is smooth and regular. There are no irregularities that could be interpreted as a "cut" or "tear".

12. May 2, 2017

### Staff: Mentor

Several posts have been removed and the thread is now re-opened.

13. May 9, 2017

### Grinkle

Is this line of thinking what drives theorizing about gravitons? Its hard for me to picture without bringing some graviton-esque things in there somewhere ....

14. May 10, 2017

### Staff: Mentor

I don't know. Both the classical electromagnetic field and the curvature of spacetime according to GR are non-quantum descriptions. How you get from there to gravitons probably requires looking into quantum theory.

15. May 10, 2017

### Staff: Mentor

Meaning, the analogy with electromagnetism? Sort of, in the sense that, since we now have quantum field theories of the other fundamental interactions, the natural expectation is that there would also be a quantum field theory of gravity. For reasons which are typically addressed as exercises in GR textbooks (for example, MTW does it in a fairly early chapter), a quantum field theory of gravity, along the lines of our other QFTs, would have to be a theory of a massless spin-2 field (whereas the QFT of electromagnetism is a theory of a massless spin-1 field); the particle associated with excitations of this field is called the "graviton".

This sort of theory was investigated in the 1960s and early 1970s by Feynman, Deser, and others. It has been discussed on this forum before, but not recently AFAIK.

16. May 11, 2017

### Chronos

What does 'cut a hole' in spacetime actually mean? Evidently you view spacetime as some kind of quilt over the mattress of some grander reality. That's the kind of nonsense I would expect from a multiversalist. Apologies for not better disguising my contempt.