l just read about wormholes and black holes. They're simply hypothetical structures in the universe predicted by physicists. My question is, what is the basis of their prediction? Who are the people who just wake up one day and predict "There is a hole that sucks all of the matter and light into it" and "There is an alternate universe once we travel through this tunnel called the wormhole". It seems too random and frivolous to me because I can wake up one day and speculate about the existence of something weird too. I am no expert in such abstract physics (Heck, I'm still applying for college). Any qualitative or quantitative explanation would be of much help.
Asked
Active
Viewed 289 times
5 Answers
4
The prediction of Black holes is very very old, you do not need General Relativity to start to think about it. John Michell and France’s Pierre-Simon Laplace both independently postulated the existence of “non-luminous bodies” (in 1784 and 1796 respectively).$^1$
If you think that light has a velocity and you know a bit about gravity, you will conclude that you can have a body with a escape velocity equal or bigger than the speed of light. By conservation of energy you can deduce the escape velocity$^2$:
$$v=\sqrt{\frac{2GM}{r}}$$ where $G$ is the universal gravitational constant, $M$ the mass of the body to be escaped, and $r$ the distance from the center of mass of the body to the object.
References
MoYavar
- 142
1
You have to remember that the people who originally came up with these ideas were very aware that their careers would likely be adversely affected by them, and they would lose peer respect, which most people would try to avoid.
A lot of physics is spent specifically trying to refute idea, especially weird ones. So the people behind these ideas had to work, often for literally years, to think of ways to justify them using known mathematics and experimental evidence.
They are speculative ideas, but it is usually a TV program that dramatises them with claims and CGI effects, and ignores the caveats, peer review procedures and cautious wording included in the original papers.
1
I think this question is really a history of science one, but here is an answer about black holes in the context of GR (there is a famous earlier prediction in the context of Newtonian gravitation but that is not, I think relevant to GR).
One of the things you do with a theory is look for 'exact solutions': solutions to the equations which you can write down on a bit of paper rather than having to do some huge numerical approximation. Before electronic computers exact solutions were even more important than they still are because numerical approximation was extremely laborious. If you are lucky you will find exact solutions which are good approximations to real physical situations.
And one of the early exact solutions to GR was the Schwarzschild metric, published in 1916 (so within a year of the publication of GR itself). This describes a spherically-symmetric vacuum (no matter or EM fields), which might be taken to be a good approximation to the gravitational field outside a nonrotating star (inside the star there is obviously not a vacuum!).
Fairly soon it was shown that the Schwarzschild solution was the unique spherically-symmetric vacuum solution with no charge (there are other solutions for rotating objects &c: I will concentrate on Schwarzschild here to make this answer shorter). This makes it terribly useful, of course: it means that the field around nonrotating stars (no stars have much charge!) is Schwarzschild to a really good approximation, however it gets stitched onto the solution inside the star.
But the Schwarzschild solution goes all the way down, and there was some theoretical interest in what things would be like if the star became smaller and more dense and, ultimately, if the star was not there at all. And there was a bunch of work done on this: something very odd happens at $r = 2GM/c^2$ for insance and for a long time it was thought that the solution broke down there, while in fact it doesn't.
And out of this theoretical work came a model of this toy object which had an event horizon and a singularity (where the solution really does break down) and a journalist came up with the term 'black hole' which has, obviously, stuck. All of this took until the mid 1950s-1960s: the start of the GR renaissance.
(Note here that black holes do not 'suck matter and light in': far from the event horizon a black hole just behaves like a star. If our Sun was replaced by a black hole the Earth would continue to orbit happily with no difference at all (the absence of sunlight would be a problem for us, as might the X-ray radiation if it had an accretion disk)).
These things were, I think, seen as theoretically interesting, but not actually physically plausible objects. Certainly understanding the Schwarzschild and related solutions well outside the horizon was very interesting and useful since these solutions describe things like the Solar system.
Three things then happened in parallel (some of them starting well before the above-described theoretical work on black hole solutions).
Firstly good models of stellar evolution started to get sorted out: I am very far from an expert on this but I think that Chandrasekhar was the big early name in this field. One of the things that was interesting was what the end-states of stars looked like: what happened to them after they had burned all their fuel. And massive stars, particularly, were interesting. And one awkward thing was that there is an upper limit on the mass of the end-state object, called the Chandresekhar limit which is about 1.4 times the mass of the Sun, beyond which they will collapse. But there are stars much more massive than this: what happens to them? Well, it turns out that they collapse to a really bizarre thing -- a neutron star. But this too has a limit, the Tolman–Oppenheimer–Volkoff limit which is somewhere in the range 1.5-3.0 solar masses. And what happens to things which are heavier than that?
Secondly a couple of mathematical / theoretical physicists proved some rather general theorems about GR. And these theorems are really awkward: they say that, for quite general starting conditions, singularities will form which, really, means that there will be collapse, and there will be black holes as the end states (the theorems do not quite tie the nature of the singularies down and there has been a bunch of later work on this, although I believe it is still an open question). The people who did this were Roger Penrose and Stephen Hawking and the results are the Penrose-Hawking singularity theorems, and they were proved in the 1960s. These are pretty awkward results.
Thirdly and most importantly people started to find completely extraordinary objects observationally. These discoveries really started in the 1960s, and they started then because people became able to make observations from very high in the atmosphere (and, quite soon, from space) where X-rays were not shielded, and these objects are typically active X-ray sources which were not previously visible.
The most famous early one of these was Cygnus X-1 which is a bright X-ray source, initially discovered by detectors on sounding rockets high in the atmosphere (its discovery predates X-ray obbserving satellites). There are various things that people worked out about this source (and others, of course).
- It's in a binary system with a star whose mass can be inferred from the things we know about stars. This means that the object itself has a mass which can be inferred from the orbital period and the mass of its companion. This mass is about 15 solar masses.
- Its X-ray emissions vary several times a second, and this limits its size: for the emissions to vary in time $t$ the size of the object must be small enough that light can traverse it in less than $t$. For Cygnus X-1 the limit was about $10^8\,\mathrm{m}$ -- the diameter of the Sun is about $10^9\,\mathrm{m}$.
So whatever this thing is it is more than 10 solar masses, and less than a tenth the diameter of the Sun. Later observations showed variations in the X-ray emissions down to millisecond timescales, limiting its size much further.
In the 1970s (before all of this was known), Stephen Hawking and Kip Thorne made a bet, Hawking betting that Cygnus X-1 would not turn out to be a black hole. He lost that bet: combined with the models of stellar evolution I talked about above, there's only one candidate object for Cygnus X-1: it's almost certainly a black hole. Hawking conceded in about 1990 I think.
So these toy, absurd, theoretical objects turn out to be real, and in fact pervasive. I think this is one of the most extraordinary stories in physics in the last hundred years.
Note I have talked only about black holes here: wormholes are still, I think, toy models. Although the lesson of black holes is, perhaps, that toy models should be taken quite seriously.
Note also that there is a theoretical difference between the Schwarzschild solution (which is static) and the solution describing collapse: that's interesting in theory, but in practice it makes no difference at all to anyone outside the object's horizon (or where the horizon would be if it was a Schwarzschild solution). I am quite happy to call both kinds of objects 'black holes'.
0
Einstein presented his field equation in November 1915. They describe the motion of matter or particles according to the curvature of spacetime in 3 space plus one time dimensions. Karl Schwarzschild was a physicist who at the time was serving in the German army against the Russians in Poland. He worked a solution to the Einstein field equations within months after reading Einstein's paper. This solution was for a spherically symmetric gravity field. This solution far from the center of the source approximates Newtonian gravitation. This solution lead to the black hole.
Schwarzschild never had any idea this was a black hole, even though it had some odd features. He died shortly after of an auto immune disorder he developed. The solution was considered accurate for large distances, but for a compact object it seemed pathological. It was Oppenheimer and Snyder in 1939 who took this a big step further by demonstrating that once a particle passed the region $r = 2GM/c^2$ its dynamics could not be observed. This research did not go as far as it could have for not long after Oppenheimer was developing a little device that on July 16, 1945 changed the world forever and months later was used on Japan to end the World War II.
Leap forwards to the 1960s and Wheeler, Penrose and Hawking did all sorts of work to illlustrate that black holes were a theoretical possibility. In 1972 a small X-ray detecting satellite found the signature of a black hole in Cygnus X-1. Black holes entered the lexicon of astrophysics at this point. Based on their theoretically derived properties and phenomenology predicted from them we have a high level of confidence for having detected them. The recent detection of gravitational radiation is further evidence for the existence of black holes.
Wormholes are related to black holes. It was recognized that if one crossed the event horizon of a black hole that the horizon in effect splits. The past horizon in your universe is continuous with the future horizon in another universe, or in some region elsewhere. This seems to be some sort of connection between two regions, and it is called the Einstein-Rosen bridge. This other region however is something you can see into, but you can't enter. You can witness it from the interior of this trapping region that is dragging you inexorably towards a singularity. Realistically a standard stellar generated black hole you can't enter because you would be pulled apart by tidal forces. You could enter a galactic superpmassive black hole and endure in the interior for hours or even days.
The black hole is then a sort of non-traversable black hole. You can from the black hole interior see into some other region of the universe or maybe into a different universe entirely. You can't however enter this other region. Consequently people played with adjusting black hole metrics so you can cross into some other region. This is the traversable wormhole, and it permits one to use local symmetries of general relativity to violate global symmetries that make up special relativity. You also have negative vacuum energy, which means that quantum states are not bounded below. Quantum mechanics provides a minimum energy to systems, such as the $S^1$ shell of the hydrogen atom; the electron goes no further down to lower energy. If you have negative energy states, which violate the energy conditions of Penrose and Hawking, then some strange things can happen. It means that a quantum field that is the source of this spacetime geometry can fall into ever lower energy levels and emit an arbitrary amount of radiation. The wormhole is also odd in that it permits time travel.
I doubt that wormholes exist in our universe. Spacetimes with negative energy vacuum are however interesting. The anti-de Sitter spacetime has negative energy and this may be the inflationary spacetime that generates cosmologies. Just as the quantum field that generates the wormhole could generate and endless flood of radiation the anti-de Sitter spacetime can generate an arbitrary number of positive energy valued cosmologies, such as the one we observe. So in a theoretical sense of pure study oddball spacetimes such as wormholes are of some value. However, it is not likely wormholes exist in our observable universe or that we will be zipping across the universe in them.
LC
Lawrence B. Crowell
- 13,005
-1
Black holes are real, look it up in Wikipedia, or google it. Someone didn't just wake up in the middle of the night and invent. It was developed as a possible configuration (called solution by physicists, because you have to solve equations of) General Relativity, which show how curved space and time (called spacetime) arises because of the gravitation of massive bodies (or energy). The fact that gravity is simply the curvature of spacetime has been known since 1915's Einstein publication of General Relativity, with many confirmed observations over the years verifying the theory.
The solution was found in the 1910's but depending on various conditions it can represent the spacetime (or as is also understood) the gravitation outside of a spherical body. If that body is too massive it turns out it collapses and causes enough gravitation that from the inside (more in a minute) nothing can escape, not even light. Thus it looks black, and a physicist with some sense of humor called it black hole.
A lot of astrophysical are thought pretty seriously to be black holes, and they are often detected by the gravity they impose (outside the 'hole') on a companion star which revolves around it. In 2015 a gravitational observatory called LIGO detected for the first time gravitational waves which two black holes merging emitted before they became one (those waves were generated outside, sort of in between the holes because of the very strong and rapidly changing gravitational fields, or in other words, changing spacetime). Since then they've detected tow other pairs of black holes merging. It is strongly believed that the core of our galaxy, and many other galaxies, are supermassive black holes.
Google and in wilipedia read all about it: https://en.wikipedia.org/wiki/Black_hole
Wormholes you can also find in Wikipedia, at https://en.m.wikipedia.org/wiki/Wormhole But they are stranger than black holes and there is no observation that indicates they can really happen. It is hypothetical, comes from a variation of a solution also, but calculations I dictate that even if they existed they are totally unstable and decay very fast, almost instantaneously. The only way to keep them there seems to be if there was some negative mass particles which have never been found, and many physicists (but not all) think they are impossible.
BTW, there's lots of other strange things in physics, those two are not the only ones. We know for instance that quantum physics says, and it has been confirmed, that microscopic elementary particles can be in two different places at once. Read, look for the Heisenberg uncertainty principle and why can not determine where anything is exactly. Also will tell you how a single particle can go through two little slits at the same time and interfere with itself.
Finally, this site will want you to read up and ask better questions, not something you can just simply look up easily in Google or Wikipedia. If you don't do some preliminary research and then you can ask what you do not understand specifically, your questions might just be unacceptable and deleted. So hopefully you can go by the rules and enjoy finding out more about physics and the mysteries of it in this site.
Bob Bee
- 14,246