# Can A Black Hole Exist?

In 1939, Robert Oppenheimer and other concluded that a certain neutron stars could collapse to black holes and no known law of physics would intervene. As far as I can tell, there is no observational data that shows that an event horizon of a black hole exists.

Assuming that a black hole with an event horizon exists, then there are all kinds of mathematics that explain what is observed externally, approaches the event horizon. It can be explained what an infalling observer sees as they approach the event horizon. However, what happens when an infalling observer passes the event horizon, the result is a mathematical absurdity, in that the observer would observe the universe outside the event horizon as beyond "the end of time."

It seems that passing through an event horizon is as mathematically absurd as an object with non-zero mass reaching and surpassing the speed of light, as it would require more than infinite amount of energy in the latter, or more than an infinite amount of time in former.

So, can an event horizon even "exist" in the first place, if "existing" means within the bounds of any moment after the big bang and before an infinite amount of time from any point within the universe.

It seems that the existence of black holes seems to be well accepted, however faster than than light travel does not under the current understanding of the physics of relativity.

• Yes, in GTR it can. Related, possibly duplicate: Would time go by infinitely fast when crossing the event horizon of a black hole? You're making similar erroneous assumptions here about "the end of time." – Stan Liou Dec 23 '14 at 9:17
• I have addressed the point you make about observational evidence for event horizons. If you really want to to ask theoretical questions about black holes, you should separate them off and ask your question on Physics SE (if you want a much wider audience of potential responders). – Rob Jeffries Dec 23 '14 at 11:01
• "However, what happens when an infalling observer passes the event horizon, the result is a mathematical absurdity, in that the observer would observe the universe outside the event horizon as beyond "the end of time."" -- this is incorrect. Whether beyond the event horizon or not, the infalling observer would observe photons coming from the universe outside the event horizon. – Johannes Dec 27 '14 at 13:37
• You may be interested to see this answer: physics.stackexchange.com/questions/21319/… – Anixx Jul 11 '15 at 20:49
• Related question: astronomy.stackexchange.com/questions/28945/… – user25329 Dec 30 '18 at 22:45

There is evidence that both black holes and their event horizons exist.

The primary evidence for stellar-mass black holes arises from observations of the dynamics of binary systems. What has been found, for at least 20 binary systems, is that the optically visible star has a dark companion, that is usually more massive, and more massive than can possibly be supported as a neutron star or any other degenerate configuration ($>3M_{\odot}$ and mostly $>5M_{\odot}$ - see Narayan & McClintock (2014) for an excellent, accessible review).

Supermassive black holes at the centres of galaxies can also be "weighed" using the dynamics of gas, or in the case of our Galaxy, actually watching the stars orbiting something that must have a mass of $4.4\times10^{6}M_{\odot}$, yet is remarkably compact and emits little or no radiation.

The evidence for event horizons is more circumstantial, but not absent. In any case, absence of evidence would not be evidence of absence - event horizons are difficult to prove; they are small and far away, with elusive observational signatures. The lack of an event horizon could only be accommodated by inventing something even more bizarre than a black hole.

The most compelling argument for an event horizon is found in sources that accrete from their surroundings, either in a binary system or at the centres of galaxies (see Narayan & McClintock 2008; read particularly section 4). In some circumstances, the accretion flow can become radiatively inefficient (dark), in which case one expects that if the accretion flow reaches a "surface", it will radiate its kinetic energy strongly as it thermalises on impact. On the other hand, a black hole event horizon can swallow such flows without trace. The prediction is, that in their low-states, transient black hole accretors will be much less luminous than their neutron star counterparts, and this is what is found (by a factor of 100).

Other evidence is that quiescent neutron star binaries show hot, thermal emission, presumed to be from the neutron star surface. None has been seen from the black hole binaries. Type I X-ray bursts are seen in neutron star binaries, caused by an accumulation of matter and subsequent nuclear ignition on their surface. The black hole candidates do not show these bursts. The accretion rate onto the supermassive black hole in the centre of the Galaxy should produce thermal radiation at any "surface" that would be quite visible in the infrared part of the spectrum. No such detection has been made.

All of these observations are best explained if the black hole has no surface and can simply make the accreted energy "disappear" inside its event horizon. Narayan & McClintock conclude that these lines of evidence are "impervious to counterarguments that invoke strong gravity or exotic stars".

Yes they do exist. General relativity predicts that when a massive enough star dies, it's core collapses into a black hole in a finite amount of time and space is continuously flowing into the black hole. If you fall into a supermassive black hole, the tidal forces you experience will be small and you will pass the event horizon in a finite amount of time so you will not see a time very far in the future before you pass the event horizon. It does not work the other way. An outside observer will never observe you reach the event horizon.