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Black holes are perhaps the strangest objects predicted by Einstein's theory of General Relativity, objects so dense that gravity reigns supreme, and not even light can escape beyond a certain distance, known as the event horizon. The actual existence of black hole event horizons has not been proved, but some clever observations made by astronomers at the University of Texas at Austin and Harvard University have tested the alternative hypothesis: instead of an event horizon, there might instead be a solid surface to a black hole that objects colliding against it will hit. They found results that show that this alternative can't be true, and that an event horizon as predicted by GR is more likely. ScienceDaily has an article:
Astronomers at The University of Texas at Austin and Harvard University have put a basic principle of black holes to the test, showing that matter completely vanishes when pulled in. Their results constitute another successful test for Albert Einstein's General Theory of Relativity.
Most scientists agree that black holes, cosmic entities of such great gravity that nothing can escape their grip, are surrounded by a so-called event horizon. Once matter or energy gets close enough to the black hole, it cannot escape — it will be pulled in. Though widely believed, the existence of event horizons has not been proved.
"Our whole point here is to turn this idea of an event horizon into an experimental science, and find out if event horizons really do exist or not," said Pawan Kumar, a professor of astrophysics at The University of Texas at Austin.
Supermassive black holes are thought to lie at the heart of almost all galaxies. But some theorists suggest that there's something else there instead — not a black hole, but an even stranger supermassive object that has somehow managed to avoid gravitational collapse to a singularity surrounded by an event horizon. The idea is based on modified theories of General Relativity, Einstein's theory of gravity.
While a singularity has no surface area, the noncollapsed object would have a hard surface. So material being pulled closer — a star, for instance — would not actually fall into a black hole, but hit this hard surface and be destroyed.
The team figured out what a telescope would see when a star hit the hard surface of a supermassive object at the center of a nearby galaxy: The star's gas would envelope the object, shining for months, perhaps even years.
Once they knew what to look for, the team figured out how often this should be seen in the nearby universe, if the hard-surface theory is true.
[...] "Given the rate of stars falling onto black holes and the number density of black holes in the nearby universe, we calculated how many such transients Pan-STARRS should have detected over a period of operation of 3.5 years. It turns out it should have detected more than 10 of them, if the hard-surface theory is true," Lu said.
They did not find any.
"Our work implies that some, and perhaps all, black holes have event horizons and that material really does disappear from the observable universe when pulled into these exotic objects, as we've expected for decades," Narayan said. "General Relativity has passed another critical test."
The full text of the original paper "Stellar disruption events support the existence of the black hole event horizon" (DOI: 10.1093/mnras/stx542) is available open access from the Monthly Notices of the Royal Astronomical Society.
Further evidence for or against the existence of black hole event horizons will have to wait for the Event Horizon Telescope, which is due to release its first results later this year.
(Score: 2) by AthanasiusKircher on Saturday June 03 2017, @05:31AM (1 child)
It really depends on what you mean by "not reach the event horizon in finite time." I assume you're talking about an external observer here. Yes, basically time dilation as viewed from a distant observer basically becomes infinite at the event horizon. But the idea of being able to see an astronaut "frozen in time forever" at the event horizon simply is inaccurate. Why? Because really what time dilation affects is the rate of release of photons that will ultimately reach the distant observer. Since the number of photons emitted from the falling astronaut before crossing the event horizon is finite, there is a finite future time when the "last photon" will be observed.
But from a more practical standpoint, a distant observer would just see an exponential decay in the number of photons from the source, and this decay will happen quite rapidly. As discussed here [ucr.edu], for a somewhat small black hole, the final approach and "dimming" to basically nothing will likely happen in a matter of seconds or less. For very large black holes, it could be longer, but still not "until the end of universe" kind of scale.
By "beyond the event horizon" I assume you mean at a greater distance outside the black hole? Or do you mean inside the event horizon? Anyhow, the reason all the math breaks down at the event horizon has to do with the speed of light. A common way of discussing this is to say that gravity is so strong at the event horizon that beyond that escape velocity exceeds the speed of light. (That's a little imprecise, but it gets the general idea across.) Light paths inside the event horizon necessarily curve inward. Those immediately outside can curve outward, but it gets less possible the closer you get (and time dilation will cause escaping photons to take ever longer). The event horizon is basically defined as the dividing point between these two zones.
This, again, is a common misconception. The link above explains it some more. But for most "normal" black holes, the astronaut would likely see some time acceleration upon approach. But the astronaut actually will pass the event horizon at a subjective "normal" rate of speed, and just as the emitted photons are limited, so are those which can arrive to the astronaut from faraway events in the future. For a somewhat more technical explanation, see answers here [stackexchange.com].
Note there are unusual circumstances with rotating black holes that could theoretically expand this, especially with exotic stuff like wormholes. But the standard scenario for falling into a black hole doesn't allow you to see the entire history of the universe.
For large black holes, one could easily pass the event horizon without being "spaghettified" or possibly harmed at all (depending on what one thinks goes on with the potential "firewall" around the event horizon). Tidal forces for small black holes will rip you apart before you get there, but for a sort of "galactic core" size black hole, the tidal forces might barely be noticeable at the event horizon.
(Score: 2) by melikamp on Saturday June 03 2017, @07:05PM
Not to argue, but just to understand... If we think about the EM wave emitted, not photons, there seems to be practically infinite time to detect the wave.