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posted by Fnord666 on Friday June 02 2017, @05:51PM   Printer-friendly
from the abandon-all-hope-ye-who-enter-here dept.

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.


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  • (Score: 2) by bob_super on Friday June 02 2017, @06:43PM (2 children)

    by bob_super (1357) on Friday June 02 2017, @06:43PM (#519500)

    I think it's the opposite.
    Time slows for the person going over the event horizon (ignoring how very dead they are).
    Time flows at the faster rate for the distant low-grav observer seeing the object cross the event horizon.

    https://simple.wikipedia.org/wiki/Time_dilation [wikipedia.org]

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  • (Score: 0) by Anonymous Coward on Friday June 02 2017, @07:30PM

    by Anonymous Coward on Friday June 02 2017, @07:30PM (#519531)

    Yes, Brian Greene explained it best to me in his book on String Theory (which I still don't understand, but hey, he helped me understand GR).

    Think of gravity as centrifugal force, if you are far away from the center you will feel a stronger pull to the outside (at a given angular speed).

    Note that the further out from the center you are the the further you have to go each rotation, and thus the further light travels with you. Thus, to preserve the speed of light, as you get further out time goes slower.

    Likewise, as you approach a massive object, you get pulled more strongly by that object, and time goes slower for you.

  • (Score: 2) by AthanasiusKircher on Saturday June 03 2017, @05:37AM

    by AthanasiusKircher (5291) on Saturday June 03 2017, @05:37AM (#519743) Journal

    This is incorrect. Your statements are accurate for a 3rd party observer looking at two people, one close to the event horizon and one far away. The one close to the event horizon will appear to have clocks moving slowly; the one far away will appear to have clocks moving fast. The subjective experience of each, however, is a different question. Since the person near the black hole has slow moving clocks, far away stuff appears to move even faster comparatively. But subjective experience of time doesn't "slow down" -- the clock still seems to tick at a "normal" rate for the person near the black hole, and thus they pass through the event horizon relatively quickly (from their perspective of time) compared to what's observed from a distance.