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.
The Event Horizon Telescope (EHT) is finally ready to take a picture of Sagittarius A*. From April 5th to 14th this year, the virtual telescope that's been in the making for the past two decades will peer into the supermassive black hole in the center of our galaxy. EHT is actually an array of radio telescopes located in different countries around the globe, including the Atacama Large Millimeter/submillimeter Array in Chile.
By using a technique called very-long-baseline interferometry, the EHT team turns all the participating observatories into one humongous telescope that encompasses the whole planet. We need a telescope that big and powerful, because Sagittarius A* is but a tiny pinprick in the sky for us. While scientists believe it has a mass of around four million suns, it also only measures around 20 million km or so across and is located 26,000 light-years away from our planet. The EHT team says it's like looking at a grapefruit or a DVD on the moon from Earth.
To prepare the participating observatories, the team equipped them with atomic clocks for the most precise time stamps and hard-drive modules with enormous storage capacities. Since the scientists are expecting to gather a colossal amount of data, they deployed enough modules to match the capacity of 10,000 laptops. Those hard drives will be flown out to the MIT Haystack Observatory, where imaging algorithms will make sense of EHT's data, once the observation period is done.