Do stars fall quietly into black holes, or crash into something utterly unknown?

Do stars fall quietly into black holes, or crash into something utterly unknown?
This artist's impression shows a star crossing the event horizon of a supermassive black hole located in the center of a galaxy. The black hole is so large and massive that tidal effects on the star are negligible, and the star is swallowed whole. The effects of gravitational lensing distorting the light of the star are not shown here. Credit: Mark A. Garlick/CfA

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 . 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.

Do stars fall quietly into black holes, or crash into something utterly unknown?
The first in a sequence of two artist's impressions that shows a huge, massive sphere in the center of a galaxy, rather than a supermassive black hole. Here a star moves towards and then smashes into the hard surface of the sphere, flinging out debris. The impact heats up the site of the collision. Credit: Mark A. Garlick/CfA

Kumar, his graduate student Wenbin Lu, and Ramesh Narayan, a theorist from the Harvard-Smithsonian Center for Astrophysics, have come up with a test to determine which idea is correct.

"Our motive is not so much to establish that there is a hard surface," Kumar said, "but to push the boundary of knowledge and find concrete evidence that really, there is an event around black holes."

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.

"We estimated the rate of stars falling onto ," Lu said. "Nearly every galaxy has one. We only considered the most massive ones, which weigh about 100 million solar masses or more. There are about a million of them within a few billion light-years of Earth."

Do stars fall quietly into black holes, or crash into something utterly unknown?
In this second artist's impression a huge sphere in the center of a galaxy is shown after a star has collided with it. Enormous amounts of heat and a dramatic increase in the brightness of the sphere are generated by this event. The lack of observation of such flares from the center of galaxies means that this hypothetical scenario is almost completely ruled out. Credit: Mark A. Garlick/CfA

They then searched a recent archive of telescope observations. Pan-STARRS, a 1.8-meter telescope in Hawaii, recently completed a project to survey half of the northern hemisphere sky. The telescope scanned the area repeatedly during a period of 3.5 years, looking for "transients"—things that glow for a while and then fade. Their goal was to find transients with the expected light signature of a star falling toward a supermassive object and hitting a hard surface.

"Given the rate of stars falling onto black holes and the number density of black holes in the , 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, 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."

Now the team is proposing to improve the test with an even larger telescope: the 8.4-meter Large Synoptic Survey Telescope (LSST, now under construction in Chile). Like Pan-STARRS, LSST will make repeated surveys of the sky over time, revealing transients—but with much greater sensitivity.

This research has been published in the June issue of the journal Monthly Notices of the Royal Astronomical Society.


Explore further

Test of general relativity could potentially generate new gravitational models

Citation: Do stars fall quietly into black holes, or crash into something utterly unknown? (2017, May 30) retrieved 22 April 2019 from https://phys.org/news/2017-05-stars-fall-quietly-black-holes.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.
2214 shares

Feedback to editors

User comments

RNP
May 30, 2017
An open access copy of the paper can be found here; https://arxiv.org...0023.pdf

May 30, 2017
Let's add a bit more science to this collision. The presence of the star in proximity of black hole actually displaces the event horizon because of it's gravitational pull. So to increase your understanding of this event, consider the case where the star is itself a black hole (e.g. of similar mass). What happens when their event horizons "collide"? Well actually, the event horizons will take on different shapes and may not collide, depending on their relative masses. In fact, they can pass extremely close to each other with neither black hole fully absorbing the other. It is possible for some matter to transfer from one to the other. And their relative velocities will play a big role as well, and even a bigger role for the case when the star is not a black hole. Consider even it this case whether matter can transfer from the black hole to the star as the event horizon changes shape!

May 30, 2017
I have tried to put my head around this for years ... because time slows down as you approach the event horizon till it stops at the event horizon there cannot yet be a singularity in our timeline of our universal space-time. All black holes are things where something is just about to happen but that last second that is required to merge never actually passes because it is frozen in the space-time that has stopped (from our vantage point) waiting for the last instance before time ends for it to happen. So no blackholes have ever merged and nothing has yet fallen through an even horizon... time stops just before and waits for the end of time to happen.

Naturally for the things going through the event horizon they do not notice anything till they are crushed in an instant... but for us outside in our timeline of space-time those things (merging or falling through) cannot happen until the end of time happens simultaneously.

May 30, 2017
This comment has been removed by a moderator.

May 30, 2017
I have tried to put my head around this for years ... because time slows down as you approach the event horizon till it stops at the event horizon there cannot yet be a singularity in our timeline of our universal space-time.

Addendum – this is only true in the reference frame of an external observer (mathematically precisely, one sitting at potential zero, at infinite distance from any gravity sources). But collapse models predict that event horizon appears at a distinct point in time at the centre (in the singularity, which is instantly pushed into infinite time by external frame of reference), and only the non-singular part of the black hole (which is 100%) actually has to fall into it.

But there is one more strange thing happening. Once you pass the event horizon, the radius coordinate becomes time-like. This is the mechanism that forces movement towards the centre for objects that have fallen in (in their reference frame); as movement in time has fixed direction.

May 30, 2017
In fact, they can pass extremely close to each other with neither black hole fully absorbing the other. It is possible for some matter to transfer from one to the other.

I doubt this. For matter to remain stable it relies on the interchange of force carriers (W/Z bosons, gluons). These force carriers have masses and so are bound by the 'slower than light' rule. Once you're inside an event horizon I can see no way that this interchange e.g. between quarks in a proton can still happen, because it would require the force carriers to be able to move faster than light (anything further 'in' - even if it's only a fraction of a proton's width - is effectively behind an event horizon of its own with regards to anything further 'out' once you pass the event horizon)

Two perfectly same mass black holes could come arbitrarily close, but still have respective event horizons between them. If either is only slightly bigger than the other that one will swallow the other.

May 30, 2017
This comment has been removed by a moderator.

May 30, 2017
It doesn't prove in any way that event horizons exist. It just tends to prove that spherical hard surfaces are unlikely. Event horizons are simply impossible because information must be conserved. Possible solution would be extreme Kerr black holes with a real ring, where matter is spacetime...

May 30, 2017
This comment has been removed by a moderator.

May 30, 2017
they cannot fuse or merge in our time-line until the end of our timeline as external observers. Since we are here as external observers we can never see or experience the existence of a merge or fuse of event horizons because to us they would only get very close and freeze in time but never actually merge or touch in our time-lines until the end of our time-line.

So for (as far as we can ever tell)... us here and now no black hole has ever merged and nothing has passed any event horizon from today all the way back to the Big Bang... they all get close but then freeze before then can actually merge or touch.

May 30, 2017
This comment has been removed by a moderator.

May 30, 2017
At infinity time minus one second after the Big Bang all of the blackholes that started to be created would still not have yet merged from our perspective of observers looking from outside of their event horizons.

May 30, 2017
Dark matter is quite simply speed. It has mass and can interact with gravity through any black hole, or infinite density object, but does not interact in any way with the electromagnetic spectrum.

Add up the speed of objects as they fall into a black hole, minus the acceleration due to gravity, and voila, the missing speed comes back out.

The speed essentially adds to the spin of the space around the galactic core. Call it gravitic spin.

May 30, 2017
"The black hole is so large and massive that tidal effects on the star are negligible, and the star is swallowed whole."

This part seems nonsensical to me. Larger/massive objects produce greater tidal gravity effects. Why would the star *not* be pulled apart by tidal effects at a given distance?

May 30, 2017
At the even horizon (assumed hard surface), an object travelling at the speed of light can't escape. Since any ordinary matter will be traveling at a hefty fraction of the speed of light towards the event horizon, it is supposed to somehow stop and reverse direction at nearly the speed of light?

There won't be much to splash since it is all heading the wrong direction at relativistic speeds. I don't think there are any mechanisms that would stop relativistic matter in its tracks and reverse direction. The amount of energy needed (mass equivalent) would only add to the problem.

May 30, 2017
you can travel as fast as you like towards a black hole. you will never reach the event horizon according to our perspective as observers not travelling with you. This entire article is pointless since these things cannot happen from our perspective as observers watching the events. All we can ever see is everything stop just shy of entering the event horizon.

so all of these discussions about what happens after the timeline ends of the observer is not much different than speculating about what happened before the big bang.

I can say that one unicorn barfed to start the big bang and another unicorn ate it at the end of time.

What I am saying it that science starts at the big bang and ends at the event horizon of all black holes (this includes the edge of the universe which is an event horizon in reverse due to our point of reference). Everything beyond these points is the realm of pseudoscience.

May 30, 2017
I believe TopCat22 has captured the essence of my argument as well. It is really irrelevant from the outside observer's perspective exactly what happens at the event horizon. All information at that point is lost forever (except as noted by Hawking's proposal of this information being somehow recovered by hawking radiation). From the outside observer's perspective time slows down as the gravity field becomes more and more intense, until at the event horizon it becomes infinitely slow. This doesn't mean that time is slowed at all from the perspective of the infalling matter of course. So whether it accumulates on a "hard surface" or flows past the event horizon is essentially unknowable, at least from an experimental or observational standpoint.

May 30, 2017
We get another three "artist's impression" pictures that have nothing to do with reality and only serve to reinforce misconceptions.

May 31, 2017
Apparently the 'artist' knew nothing about tidal forces. A star diving into a black hole would be shredded to a plasma cloud as it approaches the event horizon. Well-known images of a dwarf star cannibalizing its companion in a close binary system would be much closer to the truth.

May 31, 2017
"The black hole is so large and massive that tidal effects on the star are negligible, and the star is swallowed whole."

This part seems nonsensical to me. Larger/massive objects produce greater tidal gravity effects. Why would the star *not* be pulled apart by tidal effects at a given distance?

Because the infalling star is in a gravity well with a less abrupt gradient. The effect of the gravity on its closest part is not so much stronger than the effect on its furthest part that it is pulled apart, or pulled into a string.

Jun 01, 2017
The paper is an excellent use of null hypothesis tests. By providing evidence against the existence of a physical event horizon, it reinforces the concept that the event horizon is a mathematical concept and not a real object.

Jun 01, 2017
Considering the sizes of supermassive black holes we have seen eat stars by ripping them apart first, I am not sure how credible this is. I'd accept arguments to the contrary but they'd have to be backed up with solid math and some references.

Jun 02, 2017
... I don't think there are any mechanisms that would stop relativistic matter in its tracks and reverse direction. The amount of energy needed (mass equivalent) would only add to the problem.

It wouldn't "stop in in it's tracks". It would pull it into an inbound trajectory curve...

Jun 02, 2017
@DS @WG

Maybe spooky action not at a distance? Could it be that a cascade effect for the stars that go directly to black star not supernova?

Thanks in advance.

Not quite sure what yer asking, Hat. Could you maybe re-describe?

Jun 02, 2017
This comment has been removed by a moderator.

Jun 02, 2017
@Dingbone
I think we're in agreement, hopefully I can explain.

The authors asserting that a physical representation/solid surface of event horizons will produce transients, have produced the null hypothesis that there is no physical or solid surface of the black hole. https://simple.wi...pothesis

Finding no transients then disproves the assertion, and proves the null hypothesis. Showing that it is not a physical or solid surface. Which is what you also are saying, if I understood correctly.

Jun 02, 2017
This comment has been removed by a moderator.

Jun 02, 2017
@DS Maybe spooky action not at a distance? Could it be that a cascade effect for the stars that go directly to black star not supernova?
Nawww, all you need here is gravity. If the nuclear reaction sputters too much, it will collapse and what happens after that is inside the event horizon. The observation in the article indicates (does not definitively show, but we know of no other predicted process) that it can happen just like this. It's actually a rather startling observation, but not totally groundbreaking.

Thanks in advance.
Sure man. Ask questions if I'm not clear for you.

Jun 02, 2017
... I don't think there are any mechanisms that would stop relativistic matter in its tracks and reverse direction. The amount of energy needed (mass equivalent) would only add to the problem.

It wouldn't "stop in in it's tracks". It would pull it into an inbound trajectory curve...
It would also spaghettify it, for known masses of supermassive BH. A BH with enough mass to allow a star to approach the EH has not been discovered, AFAIK. I am willing to be corrected.

Jun 02, 2017
Wrong thread.

Jun 06, 2017
If we would observe time dilation associated with a mass approaching the event horizon of a black hole, with time stopping at the EH, must we then conclude that no black hole has ever consumed any matter? If so, how do we explain the huge black holes that are presumed to occupy the center of galaxies? Were they made that way at birth? Consider also a star approaching a black hole that has mass and event horizon diameter much smaller than that of the star. Would the black hole accelerate much more than the star in this collision, and would the black hole penetrate the star like a bullet, with the bulk of the star staying outside the event horizon, and with the black hole punching out a plug of some sort from the star, with this plug then kind of stuck to the front part of the black hole??? This all can get quite confusing.

Jun 06, 2017
The theory is, the material at the center of a supernova gets compressed until it forms an event horizon. And in fact, recent results reported here on physorg indicate that massive enough stars can form a black hole without a supernova. Once there's enough mass inside a small enough radius, an event horizon forms. After that it sucks everything around it in; your time dilation idea works after that, but it's already a black hole.

To disprove this you will need a quantum theory of gravity. Are you proposing one?

Jun 11, 2017
Let's consider just one of the questions that puzzle me. It's my understanding that, if we were to observe from a safe distance the interaction between a black hole and say, an asteroid caught in its gravitation grip, the asteroid would accelerate toward the black hole to tremendous speeds, and at some point, we would notice time dilation associated with the asteroids motion, because of Special Relativity. Namely, it's approach speed to the EH, compared to our reference, would slow down and just before the EH, would essentially become zero. If that's all true, wouldn't we then conclude that no object could penetrate the EH of a black hole in finite time? I would appreciate if anyone could explain any false reasoning in this scenario.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more