# As matter approaches a black hole, does it speed up?

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If so, how do we know it speeds up? Doesn't time slow down as gravity increases? If time slows down around a black hole, is it possible matter doesn't actually speed up?

The answer is neither yes or no or possibly both.

Take a simple example. If something falls freely towards a black hole along a radial path, and is observed by someone who is far from the black hole, its velocity (according to the distant observer) is given by $$v = -left(1 - frac{r_s}{r} ight)left(frac{r_s}{r} ight)^{1/2}c, ,$$ (e.g. see chapter 6 of Exploring Black Holes by Taylor, Wheeler & Bertschinger - freely available) where $$r_s$$ is the Schwarzschild radius and the negative sign just indicates an inward velocity with $$r$$ decreasing.

If you plot this function (see Fig.2 in Ch.6 of Taylor et al. - freely available) you will see that initially the magnitude of the velocity increases as $$r$$ decreases, but as $$r ightarrow r_s$$ then $$v ightarrow 0$$ and the falling object appears to come to a standstill (actually, because the light from the object is gravitationally redshifted, this may not actually be observed). However, if the velocity first increases and then slows to a standstill, then it must go through a maximum!

The maximum observed speed in this scenario is achieved at $$r=3r_s$$ and is $$0.384c$$.

Of course this story is different for different observers. If you are the falling object then your speed just keeps increasing through the event horizon and towards the singularity. On the other hand, an observer who was somehow able to hover just above the event horizon would measure the falling object's speed as just below $$c$$ as it passed.

The dilation of time is only relevant from the perspective of someone far away from the black hole. Close to the black hole time is still progressing forward at what would appear to be a normal rate to someone who is close to the black hole. The movie Interstellar had a great depiction this phenomenon, with the astronauts Copper and Brand on Miller's planet, near the black hole, spending only a few hours, but the astronaut Romilly aging decades as he remained far from the planet. Copper and Brand didn't experience any change in the passage of time, from their perspective.

Matter falling into a black hole would not experience any change in its perspective of time, so would not appear to change speed, other than what would be expected by the gravitational attraction.

## Astronomers Peer Through the Fog at Milky Way’s Supermassive Black Hole –“Is It Emitting a Jet Angled Toward Earth?”

In 2019 astronomers lifted the veil on the monster black hole called Sagittarius A* (Sgr A*) at the heart of our Milky Way Galaxy. Using computer modelling, the scientists simulated the material inside the thick cloud of plasma, dust and gas surrounding Sgr A*. The results pointed to the possibility of a relativistic jet coming from the supermassive black hole with an inclination that is aligned with Earth’s viewing point.

An international team of astronomers recently used an Interferometric technique which combines many telescopes to form a virtual telescope the size of Earth to map out the exact properties of Sgr A*. Using telescopes including the Atacama Large Milimeter/submilimeter Array (ALMA) in northern Chile, an image was produced with a resolution that allowed us to peer through the fog surrounding the supermassive black hole.

Top left: simulation of Sgr A* at 86 GHz. Top right: simulation with added effects of scattering. Bottom right: scattered image from the observations, this is how we see Sgr A* on the sky. Bottom left: the unscattered image, after removing the effects of scattering in our line of sight, this is how Sgr A* really looks like. Credit: S. Issaoun, M. Mościbrodzka, Radboud University/ M. D. Johnson, CfA

To their surprise, they discovered that Sgr A*s emission is coming from an extremely narrow area of the sky — just one 300 milllionth of a degree. The emission also appeared to have a symmetrical shape. And, since black holes don’t emit detectable radiation on their own, the source is most likely one of two things.

Sgr A* –A disk of Infalling Gas or a Jet?

“This may indicate that the radio emission is produced in a disk of infalling gas rather than by a radio jet,” said astrophysicist Sara Issaoun of Radboud University in The Netherlands and a member of the EHT collaboration. “However, that would make Sgr A* an exception compared to other radio emitting black holes. The alternative could be that the radio jet is pointing almost at us.”

Since the 2019 observations, wrote Sera Markoff, American astrophysicist and professor of theoretical high energy astrophysics at the University of Amsterdam in an email to The Daily Galaxy, “ Sgr A* does not, as far as we can tell, have relativistic jets at the moment, or at least nothing like those we see from our other Event Horizon Telescope (EHT) source galaxy M87. Those would be impossible to hide! It might have weak jets pointing towards Earth, but the point is that it wouldn’t matter since they are so weak they can’t even seem to make it outside of the Galactic center.”

The M87 Jet �,000 Light Years Long at Radio Wavelengths

Compared to the ambiguous existence of the Sgr A* jet, the M87 jet (shown above), shoots out 5,000 light-years at optical wavelengths (100,000 light years at radio wavelengths), traveling at close to the cosmic speed limit. Using Chandra observations , researchers have seen that sections of the jet are moving at nearly the speed of light. When matter gets close enough to a black hole, it enters into a swirling pattern called an accretion disk. Some material from the inner part of the accretion disk falls onto the black hole and some of it is redirected away from the black hole in the form of narrow beams, or jets, of material along magnetic field lines. Because this infall process is irregular, the jets are made of clumps or knots that can sometimes be identified with Chandra and other telescopes.

EHT observed M87 over six days in April 2017, giving a snapshot of the black hole. The Chandra observations investigate ejected material within the jet that was launched from the black hole hundreds and thousands of years earlier. “It’s like the EHT is giving a close-up view of a rocket launcher,” said the CfA’s Paul Nulsen, “and Chandra is showing us the rockets in flight.”

Existence of Sgr A* Jets Debated

“Theoretically speaking,” Markoff added, “Sgr A* has all the conditions to launch weak jets, so many of us suspect they are present, just difficult to detect because the Galactic center is a very complicated region with lots of confusing features that could hide weak jets.”

“Since we cannot prove the existence of jets there has been controversy for years,” Markoff wrote to The Daily Galaxy, “The radio spectrum looks very much like that of other supermassive black holes that are weakly accreting like those in nearby galaxies such as the weak jet in M81, which is almost a twin of our Galactic center but a bit higher power. Also the variability pattern moves from high to low frequency, which is the opposite of what you would expect for infalling gas, because high frequency light comes from more compact regions. Thus stuff moving outwards from compact regions near the black hole, as in a jet, would show high to low frequency ‘waves’ of variability, and we do see that.

EHT and Radio VLBI Will Help Continue Lifting the Veil

“I do think EHT and radio VLBI in general will help us sort this out,” Markoff concludes, “but I would not bet that our single epoch of observations from 2017 will be enough. Most likely we will need several years of observations to build up enough certainty, as well as reliable ‘movies’ of what’s going on in the source. Not only to resolve the question of if there is a jet, but which direction is it pointing, and does that direction line up with the black hole spin (it doesn’t have to!)?”

Markoff is a member of the Event Horizon Telescope team that produced the first ever image of the massive now-iconic black hole at the center of M87 described by scientists on April 10, 2019 at the press conference in Brussels where the photograph was revealed as the “ Gates of Hell” and the “End of Spacetime” . An image described as “fathomless dark creations of the Universe” — equal to the famous “Earthrise” photo taken by Apollo 8 astronaut Bill Anders in December 1968.

“While it’s possible that Sgr A* drives a relativistic jet,” says Daryl Haggard, Associate Professor of Physics at McGill University in the McGill Space Institute told The Daily Galaxy, “but if it’s there, it’s nowhere near as powerful as the one we have beautifully imaged in M87. This doesn’t necessarily mean that Sgr A* is an exception,” Haggard adds, “not all supermassive black holes drive powerful jets, but it is a bit of a puzzle – theoretically we think a jet should be there but we haven’t convincingly detected one yet. It’s definitely the case that a substantial portion of the radio and sub-mm emission from Sgr A* comes from the hot plasma swirling around the black hole. Stay tuned though, observing Sgr A* never gets boring and we’ll have more rich data to share soon!”

Haggard leads multi-wavelength, time domain studies of growing supermassive black holes, including Sagittarius A* and M87. She was a member of the Event Horizon Telescope collaboration in reporting the first direct image of the M87 black hole’s shadow in 2019 and the EHT team receiving the 2020 Breakthrough Prize in Fundamental Physics.

“A One-way Door Out of Our universe,” –How the Scientists Described the 2019 Image

We gave humanity its first view of a black hole — “a one-way door out of our universe,” said EHT project director Sheperd S. Doeleman of the Center for Astrophysics, of the image of the massive black hole at the center of elliptical galaxy M87. “This is a landmark in astronomy, an unprecedented scientific feat accomplished by a team of more than 200 researchers.”

The M87 black hole really is a monster, observed Ellie Mae O’Hagan for The Guardian. “Everything unfortunate enough to get too close to it falls in and never emerges again, including light itself. It’s the point at which every physical law of the known universe collapses. Perhaps it is the closest thing there is to hell: it is an abyss, a moment of oblivion.”

Astrophysicist Janna Levin author of “Black Hole Blues” with Columbia University noted for The Guardian that we are actually seeing the black hole as it was 55 million years ago, because it’s so far away the light takes that long to reach us. “Over those eons, we emerged on Earth along with our myths, differentiated cultures, ideologies, languages and varied beliefs,” she says. “Looking at M87, I am reminded that scientific discoveries transcend those differences.”

The Daily Galaxy, Jackie Faherty, astrophysicist, Senior Scientist with AMNH via Radboud University and Sera Markoff, University of Amsterdam and Daryl Haggard, McGill University. Jackie was formerly a NASA Hubble Fellow at the Carnegie Institution for Science.

Image at the top of the page: Shutterstock License

The Galaxy Report newsletter brings you twice-weekly news of space and science that has the capacity to provide clues to the mystery of our existence and add a much needed cosmic perspective in our current Anthropocene Epoch.

## Is matter being destroyed forever in black holes?

Is matter being destroyed forever in black holes? If so, this would mean that the universe is losing matter constantly and in a few billion years will disappear altogether!

I realize that some astronomers feel that the matter is being sucked into another universe, but there is really no proof of that - yet.

### #2 star drop

I would think that since its gravity can be felt that the matter is still in this universe.

### #3 shawnhar

Isn't it true that from our reference point, nothing ever actually enters a black hole? Since time slows down and completely stops at the event horizon?

### #4 GJJim

Black holes are still in the universe. Matter accreting adds to the gravity and entropy of a black hole, it hasn't gone anywhere.

Edited by GJJim, 31 October 2014 - 02:42 PM.

### #5 Rick Woods

My understanding is that matter can't be destroyed, only converted to energy.

### #6 maugi88

I believe it is true that the matter is just swallowed and made part of the black hole. So it is still there, but is the information still there? The "what" that matter was?

### #7 GJJim

I believe it is true that the matter is just swallowed and made part of the black hole. So it is still there, but is the information still there? The "what" that matter was?

One of the ongoing food fights among cosmologists is the relationship between gravity and entropy. Claude Shannon's seminal work showed the equivalence of information and entropy. If the entropy of information (encoded in matter) is converted to gravitational entropy in a black hole, then everything is hunky dory.

### #8 shawnhar

Why am I still hung up on the "nothing ever really falls into a black hole" thing? From the outside, no information is lost so no entropy issues.

### #9 GJJim

Why am I still hung up on the "nothing ever really falls into a black hole" thing? From the outside, no information is lost so no entropy issues.

Is that wrong?

The time dilation you are thinking off is from the reference point of the object falling into the black hole. Outside observers would see the object accelerate and vanish into the black hole.

### #10 shawnhar

That's the opposite of what Krause and Kaku are saying in the video. They said we can never observe an object going into a black hole because it would slow down more and more from our reference and just basically stop forever.

Edited by shawnhar, 01 November 2014 - 11:56 AM.

### #11 GJJim

That's the opposite of what Krause and Kaku are saying in the video. They said we can never observe an object going into a black hole because it would slow down more and more from our reference and just basically stop forever.

The light emitted from an object falling into a black hole is red shifted as it approaches the event horizon. An outside observer would see it get redder and dim to invisibility within a few seconds. The object never "stops" falling.

### #12 StarWars

PBS: Seeing Stars

Seen this on my PBS they show a massive black hole in different wave lengths of light spraying out stuff at the poles.

### #14 Pess

That's the opposite of what Krause and Kaku are saying in the video. They said we can never observe an object going into a black hole because it would slow down more and more from our reference and just basically stop forever.

Do not confuse observing with reality. From outside the Hole we 'observe' by detecting photons reflected off an object headed toward the BH.

As the object gets closer to the BH, the gravity gradient builds. Thus the photons are red-shifted as we continue to watch.

Once the object gets near to the Event Horizon the object would be seen as traveling slower and slower as the photons are red shifted more and more. Eventually the red-shift would be so intense that billions of years would pass without detectable movement. So for all practical purposes, we would never see the object 'disappear'. That is provided we have optical equipment that could render visible such extreme red-shifted photons.

For the traveler though, it is a different story. Time, for them, appears normal and they would instantaneously (from their perspective) cross the Event Horizon. And if the ship survived the Tidal forces, they would not notice much difference.

Remember, photons can easily cross the EH from the outside, so they (theoretically) can look out the back window and still see us watching them. Of course, you have to take into account all the high energy photons caught behind the Event Horizon with you. They can't escape but they can 'orbit' the matter way down inside the EH. I imagine all the high energy photons circling the Black Hole and within the EH might make a pretty energetic (and lethal) environment.

Pesse (Black Holes & my Financial obligations share very similar physical characteristics!) Mist

### #15 shawnhar

For the traveler though, it is a different story. Time, for them, appears normal and they would instantaneously (from their perspective) cross the Event Horizon. And if the ship survived the Tidal forces, they would not notice much difference.

Remember, photons can easily cross the EH from the outside, so they (theoretically) can look out the back window and still see us watching them. Of course, you have to take into account all the high energy photons caught behind the Event Horizon with you. They can't escape but they can 'orbit' the matter way down inside the EH. I imagine all the high energy photons circling the Black Hole and within the EH might make a pretty energetic (and lethal) environment.

Pesse (Black Holes & my Financial obligations share very similar physical characteristics!) Mist

Yeah but wouldn't the traveler see the entire universe speed up faster and faster through that back window, so everything expands and fades to nothing and the universe effectively ends BEFORE they cross the EV?

### #16 AR6

Why am I still hung up on the "nothing ever really falls into a black hole" thing? From the outside, no information is lost so no entropy issues.

Is that wrong?

The time dilation you are thinking off is from the reference point of the object falling into the black hole. Outside observers would see the object accelerate and vanish into the black hole.

I think you've said it backwards. When viewed from a distance, the object would appear to fall into the black hole at an ever decreasing rate, then for all intents and purposed from our perspective, just stop. After all, the object's time is becoming ever slower, while our time is speeding along like normal, so that nanosecond when it slips over the edge, in it's time frame, would potentially be millions or billions of years in our time frame. So while it never actually stops, we stop having enough time to see it move.

Edited by AR6, 23 November 2014 - 12:36 PM.

### #17 Pess

For the traveler though, it is a different story. Time, for them, appears normal and they would instantaneously (from their perspective) cross the Event Horizon. And if the ship survived the Tidal forces, they would not notice much difference.

Remember, photons can easily cross the EH from the outside, so they (theoretically) can look out the back window and still see us watching them. Of course, you have to take into account all the high energy photons caught behind the Event Horizon with you. They can't escape but they can 'orbit' the matter way down inside the EH. I imagine all the high energy photons circling the Black Hole and within the EH might make a pretty energetic (and lethal) environment.

Pesse (Black Holes & my Financial obligations share very similar physical characteristics!) Mist

Yeah but wouldn't the traveler see the entire universe speed up faster and faster through that back window, so everything expands and fades to nothing and the universe effectively ends BEFORE they cross the EV?

Time dilation or contraction is a function of relative speed differences between two reference frames. It is not dependent on proximity to an Event Horizon. So, hypothetically, you could have all your thrust directed towards slowing you down so that you cross the Event Horizon at, say, 5mph.

Looking out your rear window everything should appear more or less normal.

Now, depending on the size of the BH, there would be some weird light bending going on, but straight back behind you things should be fairly clear even after you pass the EH, although I imagine you'd need some really powerful filters once you pass the EH to make out anything.

As an aside, current theory suggests that information, once past the EH, is lost forever. The mass is obviously still there since it still contributes to the gravitational field, but you can't infer anything about the mass that went in once it is 'in'.

I says this with a caveat: Recent ideas about BH's suggest that this may not be the case. The simplest idea (very simple actually) is the case of hawking Radiation. There is the idea that as a virtual particle pair appears next to an EH, one particle is snatched away into the BH while the paired particle thus goes from virtual to real and speeds off into space. It takes energy to transition from a 'Virtual' particle to a 'real' particle and that energy is siphoned off from the BH.

Some have suggested that by measuring aspects of these particles of Hawking radiation we can infer information about the lost pair. It is also interesting that Black Holes can completely evaporate through mass & energy loss via Hawking radiation over time.

In fact, recent theory tossed out there suggest that BH's don't even have Event Horizons but merely Gray horizons that reflect a gradient of energy levels.

## Do black holes swallow dark matter?

We know dark matter is only strongly affected by gravity but has mass- do black holes interact with dark matter? Could a black hole swallow dark matter and become more massive?

We don't really know what dark matter is.

The prevailing hypothesis is that it's some kind of particle that only interacts gravitationally (well, for the most part). If that's the case, then yes, black holes should definitely be able to swallow that stuff up.

Under that same assumption, it should be noted that dark matter will probably not form an accretion disk, nor would it care about an existing accretion disk. So dark matter particles would just describe conic curves around the black hole. If the curves happen to intersect the event horizon, the particles will be captured. Otherwise no capture will occur. (with some corrections to those trajectories due to general relativity)

If it turns out that dark matter is not particulate stuff, then all of the above does not apply.

## Significant Black Hole Stories

By analyzing a supercomputer simulation of gas flowing into a black hole, the team finds they can reproduce a range of important X-ray features long observed in active black holes. Jeremy Schnittman, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Md., led the research.

Black holes are the densest objects known. Stellar black holes form when massive stars run out of fuel and collapse, crushing up to 20 times the sun's mass into compact objects less than 75 miles (120 kilometers) wide.

Gas falling toward a black hole initially orbits around it and then accumulates into a flattened disk. The gas stored in this disk gradually spirals inward and becomes greatly compressed and heated as it nears the center, ultimately reaching temperatures up to 20 million degrees Fahrenheit (12 million C), or some 2,000 times hotter than the sun's surface. It glows brightly in low-energy, or soft, X-rays.

For more than 40 years, however, observations show that black holes also produce considerable amounts of "hard" X-rays, light with energy tens to hundreds of times greater than soft X-rays. This higher-energy light implies the presence of correspondingly hotter gas, with temperatures reaching billions of degrees.

The new study involves a detailed computer simulation that simultaneously tracked the fluid, electrical and magnetic properties of the gas while also taking into account Einstein's theory of relativity. Using this data, the scientists developed tools to track how X-rays were emitted, absorbed, and scattered in and around the disk.

The study demonstrates for the first time a direct connection between magnetic turbulence in the disk, the formation of a billion-degree corona above and below the disk, and the production of hard X-rays around an actively "feeding" black hole.

Catching gravitational waves from some of the strongest sources — colliding black holes with millions of times the sun's mass — will take a little longer. These waves undulate so slowly that they won't be detectable by ground-based facilities. Instead, scientists will need much larger space-based instruments, such as the proposed Laser Interferometer Space Antenna, which was endorsed as a high-priority future project by the astronomical community.

A team that includes astrophysicists at NASA's Goddard Space Flight Center in Greenbelt, Md., is looking forward to that day by using computational models to explore the mergers of supersized black holes. Their most recent work investigates what kind of "flash" might be seen by telescopes when astronomers ultimately find gravitational signals from such an event.

To explore the problem, a team led by Bruno Giacomazzo at the University of Colorado, Boulder, and including Baker developed computer simulations that for the first time show what happens in the magnetized gas (also called a plasma) in the last stages of a black hole merger.

In the turbulent environment near the merging black holes, the magnetic field intensifies as it becomes twisted and compressed. The team suggests that running the simulation for additional orbits would result in even greater amplification.

The most interesting outcome of the magnetic simulation is the development of a funnel-like structure — a cleared-out zone that extends up out of the accretion disk near the merged black hole.

The most important aspect of the study is the brightness of the merger's flash. The team finds that the magnetic model produces beamed emission that is some 10,000 times brighter than those seen in previous studies, which took the simplifying step of ignoring plasma effects in the merging disks.

Using data from NASA's Rossi X-ray Timing Explorer (RXTE) satellite, an international team has uncovered a dozen instances where X-ray signals from active galaxies dimmed as a result of a cloud of gas moving across our line of sight. The new study triples the number of cloud events previously identified in the 16-year archive.

The study is the first statistical survey of the environments around supermassive black holes and is the longest-running AGN-monitoring study yet performed in X-rays. Scientists determined various properties of the occulting clouds, which vary in size and shape but average 4 billion miles (6.5 billion km) across – greater than Pluto's distance from the sun — and twice the mass of Earth. They orbit a few light-weeks to a few light-years from the black hole.

## The Stuff Falling into This Black Hole Is Moving at Almost 56,000 Miles a Second!

A glob of material the size of Earth is getting sucked into a black hole at nearly one-third the speed of light, a new study reports.

The speed of light in a vacuum is 186,282 miles (299,792 kilometers) per second, and, according to Einstein's theory of special relativity, that's the top speed for anything traveling in our universe. So, something zipping at a third the speed of light is moving nearly 56,000 miles (90,000 km) per second — fast enough to circle Earth twice in that brief time.

The newly observed infall event occurred in the galaxy PG211+143, which is more than 1 billion light-years away from Earth. Astronomers spotted it using the European Space Agency's XMM-Newton space telescope, which observes the universe in X-ray light. [Images: Black Holes of the Universe]

"We were able to follow an Earth-sized clump of matter for about a day, as it was pulled towards the black hole, accelerating to a third of the velocity of light before being swallowed up by the hole," study lead author Ken Pounds, a space physicist at the University of Leicester in England, said in a statement.

The matter reached such incredible speeds because black holes have extremely strong gravitational fields, so strong that even light cannot escape once it goes beyond a critical boundary known as the "event horizon." (That's why they're called black holes.)

There are several types of black holes. The most massive kind, called a supermassive black hole, resides at the core of most if not all galaxies, including our own Milky Way.

If there's enough matter falling into a supermassive black hole, the area shines in superbright X-rays that are visible for long distances. These objects are called quasars, or active galactic nuclei. However, most black holes are too compact to pull such material — which is mostly gas — in immediately. Instead, the stuff orbits the black hole, forming an "accretion disk" as it spirals closer. Eventually, the gas is moving so fast that it gets extremely hot and luminous, generating radiation that we can often see from Earth.

"The orbit of the gas around the black hole is often assumed to be aligned with the rotation of the black hole, but there is no compelling reason for this to be the case," University of Leicester representatives wrote in the same statement.

"In fact, the reason we have summer and winter is that the Earth's daily rotation does not line up with its yearly orbit around the sun," they added. "Until now, it has been unclear how misaligned rotation might affect the infall of gas. This is particularly relevant to the feeding of supermassive black holes, since matter — interstellar gas clouds or even isolated stars — can fall in from any direction."

Members of the study team think the gas is indeed misaligned with the black hole's rotation in PG211+143. In such situations, accretion disks can be twisted and torn some of the various pieces can then slam into each other, "canceling out" their rotation and allowing some gas to zoom directly toward the black hole, rather than swirl around it.

If misaligned disks are common, it could help explain why black holes from the early universe grew big so quickly. Such black holes would spin relatively slowly, allowing them nab more gas in a shorter amount of time than previously thought, researchers said.

The new study was published this month in the journal Monthly Notices of the Royal Astronomical Society.

## How to get from Universe A to Universe B in one piece

To the right is the Penrose diagram for a charged or rotating black hole. One of the first things I want to point out is the nature of the singularity. I told you earlier that the singularity was a place in time but that was only for the static black hole! See here, the singularity is a definite place, and places can be avoided as long as you don't have to go the speed of light to do it.

This is the road map for jumping from one universe to another. Say that purple worldline is me in my 2085 Ford Tempo rocket (with mismatching red paint). I want to travel somewhere using the super-massive rotating black hole right in front of me. I take the time to perch at the lip of the gravity well and at a small angle to one of the poles of the axis of rotation of the black hole. (My Tempo is impossibly well-shielded against radiation.) Armed with the might of relativity (and some auto insurance), I accelerate my Tempo towards the outer event horizon and dive into the 'well. As I fall, I'm trading gravitational potential energy for kinetic energy, and I end up going quite fast as I cross the outer event horizon. The instant I reach Rs, my engines cut off just as I preprogrammed them to do.

This particular galactic black hole is rotating very quickly, so I very quickly cross the inner event horizon. Since the two event horizons are nearly on top of one another and since I cut my engines before I entered the realm between them, I do not experience any tidal unpleasantness. A very curious thing happens when I cross the outer event horizon. The singularity becomes an unavoidable place in time---it becomes my future---as the time axis and the space axis of my spacetime diagram exchange places. As I cross the inner event horizon, time and space resume their normal axes on my spacetime diagram, and the singularity becomes a place in space.

I should remind you that I'm rocketing along at a speed close to light. I blaze across the inner event horizon and shoot right through the center of the ring singularity. Oooh, confusing statement. The singularity appears to me as a round window. If the singularity emits any light on its own, I would see that as the frame of the window. Inside that window. is reminiscent of what you see when you reflect one mirror into another: a hallway of mirrors arching into infinity. The smaller the angle between my approach and the axis of rotation, the more mirrors I see. What I see in the window of the singularity is the same but, instead of mirrors, I see an infinite number of locations.

There is only one restriction on where I may go with a rotating black hole: to enter a black hole means to leave a black hole. Black holes are rather like subway terminals in that sense if you walk down the stairs to take a train, you've got to walk back up the stairs when you exit. You can only exit at locations with those stairs. You could not use a black hole to pop out right next to earth, 1940, because there were no black holes right next to earth at that time.

I shoot through the very center of the window, nearly orthogonal to (perpendicular to) the window (nearly because I approached nearly parallel to the axis of rotation. I recross both event horizons, one after the other, and leave the black hole at a speed close to that of light. I gained all my speed entering the gravitational well, now I lose it all leaving the well. I coast away from the black hole's gravity well at the same speed I entered, the mirror-image of my worldline when I entered the gravity well --- which kinda means I end up perched at the lip of the gravity well, again, with the option to fall back in or to leave and explore.

This universe-jumping is a fun thing to think about, but I always get edgy when considering the idea of innocently wandering into a whole different universe. I mean, the only things that define our universe are our "laws" (axioms, theories --- as you please) of physics. The speed of light in a vacuum is 3x10 8 m/s. Electrons have such and such weight and charge. The distribution of matter formed just after the big bang favored matter over antimatter (just). The universe expanded at such a rate that stars formed, some of which were conducive to the formation of planets. In another universe, the numbers for these laws might differ somewhat --- or the laws could be completely different! Recall all that dust and gas falling into the black hole as innocent little me attempts to leave the gravity well? Suppose the universe I just entered is one where antimatter is the dominant type of matter --- and here's little me and my rocket, made entirely of matter. Imagine my surprise as a tiny clump of anti-hydrogen atoms wisps against my Tempo's fender. Boom! Tremendous explosion and lots of energy released, and that's the end of my traveling days.

The other problem is that this situation is completely theoretical. The Kerr solution is very unstable. The mere approach of a rocket to the outer event horizon (let alone one diving across said horizon), will destabilize the black hole and make it fatal for the rocket attempting to travel through it. I'm sorry, it sounds like a fun way to explore, but that is the way things work.

## Astronomers catch a black hole shredding a star to pieces

This illustration of a recently observed tidal disruption, named ASASSN-14li, shows a disc of stellar debris around the black hole at the upper left. A long tail of ejected stellar debris extends to the right, far from the black hole. The X-ray spectrum obtained with NASA’s Chandra X-ray Observatory (seen in the inset box) and ESA’s XMM-Newton satellite both show clear evidence for dips in X-ray intensity over a narrow range of wavelengths. These dips are shifted toward bluer wavelengths than expected, providing evidence for a wind blowing away from the black hole. Image credit: NASA/CXC/M. Weiss. When a star comes too close to a black hole, the intense gravity of the black hole results in tidal forces that can rip the star apart. In these events, called tidal disruptions, some of the stellar debris is flung outward at high speeds, while the rest falls toward the black hole. This causes a distinct X-ray flare that can last for years.

A team of astronomers, including several from the University of Maryland, has observed a tidal disruption event in a galaxy that lies about 290 million light-years from Earth. The event is the closest tidal disruption discovered in about a decade, and is described in a paper published in the 22 October 2015 issue of the journal Nature.

“These results support some of our newest ideas for the structure and evolution of tidal disruption events,” said study co-author Coleman Miller, professor of astronomy at UMD and director of the Joint Space-Science Institute. “In the future, tidal disruptions can provide us with laboratories to study the effects of extreme gravity.”

The optical light All-Sky Automated Survey for Supernovae (ASAS-SN) originally discovered the tidal disruption, known as ASASSN-14li, in November 2014. The event occurred near a supermassive black hole at the centre of the galaxy PGC 043234. Further study using NASA’s Chandra X-ray Observatory, NASA’s Swift Gamma-ray Burst Explorer and the European Space Agency’s XMM-Newton satellite provided a clearer picture by analysing the tidal disruption’s X-ray emissions.

“We have seen evidence for a handful of tidal disruptions over the years and have developed a lot of ideas of what goes on,” said lead author Jon Miller, a professor of astronomy at the University of Michigan. “This one is the best chance we have had so far to really understand what happens when a black hole shreds a star.”

After a star is destroyed by a tidal disruption, the black hole’s strong gravitational forces draw in most of the star’s remains. Friction heats this infalling debris, generating huge amounts of X-ray radiation. Following this surge of X-rays, the amount of light decreases as the stellar material falls beyond the black hole’s event horizon &mdash the point beyond which no light or other information can escape.

Gas often falls toward a black hole by spiralling inward and forming a disc. But the process that creates these disc structures, known as accretion discs, has remained a mystery. By observing ASASSN-14li, the team of astronomers was able to witness the formation of an accretion disc as it happened, by looking at the X-ray light at different wavelengths and tracking how those emissions changed over time.

The researchers determined that most of the X-rays are produced by material that is extremely close to the black hole. In fact, the brightest material might actually occupy the smallest possible stable orbit. But astronomers are equally interested to learn what happens to the gas that doesn’t get drawn past the event horizon, but instead is ejected away from the black hole.

“The black hole tears the star apart and starts swallowing material really quickly, but that’s not the end of the story,” said study co-author Jelle Kaastra, an astronomer at the Institute for Space Research in the Netherlands. “The black hole can’t keep up that pace so it expels some of the material outwards.”

The X-ray data also suggest the presence of a wind moving away from the black hole, carrying stellar gas outward. However, this wind does not quite move fast enough to escape the black hole’s gravitational grasp. A possible explanation for the low speed of this wind is that gas from the disrupted star follows an elliptical orbit around the black hole, and travels slowest when it reaches the greatest distance from the black hole at the far ends of this elliptical orbit.

“This result highlights the importance of multi-wavelength observations,” explained study co-author Suvi Gezari, an assistant professor of astronomy at UMD. “Even though the event was discovered with an optical survey telescope, prompt X-ray observations were key in determining the characteristic temperature and radius of the emission and catching the signatures of an outflow.”

Astronomers are hoping to find and study more events like ASASSN-14li so they can continue to test theoretical models about how black holes affect their nearby environments, while learning more about what black holes do to any stars or other bodies that wander too close.

## A Black Hole Myth

Much of the modern folklore about black holes is misleading. One idea you may have heard is that black holes go about sucking things up with their gravity. Actually, it is only very close to a black hole that the strange effects we have been discussing come into play. The gravitational attraction far away from a black hole is the same as that of the star that collapsed to form it.

Remember that the gravity of any star some distance away acts as if all its mass were concentrated at a point in the center, which we call the center of gravity. For real stars, we merely imagine that all mass is concentrated there for black holes, all the mass really is concentrated at a point in the center.

So, if you are a star or distant planet orbiting around a star that becomes a black hole, your orbit may not be significantly affected by the collapse of the star (although it may be affected by any mass loss that precedes the collapse). If, on the other hand, you venture close to the event horizon, it would be very hard for you to resist the “pull” of the warped spacetime near the black hole. You have to get really close to the black hole to experience any significant effect.

If another star or a spaceship were to pass one or two solar radii from a black hole, Newton’s laws would be adequate to describe what would happen to it. Only very near the event horizon of a black hole is the gravitation so strong that Newton’s laws break down. The black hole remnant of a massive star coming into our neighborhood would be far, far safer to us than its earlier incarnation as a brilliant, hot star.

Time machines are one of the favorite devices of science fiction. Such a device would allow you to move through time at a different pace or in a different direction from everyone else. General relativity suggests that it is possible, in theory, to construct a time machine using gravity that could take you into the future.

Let’s imagine a place where gravity is terribly strong, such as near a black hole. General relativity predicts that the stronger the gravity, the slower the pace of time (as seen by a distant observer). So, imagine a future astronaut, with a fast and strongly built spaceship, who volunteers to go on a mission to such a high-gravity environment. The astronaut leaves in the year 2222, just after graduating from college at age 22. She takes, let’s say, exactly 10 years to get to the black hole. Once there, she orbits some distance from it, taking care not to get pulled in.

She is now in a high-gravity realm where time passes much more slowly than it does on Earth. This isn’t just an effect on the mechanism of her clocks—time itself is running slowly. That means that every way she has of measuring time will give the same slowed-down reading when compared to time passing on Earth. Her heart will beat more slowly, her hair will grow more slowly, her antique wristwatch will tick more slowly, and so on. She is not aware of this slowing down because all her readings of time, whether made by her own bodily functions or with mechanical equipment, are measuring the same—slower—time. Meanwhile, back on Earth, time passes as it always does.

Our astronaut now emerges from the region of the black hole, her mission of exploration finished, and returns to Earth. Before leaving, she carefully notes that (according to her timepieces) she spent about 2 weeks around the black hole. She then takes exactly 10 years to return to Earth. Her calculations tell her that since she was 22 when she left the Earth, she will be 42 plus 2 weeks when she returns. So, the year on Earth, she figures, should be 2242, and her classmates should now be approaching their midlife crises.

But our astronaut should have paid more attention in her astronomy class! Because time slowed down near the black hole, much less time passed for her than for the people on Earth. While her clocks measured 2 weeks spent near the black hole, more than 2000 weeks (depending on how close she got) could well have passed on Earth. That’s equal to 40 years, meaning her classmates will be senior citizens in their 80s when she (a mere 42-year-old) returns. On Earth it will be not 2242, but 2282—and she will say that she has arrived in the future.

Is this scenario real? Well, it has a few practical challenges: we don’t think any black holes are close enough for us to reach in 10 years, and we don’t think any spaceship or human can survive near a black hole. But the key point about the slowing down of time is a natural consequence of Einstein’s general theory of relativity, and we saw that its predictions have been confirmed by experiment after experiment.

Such developments in the understanding of science also become inspiration for science fiction writers. Recently, the film Interstellar featured the protagonist traveling close to a massive black hole the resulting delay in his aging relative to his earthbound family is a key part of the plot.

Science fiction novels, such as Gateway by Frederik Pohl and A World out of Time by Larry Niven, also make use of the slowing down of time near black holes as major turning points in the story. For a list of science fiction stories based on good astronomy, you can go to www.astrosociety.org/scifi.

## Professor makes black hole breakthroughs, ballads

(CNN) -- "Attracted by your gravity, your body's so compact / Pulling me inward, prepare for close contact," Boston University astronomer Alan Marscher sings in his song about a deep-space object known as a black hole.

Alan Marscher, professor at Boston University, sings about black holes and other astronomy concepts.

Marscher once used other rock groups' songs to illustrate scientific concepts for his students, such the Einsteinian "'39" by Queen.

Then he began writing his own songs tailored to specific lectures like "Superluminal Lover," a black hole ballad full of physics and innuendo. Watch him sing "Superluminal Lover" »

The song may not have won him much fame, but an international team of researchers that Marscher leads has just published some breakthrough research on the same black hole phenomena he sings about.

Black holes are somewhat like vacuum cleaners in space. These collapsed stars suck in anything and everything in their immediate vicinities and don't let anything escape, not even light.

The vacuum cleaner idea of a black hole isn't perfect. Astronomers have also detected jet streams of particles traveling at nearly the speed of light, as well as X-rays and gamma rays, shooting out from black holes.

Using radio telescopes set up all over the world, Marscher and colleagues studied a black hole nearly 1 billion light years away (one light year is about 5.9 trillion miles). They found evidence supporting one theory of why the black hole has these jet streams.

As matter falls into a black hole, it swirls around like water going down a drain, Marscher said. The closer things get to the black hole, the faster they begin to orbit.

The magnetic field then twists, like a spring that coils up, he said. This magnetic field propels particles along the black hole's rotational poles.

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Marscher's team found direct evidence to support this explanation, detecting the twisted magnetic field and the polarized light that results from it.

"This paper represents a significant advance in the field," said Marc Lacy, an associate research scientist at Caltech's Spitzer Science Center. "The authors make a convincing case that what they'd see is what you'd expect to see in this model of jet formation."

Lukasz Stawarz, a researcher at the Stanford Linear Accelerator Center, said Marscher's team's observations contribute significantly to our understanding of this type of black hole and provide evidence of a model that had little confirmation before.

"Observations reported by Alan Marscher and collaborators, although not definitive, are very convincing and novel in this respect," he said.

The researchers eagerly await results from NASA's Gamma-ray Large Area Space Telescope, a space observatory that may provide even more insight into black holes' jet streams. The spacecraft will launch no later than June 3, NASA said.

Black holes cannot be seen because they do not emit any light, but astronomers have found substantial evidence of them. Still, no existing telescope is powerful enough to observe exactly what goes on so close to a black hole, Lacy said.

"These are very rare objects, and so it's not until we look a great distance away that we see one whose jet pointing almost right at us," Marscher said. "Then, when we see a jet pointing almost right at us, the jet beams its radiation, like a halogen flashlight."

Although astronomers have detected black holes only in deep space, there is speculation that a black hole could be generated at the Large Hadron Collider, the multibillion-dollar particle accelerator under development at the European Organization for Nuclear Research in Geneva, Switzerland.

The idea that a black hole could emerge in these experiments is far-fetched, Marscher said. But even if the accelerator did create a black hole, it wouldn't necessarily be harmful, he said. See what's planned for the collider »

"If you made a little tiny black hole in a laboratory, it wouldn't have that much gravity. It wouldn't suck in everything that's on the Earth it would just suck in stuff that's within, say, a few millimeters of it," he said. "It wouldn't be the devastating danger that science-fiction writers would say, because it'd be a real tiny mass."

Still, even a laboratory-made black hole shouldn't be kept around for long. By its nature of sucking things up, it could just grow and grow, accumulating more mass and more power to pull in more things.

"I think I would put it into something that had a lot of mass and then just toss it off into space, so it wouldn't come into contact with very much matter so it wouldn't grow." Marscher said.

From the time he was a pre-teen, he was interested in astronomy. But he didn't think he could do it for a living, so he signed up for engineering at Cornell University.

Still, he ended up in astronomy, despite the low odds of making it into graduate school and earning a faculty position.

"My philosophy has always been that even if you have only a low probability of succeeding in a career, you should try anyway, so that you don't wake up when you're middle-aged and wonder what could have been," he said.

Though he was in a rock band in high school, Marscher wasn't too tempted to become a professional musician. He wrote his dozen science songs to complement his teaching in a course called "The Evolution of the Physical Universe and of the Earth," part of Boston University's core curriculum for undergraduates.

"Anybody's attention span during an hourlong period doesn't really focus on someone just lecturing," he said. The music "really does help to liven up the lectures."