Black Holes

There Is Sound In Space, Thanks To Gravitational Waves

Merging black holes are one class of objects that creates gravitational waves of certain frequencies and amplitudes. Thanks to detectors like LIGO, we can 'hear' these sounds as they occur.

It’s long been said that there’s no sound in space, and that’s true, to a point. Conventional sound requires a medium to travel through, and is created when particles compress-and-rarify, making anything from a loud “bang” for a single pulse to a consistent tone for repeating patterns. In space, where there are so few particles that any such signals die away, even solar flares, supernovae, black hole mergers, and other cosmic catastrophes go silent before they’re ever heard. But there’s another type of compression-and-rarefaction that doesn’t require anything other than the fabric of space itself to travel through: gravitational waves. Thanks to the first positive detection results from LIGO, we’re hearing the Universe for the very first time.

Two merging black holes. The inspiral results in the black holes coming together, while gravitational waves carry the excess energy away. The background spacetime is distorted as a result.

Two merging black holes. The inspiral results in the black holes coming together, while gravitational waves carry the excess energy away. The background spacetime is distorted as a result.

Gravitational waves were something that needed to exist for our theory of gravity to be consistent, according to General Relativity. Unlike in Newton’s gravity, where any two masses orbiting one another would remain in that configuration forever, Einstein’s theory predicted that over long enough times, gravitational orbits would decay. For something like the Earth orbiting the Sun, you’d never live to experience it: it would take 10^150 years for Earth to spiral into the Sun. But for more extreme systems, like two neutron stars orbiting one another, we could actually see the orbits decaying over time. In order to conserve energy, Einstein’s theory of gravity predicted that energy must be carried away in the form of gravitational waves.

As two neutron stars orbit each other, Einstein's theory of general relativity predicts orbital decay, and the emission of gravitational radiation.

As two neutron stars orbit each other, Einstein’s theory of General Relativity predicts orbital decay, and the emission of gravitational radiation. The former has been observed very precisely for many years, as evidenced by how the points and the line (GR prediction) match up so very well.

These waves are maddeningly weak, and their effects on the objects in spacetime are stupendously tiny. But if you know how to listen for them — just as the components of a radio know how to listen for those long-frequency light waves — you can detect these signals and hear them just as you’d hear any other sound. With an amplitude and a frequency, they’re no different from any other wave. General Relativity makes explicit predictions for what these waves should sound like, with the largest wave-generating signals being the easiest ones to detect. The largest amplitude sounds all? It’s the inspiral and merging “chirp” of two black holes that spiral into one another.

In September of 2015, just days after advanced LIGO began collecting data for the first time, a large, unusual signal was spotted. It surprised everyone, because it would have carried so much energy in just a short, 200 millisecond burst, that it would have outshone all the stars in the observable Universe combined. Yet that signal turned out to be robust, and the energy from that burst came from two black holes — of 36 and 29 solar masses — merging into a single 62 solar mass one. Those missing three solar masses? They were converted into pure energy: gravitational waves rippling through the fabric of space. That was the first event LIGO ever detected.

The signal from LIGO of the first robust detection of gravitational waves. The waveform is not just a visualization; it's representative of what you'd actually hear if you listened properly.

The signal from LIGO of the first robust detection of gravitational waves. The waveform is not just a visualization; it’s representative of what you’d actually hear if you listened properly.

Now it’s over a year later, and LIGO is presently on its second run. Not only have other black hole-black hole mergers been detected, but the future of gravitational wave astronomy is bright, as new detectors will open up our ears to new types of sounds. Space interferometers, like LISA, will have longer baselines and will hear lower frequency sounds: sounds like neutron star mergers, feasting supermassive black holes, and mergers with highly unequal masses. Pulsar timing arrays can measure even lower frequencies, like orbits that take years to complete, such as the supermassive black hole pair: OJ 287. And combinations of new techniques will look for the oldest gravitational waves of all, the relic waves predicted by cosmic inflation, all the way back at the beginning of our Universe.

Gravitational waves generated by cosmic inflation are the farthest signal back in time humanity can conceive of potentially detecting. Collaborations like BICEP2 and NANOgrav may indirectly do this in the coming decades.

Gravitational waves generated by cosmic inflation are the farthest signal back in time humanity can conceive of potentially detecting. Collaborations like BICEP2 and NANOgrav may indirectly do this in the coming decades.

There’s so much to hear, and we’ve only just started listening for the first time. Thankfully, astrophysicist Janna Levin — author of the fantastic book, Black Hole Blues and Other Songs from Outer Space — is poised to give the public lecture at Perimeter Institute tonight, May 3rd, at 7 PM Eastern / 4 PM Pacific, and it will be live-streamed here and live-blogged by me in real time! Join us then for even more about this incredible topic, and I can’t wait to hear her talk.

 

The Universe is out there, waiting for you to discover it

Ethan SiegelEthan Siegel, Contributor

Merging black holes are one class of objects that creates gravitational waves of certain frequencies and amplitudes. Thanks to detectors like LIGO, we can 'hear' these sounds as they occur.

Merging black holes are one class of objects that creates gravitational waves of certain frequencies and amplitudes. Thanks to detectors like LIGO, we can ‘hear’ these sounds as they occur.

It’s long been said that there’s no sound in space, and that’s true, to a point. Conventional sound requires a medium to travel through, and is created when particles compress-and-rarify, making anything from a loud “bang” for a single pulse to a consistent tone for repeating patterns. In space, where there are so few particles that any such signals die away, even solar flares, supernovae, black hole mergers, and other cosmic catastrophes go silent before they’re ever heard. But there’s another type of compression-and-rarefaction that doesn’t require anything other than the fabric of space itself to travel through: gravitational waves. Thanks to the first positive detection results from LIGO, we’re hearing the Universe for the very first time.

Two merging black holes. The inspiral results in the black holes coming together, while gravitational waves carry the excess energy away. The background spacetime is distorted as a result.

Two merging black holes. The inspiral results in the black holes coming together, while gravitational waves carry the excess energy away. The background spacetime is distorted as a result.

Gravitational waves were something that needed to exist for our theory of gravity to be consistent, according to General Relativity. Unlike in Newton’s gravity, where any two masses orbiting one another would remain in that configuration forever, Einstein’s theory predicted that over long enough times, gravitational orbits would decay. For something like the Earth orbiting the Sun, you’d never live to experience it: it would take 10^150 years for Earth to spiral into the Sun. But for more extreme systems, like two neutron stars orbiting one another, we could actually see the orbits decaying over time. In order to conserve energy, Einstein’s theory of gravity predicted that energy must be carried away in the form of gravitational waves.

As two neutron stars orbit each other, Einstein's theory of general relativity predicts orbital decay, and the emission of gravitational radiation.

As two neutron stars orbit each other, Einstein’s theory of General Relativity predicts orbital decay, and the emission of gravitational radiation. The former has been observed very precisely for many years, as evidenced by how the points and the line (GR prediction) match up so very well.

These waves are maddeningly weak, and their effects on the objects in spacetime are stupendously tiny. But if you know how to listen for them — just as the components of a radio know how to listen for those long-frequency light waves — you can detect these signals and hear them just as you’d hear any other sound. With an amplitude and a frequency, they’re no different from any other wave. General Relativity makes explicit predictions for what these waves should sound like, with the largest wave-generating signals being the easiest ones to detect. The largest amplitude sounds all? It’s the inspiral and merging “chirp” of two black holes that spiral into one another.

In September of 2015, just days after advanced LIGO began collecting data for the first time, a large, unusual signal was spotted. It surprised everyone, because it would have carried so much energy in just a short, 200 millisecond burst, that it would have outshone all the stars in the observable Universe combined. Yet that signal turned out to be robust, and the energy from that burst came from two black holes — of 36 and 29 solar masses — merging into a single 62 solar mass one. Those missing three solar masses? They were converted into pure energy: gravitational waves rippling through the fabric of space. That was the first event LIGO ever detected.

The signal from LIGO of the first robust detection of gravitational waves. The waveform is not just a visualization; it's representative of what you'd actually hear if you listened properly.

The signal from LIGO of the first robust detection of gravitational waves. The waveform is not just a visualization; it’s representative of what you’d actually hear if you listened properly.

Now it’s over a year later, and LIGO is presently on its second run. Not only have other black hole-black hole mergers been detected, but the future of gravitational wave astronomy is bright, as new detectors will open up our ears to new types of sounds. Space interferometers, like LISA, will have longer baselines and will hear lower frequency sounds: sounds like neutron star mergers, feasting supermassive black holes, and mergers with highly unequal masses. Pulsar timing arrays can measure even lower frequencies, like orbits that take years to complete, such as the supermassive black hole pair: OJ 287. And combinations of new techniques will look for the oldest gravitational waves of all, the relic waves predicted by cosmic inflation, all the way back at the beginning of our Universe.

Gravitational waves generated by cosmic inflation are the farthest signal back in time humanity can conceive of potentially detecting. Collaborations like BICEP2 and NANOgrav may indirectly do this in the coming decades.

Gravitational waves generated by cosmic inflation are the farthest signal back in time humanity can conceive of potentially detecting. Collaborations like BICEP2 and NANOgrav may indirectly do this in the coming decades.

There’s so much to hear, and we’ve only just started listening for the first time. Thankfully, astrophysicist Janna Levin — author of the fantastic book, Black Hole Blues and Other Songs from Outer Space — is poised to give the public lecture at Perimeter Institute tonight, May 3rd, at 7 PM Eastern / 4 PM Pacific, and it will be live-streamed here and live-blogged by me in real time! Join us then for even more about this incredible topic, and I can’t wait to hear her talk.


The live blog will begin a few minutes prior to 4:00 PM Pacific; join us here and follow along!

The warping of spacetime, in the General Relativistic picture, by gravitational masses.

The warping of spacetime, in the General Relativistic picture, by gravitational masses.

3:50 PM: It’s ten minutes until showtime, and to celebrate, here are ten fun facts (or as many as we can get in) about gravity and gravitational waves.

1.) Instead of “action at a distance,” where an invisible force is exerted between masses, general relativity says that matter and energy warp the fabric of spacetime, and that warped spacetime is what manifests itself as gravitation.

2.) Instead of traveling at infinite speed, gravitation only travels at the speed of light.

3.) This is important, because it means that if any changes occur to a massive object’s position, configuration, motion, etc., the ensuing gravitational changes only propagate at the speed of light.

Computer simulation of two merging black holes producing gravitational waves.

Computer simulation of two merging black holes producing gravitational waves.

3:54 PM: 4.) This means that gravitational waves, for example, can only propagate at the speed of light. When we “detect” a gravitational wave, we’re detecting the signal from when that mass configuration changed.

5.) The first signal detected by LIGO occurred at a distance of approximately 1.3 billion light years. The Universe was about 10% younger than it is today when that merger occurred.

Ripples in spacetime are what gravitational waves are.

Ripples in spacetime are what gravitational waves are.

6.) If gravitation traveled at infinite speed, planetary orbits would be completely unstable. The fact that planets move in ellipses around the Sun mandates that if General Relativity is correct, the speed of gravity must equal the speed of light to an accuracy of about 1%.

3:57 PM: 7.) There are many, many more gravitational wave signals than what LIGO has seen so far; we’ve only detected the easiest signal there is to detect.

8.) What makes a signal “easy” to see is a combination of its amplitude, which is to say, how much it can deform a path-length, or a distance in space, as well as its frequency.

A simplified illustration of LIGO's laser interferometer system.

A simplified illustration of LIGO’s laser interferometer system.

9.) Because LIGO’s arms are only 4 kilometers long, and the mirrors reflect the light thousands of times (but no more), that means LIGO can only detect frequencies of 1 Hz or faster.

Earlier this year, LIGO announced the first-ever direct detection of gravitational waves. By building a gravitational wave observatory in space, we may be able to reach the sensitivities necessary to detect a deliberate alien signal.

Earlier this year, LIGO announced the first-ever direct detection of gravitational waves. By building a gravitational wave observatory in space, we may be able to reach the sensitivities necessary to detect a deliberate alien signal.

10.) For slower signals, we need longer lever-arms and greater sensitivities, and that will mean going to space. That’s the future of gravitational wave astronomy!

4:01 PM: We made it! Time to begin and introduce Janna Levin! (Pronounce “JAN-na”, not “YON-na”, if you were wondering.)

The inspiral and merger of the first pair of black holes ever directly observed.

The inspiral and merger of the first pair of black holes ever directly observed.

4:05 PM: Here’s the big announcement/shot: the first direct recording of the first gravitational wave. It took 100 years after Einstein first put forth general relativity, and she’s playing a recording! Make sure you go and listen! What does it mean to “hear” a sound in space, after all, and why is this a sound? That’s the purpose, she says, of her talk.

The galaxies Maffei 1 and Maffei 2, in the plane of the Milky Way, can only be revealed by seeing through the Milky Way's dust. Despite being some of the closest large galaxies of all, they were not discovered until the mid-20th century.

The galaxies Maffei 1 and Maffei 2, in the plane of the Milky Way, can only be revealed by seeing through the Milky Way’s dust. Despite being some of the closest large galaxies of all, they were not discovered until the mid-20th century.

4:08 PM: If you consider what’s out there in the Universe, we had no way of knowing any of this at the time of Galileo. We were thinking about sunspots, Saturn, etc., and were completely unable to conceive of the great cosmic scales or distances. Forget about “conceiving of other galaxies,” we hadn’t conceived of any of this!

 

4:10 PM: Janna is showing one of my favorite videos (that I recognize) from the Sloan Digital Sky Survey! They took a survey of 400,000 of the nearest galaxies and mapped them in three dimensions. This is what our (nearby) Universe looks like, and as you can see, it really is mostly empty space!

The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star class shown above it, in kelvin.

The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star class shown above it, in kelvin.

4:12 PM: She makes a really great point that she totally glosses over: only about 1-in-1000 stars will ever become a black hole. There are over 400 stars within 30 light years of us, and zero of them are O or B stars, and zero of them have become black holes. These bluest, most massive and shortest-lived stars are the only ones that will grow into black holes.

The identical behavior of a ball falling to the floor in an accelerated rocket (left) and on Earth (right) is a demonstration of Einstein's equivalence principle.

The identical behavior of a ball falling to the floor in an accelerated rocket (left) and on Earth (right) is a demonstration of Einstein’s equivalence principle.

4:15 PM: When you consider “where did Einstein’s theory come from,” Janna makes a great point: the idea of the equivalence principle. If you have gravity, you might consider that you feel “heavy” in your chair, for example. But this reaction that you have is the exact same reaction you’d feel if you were accelerating, rather than gravitating. It’s not the gravity that you feel, it’s the effects of the matter around you!

4:17 PM: The band OKGO did a video flying in the vomit comet. Janna can’t show the whole thing, with audio, for copyright reasons, and highly recommends it. Luckily for you, thanks to the internet… here it is! Enjoy at your leisure!

To travel once around Earth's orbit in a path around the Sun is a journey of 940 million kilometers.

To travel once around Earth’s orbit in a path around the Sun is a journey of 940 million kilometers.

4:19 PM: There’s another huge revelation for gravity: the way we understand how things work comes from watching how things fall. The Moon is “falling” around the Earth; Newton realized that. But the Earth is falling around the Sun; the Sun is “falling” around the galaxy; and atoms “fall” here on Earth. But the same rule applies to them all, so long as they’re all in free-fall. Amazing!

Black holes are something the Universe wasn't born with, but has grown to acquire over time. They now dominate the Universe's entropy.

Black holes are something the Universe wasn’t born with, but has grown to acquire over time. They now dominate the Universe’s entropy.

4:21 PM: Here’s a fun revelation: stop thinking of a black hole as collapsed, crushed matter, even though that might be how it originated. Instead, think about it as simply a region of empty space with strong gravitational properties. In fact, if all you did was assign “mass” to this region of space, that would perfectly define a Schwarzschild (non-charged, non-rotating) black hole.

The supermassive black hole (Sgr A*) at the center of our galaxy is shrouded in a dusty, gaseous environment. X-rays and infrared observations can partially see through it, but radio waves might finally be able to resolve it directly.

The supermassive black hole (Sgr A*) at the center of our galaxy is shrouded in a dusty, gaseous environment. X-rays and infrared observations can partially see through it, but radio waves might finally be able to resolve it directly.

4:23 PM: If you were to fall into a black hole the mass of the Sun, you’d have about a microsecond, from crossing the event horizon (according to Janna) until you were crushed to death at the singularity. This is consistent with what I once calculated, where, for the black hole at the center of the Milky Way, we’d have about 10 seconds. Since the Milky Way’s black hole is 4,000,000 times as massive as our Sun, the math kind of works out!

Joseph Weber with his early-stage gravitational wave detector, known as a Weber bar.

Joseph Weber with his early-stage gravitational wave detector, known as a Weber bar.

4:26 PM: How would you detect a gravitational wave? Honestly, it would be like being on the surface of the ocean; you’d bob up and down along the surface of space, and there was a big argument in the community as to whether these waves were real or not. It wasn’t until Joe Weber came along and decided to try and measure these gravitational waves, using a phenomenal device — an aluminum bar — that would vibrate if a rippling wave “plucked” the bar very slightly.

Weber saw many such signals that he identified with gravitational waves, but these, unfortunately, were never reproduced or verified. He was, for all of his cleverness, not a very careful experimenter.

4:29 PM: There’s a good question from Jon Groubert on twitter: “I have a question about something she said – there is something inside a black hole, isn’t there? Like a heavy neutron star.” There should be a singularity, which is either point-like (for a non-rotating singularity) or a one-dimensional ring (for a rotating one), but not condensed, collapsed, three-dimensional matter.

Why not?

Because in order to remain as a structure, a force needs to propagate and be transmitted between particles. But particles can only transmit forces at the speed of light. But nothing, not even light, can move “outward” towards the exit of a black hole; everything moves towards the singularity. And so nothing can hold itself up, and everything collapses into the singularity. Sad, but the physics makes this inevitable.

From left to right: the two LIGO detectors (in Hanford and Livingston, US) and the Virgo detector (Cascina, Italie).

From left to right: the two LIGO detectors (in Hanford and Livingston, US) and the Virgo detector (Cascina, Italie).

4:32 PM: After Weber’s failures (and fall from fame), the idea of LIGO came along by Rai Weiss in the 1970s. It took more than 40 years for LIGO to come to fruition (and over 1,000 people to make it happen), but the most fantastic thing was that it was experimentally possible. By making two very long lever-arms, you could see the effect of a passing gravitational wave.

 

 

4:34 PM: This is my favorite video illustrating what a gravitational wave does. It moves space itself (and everything in it) back and forth by a tiny amount. If you have a laser interferometer set up (like LIGO), it can detect these vibrations. But if you were close enough and your ears were sensitive enough, you could feel this motion in your eardrum!

4:35 PM: I’ve got some really good headphones, Perimeter, but unfortunately I can’t hear the different gravitational wave model signals that Janna is playing!

The LIGO Hanford Observatory for detecting gravitational waves in Washington State, USA.

The LIGO Hanford Observatory for detecting gravitational waves in Washington State, USA.

4:38 PM: It’s funny to think that this is the world’s most advanced vacuum, inside the LIGO detectors. Yet birds, rats, mice, etc., are all under there, and they chew their way into almost the vacuum chamber that the light travels through. But if the vacuum had been broken (it’s been constant since 1998), the experiment would have been over. In Louisiana, hunters shot at the LIGO tunnels. It’s horrifying how sensitive and expensive this equipment is, but yet how fragile it all is, too.

4:41 PM: Janna is doing a really great job telling this story in a suspenseful but very human way. We only saw the final few orbits of two orbiting black holes, drastically slowed down in the above movie. They were only a few hundred kilometers apart, those final four orbits took 200 millisecond, and that’s the entirety of the signal that LIGO saw.

 

4:43 PM: If you’re having trouble listening/hearing the events in the talk, listen to this video (above), in both natural pitch and increased pitch. The smaller black holes (roughly 8 and 13 solar masses) from December 26, 2015, are both quieter and higher pitched than the larger ones (29 and 36 solar masses) from September 14th in the same year.

4:46 PM: Just a little correction: Janna says this was the most powerful event ever detected since the Big Bang. And that’s only technically true, because of the limits of our detection.

When we get any black hole mergers, approximately 10% of the mass of the least massive black hole in a merger pair gets converted into pure energy via Einstein’s E = mc2. 29 solar masses is a lot, but there are going to be black holes of hundreds of millions or even billions of solar masses that have merged together. And we have proof.

The most massive black hole binary signal ever seen: OJ 287.

The most massive black hole binary signal ever seen: OJ 287.

4:49 PM: This is OJ 287, where a 150 million solar mass black hole orbits an ~18 billion solar mass black hole. It takes 11 years for a complete orbit to occur, and General Relativity predicts a precession of 270 degrees per orbit here, compared to 43 arc seconds per century for Mercury.

4:51 PM: Janna did an incredible job ending on time here; I’ve never seen an hour talk actually end after 50 minutes at a Perimeter public lecture. Wow!

The Earth as viewed from a composite of NASA satellite images from space in the early 2000s.

The Earth as viewed from a composite of NASA satellite images from space in the early 2000s.

4:52 PM: What would happen if Earth got sucked up into a black hole? (Q&A question from Max.) Although Janna’s giving a great answer, I’d like to point out that, from a gravitational wave point of view, Earth would be shredded apart, and we’d get a “smeared out” wave signal, that would be a much noisier, static-y signal. Once Earth got swallowed, the event horizon would grow just a tiny bit, as an extra three millionths of a solar mass increased the black hole’s radius by just that tiny, corresponding amount.

4:55 PM: What a fun talk, a great and snappy Q&A session, and a great experience overall. Enjoy it again and again, because the video of the talk is now embedded as a permalink. And thanks for tuning in!

A black hole 12 billion times more massive than our Sun has been detected

It’s so big, we need new physics to explain it.

Step aside regular black holes, astronomers have detected an ancient black hole that’s so incredibly massive and luminous, it defies our understanding of the early Universe.

In fact its quasar, the shining object produced by a supermassive black hole, is 420 trillion times more luminous than our Sun – despite forming only around 900 million years after the birth of the Universe. This makes it the brightest object ever spotted in the ancient Universe. It’s so impossibly bright that it’s almost, well impossible.

“How could we have this massive black hole when the universe was so young? We don’t currently have a satisfactory theory to explain it,” the lead researcher, Xue-Bing Wu, from Peking University in China and the Kavli Institute of Astronomy and Astrophysics in the US, told Rachel Feltman from The Washington Post.

The quasar, known as SDSS JO100+2802, is located around 12.8 billion light years away from Earth, and was spotted by the Sloan Digital Sky Survey, before being verified by three Earth-bound telescopes.

Reporting in Nature, the team explains that for the black hole to reach such a massive size in less than a billion years, it would have been constantly sucking in interstellar mass at its maximum rate. But this doesn’t fit with our current understanding of black hole growth, which states that the process is limited by energy that blasts out of the quasar as the black hole heats up.

Following that hypothesis, and also factoring in the limited amount of matter available in the early Universe, it makes it extremely difficult for scientists to explain how the supermassive black hole exists – and how it came to be 12 billion times more massive than our Sun.

“With this supermassive black hole, very early in the Universe, that theory cannot work,” Fuyan Bian from the Australian National University, who was involved in the research, told Genelle Weule and Stuart Gary for ABC Science. “It’s time for a new hypothesis and for some new physics.”

But even though we’re still not sure how the mega quasar is possible, it’s going to be a useful tool for finding other objects in the Universe.

“This quasar is very unique. Just like the brightest lighthouse in the distant universe, its glowing light will help us probe more about the early Universe,” said Wu in a press release.

Sources: ABC ScienceThe Washington Post

Read this next: The masses of black holes are more predictable than we thought

Infinity and Beyond: The Ultimate Test. Are we living in a Multiverse

In a Multiverse, What Are the Odds?

Testing the multiverse hypothesis requires measuring whether our universe is statistically typical among the infinite variety of universes. But infinity does a number on statistics.

To account for our incredible luck, leading cosmologists like Alan Guth and Stephen Hawking envision our universe as one of countless bubbles in an eternally frothing sea. This infinite “multiverse” would contain universes with constants tuned to any and all possible values, including some outliers, like ours, that have just the right properties to support life. In this scenario, our good luck is inevitable: A peculiar, life-friendly bubble is all we could expect to observe.

Many physicists loathe the multivere hypothesis, deeming it a cop-out of infinite proportions. But as attempts to paint our universe as an inevitable, self-contained structure falter, the multiverse camp is growing.

The problem remains how to test the hypothesis. Proponents of the multiverse idea must show that, among the rare universes that support life, ours is statistically typical. The exact dose of vacuum energy, the precise mass of our underweight Higgs boson, and other anomalies must have high odds within the subset of habitable universes. If the properties of this universe still seem atypical even in the habitable subset, then the multiverse explanation fails.

But infinity sabotages statistical analysis. In an eternally inflating multiverse, where any bubble that can form does so infinitely many times, how do you measure “typical”?

Guth, a professor of physics at the Massachusetts Institute of Technology, resorts to freaks of nature to pose this “measure problem.” “In a single universe, cows born with two heads are rarer than cows born with one head,” he said. But in an infinitely branching multiverse, “there are an infinite number of one-headed cows and an infinite number of two-headed cows. What happens to the ratio?”

For years, the inability to calculate ratios of infinite quantities has prevented the multiverse hypothesis from making testable predictions about the properties of this universe. For the hypothesis to mature into a full-fledged theory of physics, the two-headed-cow question demands an answer.

Eternal Inflation

As a junior researcher trying to explain the smoothness and flatness of the universe, Guth proposed in 1980 that a split second of exponential growth may have occurred at the start of the Big Bang. This would have ironed out any spatial variations as if they were wrinkles on the surface of an inflating balloon. The inflation hypothesis, though it is still being tested, gels with all available astrophysical data and is widely accepted by physicists.

Play this video

In the years that followed, Guth and several other cosmologists reasoned that inflation would almost inevitably beget an infinite number of universes. “Once inflation starts, it never stops completely,” Guth explained. In a region where it does stop — through a kind of decay that settles it into a stable state — space and time gently swell into a universe like ours. Everywhere else, space-time continues to expand exponentially, bubbling forever.

Each disconnected space-time bubble grows under the influence of different initial conditions tied to decays of varying amounts of energy. Some bubbles expand and then contract, while others spawn endless streams of daughter universes. The scientists presumed that the eternally inflating multiverse would everywhere obey the conservation of energy, the speed of light, thermodynamics, general relativity and quantum mechanics. But the values of the constants coordinated by these laws were likely to vary randomly from bubble to bubble.

Paul Steinhardt, a theoretical physicist at Princeton University and one of the early contributors to the theory of eternal inflation, saw the multiverse as a “fatal flaw” in the reasoning he had helped advance, and he remains stridently anti-multiverse today. “Our universe has a simple, natural structure,” he said in September. “The multiverse idea is baroque, unnatural, untestable and, in the end, dangerous to science and society.”

Steinhardt and other critics believe the multiverse hypothesis leads science away from uniquely explaining the properties of nature. When deep questions about matter, space and time have been elegantly answered over the past century through ever more powerful theories, deeming the universe’s remaining unexplained properties “random” feels, to them, like giving up. On the other hand, randomness has sometimes been the answer to scientific questions, as when early astronomers searched in vain for order in the solar system’s haphazard planetary orbits. As inflationary cosmology gains acceptance, more physicists are conceding that a multiverse of random universes might exist, just as there is a cosmos full of star systems arranged by chance and chaos.

“When I heard about eternal inflation in 1986, it made me sick to my stomach,” saidJohn Donoghue, a physicist at the University of Massachusetts, Amherst. “But when I thought about it more, it made sense.”

One for the Multiverse

The multiverse hypothesis gained considerable traction in 1987, when the Nobel laureate Steven Weinberg used it to predict the infinitesimal amount of energy infusing the vacuum of empty space, a number known as the cosmological constant, denoted by the Greek letter Λ (lambda). Vacuum energy is gravitationally repulsive, meaning it causes space-time to stretch apart. Consequently, a universe with a positive value for Λ expands — faster and faster, in fact, as the amount of empty space grows — toward a future as a matter-free void. Universes with negative Λ eventually contract in a “big crunch.”

Physicists had not yet measured the value of Λ in our universe in 1987, but the relatively sedate rate of cosmic expansion indicated that its value was close to zero. This flew in the face of quantum mechanical calculations suggesting Λ should be enormous, implying a density of vacuum energy so large it would tear atoms apart. Somehow, it seemed our universe was greatly diluted.

Weinberg turned to a concept called anthropic selection in response to “the continued failure to find a microscopic explanation of the smallness of the cosmological constant,” as he wrote in Physical Review Letters (PRL). He posited that life forms, from which observers of universes are drawn, require the existence of galaxies. The only values of Λ that can be observed are therefore those that allow the universe to expand slowly enough for matter to clump together into galaxies. In his PRL paper, Weinberg reported the maximum possible value of Λ in a universe that has galaxies. It was a multiverse-generated prediction of the most likely density of vacuum energy to be observed, given that observers must exist to observe it.

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A decade later, astronomers discovered that the expansion of the cosmos was accelerating at a rate that pegged Λ at 10−123 (in units of “Planck energy density”). A value of exactly zero might have implied an unknown symmetry in the laws of quantum mechanics — an explanation without a multiverse. But this absurdly tiny value of the cosmological constant appeared random. And it fell strikingly close to Weinberg’s prediction.

“It was a tremendous success, and very influential,” said Matthew Kleban, a multiverse theorist at New York University. The prediction seemed to show that the multiverse could have explanatory power after all.

Close on the heels of Weinberg’s success, Donoghue and colleagues used the same anthropic approach to calculate the range of possible values for the mass of the Higgs boson. The Higgs doles out mass to other elementary particles, and these interactions dial its mass up or down in a feedback effect. This feedback would be expected to yield a mass for the Higgs that is far larger than its observed value, making its mass appear to have been reduced by accidental cancellations between the effects of all the individual particles. Donoghue’s group argued that this accidentally tiny Higgs was to be expected, given anthropic selection: If the Higgs boson were just five times heavier, then complex, life-engendering elements like carbon could not arise. Thus, a universe with much heavier Higgs particles could never be observed.

Until recently, the leading explanation for the smallness of the Higgs mass was a theory called supersymmetry, but the simplest versions of the theory have failed extensive tests at the Large Hadron Collider near Geneva. Although new alternatives have been proposed, many particle physicists who considered the multiverse unscientific just a few years ago are now grudgingly opening up to the idea. “I wish it would go away,” said Nathan Seiberg, a professor of physics at the Institute for Advanced Study in Princeton, N.J., who contributed to supersymmetry in the 1980s. “But you have to face the facts.”

However, even as the impetus for a predictive multiverse theory has increased, researchers have realized that the predictions by Weinberg and others were too naive. Weinberg estimated the largest Λ compatible with the formation of galaxies, but that was before astronomers discovered mini “dwarf galaxies” that could form in universes in which Λ is 1,000 times larger. These more prevalent universes can also contain observers, making our universe seem atypical among observable universes. On the other hand, dwarf galaxies presumably contain fewer observers than full-size ones, and universes with only dwarf galaxies would therefore have lower odds of being observed.

Researchers realized it wasn’t enough to differentiate between observable and unobservable bubbles. To accurately predict the expected properties of our universe, they needed to weight the likelihood of observing certain bubbles according to the number of observers they contained. Enter the measure problem.

Measuring the Multiverse

Guth and other scientists sought a measure to gauge the odds of observing different kinds of universes. This would allow them to make predictions about the assortment of fundamental constants in this universe, all of which should have reasonably high odds of being observed. The scientists’ early attempts involved constructing mathematical models of eternal inflation and calculating the statistical distribution of observable bubbles based on how many of each type arose in a given time interval. But with time serving as the measure, the final tally of universes at the end depended on how the scientists defined time in the first place.

Berkeley physicist Raphael Bousso, 43, extrapolated from the physics of black holes to devise a novel way of measuring the multiverse, one that successfully explains many of our universe’s features.

“People were getting wildly different answers depending on which random cutoff rule they chose,” said Raphael Bousso, a theoretical physicist at the University of California, Berkeley.

Alex Vilenkin, director of the Institute of Cosmology at Tufts University in Medford, Mass., has proposed and discarded several multiverse measures during the last two decades, looking for one that would transcend his arbitrary assumptions. Two years ago, he and Jaume Garriga of the University of Barcelona in Spainproposed a measure in the form of an immortal “watcher” who soars through the multiverse counting events, such as the number of observers. The frequencies of events are then converted to probabilities, thus solving the measure problem. But the proposal assumes the impossible up front: The watcher miraculously survives crunching bubbles, like an avatar in a video game dying and bouncing back to life.

In 2011, Guth and Vitaly Vanchurin, now of the University of Minnesota Duluth,imagined a finite “sample space,” a randomly selected slice of space-time within the infinite multiverse. As the sample space expands, approaching but never reaching infinite size, it cuts through bubble universes encountering events, such as proton formations, star formations or intergalactic wars. The events are logged in a hypothetical databank until the sampling ends. The relative frequency of different events translates into probabilities and thus provides a predictive power. “Anything that can happen will happen, but not with equal probability,” Guth said.

Still, beyond the strangeness of immortal watchers and imaginary databanks, both of these approaches necessitate arbitrary choices about which events should serve as proxies for life, and thus for observations of universes to be counted and converted into probabilities. Protons seem necessary for life; space wars do not — but do observers require stars, or is this too limited a concept of life? With either measure, choices can be made so that the odds stack in favor of our inhabiting a universe like ours. The degree of speculation raises doubts.

The Causal Diamond

Bousso first encountered the measure problem in the 1990s as a graduate student working with Stephen Hawking, the doyen of black hole physics. Black holes prove there is no such thing as an omniscient measurer, because someone inside a black hole’s “event horizon,” beyond which no light can escape, has access to different information and events from someone outside, and vice versa. Bousso and other black hole specialists came to think such a rule “must be more general,” he said, precluding solutions to the measure problem along the lines of the immortal watcher. “Physics is universal, so we’ve got to formulate what an observer can, in principle, measure.”

This insight led Bousso to develop a multiverse measure that removes infinity from the equation altogether. Instead of looking at all of space-time, he homes in on a finite patch of the multiverse called a “causal diamond,” representing the largest swath accessible to a single observer traveling from the beginning of time to the end of time. The finite boundaries of a causal diamond are formed by the intersection of two cones of light, like the dispersing rays from a pair of flashlights pointed toward each other in the dark. One cone points outward from the moment matter was created after a Big Bang — the earliest conceivable birth of an observer — and the other aims backward from the farthest reach of our future horizon, the moment when the causal diamond becomes an empty, timeless void and the observer can no longer access information linking cause to effect.

The infinite multiverse can be divided into regions called causal diamonds that range from large and rare with many observers, left, to small and common with few observers, right. In this scenario, causal diamonds like ours should be large enough to give rise to many observers but small enough to be relatively common.

Bousso is not interested in what goes on outside the causal diamond, where infinitely variable, endlessly recursive events are unknowable, in the same way that information about what goes on outside a black hole cannot be accessed by the poor soul trapped inside. If one accepts that the finite diamond, “being all anyone can ever measure, is also all there is,” Bousso said, “then there is indeed no longer a measure problem.”

In 2006, Bousso realized that his causal-diamond measure lent itself to an evenhanded way of predicting the expected value of the cosmological constant. Causal diamonds with smaller values of Λ would produce more entropy — a quantity related to disorder, or degradation of energy — and Bousso postulated that entropy could serve as a proxy for complexity and thus for the presence of observers. Unlike other ways of counting observers, entropy can be calculated using trusted thermodynamic equations. With this approach, Bousso said, “comparing universes is no more exotic than comparing pools of water to roomfuls of air.”

Using astrophysical data, Bousso and his collaborators Roni Harnik, Graham Kribs and Gilad Perez calculated the overall rate of entropy production in our universe, which primarily comes from light scattering off cosmic dust. The calculation predicted a statistical range of expected values of Λ. The known value, 10-123, rests just left of the median. “We honestly didn’t see it coming,” Bousso said. “It’s really nice, because the prediction is very robust.”

Making Predictions

Bousso and his collaborators’ causal-diamond measure has now racked up a number of successes. It offers a solution to a mystery of cosmology called the “why now?” problem, which asks why we happen to live at a time when the effects of matter and vacuum energy are comparable, so that the expansion of the universe recently switched from slowing down (signifying a matter-dominated epoch) to speeding up (a vacuum energy-dominated epoch). Bousso’s theory suggests it is only natural that we find ourselves at this juncture. The most entropy is produced, and therefore the most observers exist, when universes contain equal parts vacuum energy and matter.

In 2010 Harnik and Bousso used their idea to explain the flatness of the universe and the amount of infrared radiation emitted by cosmic dust. Last year, Bousso and his Berkeley colleague Lawrence Hall reported that observers made of protons and neutrons, like us, will live in universes where the amount of ordinary matter and dark matter are comparable, as is the case here.

“Right now the causal patch looks really good,” Bousso said. “A lot of things work out unexpectedly well, and I do not know of other measures that come anywhere close to reproducing these successes or featuring comparable successes.”

The causal-diamond measure falls short in a few ways, however. It does not gauge the probabilities of universes with negative values of the cosmological constant. And its predictions depend sensitively on assumptions about the early universe, at the inception of the future-pointing light cone. But researchers in the field recognize its promise. By sidestepping the infinities underlying the measure problem, the causal diamond “is an oasis of finitude into which we can sink our teeth,” said Andreas Albrecht, a theoretical physicist at the University of California, Davis, and one of the early architects of inflation.

Kleban, who like Bousso began his career as a black hole specialist, said the idea of a causal patch such as an entropy-producing diamond is “bound to be an ingredient of the final solution to the measure problem.” He, Guth, Vilenkin and many other physicists consider it a powerful and compelling approach, but they continue to work on their own measures of the multiverse. Few consider the problem to be solved.

Every measure involves many assumptions, beyond merely that the multiverse exists. For example, predictions of the expected range of constants like Λ and the Higgs mass always speculate that bubbles tend to have larger constants. Clearly, this is a work in progress.

“The multiverse is regarded either as an open question or off the wall,” Guth said. “But ultimately, if the multiverse does become a standard part of science, it will be on the basis that it’s the most plausible explanation of the fine-tunings that we see in nature.”

Perhaps these multiverse theorists have chosen a Sisyphean task. Perhaps they will never settle the two-headed-cow question. Some researchers are taking a different route to testing the multiverse. Rather than rifle through the infinite possibilities of the equations, they are scanning the finite sky for the ultimate Hail Mary pass — the faint tremor from an ancient bubble collision.

Black Hole In Movie Interstellar Is Most Accurate Ever In Any Movie

The new movie Interstellar not only promises to be a top-notch science-fiction movie, it may have actually advanced our understanding of black holes.

This came as quite a wonderful shock to me. All too often, scientific accuracy in the movies takes a back seat or even a trunk seat to such considerations as plot development, dazzling eye-candy etc. Often, movie-makers are not even aware that real science is often more amazing than the lame made-up ideas they come up with.

This is one of the reasons that I will always appreciate director, screenwriter, and producer Chris Nolan. You have almost certainly seen some of his films which is a very safe prognostication considering that list includes the likes of Memento, Inception, and of course, The Batman Dark Knight trilogy. Describing the reason for scientific accuracy in Interstellar Nolan said this:

“The initial impetus for the project had been to say why not examine real possibilities, why not actually look at the real science there”

Why not indeed.

The genesis of that thought occurred when Chris joined the movie project and starting re-writing the script that his brother Jonathan had written. The gist of the movie involves a dystopian future in which humanity is at the brink of extinction due to global GMO crop failures (I made the GMO part up). Matthew McConaughey’s erstwhile astronaut character is ushered out of retirement to make a bold attempt at finding another solar system that the rag-tag remnants of homo-sapiens could migrate to.

Science is clearly an integral component of this movie and Nolan wanted to properly wrap his head around it. To do so he started having talks with theoretical astro-physicist Kip Thorne. Kip was more than qualified to be the scientific advisor for the movie having been the The Feynman Professor of theoretical Physics at Caltech and a prominent expert at determining the implications for astrophysical phenomena of Einstein’s General Relativity.

Due to various plot developments, the movie required time dilation to occur so that different characters could experience the flow of time at significantly different rates relative to each other. Kip of course suggested a black hole to pull that off (relativistic velocities would also suffice btw). For this specific plot element, Nolan asked Paul Franklin, senior supervisor of the Academy Award-winning special effects house Double Negative, to work with Kip to create the black hole. Franklin was an excellent choice not only for his obvious talent but since he clearly shared Nolan’s love of scientific accuracy. I say that because Franklin said this:

“Science fiction always wants to dress things up, like it’s never happy with the ordinary universe”

He hit the nail on the head with that one.

Thorne's light distortion diagram

Kip could at that point have just given Frankin some ideas how to accurately represent a black hole. Instead he gave him page after page of equations that could be fed into a rendering program. The problem was, no renderer they had could use them because the ray-tracing algorithms they employed to build up images and reflections are based on light traveling in straight lines. Into order to deal with the curved paths of light from gravitational lensing around a black hole the team had to write a whole new renderer.

Once they were ready it was time to render the images based on Kip’s equations….and render they did. Some images were so number-crunchy that it took 100 hours to create just one. After 30 people and thousands of computers toiled for a year they finally had the 800 terabyte result. When the team looked at what they had wrought though, they thought there was a bug in the program. When Kip looked at it though he said:

“Why, of course. That’s what it would do.”

What they saw was the most accurate portrayal of a black hole in cinema ever. It was different enough from expectation that the-black-hole-movieeven Kip said he never would have expected it. The images showed that the light emitted from the accretion disk of swirling matter orbiting the black hole would have its light gravitationally distorted such that a halo of light appeared not only above and below but also in a line in front of it as well. This lensing of light has been used for years to see distant background objects like galaxies but now we were seeing what that lensing does to the light generated nearby the black hole as well.

So, not only has a science fiction movie been instrumental in advancing scientific research, the icing on the cake is that Kip thinks he should be able to get a couple bona-fide papers out of it as well. One for the astrophysics community and one for the special effects community.  Like any advances in science, I’m curious to see what peer review has to say about his results. I was actually bummed a bit when I came across a comment about this news item which said that we’ve had accurate scientific representations of accretion disk black holes for a while now even if we’ve not had accurate cinematic representations of them. On the other hand Kip Thorne seemed genuinely surprised at what his equations produced and if anyone is aware of this topic he’d be the guy.

Regardless how this plays out, I hope other movie makers take note that it makes sense to integrate real science into movies as often as possible, if for no other reason that it could get you tons of  free publicity ( and maybe a scientific paper)

 

Video: Science of the Movie

Press Release

Movie Trailer

Image Credit: Paramount

The Largest Black Holes in the Universe

Our Milky Way may harbor millions of black holes… the ultra dense remnants of dead stars. But now, in the universe far beyond our galaxy, there’s evidence of something far more ominous. A breed of black holes that has reached incomprehensible size and destructive power. Just how large, and violent, and strange can they get?

A new era in astronomy has revealed a universe long hidden to us. High-tech instruments sent into space have been tuned to sense high-energy forms of light — x-rays and gamma rays — that are invisible to our eyes and do not penetrate our atmosphere. On the ground, precision telescopes are equipped with technologies that allow them to cancel out the blurring effects of the atmosphere. They are peering into the far reaches of the universe, and into distant caldrons of light and energy. In some distant galaxies, astronomers are now finding evidence that space and time are being shattered by eruptions so vast they boggle the mind.

We are just beginning to understand the impact these outbursts have had on the universe: On the shapes of galaxies, the spread of elements that make up stars and planets, and ultimately the very existence of Earth. The discovery of what causes these eruptions has led to a new understanding of cosmic history. Back in 1995, the Hubble space telescope was enlisted to begin filling in the details of that history. Astronomers selected tiny regions in the sky, between the stars. For days at a time, they focused Hubble’s gaze on remote regions of the universe.

These hubble Deep Field images offered incredibly clear views of the cosmos in its infancy. What drew astronomers’ attention were the tiniest galaxies, covering only a few pixels on Hubble’s detector. Most of them do not have the grand spiral or elliptical shapes of large galaxies we see close to us today.

Instead, they are irregular, scrappy collections of stars. The Hubble Deep Field confirmed a long-standing idea that the universe must have evolved in a series of building blocks, with small galaxies gradually merging and assembling into larger ones.

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