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Ethan Siegel
43,911 followers -
Science writer, professor and theoretical astrophysicist
Science writer, professor and theoretical astrophysicist

43,911 followers
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“Everything you read about a black indicates that “nothing, not even light, can escape them”. Then you read that there is Hawking radiation, which “is blackbody radiation that is predicted to be released by black holes”. Then there are relativistic jets that “shoot out of black holes at close to the speed of light”. Obviously, something does come out of black holes, right?”

When it comes to black holes, the cardinal rule is that there exists an event horizon: a region from which nothing inside can ever escape. Once you cross over, you can never get out. No matter how fast you move, how quickly or what direction you accelerate in, or even if you travel at the speed of light, your inevitable destiny lies at the central singularity. So how, then, are things like relativistic jets and Hawking radiation emitted from black holes? The key to understanding them lies in examining the conditions that occur outside the event horizon, in the region near (but not exactly at) the black hole itself. This is the critical environment where spacetime is curved, matter achieves relativistic speeds, and the quantum fields themselves are affected by relativity.

Hawking radiation and relativistic jets may be real, but they don’t break the laws of physics to exist! Find out how they really do escape on this edition of Ask Ethan.
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“But if you continue your fall towards the event horizon, you’ll eventually see the starlight compress down into a tiny dot behind you, changing color into the blue due to gravitational blueshifting. At the last moment before you cross over into the event horizon, that dot will become red, white, and then blue, as the cosmic microwave and radio backgrounds get shifted into the visible part of the spectrum for your last, final glimpse of the outside Universe, still assuming that nothing else falls in with you.”

When you fall into a black hole, terrible things happen to you. Your atoms get stretched apart in the terrifying phenomenon of spaghettification, you get sucked inevitably into the central singularity, and the entire outside Universe goes dark. But it doesn’t go dark all at once; instead, the front of the Universe gets cut off, but all the light paths converge directly behind you. What you see, as a couple of the videos within the article show, is that the entire outside Universe gets encoded in a light path that decreases to a shrinking circle directly behind you. As you try to avoid the central singularity, maximizing your survival time, you discover a terrifying phenomenon from inside the event horizon: that the singularity is everywhere, in all directions. At the last, you see the blueshifted, leftover glow from the Big Bang… and then nothing but darkness.

My words cannot do it justice (although I try!), but you’ll want to see the visualizations for yourself. They’re spectacular, and best of all, they’re accurate! Come see what you would see if you had the misfortune to fall into a black hole.
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“We think of space as being an empty place, but the truth is that there are dust grains, particles, neutral atoms, ions, and cosmic rays zipping through the entirety of the galaxy, even when there are no stars. As an object moves through space, circling the galaxy at hundreds of kilometers per second (and moving relative to most other objects at tens of kilometers per second), it’s constantly bombarded by large numbers of small, fast-moving bits of matter. Just as water and sand will smooth out and erode pebbles and cobbles in the ocean here on our world, the cosmic equivalent — the interstellar medium — will have the same effect over extremely long timescales on ejected icy bodies.”

When scientists discovered ‘Oumuamua last year, they were surprised to find that it not only originated from outside our Solar System, but possessed bizarre properties we had never seen before. It was extremely elongated, tumbled irregularly, and had a never-before-seen composition: a carbon crust over an icy interior. Despite heating up to 550 °F (290 °C), it never developed a tail, a coma, or showed any ejecta. Many have proposed exotic or recent origins for this interstellar interloper, but in this case, simplicity rules: it may just be a cosmic pebble in the galactic sea. The interstellar medium is full of particles, and ‘Oumuamua, like most interstellar objects, should move at about 0.01% the speed of light through the galaxy. Over time, it should be worn down in exactly the fashion we see. As we discover more objects with an origin beyond our Solar System, we fully expect they’ll appear quite similar to this one.

How was ‘Oumuamua shaped? Likely by cosmic particles, rather than anything exotic. Come find out the science behind how.
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“If all we had done was look at the automated signals, we would have gotten just one “single-detector alert,” in the Hanford detector, while the other two detectors would have registered no event. We would have thrown it away, all because the orientation was such that there was no significant signal in Virgo, and a glitch caused the Livingston signal to be vetoed. If we left the signal-finding solely to algorithms and theoretical decisions, a 1-in-10,000 coincidence would have stopped us from finding this first-of-its-kind event. But we had scientists on the job: real, live, human scientists, and now we’ve confidently seen a multi-messenger signal, in gravitational waves and electromagnetic light, for the very first time.”

Imagine the scene: it’s mid-August, 2017, and the Virgo detector has just joined the twin LIGO detectors barely two weeks ago. Amazingly, on August 14th, you’ve seen a gravitational wave signal in all three detectors; another black hole-black hole merger. Then, all of a sudden, even though the LIGO detectors are set to shut down later in the month, an extraordinarily significant signal goes off… but only in one detector. The LIGO Hanford detector sees a signal with a false-alarm probability of just one part in 300 billion; a slam dunk. Yet both LIGO Livingston and Virgo see nothing. A non-coincident signal should automatically be rejected, but somehow, one of the young researchers working on the project thought to check the Livingston data by hand… and that was where the secret lay.

LIGO’s greatest discovery, of two merging neutron stars, almost was overlooked. Thankfully, the hands-on nature of the scientists working on gravitational waves were able to turn this into the discovery of the century! (So far!)
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"Even so, because of its ability to measure light to high sensitivity far into the infrared, there's a remarkable hope for determining whether these worlds have atmosphere regardless of any other measurements. As planets orbit their star, we see different phases: a full phase when it's on the far side of the star; a new phase when it's on the near side, and everything in between. Based on the temperature of the world at night, we'll receive different amounts of infrared light from the "dark" side that faces away from the Sun. Even without a transit, James Webb should be able to measure this."

The overwhelming majority of Earth-sized, potentially habitable planets that Kepler found are in orbit around red dwarf stars. In many ways, this is great: red dwarf stars are stable, temperature-wise, for longer than our Sun. Their planets are easier to detect, and they will be the first Earth-sized ones we can measure the atmospheres of directly. But even if we can't make those measurements with James Webb, we'll be able to learn whether they have atmospheres or not via a different method: by measuring the infrared radiation coming from the planets themselves in various phases. Just as we can measure the presence of Venus' atmosphere from the hot, infrared radiation emanating from it even on the night side, we can make those same measurements with James Webb of other Solar Systems. By time the early 2020s roll around, we'll have our first answers to this longstanding debate.

Many scientists think that Earth-sized planets around M-class stars will have no atmospheres left; others think there's a chance they survive. Here's how James Webb will find out!
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"But my favorite part of this episode is the clear dichotomy between two opposing principles playing out in our society today: the battle between diversity and purity. How do you strike the balance between maintaining your cultural identity, learning about and respecting your heritage, while simultaneously integrating with a larger, more diverse community that doesn't always represent you? The mirror Universe version of Voq has put a tremendous amount of thought into this, delivering some of the best lines of the show so far, lines that could be spoken with respect to confederate statues; to protesting the government; to peaceful assembly; to the silencing of dissent. It is the power of Star Trek to hold a mirror up to society and show us issues that are extremely personal to us in a new light that help give society hope."

Star Trek is famed for brilliant new technologies and scientific applications. In this episode alone, questions are raised about the nature of consciousness, connections in the mind, how entanglement works biologically, and whether quantum effects extend to different Universes. Biological questions about fusing multiple species together into a single organism, and questions of autonomy and identity permeate almost every scene of this episode. But the greatest point of all isn't about any of these, nor is it about the trials and tribulations the characters face: it's about the opposing ideologies of diversity and purity. How can you "remain Klingon" when you partner with Vulcans and Andoorians? How can you keep your honor intact when the only unifier you have is a common enemy? How can you espouse protecting your culture when you aren't even speaking your native language?

Star Trek attempts to answer the question that America struggles to answer even today, and explores a theme we all sorely need to think more about. The latest episode of Discovery has me pondering it long after my watch (and re-watch) has ended.
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“I would like to read or hear some on what scientists are planning to do next. What’s in the pipeline, on the drawing board or just an idea up for discussion?”

If it seems like there are deeper views, revolutionary finds, small advances, and better constraints coming out in science all the time, that’s because this is exactly what scientists are doing. While you may commonly hear about some of the greatest machines and observatories of all-time like the LHC and Hubble, the reality is that there are dozens of observatories and experiments all working together to unveil the secrets of the Universe. Want a taste of what’s coming in the future, and how it’s going to push forward and change what we know even more significantly? This past week, I had the pleasure to attend the latest American Astronomical Society meeting, and one of the things I was able to learn from this is the roadmap over the next few years and decades for NASA, the NSF, and other physics-and-astronomy based agencies. The future is bright, and that it’s being done on a stagnant budget is all the more impressive.

Come learn what we all have to look forward to, and imagine what we could do if we truly invested in science, all on this week’s Ask Ethan!
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“With SETI focusing solely on electromagnetic signatures, we may, at present, be looking for the cosmic equivalent of smoke signals in a cellphone-filled world. But this likely won’t be the case for long. As our technology continues to advance, our knowledge of what to look for will advance along with it. And perhaps someday — perhaps even someday soon — the Universe may have the most pleasant surprise of all in store for us: the news that we aren’t alone, after all.”

The question of whether alien life exists has troubled humanity for millennia, and that question is still one without a known answer. We have lots of reasons to expect that life is plentiful, but also many reasons to think that it’s extremely difficult to create life in the first place, to evolve complex life, and to arrive at a technologically advanced state. In the entirety of the Universe, so far, we only have one instance of success for each! But despite all that we know is out there, we aren’t looking for alien life in many of the arenas where we’d expect signals to exist. Our radio broadcasts, which we originally thought aliens would be bound to use, are far fewer than they were a generation ago; it might be foolish to expect aliens to broadcast what SETI is looking for. Instead, there might be other signals we should be seeking instead.

Are aliens really out there? If they are, we haven’t found them yet. Here are some less common possibilities that perhaps we should be looking at.
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“As our data sets improve, we should begin to measure the internal motions of large numbers of galaxies like this, which will answer many questions and raise others. Do most/all galaxies at these early stages rotate in a whirlpool-like plane? Is there a variety and multiple sets of populations that exhibit different behaviors? What are the actual effects of gas infall, supernovae, and small-scale motions? What is the velocity profile of these rotation curves, and can they teach us anything about the interplay of radiation, normal matter, and dark matter?

While we hope to learn these answers, we can now ask these questions sensibly in the aftermath of having measured the movement and internal motions of a galaxy so far away. At least for the first two, they rotate very similarly to their much older cousins, a quite unexpected result. Thanks to ALMA, we’re taking those coveted next steps into the final frontier.”

It wasn’t supposed to be this way. When you form galaxies in the very young Universe, it’s supposed to be a chaotic, turbulent place. Sure, you have gravitation, pulling matter in and creating a pancake-like shape. But then you form stars, and everything goes haywire. Supernovae go off, gas falls in, protogalaxies merge and get swallowed, motions get stirred up, and turbulence should permeate the galaxy. It ought to take billions of years for them to quiet down into a Milky Way-like whirlpool. Well, for the first time, owing to ALMA and Renske Smit’s team, the internal motions of galaxies less than a billion years old were measured, and – surprise! – their movement is smooth and not chaotic at all.

They’re less than a billion years old. And, thanks to ALMA observing them, they might finally pave the way to understand how galaxies form altogether.
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"If either photons, neutrinos, or some new type of dark radiation (that interacts with dark matter but not any of the normal particles) has a non-zero cross-section with dark matter, it could bias measurements of the Hubble rate to an artificially low value, but only for one type of measurement: the kind that you get from measuring these leftover relics. If interactions between dark matter and radiation are real, they might not only explain this cosmic controversy, but could be our first hint of how dark matter might directly interact with other particles. If we're lucky, it could even give us a clue to how to finally see dark matter directly."

One of the biggest controversies in physics today is over the expanding Universe. Despite attempts to measure the Hubble rate for nearly 100 years, we still don't know exactly how fast the Universe expands. Two independent classes of methods, from the cosmic distance ladder and the Big Bang's leftover relic, give two very precise and incompatible results: 73 km/s/Mpc and 67 km/s/Mpc, respectively. There's always the possibility that one class of methods gives a biased answer, and we simply haven't uncovered the bias. But it's also possible that new physics is responsible, that both teams are right, and that the discrepancy is a hint of the next great leap forward in our understanding of the fundamental properties of the Universe itself.

One exciting possibility is that dark matter has a new interaction with radiation: either photons, neutrinos, or a new type of 'dark radiation.' Come learn more about it today!
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