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Ethan Siegel
43,999 followers -
Science writer, professor and theoretical astrophysicist
Science writer, professor and theoretical astrophysicist

43,999 followers
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“It’s true and remarkable that the positrons implied by the HAWC data explain only 1% of the positrons observed by the other experiments, indicating that something else is responsible. When you see an observation that our conventional ideas can’t account for, like an excess of astrophysical positrons, keep in the back of your head that it might be dark matter, exhibiting the long-sought-after interaction properties that have eluded us so far. But it’s far more likely that some other astrophysical process is accelerating conventional, known particles to produce these effects. When you have a mystery in science, keep your mind open to a revolution, but place your bets on the mundane. And never, ever believe the hype that claims otherwise.”

When there’s something you can’t explain in space, there’s always an explanation. Before you posit some new, groundbreaking physics, however, the smart move is to examine the mundane explanations, like the matter and the objects we already know must exist. In a recent study, scientists sought to explain the positron/gamma-ray excess seen by astrophysical observatories like PAMELA, AMS, and Fermi by measuring the positrons from a couple of pulsars. While they saw a signal that was consistent with some positron emission, it was too little and too weak to be the answer. So if it isn’t these nearby pulsars, what is it? There could be a great many other possibilities, such as microquasars, magnetars, younger or older pulsars, or supernova remnants, among others. So why go straight to ‘dark matter’ as the primary explanation? There’s no good reason for it at all.

This is the problem when you believe what you wish was true instead of what was likely true, and you can’t distinguish between them. Come get the real story instead!

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“Yet at their core, these two were practically tailor-made to collaborate with one another. Wheeler’s wild ideas always contained components that were spectacularly wrong and unworkable, but often contained a kernel of deep truth that would pave the road to an understanding that was otherwise unachievable. The idea of a path integral, the essential tool used to calculate physical observables in quantum field theory, came about from Wheeler’s insistence on a sum over histories, but it was Feynman who worked out the details correctly, and applied them properly to our physical Universe.

Feynman’s ability to connect the wild ideas to the physical Universe, never far afield from what could be measured, was the perfect complement to Wheeler’s imagination. Together and separately, they took on gravitation, the quantum nature of reality, and even space and time itself. And as much as any physicist ever did, they not only took these ideas on; they won.”

In popular culture, Richard Feynman is revered as a non-conformist/genius, whose bongo-playing, carefree antics are as notable as his groundbreaking physics research. John Wheeler, renowned for his contributions to General Relativity, gravitation, and information theory, has no similar stories from his personal lives. Yet professionally, these two complemented one another in ways that were unimaginable to an outside onlooker: Wheeler’s imagination ran wildly into the speculative and unworkable, while Feynman was always dragging things back to observable and measurable quantities. In the end, both are remembered as towering figures in physics in the 20th century, on par with names like Bohr, Dirac, Pauli, and Heisenberg. In a sweeping new book, Paul Halpern takes an in-depth look at the scientific and personal lives of these two physicists, who first met in 1939 and spent the next five decades revolutionizing our conception of the Universe.

Get the full book review here, and if you’re at all interested in picking up a copy for yourself (or the physics enthusiast in your life), follow the links and go get a copy of The Quantum Labyrinth!

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“It’s why theories like quantum field theory and general relativity are so powerful: even after all these decades, they’re still making new predictions that are being successfully borne out by experiment. It’s why dark matter is here to stay, as its successful predictions include the speeds of galaxy pairs, the large-scale cosmic web, the fluctuations in the CMB, baryon acoustic oscillations, gravitational lensing and more. It’s why cosmic inflation— with its successful predictions including superhorizon fluctuations, the acoustic peaks in the Big Bang’s leftover glow, the departure from scale invariance, etc. — is the leading theory for the origin of the Big Bang. And it’s why their alternatives are so thoroughly fringe.”

It’s tempting to view science as a steady progression forward, where new evidence comes to light, eliminating some hypotheses while preserving others. But the truth is a lot murkier than that, as scientists routinely tweak and alter their hypothesis in detail to match the current, favored observations. When dark energy was discovered through supernovae, for instance, some thought it might be an effect of dust instead. When the color of the light didn’t match up, they suggested a new form of dust instead: grey dust. And when the distant observations showed that grey dust was insufficient, they coined a new term: replenishing grey dust. While constantly tweaking your theory to bring it in line with observations may be unpalatable, it’s what theorists do all the time. It’s why old, discredited scientific ideas can never die, not as long as someone’s still willing to work on them.

The notion that science advances one funeral at a time comes from this exact behavior. Like it or not, it’s all part of the scientific process.

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"Global dimming could someday provide a geoengineering solution to counteract the effects of global warming, if our environmentally-minded efforts to curb our emissions fail. The discovery of the haze-temperature connection on Pluto demonstrates, for the first time, that there are actual cases out there where this type of effect can serve to reduce the worldwide temperature by far more than humanity's emissions have increased it. On the surface, it provides a new hope for geoengineering scenarios to mitigate global warming. However, there are side effects associated with having pollutants and potentially toxic particulates in our clouds, and therefore our rainwater. We must ensure, before embarking down such a path, that the cure isn't worse than the disease."

When NASA's New Horizons arrived at Pluto, it discovered a slew of wonderful surprises and sights. It found reddish-brown terrain, ices, a complex atmosphere with hazes, and a temperature surprise: it was 30 K (54 °F) cooler than we would have expected. There are only three factors that normally control a world's temperature: its distance from the Sun, its reflectivity, and the gases present in its atmosphere. But on Pluto, the hazes play an incredible role, as a new paper out today reveals. These hazes, made of complex hydrocarbons formed from ionized gases like nitrogen and methane, actually add heat to the atmosphere but cool down the surface, leading to a dramatic difference that's more than 10 times cooler than the heating effects of humanity's contribution to Earth's atmosphere. It makes you wonder if this might hold the key to fighting global warming here on Earth.

We may not be in control of our planet, but we're at the controls. It's up to us to make the right decisions, and learning all we can about other worlds in the Solar System is certainly helping!

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"Satisfyingly, we discover that this form of time travel also forbids the grandfather paradox! Even if the wormhole were created before your parents were conceived, there's no way for you to exist at the other end of the wormhole early enough to go back and find your grandfather prior to that critical moment. The best you can do is to put your newborn father and mother on a ship to catch the other end of the wormhole, have them live, age, conceive you, and then send yourself back through the wormhole. You'll be able to meet your grandfather when he's still very young — perhaps even younger than you are now — but it will still, by necessity, occur at a moment in time after your parents were born."

So, you want to travel back in time? It's long been considered as a trope in science fiction movies, television, and literature, but the laws of physics make traveling backwards through time very difficult. In special relativity, it's impossible, as you can only control the rate you move forward through time; the direction is non-negotiable. But in General Relativity, the curvature of space and time opens up additional possibilities. You can create a stable, traversible wormhole if some type of negative mass/energy exists, with a supermassive black hole connected to its negative mass/energy counterpart. Now, move one end of that wormhole close to the speed of light, and the two mouths age at different rates. Travel through the fast-moving end, and discover you're back at the stationary end way in the distant past... but still in the future compared to when the wormhole was created.

It's a brilliant way to achieve time travel, and as a bonus, it makes it impossible to go back in time and kill your own grandfather before you were conceived! Come learn how time travel could really, physically be possible after all.

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“But the parallel Universes part is the hardest part for me to swallow. Newly introduced in this episode, Lorca shows Stamets how all the data gathered from the spore drive shows not only the mycelium network, but doorways to parallel Universes. They build off the many-world interpretation of quantum mechanics to indicate that Universes where anything and everything that can happen does, only in some other parallel Universe. The problem is, these branches occur at an “event” in spacetime, which means they occur at a specific location in space at a specific time; you can’t simply have a “map” of a place where you can access a parallel Universe. Yet that’s how Star Trek: Discovery chooses to portray the science, and it appears that’s where they wind up at the end: in a parallel Universe that’s nowhere known.”

After nine episodes, Star Trek: Discovery reaches its mid-season hiatus with a visually spectacular battle on multiple fronts at Pahvo. Discovery engages the Klingon sarcophagus ship, Stamets faces his own mental decline to power the spore drive, Lorca orders others to uncertain fates, Burnham engages in combat at the scene of her greatest failure, and Tyler battles his own PTSD. It’s a great stage for some very compelling internal and external conflicts to play out. But it’s also all too easy. The Klingons are one-dimensional villains. There’s no ethical dilemma to obeying/disobeying orders here. Burnham exercises terrible judgment, but gets lucky in the end. And the “Gilligan’s starship” ending seems, at first glance, to be a new twist on an old plotline: Lost In Space. Which is really too bad, because that’s one of the classic counterexamples I use to show what Star Trek, as a franchise, is not all about.

There’s a lot of potential in Discovery, but it has some growing to do, on both the science and the fiction fronts, if it wants to go down as one of the greats.

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"If two black holes merge, is it possible for matter that was within the event horizon of one black hole to escape? Could it escape and migrate to the other (more massive black hole)? What about escape to outside of both horizons?"

Imagine that you’ve got two black holes about to merge in space. They’re radiating energy away, spacetime itself is at its most distorted, and perhaps you have particles just crossing over the event horizon for the first time. Is there any way you could configure it to have them escape, or to have a particle jump from one event horizon to another? The situation is incredible, and involves some of the strongest gravitational fields ever considered in the Universe. But numerical relativists are up to the challenge of simulating these spacetimes, and we can see what happens! Believe it or not, even as energy is radiated away and the total mass drops, the event horizons themselves never decrease in size, and the total volume encapsulating the “no-escape zone” only increases as time goes on!

Come learn the reason why matter can’t escape the event horizon, even during a black hole merger, on this week’s Ask Ethan!

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“Unsurprisingly, this result and this technique is controversial, and some of the other researchers dispute Livermore et al.’s findings. This is normal in science; discourse about different techniques, assumptions, and methodologies are key to uncovering systematic errors. When those are, at last, sorted out, we’ll have our best-ever understanding of the faintest, most distant galaxies we’ve ever seen.

But until that day comes, there are going to be arguments and disagreements. There are going to be different models, different results, different conclusions, and disparate ways of modeling the galaxy clusters in question, including models that vary from one cluster to the next. Right now, there are five independent teams working on this exact class of problems, including Hakim Atek’s team, Livermore’s, and Rychard Bouwens’. Bouwens has been professionally critical of Livermore’s work in the past; he thinks her galaxy sizes are too large. Of course, Livermore believes that Bouwens’ has made incorrect assumptions himself: about lensing, her field of expertise. Debates and discourse around these professional disagreements are normal, and usually play out in person, at conferences, and in the professional literature.

But in a new paper out this week, Bouwens demolished the line between professional disagreement and personal bullying, and did so in spectacular fashion.”

Disagreements in scientific fields are nothing new. They not only occur all the time, they need to occur in order to drive the field forward. When you’re at the frontiers of knowledge, there are going to be arguments about how to model certain effects, which calculations are important and when, what properties unknown objects have, etc. And different people have different scientific toolkits, areas of expertise, and opinions as to how best approach the problem. But there’s an unspoken rule that people treat each other with respect, and not to bully, harass, or put them down in the heat of an argument. Over the past two years, with the advent of the Hubble Frontier Fields, the most distant, faint galaxies ever discovered have been revealed through gravitational lensing. But what are the properties and distribution of these galaxies? That’s what a number of researchers are trying to figure out. But one Rychard Bouwens has taken his scientific passion into the realm of the personal, and went so far to personally attack another scientist (a junior, woman scientist) in a harassing and bullying fashion in a paper he submitted to a leading professional journal.

This is behavior we must stop. No one should ever be subject to this kind of treatment. Not in my field. Not on all our watches.

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“When your cluster is undisturbed, the gravitational effects are located where the matter is distributed. It’s only after a collision or interaction has taken place that we see what appears to be a non-local effect. This indicates that something happens during the collision process to separate normal matter from where we see the gravitational effects. Adding dark matter makes this work, but non-local gravity would make differing before-and-after predictions that can’t both match up, simultaneously, with what we observe.

Interestingly, this argument has been made for over a decade, now, with no satisfactory counterargument coming from detractors of dark matter. It isn’t the displacement of gravitation from normal matter that “proves” dark matter exists, but rather the fact that the displacement only occurs in environments where dark matter and normal matter would be separated by astrophysical processes. This is a fundamental issue that must be addressed, if alternatives to dark matter are to be taken seriously as complete theories, rather than ideas in their infancy. That time is not yet at hand.”

Recently, a paper came out challenging alternative theories to dark matter and claiming that many of them were invalid. The basis for that argument? That those theories predict different arrival times for gravitational waves and light waves from a neutron star merger, when we saw them arrive practically simultaneously. One of those theories, MOG, claims to survive, but it’s already been discredited for another reason that’s discussed far less frequently: the Bullet Cluster. When the apparent effects of gravitation are well-separated in space from where we see the matter, you require non-locality to save your theory. MOG is a non-local theory of gravity, so you might think everything is fine. But if gravitational effects aren’t where the matter is located, we’d expect to see these non-local effects in clusters that are in a pre-merger state, and those don’t exist.

Can a theory like MOG survive in this context? I don’t believe so. The Bullet Cluster proves dark matter exists, but not for the reason most physicists think!

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“In order to create a particle with a heavy quark (strange, charm, bottom, etc.) in it, you have to collide other particles together at extremely high energies: enough to make equal amounts of matter and antimatter. Assuming you then make the two baryons you need (two charmed or two bottomed baryons, for instance), you must then have them interact under the right conditions — fast and energetic, but not too fast or too energetic — to cause that fusion reaction. And then, at last, you get that ~3-4% energy gain out.

But it cost you over 100% to make these particles in the first place! They’re also incredibly unstable, meaning they’ll decay to lighter particles on incredibly short timescales: a nanosecond or less. And, finally, when they do decay, you get 100% of your energy back, in the form of new particles and their kinetic energies. In other words, you don’t get any net energy out; you simply get out what you put in, but in a lot of different, hard-to-harness ways.”

Nuclear fusion is often hailed as the future of energy, as it converts more mass into energy via Einstein’s E = mc^2 than any other reaction we’ve ever produced in large quantities. But even though the fusion of hydrogen into helium causes such a large energy release, it’s still less than 1% of the mass you begin with. On the other hand, a new set of simulations involving a recently discovered particle indicates that, by fusing charmed baryons with one another, you can produce a doubly-charmed baryon and get up to 4% of your mass converted into energy. While many are touting this as a potential game-changer, the reality is much more sobering. Nuclear fusion is promising not just for the large yield, but because its reactants are abundant and stable, because the energy outputted is easy to harness, and the reaction is controllable. “Melting quarks” offer none of these, and as such, will never work as an energy source.

Come get the science explaining why this new discovery is so interesting, but also why it isn’t going to deliver an energy revolution anytime soon!
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