Ask Ethan: How Do Massless Particles Experience Gravity?
“Given the equation for gravity between two masses, and the fact that photons are massless, how is it possible for a mass (like a star or a black hole) to exert influence on said photon?”
You know the law of universal gravitation: you put in what any two masses are, how far apart they are from each other, and the gravitational constant of the Universe, and you can immediately know what the force is between any two objects. Set one of the masses to zero, and the force goes to zero. So why is it, then, that if you take the ultimate particle with no mass, a photon, and pass it close by a mass, its path does bend? Why do massless particles experience gravity?
To understand why, you should think about what happens if you and I start at the same place near a mass, but I’m stationary and you’re moving. How far away is that mass? What’s the “r” that goes into Newton’s equation? And who’s right: me or you?
The answer is that we both need to be right, and Newton won’t get us there. Come get the real story on gravity, and learn why, in the end, massless particles feel it, too!
The Physics Of Why Timekeeping First Failed In The Americas
“As soon as the clock arrived and was set up, it began keeping time more accurately than any timepiece before ever located on the North American continent. At least, that was what everyone assumed was happening for about a week or so. But after that amount of time, it became clear that something was amiss. The Sun and Moon weren’t rising at their predicted times, but rather were off by a bit.
Even worse, the amount that the clock was off by appeared to be getting worse over time: whatever error was at play was accumulating. Instead of these reliable, celestial events occurring at the predicted times on the clock, they were occurring earlier, according to the clock. Something was wrong. The clock was not only running slow, but appeared to be losing close to a minute per day.”
Imagine the news in 17th century America: a new form of timekeeping has been developed, and instead of an uncertainty of around 15 minutes a day (like you get with sundials), you can keep time accurately to within seconds per day. It would be an incredible advance! So you place an order to the Netherlands, where they’ve developed it, and they build you a clock. You send it across the ocean, set it up, and start it working. It seems to work great! But then you realize, after about a week, that the Sun and Moon aren’t rising and setting when they should. Something about your pristine clock is off. So you send it back, and when they start it up back in Amsterdam, it works perfectly.
Sounds like a mystery! But this mystery is something special, because the problem wasn’t with the clock, after all, but with the Earth. Come get the bizarre but educational story of how timekeeping first failed in the Americas!
Ask Ethan: Why don’t comets orbit the same way planets do?
“Why [do] comets orbit the Sun in a parabolic path, unlike planets which orbit in an elliptical one? Where do comets get the energy to travel such a long distance, from the Oort cloud to the Sun & back? Also, how could interstellar comets/asteroids come out of their parent star [system] and visit other ones?”
When we see comets in our Solar System, they can be either periodic, passing near the Sun and then extending very far away, to return many years later, or they could be a one-shot deal. But comets are driven by the same gravitational laws that drive the planets, which simply make fast-moving, nearly-circular ellipses around the Sun. So what makes these orbits so different, particularly if they’re obeying the same laws? Believe it or not, most of the would-be comets out there are moving in exactly the same nearly-circular paths, only they’re far more tenuously held by the Sun. Gravitational interactions might make small changes in their orbits, but if you’re already moving very slowly, a small change can have a very big effect!
Why don’t comets orbit the same way as the planets? Find out on this edition of Ask Ethan!
Richard Feynman And John Wheeler Revolutionized Time, Reality, And Our Quantum Universe
“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!
The Bullet Cluster Proves Dark Matter Exists, But Not For The Reason Most Physicists Think
“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!
What Would Happen If You Became Dark Matter?
“What if, instead of being made out of Standard Model particles, which experience the full suite of all the fundamental forces, we transitioned to being made out of particles which, to the best of our knowledge, interact only gravitationally? The first thing that would happen to you is that you’d no longer be bound together in any way whatsoever, and that anyone watching you would immediately see you disappear. The nuclear forces that hold your nuclei and protons together would vanish; the electromagnetic forces that caused atoms and molecules to stay together (and light to interact with you) would disappear; your cells and organs and entire body would cease to hold together.”
Here on Earth, everything we know of is made of normal matter, almost exclusively in the form of atoms. But if our understanding of the Universe is correct, there’s five times as much dark matter out there as there is normal matter. Have you ever wondered what would happen to you, a human being, if you spontaneously converted from normal matter into dark matter? Dark matter is fundamentally different from normal matter in a myriad of ways, perhaps most notably for the interactions it doesn’t have. Without the strong, weak, or electromagnetic forces acting on it, it’s effectively invisible to all types of matter and energy, including other dark matter particles. But it continues to interact gravitationally, even as the normal matter making up Earth, the solar system, and the galaxy is affected by everything else. If you gave it enough time and could watch them closely enough, you’d start to see the particles that once made you illustrate this difference.
Come find out what would happen to the particles that once made you if they instantaneously converted to dark matter. You might be surprised!
Gravitational Waves Win 2017 Nobel Prize In Physics, The Ultimate Fusion Of Theory And Experiment
“The 2017 Nobel Prize in Physics may have gone to three individuals who made an outstanding contribution to the scientific enterprise, but it’s a story about so much more than that. It’s about all the men and women over more than 100 years who’ve contributed, theoretically and experimentally and observationally, to our understanding of the precise workings of the Universe. Science is much more than a method; it’s the accumulated knowledge of the entire human enterprise, gathered and synthesized together for the betterment of everyone. While the most prestigious award has now gone to gravitational waves, the science of this phenomenon is only in its earliest stages. The best is yet to come.”
It’s official at long last: the 2017 Nobel Prize in Physics has been awarded to three individuals most responsible for the development and eventual direct detection of gravitational waves. Congratulations to Rainer Weiss, Kip Thorne, and Barry Barish, whose respective contributions to the experimental setup of gravitational wave detectors, theoretical predictions about which astrophysical events produce which signals, and the design-and-building of the modern LIGO interferometers helped make it all possible. The story of directly detecting gravitational waves is so much more, however, than the story of just these three individuals, or even than the story of their collaborators. Instead, it’s the ultimate culmination of a century of theoretical, experimental, and instrumentational work, dating back to Einstein himself. It’s a story that includes physics titans Howard Robertson, Richard Feynman, and Joseph Weber. It includes Russell Hulse and Joseph Taylor, who won a Nobel decades earlier for the indirect detection of gravitational waves. And it’s the story of over 1,000 men and women who contributed to LIGO and VIRGO, bringing us into the era of gravitational wave astronomy.
The 2017 Nobel Prize in Physics may only go to three individuals, but it’s the ultimate fusion of theory and experiment. And yes, the best is yet to come!
5 Facts We Can Learn If LIGO Detects Merging Neutron Stars
“We have already entered a new age in astronomy, where we’re not just using telescopes, but interferometers. We’re not just using light, but gravitational waves, to view and understand the Universe. If merging neutron stars reveal themselves to LIGO, even if the events are rare and the detection rate is low, it’s means we’ll have crossed that next frontier. The gravitational sky and the light-based sky will no longer be strangers to one another. Instead, we’ll be one step closer to understanding how the most extreme objects in the Universe actually work, and we’ll have a window into our cosmos that no human has ever had before.”
Two years ago, advanced LIGO turned on, and in that brief time, it’s already revealed a number of gravitational wave events. All of them, to no one’s surprise, have been merging black holes, since those are the easiest class of events for LIGO to detect. But beyond black holes, LIGO should also be sensitive to merging neutron stars. Even though the range over which LIGO can see them is much smaller, if there are enough neutron star-neutron star mergers happening, we might have a chance. A little over a week ago, a rumor broke that LIGO may have seen one, which would be a phenomenal occurrence. Not only would we have a new type of event that we detected in gravitational waves, we would, for the first time, have the capability of correlating the gravitational and electromagnetic skies. Astronomy, for the first time ever, could view the very same object in gravitational waves and through telescopes.
This is a big deal, and there are four more facts we’ll learn if LIGO sees it! Come find out what they are!
Can Moons Have Their Own Moons?
“But with all of that said, I wouldn’t expect any. The conditions to acquire and retain a moon-of-a-moon all pose extreme difficulties when you consider how many gravitationally perturbative objects there are in these gas giant systems. If I had to place bets, I’d say Iapetus and Triton were the most likely candidates for having a “moon-of-a-moon,” since they’re the farthest main satellites of their world, they’re somewhat isolated from other large masses, and the escape velocity from the surface of each of those worlds is still fairly substantial.”
From Mercury out to Neptune, most of the worlds in the Solar System have moons, with a hitherto discovered population of around 200 known ones. Yet despite all of it, we don’t know of a single instance of a moon that has its own natural satellite: a moon with a moon of its own. But that doesn’t mean it couldn’t exist! Within our own Solar System, we have many known candidates for moons that would be good fits for having their own moons, including worlds around each of the gas giants. But given the configuration of everything we see, what’s theoretically possible may not be physically realistic. There are a great many reasons why moons-with-moons wouldn’t be stable over the long term, and 4.5 billion years is certainly a long time to require survival!
We haven’t found any so far, but it’s possible for moons to have moons of their own. What would it take to make them real? Come find out!