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!
Physicists Used Einstein’s Relativity To Successfully Predict A Supernova Explosion
“When the lens and a background source align in a particular fashion, quadruple images will result. With slightly different light-travel paths, the brightness and arrival time of each image is unique. In November 2014, a quadruply-lensed supernova was observed, showcasing exactly this type of alignment. Although a single galaxy caused the quadruple image, that galaxy was part of a huge galaxy cluster, exhibiting its own strong lensing effects. Elsewhere in the cluster, two additional images of the same galaxy also appear.”
We normally think of light traveling in a straight line, but that’s only true if your space is flat. In the real Universe, mass and matter not only exist, but clump together into massive structures like galaxies, quasars, and galaxy clusters. When a background source of light passes through these foreground masses, the light can get bent and distorted into multiple images that are magnified and arrive at slightly different times. If an event occurs in one such image, we can predict, based on General Relativity, cluster dynamics, and dark matter, when that event will appear in the other images.
In November 2014, we discovered a multiply-lensed supernova, and predicted where and when it would appear in the other images. Einstein and dark matter both win again!
This Is How We Will Successfully Image A Black Hole’s Event Horizon
“Normally, the resolution of your telescope is determined by two factors: the diameter of your telescope and the wavelength of light you’re using to view it. The number of wavelengths of light that fit across your dish determines the optimal angular diameter you can resolve. Yet if this were truly our limits, we’d never see a black hole at all. You’d need a telescope the diameter of the Earth to view even the closest ones in the radio, where black holes emit the strongest and most reliably.
But the trick of very-long baseline interferometry is to view extremely bright sources, simultaneously, from identical telescopes separated by large distances. While they only have the light-gathering power of the surface area of the individual dishes, they can, if a source is bright enough, resolve objects with the resolution of the entire baseline. For the Event Horizon Telescope, that baseline is the diameter of the Earth.”
The Event Horizon Telescope is one of the best examples of international collaboration, and its necessity, in answering questions that are too big for any one nation to do alone. Part of the reason for that is geography: if you want to get the highest-resolution information possible about the Universe, you need the longest-baseline of simultaneous observations that it’s possible to make. That means, if you want to go as hi-res as possible, using the full diameter of the Earth. From the Americas to Eurasia to Africa, Australia and even Antarctica, radio astronomers are all working together to create the first image of a black hole’s event horizon.
What does it look like? Is General Relativity correct? As soon as the Event Horizon Telescope team releases their first images, we’ll know. Come watch a live-blog of a talk from their team today, and get the answers as soon as we know them!
Ask Ethan: Is Spacetime Really A Fabric?
“I’d like somebody to finally acknowledge and admit that showing balls on a bed sheet doesn’t cut it as a picture of reality.”
Okay, I admit it: visualizing General Relativity as balls on a bedsheet doesn’t make a whole lot of sense. For one, if this is what gravity is supposed to be, what pulls the balls “down” onto the bedsheet? For another, if space is three dimensional, why are we talking about a 2D “fabric” of space? And for another, why do these lines curve away from the mass, rather than towards it?
It’s true: this visualization of General Relativity is highly flawed. But, believe it or not, all visualizations of General Relativity inherently have similar flaws. The reason is that space itself is not an observable thing! In Einstein’s theory, General Relativity provides the link between the matter and energy in the Universe, which determines the geometric curvature of spacetime, and how the rest of the matter and energy in the Universe moves in response to that. In this Universe, we can only measure matter and energy, not space itself. We can visualize it how we like, but all visualizations are inherently flawed.
Come get the story of how to make as much sense as possible out of the Universe we actually have.
Ask Ethan: Which Movies Get The Science Of Time Travel Right?
“I’m admittedly a fan of time-travel movies (however they explain it). What movie makes the best case for using this plot device accurately?”
Time travel has been a staple of fiction for centuries, as the notion of either traveling forward in time to explore the future or back in time to right a past wrong have been a part of humanity’s imaginings for perhaps always. But we have explicit laws and rules for traveling through time, and how our motion through space affects it. In General Relativity, the possibilities of wormholes and closed-timelike-curves arises, opening up a whole new set of avenues for success. From Bill & Ted’s excellent adventure to Idiocracy, from Harry Potter and the Prisoner of Azkaban to Interstellar, and from Back to the Future to Groundhog Day, the science of time travel is one of the most fascinating ones out there.
Which movies get it right, and which ones get it egregiously wrong? Find out what my evaluations are on this edition of Ask Ethan!
This Simple Thought Experiment Shows Why We Need Quantum Gravity
“The description that General Relativity puts forth — that of matter telling space how to curve, and curved space telling matter how to move — needs to be augmented to include an uncertain position that has a probability distribution to it. Whether gravity is quantized or not is still an unknown, and has everything to do with the outcome of such a hypothetical experiment. How an uncertain position translates into a gravitational field, exactly, remains an unsolved problem on the road to a full quantum theory of gravity. The principles that underlie quantum mechanics must be universal, but how those principles apply to gravity, and in particular to a particle passing through a double slit, is a great unknown of our time.”
Perhaps the greatest holy grail in theoretical physics is the quest for a quantum theory of gravity. For all the gravitational phenomena we’ve ever measured, observed, or subjected to a test, General Relativity has come through with predictions that match what we’ve seen exactly. For all the other physical phenomena in the Universe, the rules of quantum field theory and the Standard Model of particle physics match up perfectly. But what would happen if we tried to apply General Relativity to an inherently quantum phenomenon? In particular, what happens if we fire a single particle, like an electron, through a double slit? What happens to that particle’s gravitational field?
Believe it or not, measuring that (or something analogous to it) would tell us whether gravity is a fundamentally quantum force or not! Come learn why this is arguably the most important, first stop on the road to quantum gravity.
Einstein Wins Again! General Relativity Passes Its First Extragalactic Test
“For the first time, we’ve been able to perform a direct test of General Relativity outside of our Solar System and get solid, informative results. The ratio of the Newtonian potential to the curvature potential, which relativity demands be equal to one but where alternatives differ, confirms what General Relativity predicts. Large deviations from Einstein’s gravity, therefore, cannot happen on scales smaller than a few thousand light years, or for masses the scale of an individual galaxy. If you want to explain the accelerated expansion of the Universe, you can’t simply say you don’t like dark energy and throw Einstein’s gravity away. For the first time, if we want to modify Einstein’s gravity on galactic-or-larger scales, we have an important constraint to reckon with.”
For many of the greatest cosmic puzzles today, you can either add two new ingredients, dark matter and dark energy, or you can seek to modify Einstein’s theory of gravity. While Einstein’s General Relativity has been confirmed spectacularly under a wide variety of circumstances, the only robust tests that are independent of dark matter or dark energy assumptions occur on scales of the Solar System or smaller. That’s only for distances that are a tiny fraction of a light year, and for masses no bigger than the Sun, which should trouble you when you’re making inferences about galaxies, clusters, or the entire Universe! But thanks to a very fortunate galactic system
a strong gravitational lens that is only 500 million light years distant
we’ve been able to put Einstein’s theory of gravity to the test for galactic masses and distance scales in the thousands of light years.
Was there ever any doubt that Einstein would win again? Here’s what happened, and here’s what it means for alternative theories of gravity!
Are Space And Time Quantized? Maybe Not, Says Science
“Incredibly, there may actually be a way to test whether there is a smallest length scale or not. Three years before he died, physicist Jacob Bekenstein put forth a brilliant idea for an experiment where a single photon would pass through a crystal, causing it to move by a slight amount. Because photons can be tuned in energy (continuously) and crystals can be very massive compared to a photon’s momentum, it ought to be possible to detect whether the “steps” that the crystal moves in are discrete or continuous. With a low-enough energy photon, if space is quantized, the crystal would either move a single quantum step or not at all.”
When it comes to the Universe, everything that’s in it appears to be quantum. All the particles, radiation, and interactions we know of are quantized, and can be expressed in terms of discrete packets of energy. Not everything, however, goes in steps. Photons can take on any energy at all, not just a set of discrete values. Put an electron in a conducting band, and its position can take on a set of continuous (not discrete) values. And so then there’s the big question: what about space and time? Are they quantized? Are they discrete? Or might they be continuous, even if there’s a fundamental quantum theory of gravity.
Surprisingly, space and time don’t need to be discrete, but they might be! Here’s what the science has to say so far.
This Is Why Physicists Think String Theory Might Be Our ‘Theory Of Everything’
“String theory offers a path to quantum gravity, which few alternatives can truly match. If we make the judicious choices of “the math works out this way,” we can get both General Relativity and the Standard Model out of it. It’s the only idea, to date, that gives us this, and that’s why it’s so hotly pursued. No matter whether you tout string theory’s successes or failure, or how you feel about its lack of verifiable predictions, it will no doubt remain one of the most active areas of theoretical physics research. At its core, string theory stands out as the leading idea of a great many physicists’ dreams of an ultimate theory.”
You don’t have to be a fan of string theory to understand why it’s such a promising area of scientific research. One of the holy grails of physics is for a quantum theory of gravitation: that describes gravity on the same footing as the other three forces, in very strong fields and at very tiny distances. Surprisingly, by looking at analogies between gravity and field theories, replacing particles with strings might be the answer.
It’s an incredibly difficult concept to understand why this would be the case without a slew of advanced mathematics, but in 2015, the world’s leading string theorist, Ed Witten, tried. That is to say, he wrote a piece for other physicists entitled, “What every physicist should know about string theory.”
But what if you want to understand it and you’re not a physicist? Then you should read this.
This Is Why The Event Horizon Telescope Still Doesn’t Have An Image Of A Black Hole
“Of all the black holes visible from Earth, the largest is at the galactic center: 37 μas.
With a theoretical resolution of 15 μas, the EHT should resolve it.
Despite the incredible news that they’ve detected the black hole’s structure at the galactic center, however, there’s still no direct image.”
Last year, data from the South Pole Telescope, a 10-meter radio telescope located at the South Pole, was added to the Event Horizon Telescope team’s overall set of information. Here we are, though, half a year later, and we still don’t have a direct image of the event horizon for the galactic center’s black hole. There aren’t any problems; the issue is that we have to successfully calibrate and error-correct the data, and that takes time and care to get it right. Science isn’t about getting the answer in the time you have to get it; it’s about getting the right answer in the time it takes to get things right. From that point of view, there’s every reason this is worth waiting for.
The Event Horizon Telescope team is on the right track; here’s where we are right now in our quest to create the first image of a black hole’s event horizon!