Category: general relativity

Ask Ethan: Why Don’t Gravitational Waves…

Ask Ethan: Why Don’t Gravitational Waves Get Weaker Like The Gravitational Force Does?

“You have stated:
1) The strength of gravity varies with the square of the distance.
2) The strength of gravity waves, as detected by LIGO, varies directly with the distance.
So the question is, how can those two be the same thing?”

Here’s a puzzling fact for you: if you get ten times as far away from a source of gravitational waves, how much less would you expect the signal to be in your gravitational wave detector? For light, brightness falls off as the inverse of the distance squared: it would be 1/100th as bright. For the gravitational force, it also falls off as the inverse of distance squared: 1/100th the force. But for gravitational waves, the signal strength only drops as the inverse of the distance; the signal would be 1/10th the original strength.

Why is this? Believe it or not, it’s mandated by physics! Come find out the deep truth behind why on this special* edition of Ask Ethan!

(* – special because I had to derive this myself; nobody gives the full explanation anywhere I can find!)

Ask Ethan: What Happens When You Fall Into A B…

Ask Ethan: What Happens When You Fall Into A Black Hole?

“If you could only know the answer to one question about the Universe, what would you ask? What would you want to know more than anything else? As we get older, most of us lose sight of the things we wondered about as children, which is why I was delighted to get a message from Eric Erb about ten questions that his son, Tristan, brought home from his 2nd grade class. Two of the biggest mysteries of all, gravity and time, dominated his curiosity. After boiling it down, here’s what he wanted to know:

I asked him just now and he wanted two questions to be answered.
1. What happens when you fall into a black hole?
2. Why/how does gravity pull us?

Let’s start at the beginning, and make sure we get to it all.”

If you fell into a black hole, you’d first approach the event horizon from a great distance away, and the light from the Universe around you would start to distort. As you got closer, the event horizon would appear to grow in size faster than you might expect, and the distortions would grow larger in magnitude and color. Time would continue to run at the same, normal speed, and you might not notice crossing the event horizon, but the spaghettifying forces on your body sure would tell you something was up.

Where would you wind up? No one is sure, but the possibilities are both fascinating and terrifying. Find out what happens when you fall into a black hole today.

Ask Ethan: How Can We Measure The Curvature Of…

Ask Ethan: How Can We Measure The Curvature Of Spacetime?

“The Universe is not simply made of point masses, but of complex, intricate objects. If we ever hope to tease out the most sensitive signals of all and learn the details that elude us today, we need to become more precise than ever. Thanks to three-atom interferometry, we can, for the first time, directly measure the curvature of space.

Understanding the Earth’s interior better than ever is the first thing we’re going to gain, but that’s just the beginning. Scientific discovery isn’t the end of the game; it’s the starting point for new applications and novel technologies. Come back in a few years; you might be surprised at what becomes possible based on what we’re learning for the first time today.”

Go out and measure how an object falls: that gives you gravitational acceleration. Go out and measure how that falling is different between two locations identical in every way except at different elevations, and you’ll measure a gravitational gradient, sufficient for telling Einstein’s theory apart from Newton’s. But if you can measure the differences in gravitational acceleration between three locations at once, you can measure changes in that gradient, and come away with an understanding of spacetime curvature.

This technique took a full 100 years from when Einstein first published General Relativity until it was performed successfully, but we’ve now done it. Here’s what it means for us, our present, and our future.

Ask Ethan: Are Gravitational Waves Themselves …

Ask Ethan: Are Gravitational Waves Themselves Affected By Gravity?

“Are gravitational waves themselves subject to gravity? That is, if a gravitational wave were to pass by a galaxy cluster, would its form get distorted (even though the wave, itself, is a distortion of space-time)? One side of me says gravitational waves are a form of energy so therefore must be affected by gravity. The other side of me says “Nah – that just doesn’t make sense!"”

Think about the fabric of space itself. All the masses and forms of energy in the Universe cause space itself to curve, while the curved space itself alters the path along which any matter or form of energy will travel. Massless particles, like photons, are bent by the fabric of space itself. But what about gravitational waves? Are they also subject to this, or does gravitation lack a self-interaction that it would require for this to be possible?

For a very long time, this was a question that was theoretical only. But over the last three years, we’ve observed a slew of gravitational waves, allowing this idea to be tested for the first time.

What were the results? Gravitational waves are affected by gravity, in at least three different observable ways. Come find out how today!

Ask Ethan: How Do Massless Particles Experienc…

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…

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…

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&r…

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 Tim…

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 Ne…

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.