Category: general relativity

Ask Ethan: Can We Really Get A Universe From N…

Ask Ethan: Can We Really Get A Universe From Nothing?

“One concept bothers me. Perhaps you can help. I see it in used many places, but never really explained. “A universe from Nothing” and the concept of negative gravity. As I learned my Newtonian physics, you could put the zero point of the gravitational potential anywhere, only differences mattered. However Newtonian physics never deals with situations where matter is created… Can you help solidify this for me, preferably on [a] conceptual level, maybe with a little calculation detail?” 

You’ve very likely heard two counterintuitive things about the Universe before. One of them is that the Universe arose from nothing, and this defies our intuition about how it’s impossible to get something from nothing. The second is that we have four fundamental forces in the Universe: gravity, electromagnetism, and the strong and weak nuclear forces. So how, then, do we account for the fact that the Universe’s expansion is accelerating? Isn’t this clearly evidence for a fifth force, one with negative gravity?

Guess what? These two counterintuitive aspects of reality are related. If you understand them both, you’re one step closer to making sense of the Universe.

Ask Ethan: Why Do Gravitational Waves Travel E…

Ask Ethan: Why Do Gravitational Waves Travel Exactly At The Speed Of Light?

We know that the speed of electromagnetic radiation can be derived from Maxwell’s equation[s] in a vacuum. What equations (similar to Maxwell’s – perhaps?) offer a mathematical proof that Gravity Waves must travel [at the] speed of light?

If you were to somehow make the Sun disappear, you would still see its emitted light for 8 minutes and 20 seconds: the amount of time it takes light to travel from the Sun to the Earth across 150,000,000 km of space. But what about gravitation? Would the Earth continue to orbit where the Sun was for that same 8 minutes and 20 seconds, or would it fly off in a straight line immediately?

There are two ways to look at this puzzle: theoretically and experimentally/observationally. From a theoretical point of view, this represents one of the most profound differences from Newton’s gravitation to Einstein’s, and demonstrates what a revolutionary leap General Relativity was. Observationally, we only had indirect measurements until 2017, where we determined the speed of gravity and the speed of light were equal to 15 significant digits!

Gravitational waves do travel at the speed of light, which equals the speed of gravity to a better precision than ever. Here’s how we know.

What Is The Smallest Possible Distance In The …

What Is The Smallest Possible Distance In The Universe?

“At present, there is no way to predict what’s going to happen on distance scales that are smaller than about 10-35 meters, nor on timescales that are smaller than about 10-43 seconds. These values are set by the fundamental constants that govern our Universe. In the context of General Relativity and quantum physics, we can go no farther than these limits without getting nonsense out of our equations in return for our troubles.

It may yet be the case that a quantum theory of gravity will reveal properties of our Universe beyond these limits, or that some fundamental paradigm shifts concerning the nature of space and time could show us a new path forward. If we base our calculations on what we know today, however, there’s no way to go below the Planck scale in terms of distance or time. There may be a revolution coming on this front, but the signposts have yet to show us where it will occur.”

If you went down to smaller and smaller distance scales, you might imagine that you’ll start to see the Universe more clearly and in higher resolution. You’ll be able to hone in on the fundamental properties of nature, and glean more information the deeper you go. This is true, but only up to a point. Beyond that, you start running into the inescapable quantum rules that govern the Universe, and that means there’s a fundamental scale at which our best laws of physics cannot be trusted any longer.

That scale is the Planck scale, and for distances, it corresponds to about 10^-35 meters. It really is a problem for physics, and it’s high time you understood why.

This Is How, 100 Years Ago, A Solar Eclipse Pr…

This Is How, 100 Years Ago, A Solar Eclipse Proved Einstein Right And Newton Wrong

“Today, May 29, 2019, marks the 100th anniversary of the day, the event, and the expedition that validated Einstein’s General Relativity as humanity’s leading theory of how gravitation works. Newton’s laws are still incredibly useful, but only as an approximation to Einstein’s theory with a limited range of validity.

General Relativity, meanwhile, has gone on to successfully predict everything from frame-dragging to gravitational waves, and still has yet to encounter an observation that conflicts with its predictions. Today marks a full century of General Relativity’s demonstrated validity, with not even a hint of how it might someday break down. Although we certainly don’t know everything about the Universe, including what a quantum theory of gravity might actually be like, today is a day for celebrating what we do know. 100 years after our first critical test, our best theory of gravity still shows no signs of slowing down.”

Happy 100th anniversary to the critical observations that demonstrated Einstein was right and Newton was wrong when it came to gravitation. There’s an incredible story to how this all occurred, and you won’t want to miss a drop of what it all means.

It took one of the longest solar eclipses in modern history for us to get there, but oh, was it worth it. Come find out why and how for yourself.

This Is Why Einstein Knew That Gravity Must Be…

This Is Why Einstein Knew That Gravity Must Bend Light

“This is the basis of Einstein’s equivalence principle: the idea that an observer cannot distinguish between an acceleration caused by gravitational or inertial (thrust) effects. In the extreme case, jumping off of a building, in the absence of air resistance, would feel the same as being completely weightless.

The astronauts aboard the International Space Station, for example, experience complete weightlessness, even though the Earth is accelerating them towards its center with about 90% of the force we experience here on its surface. Einstein later referred to this realization, which struck him in 1911, as his happiest thought. It was this idea that would lead him, after four years of further development, to publish the General theory of Relativity.”

Imagine that you were inside a closed-off elevator, and that light came in through a tiny hole from the outside. If your elevator were accelerating, you’d see that light follow a curved, bent trajectory, as your changing motion would cause the path that light took to appear that way. But from inside the elevator, you have no way of knowing whether that acceleration was due to thrust, which is an inertial effect, or gravitation. An elevator accelerating at a constant 9.8 m/s^2 from a firing thruster would be indistinguishable from one stationary on Earth’s surface, where the acceleration due to gravity is 9.8 m/s^2. If they’re indistinguishable, then gravity must bend light, the same way any other acceleration does.

This is why, nearly a century ago, Einstein never doubted what the results of the experiment that tested his theory for the first time would be. Come learn why Einstein knew that gravity must bend light!

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!