Category: relativity

This Is How Distant Galaxies Recede Away From Us At Faster-Than-Light Speeds

“All the galaxies in the Universe beyond a certain distance appear to recede from us at speeds faster than light. Even if we emitted a photon today, at the speed of light, it will never reach any galaxies beyond that specific distance. It means any events that occur today in those galaxies will not ever be observable by us. However, it’s not because the galaxies themselves move faster than light, but rather because the fabric of space itself is expanding.

In the 7 minutes it took you to read this article, the Universe has expanded sufficiently so that another 15,000,000 stars have crossed that critical distance threshold, becoming forever unreachable. They only appear to move faster than light if we insist on a purely special relativistic explanation of redshift, a foolish path to take in an era where general relativity is well-confirmed. But it leads to an even more uncomfortable conclusion: of the 2 trillion galaxies contained within our observable Universe, only 3% of them are presently reachable, even at the speed of light.

If we care to explore the maximum amount of Universe possible, we cannot afford to delay. With each passing moment, another chance for encountering intelligent life forever slips beyond our grasp.”

If you look at a galaxy, chances are you’ll see that it appears to be receding away from us, as its light is redshifted. The more distant you look, the greater the redshift, and hence, the faster the implied recession speed. But this interpretation runs into problems very quickly: by the time you’re looking at galaxies more than 13-to-15 billion light-years away, they start to appear as though they’re receding faster than the speed of light!

Impossible, you say? Sure, if you only consider special relativity. If you insist on general relativity, it all falls into place. Here’s how.

LIGO’s Lasers Can See Gravitational Waves, Even Though The Waves Stretch The Light Itself

“But this is where the puzzle comes in: if space itself is what’s expanding or compressing, then shouldn’t the light moving through the detectors be expanding or compressing too? And if that’s the case, shouldn’t the light travel the same number of wavelengths through the detector as it would have if the gravitational wave had never existed?

This seems like a real problem. Light is a wave, and what defines any individual photon is its frequency, which in turn defines both its wavelength (in a vacuum) and its energy. Light redshifts or blueshifts as the space it’s occupying stretches (for red) or contracts (for blue), but once the wave has finished passing through, the light returns to the same wavelength it was back when space was restored to its original state.

It seems as though light should produce the same interference pattern, regardless of gravitational waves.”

Have you ever thought about how gravitational wave detectors work? By passing light down two mutually perpendicular arms, reflecting them back and reconstructing an interference pattern, we can detect a passing wave by how it changes the arm-lengths of the light. But the light itself also gets compressed and expanded, and shouldn’t those effects cancel out?

Clearly, LIGO, Virgo and KAGRA all work, as many detected events bear out. But have you ever thought about how? Come get the answer today!

This Is Why The Speed Of Gravity Must Equal The Speed Of Light

“In order to get different observers to agree on how gravitation works, there can be no such thing as absolute space, absolute time, or a signal that propagates at infinite speed. Instead, space and time must both be relative for different observers, and signals can only propagate at speeds that exactly equal the speed of light (if the propagating particle is massless) or at speeds that are blow the speed of light (if the particle has mass).

In order for this to work out, though, there has to be an additional effect to cancel out the problem of a non-zero tangential acceleration, which is induced by a finite speed of gravity. This phenomenon, known as gravitational aberration, is almost exactly cancelled by the fact that General Relativity also has velocity-dependent interactions. As the Earth moves through space, for example, it feels the force from the Sun change as it changes its position, the same way a boat traveling through the ocean will come down in a different position as it gets lifted up and lowered again by a passing wave.”

According to Newtonian gravity, space and time are absolute, and the gravitational force between any two objects is defined by the distance between them. In relativity, though, different observers don’t agree on distances, which means they won’t agree on forces, accelerations, or other properties of motion from a relativistic perspective. And yet, if you use Newton’s law of gravitation to compute the orbits of Solar System objects, it gets the right answer. If you instead tried to use Newton’s laws but allowed planets to be attracted to where the Sun was in the past, you’d get the wrong answer! Does this mean that the speed of gravity is infinite?

Hardly, but you have to dive deep into relativity to understand what else is up. Thankfully, we’ve done this, and you can enjoy the answer for yourself! Here’s why the speed of gravity must equal the speed of light.

This One Puzzle Brought Physicists From Special To General Relativity

“With an average speed of 47.36 km/s, Mercury moves very slow compared to the speed of light: at 0.0158% the speed of light in a vacuum. However, it moves at this speed relentlessly, every moment of every day of every year of every century. While the effects of Special Relativity might be small on typical experimental timescales, we’ve been watching the planets move for centuries.

Einstein never thought about this; he never thought to calculate the Special Relativistic effects of Mercury’s rapid motion around the Sun, and how that might impact the precession of its perihelion. But another contemporary scientist, Henri Poincaré, decided to do the calculation for himself. When he factored in length contraction and time dilation both, he found that it led to approximately another 7-to-10 arc-seconds of orbital precession per century.“

Special Relativity was easy enough to discover in a certain sense: the Lorentz transformations, Maxwell’s equations, and the Michelson-Morley experiments had been around for decades before Einstein came along. But to go from Special Relativity to General Relativity, incorporating gravitation and the equations governing motion into the same framework, was a herculean effort. However, it was the simple identification and investigation of one puzzle, the orbit of Mercury around the Sun, that brought about Einstein’s new theory of gravity: General Relativity.

What were the key steps, and how did they help revolutionize our view of the Universe? The history is rich and spectacular, and holds a lesson for those on the frontiers of physics today.

This One Thought Experiment Shows Why Special Relativity Isn’t The Full Story

“In Einstein’s initial formulation of General Relativity way back in 1916, he mentioned the gravitational redshift (and blueshift) of light as a necessary consequence of his new theory, and the third classical test, after the precession of Mercury’s perihelion (already known at the time) and the deflection of starlight by a gravitational source (discovered during a total solar eclipse in 1919).

Although a thought experiment is an extremely powerful tool, practical experiments didn’t catch up until 1959, where the Pound-Rebka experiment finally measured a gravitational redshift/blueshift directly. Yet just by invoking the idea that energy must be conserved, and a basic understanding of particle physics and gravitational fields, we can learn that light must change its frequency in a gravitational field.”

If a photon flies through space towards Earth, it must gain energy and become bluer in nature as it approaches Earth’s surface. This idea, of a gravitational redshift or blueshift, dictates how a photon must change in energy in the presence of a gravitational field. Yet this effect, which only exists in General Relativity, could have been predicted as soon as special relativity was discovered by one simple thought experiment: to consider a particle-antiparticle pair dropped from high above the surface of the Earth, but to let the annihilation occur at varying locations.

If you considered that, you’d immediately realize how special relativity was insufficient for describing our Universe! Come learn how to reason it out for yourself today!


This Is Why Black Holes Must Spin At Almost The Speed Of Light

“Realistically, we can’t measure the frame-dragging of space itself. But we can measure the frame-dragging effects on matter that exist within that space, and for black holes, that means looking at the accretion disks and accretion flows around these black holes. Perhaps paradoxically, the smallest mass black holes, which have the smallest event horizons, actually have the largest amounts of spatial curvature near their horizons.

You might think, therefore, that they’d make the best laboratories for testing these frame dragging effects. But nature surprised us on that front: a supermassive black hole at the center of galaxy NGC 1365 has had the radiation emitted from the volume outside of it detected and measured, revealing its speed. Even at these large distances, the material spins at 84% the speed of light. If you insist that angular momentum be conserved, it couldn’t have turned out any other way.”

Have you ever wondered how black holes, ranging from a few times our Sun’s mass up to billions of times as massive, can spin so rapidly? Most black holes, as far as we can tell, are spinning very close to the speed of light: the ultimate speed limit of the Universe. Yet most stars, like our Sun, rotate extremely slowly: just once over a period of many days (or even longer).

So how does a slowly-rotating star, which goes supernova and forms a black hole, give rise to an object spinning near the cosmic speed limit? Find out today.

How To Prove Einstein’s Relativity In The Palm Of Your Hand

“If you ever doubted relativity, it’s hard to fault you: the theory itself seems so counterintuitive, and its effects are thoroughly outside the realm of our everyday experience. But there is an experimental test you can perform right at home, cheaply and with just a single day’s efforts, that allow you see the effects for yourself.

You can build a cloud chamber, and if you do, you will see those muons. If you installed a magnetic field, you’d see those muon tracks curve according to their charge-to-mass ratio: you’d immediately know they weren’t electrons. On rare occasion, you’d even see a muon decaying in mid-air. And, finally, if you measured their energies, you’d find that they were moving ultra-relativistically, at 99.999%+ the speed of light. If not for relativity, you wouldn’t see a single muon at all.

Time dilation and length contraction are real, and the fact that muons survive, from cosmic ray showers all the way down to Earth, prove it beyond a shadow of a doubt.”

Hold out the palm of your hand and turn it upwards to face the sky. Congratulations: right now, approximately 1 muon per second is passing through your hand! You might not be a very sensitive particle detector, but you can build one, in the form of a cloud chamber, for less than $100 with off-the-shelf materials. If you did, you’d be able to see these muons individually. With a little extra work, and a bit of physics, you can prove to yourself that without Einstein’s relativity, these muons wouldn’t exist!

And yet, they’re real, you can observe them yourself, and they can help you prove the truth of relativity itself. Come find out how to do it for yourself!

Ask Ethan: What’s It Like When You Fall Into A Black Hole?

“[W]hat is it like to be/fall inside a rotating black hole? This is not observable, but calculable… I have talked with various people who have done these calculations, but I am getting old and keep forgetting things.”

I get a lot of questions that people submit for Ask Ethan, but only rarely do they come to me from other scientists who tower above me in the field. This week’s question, from Event Horizon Telescope scientist extraordinaire Heino Falcke, asks me to help him visualize what it would look like if you fell into a black hole. Not just any black hole, mind you, but a realistic, rotating black hole. There’s really only one person on Earth who understands this well enough: Andrew Hamilton, who has devoted the last 15 years of his life to figuring out what it looks like and what it means when this actually happens.

So what did I do? I went and met Andrew, interviewed him, read his papers, and used his simulations to give everyone the best answer I could. I hope you love it, and I hope (even moreso) that I got it right!

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!