Category: speed of light

Ask Ethan: Can Gamma-Ray Jets Really Travel Faster Than The Speed Of Light?

“What gives? Is it really possible for gamma-rays to exceed the speed of light and thereby “reverse” time? Is the time reversal just a theoretical claim that allows these hypothetical super-light speed particles to conform with Relativity or is there empirical evidence of this phenomenon?”

Very recently, a paper came out claiming that gamma-ray bursts, and the jets that give them off, can travel faster than the speed of light. If that sounds too fantastic to be true, there’s a reason for that: particles can travel faster than light, but only in a medium, where the speed of light is less than the speed of light in a vacuum. Gamma-ray bursts, when they occur, exhibit a strange property: the signal is mostly a large peak, but when you subtract that peak out, parts of the residual signal are symmetric: if you flip it, part of it going forwards in time is identical to the remainder going backwards in time.

Sound weird? Well, we’re just getting started! Come find out the true story behind this fascinating phenomenon, and what just became our best explanation of what makes it so!

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.

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.

How Far Could A Human Travel In A Constantly-Accelerating Rocket Ship?

“Imagine that we could constantly accelerate at the same rate as Earth’s gravitational pull, 9.8 m/s2, indefinitely. While you’d initially speed up, you’ll rapidly approach the speed of light.

Owing to Einstein’s Special Relativity, time will dilate and lengths will contract. As you continue to accelerate, the distances and travel times to faraway destinations will plummet.

At the halfway mark, simply reverse your thrust to accelerate in the opposite direction for the remaining journey.

If you wanted to travel to a star that was 100 light-years away, you might think it would take you at least 100 years to get there. That might be true from the perspective of someone who remains on Earth, but for an astronaut who journeyed there at close to the speed of light, Einstein’s Special Relativity tells you that it would take far less than a century of travel. In fact, if you could accelerate at a constant rate, you could pretty much reach anywhere you wanted within 15 billion light-years of us within a human lifetime.

I even went and did the math for you here. Don’t be afraid to see how far a human could travel if we had the dream technology to get us there!

How Far Could A Human Travel In A Constantly-Accelerating Rocket Ship?

“Imagine that we could constantly accelerate at the same rate as Earth’s gravitational pull, 9.8 m/s2, indefinitely. While you’d initially speed up, you’ll rapidly approach the speed of light.

Owing to Einstein’s Special Relativity, time will dilate and lengths will contract. As you continue to accelerate, the distances and travel times to faraway destinations will plummet.

At the halfway mark, simply reverse your thrust to accelerate in the opposite direction for the remaining journey.

If you wanted to travel to a star that was 100 light-years away, you might think it would take you at least 100 years to get there. That might be true from the perspective of someone who remains on Earth, but for an astronaut who journeyed there at close to the speed of light, Einstein’s Special Relativity tells you that it would take far less than a century of travel. In fact, if you could accelerate at a constant rate, you could pretty much reach anywhere you wanted within 15 billion light-years of us within a human lifetime.

I even went and did the math for you here. Don’t be afraid to see how far a human could travel if we had the dream technology to get us there!

Ask Ethan: How Does A Photon Experience The Universe?

“Relativity says all inertial frames of reference are equally valid and true. From a photon’s point of view the entire cosmos is flattened into a two-dimensional timeless plane. Imagine I place an apple on my desk, then a while later replace it with a banana. How does the photon perceive my desk to be, when it’s all flattened into a plane without any sense of time?”

At rest, everything looks like you expect: clocks run at the same rate everywhere you look, distances are exactly as they appear, and material objects possess the color you know them to intrinsically possess. Close to the speed of light, however, all of that changes. Clocks run slower as objects move closer to the speed of light relative to you. Distances appear contracted along the direction of relative motion, including for physical objects and the fields they generate. And colors appear either redshifted or blueshifted, depending on how quickly an object moves either away from your or towards you, respectively. These effects get more and more severe the closer objects move, relative to you, to the speed of light.

But what if you reached the speed of light? What would the Universe look like from a photon’s (or any massless particle’s) point of view? The answer is most definitely not what you expect! Find out on this edition of Ask Ethan.

The Universe Has A Speed Limit, And It Isn’t The Speed Of Light

“We believe that every charged particle in the cosmos — every cosmic ray, every proton, every atomic nucleus — should limited by this speed. Not just the speed of light, but a little bit lower, thanks to the leftover glow from the Big Bang and the particles in the intergalactic medium. If we see anything that’s at a higher energy, then it either means:

1. particles at high energies might be playing by different rules than the ones we presently think they do,
2. they are being produced much closer than we think they are: within our own Local Group or Milky Way, rather than these distant, extragalactic black holes,
3. or they’re not protons at all, but composite nuclei.”

If you were to try and travel as close to the speed of light as possible, you’d never get there because of Einstein’s relativity and the fact that you have mass. But even if you pumped an arbitrary amount of energy into you, you still wouldn’t get arbitrarily close to the speed of light. Instead, you’d find that there was a barrier or cutoff just a little bit below the speed of light: about 80 femtometers-per-second below the ultimate cosmic speed limit. That’s because the leftover glow from the Big Bang, the cosmic microwave background, exists no matter where you go, and prevents you from going any faster. Even if you beat that speed, it will knock you back down below it in short order.

There’s a speed limit for matter in the Universe, and it isn’t the speed of light. Come find out the details of why today!

How Come Cosmic Inflation Doesn’t Break The Speed Of Light?

“In an inflationary Universe, any two particles, beyond a tiny fraction of a second, will see the other one recede from them at speeds appearing to be faster-than-light. But the reason for this isn’t because the particles themselves are moving, but rather because the space between them is expanding. Once the particles are no longer at the same location in both space and time, they can start to experience the general relativistic effects of an expanding Universe, which — during inflation — quickly dominates the special relativistic effects of their individual motions. It’s only when we forget about general relativity and the expansion of space, and instead attribute the entirety of a distant particle’s motion to special relativity, that we trick ourselves into believing it travels faster-than-light. The Universe itself, however, is not static. Realizing that is easy. Understanding how that works is the hard part.”

It’s true that nothing can move faster than the cosmic speed limit, the speed of light, and that no two particles can move faster than light relative to one another. So how, then, do you explain the fact that during inflation, two particles that begin a subatomic distance away from one another are, after just a tiny fraction of a second, are then billions of light years apart? The answer is because special relativity only applies, strictly, to particles that occupy the same location as one another in both space and time. If they’re separated, then the Universe is under no obligation to be static, and space is free to expand and/or contract. You cannot figure your apparent motion with special relativity alone, but need to factor in the effects of general relativity as well. And that’s where things get really weird.

If you can understand it, however, the notion of how objects appear to recede faster than light suddenly starts to make sense. Come learn how inflation doesn’t break the speed of light after all!

Five Discoveries In Fundamental Physics That Came As Total Surprises

“It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

It’s often said that advanced in physics aren’t met with “eureka!” but rather with “that’s funny,” but the truth is even stranger sometimes. Rather than the scientific method of: hypothesis, method, experiment, results, conclusion, revise, repeat, etc., many times throughout history, it’s been a series of surprise observations that have often led to our greatest leaps forward. When the speed of light was discovered not to differ when you moved with or against it, it was so revolutionary it was the only Nobel Prize ever awarded for a null result. When the gold foil experiment resulted in high-energy recoils, it surprised Rutherford so thoroughly it was the most incredible thing to ever happen to him in his life. The leftover glow from the Big Bang was discovered quite by accident; the neutrino was a crazy hypothesis that many abandoned; and the discovery of the muon, perhaps the most unexpected particle of all, literally was met with a cry of, “who ordered that?” from Nobel Laureate I.I. Rabi.

These five discoveries changed the course of physics forever, but they came as total surprises to practically everyone. Sometimes, the answer is in the place you least expect.

Comments of the Week #171: From light’s speed to proving Einstein right

“Science, just to be extremely clear, does not rely on one experiment to settle the matter, and then never perform the experiment again. No; we are constantly checking our results, gathering more data to improved precision, and looking for flaws in our predictions at the 10% level, then 1%, then 0.1%, then 0.01%, etc.

The story of scientific investigation is a story of ever-increasing precision and ever decreasing uncertainty, and one that I value and will keep telling, no matter what some (or many, or even most, sometimes) of the commenters here or elsewhere say. The scientific truth is too important, even if (and when) public opinion is against it. It’s why I’m here, and it’s what I’ve been doing — somewhat successfully, mind you — for over nine years now. In fact, when January rolls around, that will mark 10 years since the inception of Starts With A Bang. That we’re all here, thinking about the Universe and how it all works, is something worth celebrating, even when it’s difficult.”

Some of you caught me at the Star Trek Las Vegas convention this year, where thousands of Star Trek fans gathered and a tremendous time was had by practically everyone. But even while away and traveling, the science doesn’t stop, and I couldn’t help myself from sharing another dose of bonus science with you all.

Check out our comments of the week and enjoy!