Artist impression of ripples in the fabric of space-time. Some boring scientists call them gravitational waves. (Credit: R. Hurt/Caltech-JPL)

WTF, Universe? The fabric of your space-time is rippling!

Bridget Falck
Jun 21, 2017 · 6 min read

It is one of the joys of my life that I get to say “ripples in the fabric of space-time” when talking about the Universe. “The fabric of space-time” is already pretty fun to say, and now we have conclusive proof that it ripples, quite literally like a wave!

In this post, I will try to explain what the fuck space-time is, how it ripples, and why this makes astrophysicists as excited as a kid in a candy shop experiencing a sugar high that reveals a fundamental truth about existence. If you are just joining us, you might want to read about how we can see the Universe’s past, how the observable Universe is larger than you can possibly imagine, and how the infinity of existence is getting bigger.

Literal waves in the metaphorical fabric of literal space-time

What is space-time? We could say that it is the combination of the 3 dimensions of space and the 1 dimension of time into a 4-dimensional continuum, and be done. That’s it; call it a day and go for a beer. But to go beyond a simple definition of space-time, we’re gonna have to enter the realm of Einstein’s “special relativity” theory, which explores the logical consequences of having a constant speed of light.

Let’s pretend you are on a moving train and you throw a rock in the same direction. Maybe your arch-nemesis is there, stupidly sitting in front of a moving train, but you certainly can’t let the train fight your fights for you. Let’s also pretend there isn’t any wind resistance, because physics works best in ideal, unrealistic circumstances. You might expect that the total speed of the rock will be the speed of the train + the speed of your throw, which turns out to be basically true if both speeds are substantially less than the speed of light.

But what if that is too slow? Your arch-nemesis is flipping you off or talking about your mother, so you shoot your laser, which travels at the speed of light (because it is light), to kill her faster. But the speed of light must be constant and can never be exceeded, so your laser doesn’t get a boost from the moving train. You burn your arch-nemesis into a pile of ash and ponder special relativity as the train rumbles along, past gently-rolling hills.

Without going into the math, special relativity and the constancy of the speed of light have some super-odd consequences:

  • Time moves slower for things moving with respect to an observer “at rest.” The passage of time is relative.
  • Lengths of moving objects get shorter (along the direction of motion) as measured by an observer “at rest.” Distances are relative.
  • The question of whether two events are simultaneous depends on the speed of the reference frame in which these events are observed. It’s all relative, dude.

Wound up in all of this weirdness is the fact that you can’t separate time from space in order for any of this to work: the lengths of objects and the time between two events depend on relative velocities — on distance divided by time. An interval of space-time is invariant, but an interval of distance or of time is not.

Space-time diagram of light (A and B) and something slower than light (C ), if you’re into that sort of thing.

So, space and time are intertwined, and the combination is very cleverly referred to as space-time. What about this “fabric” everyone keeps talking about?

Once you add gravity to space-time you get Einstein’s “general relativity” theory, which is EVEN WEIRDER than the special version. Most importantly, space-time can have curvature. Objects with mass distort and bend space-time, and matter tends to move along the straightest possible line in this curved space-time.

For example, black holes have so much mass in such a small space that the curvature goes to infinity — they literally punch a hole in the fabric of space-time! Or not. This could be complete bullshit because we don’t have a quantum theory of gravity yet… hopefully you can live with uncertainty about whether the Universe is filled with literal holes, cause we don’t fucking know.

Other massive things, like galaxy clusters, don’t punch any potential holes, but they distort space-time so much that the images of galaxies get warped as their light travels along this extremely curved space-time to reach our telescopes. We call this gravitational lensing because gravity, represented as the curvature of space-time, acts like a lens.

The light from distant galaxies gets stretched into arcs as it travels through curved space-time. (Credit: NASA/ESA)

As I stressed last time, I want to be clear that space-time is not a thing. We talk about the “fabric of space-time” because it helps us to understand curvature by imagining a two-dimensional stretchy fabric bending and warping. But there is no real fabric. There is only the relationship between objects — intervals in space and in time.

And when you get two very massive objects spinning around each other and coming together to merge, these distortions in the metaphorical fabric of space-time propagate outward in a wave. The video below simulates what happens when two black holes merge together.

Scientists have been wrapping their heads around the idea of bending, warping, rippling space-time for decades. The “WTF are you talking about?” bumping around in your head right now continues to bump around in our heads. The only advantage scientists who study relativity have is complicated math, which means they can solve some equations and pretend that this translates into genuine understanding. (It doesn’t.)

The cool thing is, it is DEFINITELY REAL. We have seen the bending of light, and more recently, we have detected the ripples in the fabric of space-time directly! The LIGO experiment has recently detected the signal of merging binary black holes for the 3rd time. By shooting lasers for two-and-a-half miles along two arms of an interferometer and measuring a change in the length of the arms that is smaller than one-ten-thousandth the diameter of a proton (1/10¹⁹ meters), we can observe these otherwise invisible events, in real time, and measure the black holes’ masses directly via waves of gravity instead of waves of light, which is SUPER FUCKING COOL.

All of this coolness was enabled primarily by the National Science Foundation, together with other funding sources around the world and a shared belief in the value of decades of hard work with no short-term payoff purely for the chance of learning something about the world. These funding sources and values are not guaranteed. We can now point to proof that the fabric of space-time ripples like a wave, but the Universe doesn’t care whether we learn these things about it. It is up to us to collectively decide whether we value scientific knowledge so we can discover even more things about our Universe that make no fucking sense.

Luckily, most of what we learned about the Universe in the last century, and even the last decade, is weird as fuck, so we have no shortage of mysteries to contemplate. Now that we are all experts in special and general relativity theory, in the next post we will explore the global space-time curvature of the entire Universe.

Bridget Falck

Written by

Professional astrophysicist. Amateur philosopher. Human.

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