The hunt for extra dimensions sounds like science fiction.

Fortunately, with the discovery of gravitational waves, we're now living in a science fiction future.

So how many dimensions are there?

[MUSIC PLAYING] We may have mentioned once or twice that the new era of gravitational wave astronomy is going to open new windows to the universe and unlock many mysteries.

When does all of that start to happen?

Oh, it has.

We're already making definitive statements about hypotheses that were previously untestable.

Today, on "Space Time Journal Club," I want to tell you about one in particular described in a new paper, "Limits on the Number of Spacetime Dimensions from GW170817," by Pardoa, Fishbachb, Holzb, and Spergela.

The key to this breakthrough was the gravitational wave event observed in August of 2017, GW170817.

A pair of neutron stars spiralled together and merged.

These superdense remnants of dead stars churned the fabric of space and time in their death spiral.

And the LIGO and Virgo gravitational wave observatories detected the resulting ripples.

Unlike merging black holes, which are invisible, merging neutron stars explode spectacularly.

The resulting kilonova is first observed in gravitational waves and then as a gamma ray burst.

In GW170817, the flash of gamma radiation arrived 1.7 seconds after the gravitational waves.

This was followed by a glow across the electromagnetic spectrum and, ultimately, with the discovery of the distant galaxy in which the explosion happened.

Among other things, this optical identification gave a completely independent measurement of the distance traveled by the gravitational waves.

This allowed us to make some really important conclusions about how gravity travels through space.

In the case of today's paper, it allows us to measure how many dimensions that space actually has.

Yeah, that's actually a serious question.

We think of space as three-dimensional.

Add one dimension of time to give us 4D space-time, which we'll also refer to as 3-plus-1-dimensional space-time.

But adding the extra spatial dimensions beyond the usual three could actually explain a lot, from the difference between gravity and the other forces of nature to the nature of dark energy.

But before we get all hyper-dimensional, let's think a bit more about 3 plus 1D space-time and how gravity, light, and matter behave there.

Imagine a pulse of light traveling from some distant source.

We can think of light rays spreading up evenly over an expanding spherical shell.

If we see that pulse, it means our eye or our telescope intercepts some of those light rays.

The brightness of the pulse is determined by how many rays we intercept.

So as this shell expands, the light rays become more spread out.

Intensity drops proportional to the surface area of the shell, which is proportional to the square of its radius, the square of the distance to the source.

This is the famous inverse square law.

But what if we, instead, lived in 2D space?

Then the same pulse would spread out over an expanding circle, not a sphere.

It would diminish in intensity proportional to the circumference of that circle, and also proportional to the radius-- so the distance to the source, not the distance squared.

The way pulses fade in brightness depends on the number of dimensions, typically proportional to 1 over the distance to the power of the number of dimensions minus 1.

So in 4-plus-1-dimensional space-time, brightness should drop off more quickly than in 3D space.

This relationship also applies to the force felt in a gravitational field.

In our universe, gravity appears to diminish according to the inverse square law, as reflected in Newton's law of universal gravitation.

We do see slight deviations in very strong gravitational fields, like close to the sun.

But even there, Einstein's general relativity describes gravity perfectly with three spatial dimensions.

In general, general relativity in 3 plus 1 space-time does a great job at describing gravity in the large-scale universe.

But there are some things about this version of gravity that seem peculiar-- for example, its pathetic strength.

While the electromagnetic strong and weak forces are all in the same ballpark in terms of strength, gravity is vastly weaker, 10 to the power of 32 times weaker, than even the weak nuclear force.

The only reason we see so much gravity is that its range is infinite-- and unlike the nuclear forces.

And it doesn't cancel out, like the electromagnetic force.

This mismatch in strength might be because gravity is really fundamentally different to the other forces.

But that idea makes some physicists sad.

Many would like to find a "theory of everything" which merges the forces of nature into the same 眉ber force.

That means gravity has to look just like the other forces at very high energies.

It needs to be intrinsically strong, but then become weakened in the low-energy, large-scale regime of the familiar universe.

One fun way to do that is to throw in an extra spatial dimension.

If you recall, intensity drops off more quickly the more dimensions you have.

So you drain gravity into an extra dimension.

But you restrict all the other stuff in the universe-- matter, radiation, astronomers-- to only three spatial dimensions.

All of that stuff will behave relatively normally, while gravity is weakened.

Let's get a little bit more technical.

There are these theoretical objects called branes.

We can think of them as geometrical structures of potentially any number of dimensions on which the quantum field and their corresponding particles can live.

They're used in string theory, where they typically have a large number of dimensions.

11 is popular.

But in string theory, all but three spatial dimensions of the brane are inaccessible.

They're finite and coiled up on themselves, compactified, allowing us to cram them into three spatial dimensions.

But you can also flip this idea around.

You can imagine a three-dimensional brane, a 3-brane, embedded in a space-time with four spatial dimensions, where the extra dimension of space is extended rather than compact.

Most of the stuff in such a universe, including all of the fundamental forces besides gravity, would be restricted to the 3-brane.

Tune your theory just right, and you get normal physics for matter and radiation in three spatial dimensions-- for example, the usual inverse square law for light.

On some spatial scales, you even get the inverse square law for gravity.

But on other spatial scales, gravity can behave very differently.

If gravity spreads out in four dimensions rather than three, then it should become much weaker.

This can be used to explain the general weakness of the gravitational force on all but the tiniest scales.

It can also be used to explain another mysterious phenomenon, dark energy.

This is something we've gone into in great depth.

But in short, the expansion of the universe seems to be accelerating.

This is usually thought of as coming from the action of the energy of the vacuum.

But there's another way to get this type of acceleration.

In our hypothetical universe with four spatial dimensions, gravity is already weak on the scale of the solar system and the galaxy.

But it can become even weaker on larger scales.

Depending on how you tweak the theory, gravity can obey an inverse square law on galactic scales, where it's sort of coupled to the three spatial dimensions of the 3-brane.

But it starts to obey the inverse cubed law on much larger scales.

In fact, the 3-brane itself, which defines the three-dimensional structure on which our observable universe exists, can actually expand into the extra fourth spatial dimension.

To us, that would look like an accelerating expansion of the universe.

It would look like dark energy.

So how do you even test a wild idea like this?

Well, here's where we finally get back to our gravitational waves.

If the gravitational field can extend into this hypothetical extra spatial dimension, then gravitational waves should lose energy to that extra dimension as they travel through space.

Here's where I have to complicate things a tiny bit more.

But I promise, we're nearly there.

Wild light and the force of gravity appear to obey the inverse square law.

In regular 3D space, gravitational waves drop in intensity proportional to just distance, not distance squared.

But it's the same general trend.

If space has four or more dimensions, then gravitational waves should drop off in intensity faster than you'd expect in three dimensions.

So that gives us a simple test.

Just observe a gravitational wave and figure out how much its intensity dropped off over the distance traveled.

Does that match what you expect in a universe with three spatial dimensions?

If the dropping intensity was too much, then you have evidence for extra dimensions-- basic stuff, right?

All you need is a billion-dollar network of gravitational wave detectors and a way to independently measure the distance the wave traveled.

Fortunately, we have both.

We have LIGO and Virgo.

And now we also have GW170817.

The electromagnetic signal from these merging exploding neutron stars allowed us to measure its distance completely independently to the gravitational wave signal, something that's impossible with black hole mergers.

One other important factor here-- in order to determine how much intensity was lost by the gravitational wave, we need to know how intense it was when it started its journey.

A super convenient property of gravitational waves is that you can figure this out by looking at other properties of the merger event-- namely, the masses of the merging objects and the frequency of the wave combined with our independent distance measurement.

OK.

So what's our conclusion?

How many extra dimensions did we discover?

Uh, zero, precisely zero.

The gravitational wave lost the right amount of intensity for a 3-plus-1-dimensional space-time.

There was no observable leakage of gravity into extra spatial dimensions, pretty much ruling this out as an explanation for dark energy.

There still might be compactified extra dimensions.

So string theorists are OK for now.

By the way, comparison of the electromagnetic and gravitational wave arrival times also allowed us to verify that gravity really does travel pretty much exactly at the speed of light.

This ruled out or constrained various alternative theories to general relativity.

This sort of null result might sound like the less interesting outcome.

I mean, how cool would it have been to discover extra dimensions?

But don't be disappointed.

It's completely mind-blowing that we can even test these crazy ideas.

Ruling them out narrows the vast scope of possible theoretical models for our universe, bringing us closer and closer to the truth.

And apparently, that truth doesn't include a 3-brane embedded in an extended 4-plus-1-dimensional space-time.