How to Detect Extra Dimensions

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Thanks to CuriosityStream for supporting PBS Digital Studios. 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?

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 Space Time Dimensions from GW170817 by Pardo, Fishbach, Holtz and Spergel. The key to this breakthrough was the gravitational wave event observed in August of 2017, GW170817, a pair of neutron stars spiraled together and merged. These super dense 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. 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 will also refer to as 3 plus 1 dimensional space time.

But adding the extra spacial 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 hyperdimensional, 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 out 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.

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It's football season, and you can now get almost anything you need for game day, delivered with Uber Eats. What do we mean by almost? Well, you can't get a running back delivered, but you can get baby back ribs delivered. A strong defense? No. A strong deodorant? Yes. The six pack of abs? No. Six pack of beer? Yes. Get almost, almost anything for game day, delivered with Uber Eats. Official on demand food delivery partner of the NFL. Order now. Alcohol 21 plus and in select cities. Product availability varies by Regency app for details.