Skeptical Science Theater 3000: Gravitational Waves

As a physicist, I’ve heard some theories that are, to use a highly scientific term, weird. Odd. Crackpot, even, though that depends on who you ask.

Neil DeGrasse Tyson, for example, would probably be happy to enthuse about how cool [string theory/multiverse theory/whatever] is.

My husband, a brilliant and highly analytical mechanical engineer, will usually respond, “So what?”

It’s not that he lacks imagination or appreciation for fascinating ideas. It’s just that the same thought patterns that make him a good engineer also make him question whether some of these more esoteric theories are actually good for anything in the real world. Maybe you can sympathize. Sometimes you want more than “it’s a mind-blowing idea” as a reason to care about something.

With that in mind, I’d like to go beyond the “cool” factor of some popular theories, to talk a little more about everyday matters like testing theories, developing technologies, and so on. (Fair warning: None of this was my field of research in physics, and some of the worst mistakes in explanation are made by specialists working slightly outside their field. A little knowledge is a dangerous thing.)

So, with apologies to my husband for any (inaccurate) depictions of him as an unimaginative philistine, I present Skeptical Science Theater 3000!

HST3K

The hearts are there to remind us both that we love each other, even though we might be yelling a little.

Let’s start with something that isn’t at all crackpot, but got a lot of attention in the news last year, because I distinctly remember my husband’s skepticism on this subject.

Mechanical Engineer Husband (MEH): So everyone’s talking about the gravitational waves. (Note: Everyone really was. We were in grad school at the time, so we were surrounded by nerds. It was glorious.) I know basically what the findings mean, but nobody has actually told me why I should care about them.

Me: *slight splutter* The science…!!

(This was not a convincing argument.)

Me: Well, detecting gravitational waves gives us more evidence of Einstein’s theory of general relativity, and therefore more information on the likely structure of the universe.

MEH: You know, I wasn’t aware general relativity needed any help in that department. It already has some pretty convincing evidence backing it.

Me: Yes, but this is pretty cool stuff, and it’s more than just another discovery supporting general relativity!

MEH: Hmm. Well, here’s what I know already. I read that LIGO (the Laser Interferometer Gravitational-wave Observatory) first detected gravitational waves in September 2015 at two detectors in Louisiana and Washington. I’ve seen these waves described as ripples in space-time, but that seems kind of hand-wavy to me. I really hope you can explain it in more detail than that.

Me: Sure! The rippling is really a pretty accurate description. As a gravitational wave passes through a given area of space, the distances between points in that region of space actually increase and decrease. This wave of spatial distortion travels at the speed of light and can be detected by very precise measurements of relative distances, made by interferometers.

LIGO uses its two interferometers to detect minuscule changes in the distance light travels when it splits along two perpendicular arms, which are built to be exactly the same length. When the light recombines afterward, it produces an interference pattern, which affects the brightness of the detected light. If it’s brighter or dimmer than it was before it split into those two paths, that indicates that the lengths were different somehow during the travel time.

Interferometer

Not the actual LIGO interferometer. Not to scale. Not available except in WA, LA, CA, HI, and NM. Further restrictions may apply. See laws of physics for details.

Is that scientific enough?

MEH: …continue.

Me: The gravitational waves caused the light paths to vary by a fraction of a proton’s radius, which is just unimaginably small, but measurable on precision interferometers. The LIGO scientists concluded that the waves originated from the collision of two black holes, each about 30 times the mass of the Sun.

Gravitational waves from massive objects were first predicted by Albert Einstein in 1916, and there was already strong evidence for their existence. Likewise, we had observed what appear to be pairs of orbiting black holes that will eventually merge. This was the first time we’ve seen direct evidence of either gravitational waves or merging binary black holes, though.

MEH: All right, I get that this is a neat discovery, especially if you’re into astronomy. But what can anyone do with it? What does it really change for us?

Me: Well, this discovery fits with general relativity’s theory that gravity is a warping of space and time. A useful two-dimensional analogy is a flexible rubber sheet sagging under the weight of a ball. The sheet itself is still two-dimensional, but it’s curved in a third dimension that’s beyond the experience of whatever 2-D creatures might inhabit the sheet. In our three-dimensional space this warping manifests as gravity, and the more massive an object is, the more it warps the space-time immediately around it.

General Relativity

This idea of massive objects deforming the space around them has implications for what we should observe in the visible universe. We’ve confirmed many of these over the last century as our technology has vastly improved, and finally detecting gravitational waves is another big item on this list.

MEH: So, I was right? The main point of detecting gravitational waves is that it adds more evidence in favor of a theory that already has a ton of evidence in its favor?

Me: Sort of. But now that we know we can detect the waves, that opens up the possibility of observing other cosmic objects that aren’t detectable by any of our previous methods, like dark matter, which we think accounts for about 85% of the mass in our universe. So that’s actually very useful.

MEH: Psh. Call me when we can use it to invent an antigrav device. Now that’ll be useful.

Me: But the structure of the universe…I…I just…whatever. Do you want to watch Supergirl now?

MEH: Yes. Does that mean I won?

smirk and facepalm

Curse you, love of my life!

[Of course not! It may take time, but this discovery gives us the tools to explore and understand a lot more about the massive objects out there in the universe. You can find more about the basics of general relativity in this NYT interactive or good old Wikipedia.

Better yet, read the paper itself, published in Physical Review Letters in February 2016. It’s pretty short and has some useful figures.]

Stay tuned for string theory!

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