The fabric of the universe seems like it's sewn tight, but it's likely that even now, tiny ripples are stretching and compressing space and time. How can we be sure? In this week's "Ask a Physicist," we'll find out.
The other day I was quietly sitting in my office in Philadelphia, and was kind of startled to find the ground shaking underneath me. For an east-coaster, this is seriously messed up. We're just not used to the idea that something as seemingly stable as the earth can decide to suddenly thrash about.
How much stranger is it, then, that the entire universe can do the same thing? Completely independently, Pouria Molavi asked on my facebook page:
What are gravitational waves and what will discovering them mean for us?
It'll mean that we have a pretty good idea of how gravity works. It will mean that the universe is constantly shifting underneath us, the fabric of spacetime is expanding and contracting, a remnant of the beginning of time and colliding black holes. That is, if gravitational waves exist.
What's going on underneath our feet?
General relativity says that space is warped by mass and energy. I've given this a fair bit of virtual ink in this space and in my book already, but most of what we've talked about include black holes, wormholes, and other relativistic structures that just sit out there in space (or possibly not, in the case of wormholes).
Not everything is static; the universe is a very dynamic place. Stars, galaxies, and black holes are constantly flying around one another, and that means that the gravitational landscape is constantly shifting.
I'll occasionally get emails from people with ideas of how to use the dynamic universe as a way to cheat the fundamental limits of the speed of light. The idea is that a supercivilization might take individual stars, rattle them about, and cause a very slight but measurable shift in gravity far away — instantaneously!
Awesome, right? There's only one catch. That's not how gravity works.
Just like light, gravity travels at the speed of light, and that's not all they have in common. Radio waves — which are just a form of electromagnetic radiation, after all — are made by taking electrons in a transmitter and wiggling them around. You can do the same thing (more or less) with massive bodies. Two black holes spiraling in toward one another are expected to be a whopping gravitational transmitter.
But there's an important difference: electromagnetism is much stronger than gravity. Two protons repel each other electrically by a factor of about 10^36 times more strongly than they attract each other gravitationally. The tiny electrical fields in your shoe are more than sufficient to negate the gravitational pull of the entire earth. The only reason that we even notice gravity on a day-to-day basis is that most things are neutral so electricity is taken out of the game entirely. Otherwise, it'd be no contest between the two.
So while electromagnetic waves are really easy to detect (your eyes are presumably doing just that right now), gravitational waves have never been detected, at least not yet. That's unfortunate, because gravitational waves are a key prediction of general relativity, and could also turn out to tell us all sorts of cool things about the universe.
Can we see them?
To understand why gravitational wave detection is such a challenge, let me say a few words about what G-waves (as they're known on the street) do. A light wave is, at its core, nothing more than a packet of electrical and magnetic fields. A gravitational wave is more or less the same, but a packet of gravity essentially involves warping spacetime. Take two fixed points in space, and pass a gravitational wave through them. The distance between the two detectors will fluctuate ever so slightly.
To give you an idea, Laser Interferometer Gravitational Wave Observatory (or LIGO, for short), has mirrors separated by about 4 km, and a gravitational wave signal originating from colliding black holes is expected to cause the distance to oscillate by about 10^-18m — less than the size of an atomic nucleus. The idea is that you set up a few of these arms, and measure how much the detectors oscillate and with what frequency. You can then triangulate to figure out what sort of signal you have, and what direction it's coming from.
LIGO hasn't seen anything, but there is (or perhaps, was) a lot more hope for the the (potentially) upcoming Laser Interferometer Space Antenna (LISA). I'm afraid I have some bad news. Back in April, NASA announced that proposed Congressional budget cuts made it likely that we were no longer going to be able to fund LISA, except at a very low level.
LISA was one of only 2 large space missions recommended to the U.S. government for the next decade by essentially the entire astronomical community.
The European Space Agency is going to see if they can go it alone.
It's a real shame, because LISA is exactly as awesomely cool as it sounds. In the original design specs, it involves 3 satellites in an earth-trailing orbit around the sun, arranged in a triangular formation. Each of the satellites will be separated about about five million kiliometers (about 12 times the distance from the earth to the moon), and will shoot LASERs at one another in order to measure the distances with incredible precision. LISA reminds me of nothing so much as the episode of ST:TNG where Data creates a tachyon "net" to detect cloaked Romulan ships. Admit it; you know exactly what I'm talking about.
Why does it matter?
All of this seems like a hell of a lot of work to detect something that we're pretty sure exists. Why bother? For one, it's all well and good to assume that we (or Einstein) are right, and quite another to prove it.
But there's a more fundamental reason. Gravitational waves allow us to see things that we normally wouldn't be able to observe any other way. Colliding black holes and neutron stars, for example, are likely to be big sources of gravitational radiation. In 1993, Russell Hulse and Joseph Taylor won the Nobel prize for recognizing that the reason that a particular pair of pulsars was spiraling in toward one another because they were losing energy through gravitational radiation. So even though we haven't seen gravitational radiation directly, we're reasonably sure that it's out there.
Even better, though, is that we'd be able to see the beginning of the universe. Over the last decade or so, you may have enjoyed various baby pictures of the universe. The Cosmic Microwave Background is the remnant light from when the universe was about 380,000 years old. Compared to the 13.8 billion years old that the universe is now, that may seem like the very beginning, but a lot happened before the era of "recombination," when the universe became opaque. We literally can't see anything that happened before that time directly, and have to infer it through our understanding of atomic physics, relativity, and the like.
I probably don't need to tell you that the gravitational fields in the early universe were very strong, and the gravitational waves are still out there, though, like the radiation filling the universe, has lost a lot of energy since the beginning. Whether we can detect a signal from the big bang would put enormous constraints on various high-energy models of physics, and allow us the first direct probe into the beginning of time.
Dave Goldberg is the author, with Jeff Blomquist, of "A User's Guide to the Universe: Surviving the Perils of Black Holes, Time Paradoxes, and Quantum Uncertainty." (follow us on twitter, facebook, twitter or our blog.) He is an Associate Professor of Physics at Drexel University and is currently working on a new book on symmetry. Feel free to send email to [email protected] with any questions about the universe.
Images by Lena Grottling, Bill Frische, P.Uzunova, and Iaroslav Neliubov via Shutterstock