“The pages are still blank, but there is a miraculous feeling of the words being there, written in invisible ink and clamoring to become visible.” -Vladimir Nabokov
The wonderful images we take of deep space — from distant galaxies to all the stars, clusters, and nebulae within our own galaxy — all have something in common.
Light! More specifically, electromagnetic radiation. While this light isn’t always in the visible portion of the spectrum, that’s certainly the type of radiation we’re most accustomed to. And that’s unsurprising: the greatest source of energy for us is the very same as the greatest source of energy for the above cluster, NGC 3603.
The type of light that comes from these stars — like the light that comes from all stars — is highly dependent on the temperature of the star in question. The hotter a star is, the more blue, or even ultraviolet its light will be, while the cooler it is, the redder — often well into the infrared — will its light be.
But not every star is like our Sun, nor just a bit hotter, nor just a bit cooler. Some stars are literally hundreds of times more massive, while others are just a minuscule fraction of our Sun’s mass.
And hence there is a tremendous variety among the fates of the various types of stars.
And while nearly all of the stars we are familiar with get their energy source from the same place as our Sun — nuclear fusion — this is not the only energy source for the stars in our Universe.
Because in addition to nuclear reactions that give off energy, there’s also a tremendous amount of energy stored in gravitation.
As a large mass collapses or contracts, there are a couple of interesting things that do and do not happen. The spacetime outside the mass — the stuff that was outside of the initial, pre-collapse/contraction star — does not change. The energy in it doesn’t change, the curvature doesn’t change, the gravitational potential doesn’t change, and so on.
But where the spacetime, initially, was enclosed by the initial object, but isn’t after the contraction/collapse, there’s more, negative, gravitational potential energy. And that energy’s got to go somewhere.
One place it can go, in particular, is into light, which is exactly what happens for White Dwarf stars. Comparable in mass to the Sun but in size to the Earth, these stars give off large amounts of light, powered only by gravitational contraction.
For example, if the Sun were replaced with a White Dwarf star, it would still appear 400 times brighter than our Full Moon currently does!
But not every collapsed/contracted object is the only star in its Solar System; many, like our night sky’s brightest star, are in binary star systems. A binary star system is one where two stars — or star-like object — orbit one another. Over time, these orbits aren’t simply stable, they decay due to gravity, and start to inspiral in towards one another.
Only this time, as the gravitational energy decreases, it isn’t light that comes out. And I don’t mean visible light; I mean any form of light. Not X-rays, not infrared, not radio waves, nothing.
What type of radiation should come out of a system like that?
Gravitational radiation, also known as gravitational waves! These ripples should propagate through spacetime, and rather than being detectable as light, they should deform the dimensions of objects as the gravitational waves pass through them!
There should be a steadily accelerating emission of waves as two objects spiral in towards one another; the closer they get to each other, the shorter the period becomes. Then, during the merger phase, there should be a catastrophic emission of both light (as it’s very likely, especially if it’s two white dwarfs, to create a supernova) and gravitational waves, followed by a “ringdown” phase of ripples, right in the aftermath of the merger.
It’s a bold prediction by Einstein’s general relativity, to be certain. But we’ve already observed — indirectly — one important aspect of this!
By taking a look at two pulsars (collapsed neutron stars) that orbit one another, we should be able to make predictions as to how the orbital period of these two stars should decay over time! Over the past 30+ years, since the discovery of the first binary pulsar, we’ve done exactly that.
But we’ve love to be able to detect these waves directly! So what can we do to try to detect this invisible radiation in the Universe? Well, you can shoot precise lasers — of very well known wavelength — over huge distances in multiple directions. You can bounce that light off of mirrors, and send it back, where you put the light from both directions back together, and you see an interference pattern from those two directions.
Because these gravitational waves are so weak, you need an incredibly long baseline (to give you a huge number of wavelengths: you need to detect a change of about 1 part in 1028) to detect even a tiny shift in one of these distances.
We’ve got a project on Earth currently attempting to do just this: the Laser Interferometry Gravitational-wave Observatory, or LIGO.
But LIGO is stuck here on Earth, where you’re not only limited by your ability to send lasers from one location to another on Earth, you’re also limited by how well you can shield yourself from terrestrial vibrations.
If we were really serious about detecting these gravitational waves, there’s only one place to go to look: space!
And that’s exactly what the Laser Interferometer Space Antenna, or LISA, is designed to do! Unfortunately, doing anything in space is expensive, and with NASA’s budget gutted and the ESA unable to afford it on its own, this set of three spacecraft designed to orbit behind the Earth — with distances between them of five million kilometers apiece — is at least a decade away now. From the official site:
ESA has ended the study of LISA and the other concepts as partnerships at the scale proposed in the New Worlds New Horizons decadal survey (NWNH). ESA has begun a rapid definition effort that includes the formation of a new science team (to be announced shortly). That effort will identify science goals and a mission concept that can be implemented as part of an ESA-led mission launching in the early 2020’s. Revised mission concepts from the three science areas will be considered in a selection process commencing in February 2012.
The Universe is speaking — all the time — in a language we’ve never heard before. And yet, as soon as we hear it, we’re in a place where we are confident that we’ll immediately understand it!
So what’s it saying? How many — and where — are white dwarfs inspiraling? How many black hole mergers are there in distant galaxies? What does a catastrophic gravitational wave emission from a merger look like? The Universe is telling us, right now. We just need to listen to one of the last great untested predictions of general relativity, and we’ll be able to see, for the first time, the invisible radiation of the Universe: gravitational waves!