“God does not play dice with the Universe.” -Einstein
“Einstein, don’t tell God what to do.” -Neils Bohr (disputed, but awesome)
Einstein, the brilliant mind behind general relativity and the concept of “spacetime,” is making the news again this week. As you all know, gravity isn’t some mysterious invisible force traveling across space, it comes about because energy itself — most commonly in the form of mass — distorts the very fabric of space.
Of course, wrapping your head around this can prove quite difficult. Space, as we all know, is three-dimensional; how do you expect to understand it by drawing a two-dimensional surface?
I mean, honestly, it’s kind of the best we can do. It’s pretty easy to imagine what completely flat three-dimensional space would look like; just take a big 3-D grid, and there you go!
But put a mass in there, and then what? Well, think of mass as a high-powered vacuum cleaner. You turn the vacuum on, and it immediately sucks the carpet (or whatever unfortunate thing the vacuum touches) into it, distorting it and causing it to “pucker”.
Well, mass does the same thing to space. And the greater and denser your mass, the greater the range and more severe the distortion of space!
Now a black hole is an extreme example, and we are (perhaps) fortunate to not have one nearby to study relativity with. But we do have the Earth, which does a pretty respectable job distorting space close to its surface. You feel it as the force of gravity, pulling you down towards the center of the Earth, of course. But the weird, relativistic, distorted aspect of space makes things just a little bit weird.
What do I mean, weird. It means that objects change their shape a little bit thanks to the Earth’s gravity, known as the geodetic effect. If you had a gyroscope in orbit, you’d see it start to precess because of this!
But there’s more; because the Earth is rotating, there’s an extra effect on top of this, known as frame dragging, which means that an object moving with the Earth’s rotation would go slightly faster than an object moving against it.
There was an experiment designed in the 1960s to test this.
But it’s only relatively recently that the technology has advanced enough to allow us to build the necessary suspended, low-temperature, precisely smooth gyroscopes necessary to test this. And, as you may imagine, we had to do it in space. The experiment was known as Gravity Probe B.
Unsurprisingly, the results are in perfect agreement with Einstein’s prediction. In fact, the most (pleasantly) surprising thing to me, as a scientist, is that they are honestabout their experimental errors; they report significant confirmation of both of these effects, but did not perform as good of a frame-dragging test as they would have liked. (The geodetic effect was measured to an extraordinary accuracy, however,
So, for those of you keeping track, that’s another victory for Einstein’s general relativity, making the score about a bajillion to zero.
But there’s a reason we keep testing it in new ways: we won’t learn anything new unless we find something that general relativity is inadequate for. So we think about the most extreme situations for spacetime in the Universe, like very close to collapsed, massive object: black holes and neutron stars.
The proverbial “vacuum cleaner” is turned up to an ultra-high setting in this instance, and space is severely distorted. What happens if you bring a second vacuum cleaner in, very close by, and have these two compact, heavy masses orbiting one another close by?
Well, according to General Relativity, two amazing things will happen:
- These orbits will be unstable, and will decay over time. These two objects — whether they be white dwarfs, neutron stars or black holes — will eventually spiral in to one another. And…
- As they do, they will emit gravitational waves!
Now, the inspiral thing is cool enough as it is. We’ve actually observed these orbits decaying over time, because we discovered a system of two pulsing neutron stars, and their orbits are decaying!
Yes, for those of you wondering, the discoverers of this, Russell Hulse and Joseph Taylor, did in fact win the Nobel Prize for this. But the gravitational waves coming from this? That’s something we’ve never detected before. It would require, in fact, a huge set of antennae in space, looking for extraordinarily precise patterns that change the distance between them.
Luckily for us, we’ve already started building them.
The Laser Interferometer Space Antennae (LISA), a series of three spacecraft orbiting behind the Earth, would be sensitive to these gravitational waves. It was one of the major NASA science goals of this decade, scheduled to be launched in 2015. As a joint venture between NASA and the European Space Agency, LISA was supposed to finally find these gravitational waves, being the only detector designed to achieve the necessary sensitivity.
And then, last month, this happened:
ESA has ended the partnership with NASA because NASA is financially unable to participate when ESA’s funding is available (in 2015). To preserve their program, ESA’s must solicit a downscaled mission concept that does not rely on the availability of NASA funding, which they are doing.
This is according to Robin Stebbins, NASA’s project scientist for LISA. Steinn has more on this, but the gist of it is that, unless something gets done about this over the next six months, the last great unmeasured prediction of general relativity — gravitational waves — will go undetected for decades to come. We totally expect them to be there, but there’s a reason we do the experiments. If the ESA can’t figure out a way to make this work, it will be tragic to see LISA go down.
This is, quite possibly, now the last mystery of classical general relativity, and we were so close to making it happen. There are lots of reasons why it’s gone down this way, not the least of which is the James Webb Space Telescope going billions over budget, but I can’t believe it’s just going to end like this. I wrote about the importance of delivering what you promise about a year ago, and now we’re facing the consequences.
Sorry, LISA. Let us know if there’s anything we can do to save you!