In this article, we’ll take a look at gravity waves, one of the most fascinating and revolutionary ideas in astronomy. Specifically, we will talk about the LISA mission, a future project which will be a collaboration between NASA and the ESA. LISA, a single instrument composed of three spacecraft orbiting the sun, will be the first of a new generation of gravity-wave telescopes that will allow us to see the universe through new eyes. Whenever this happens, it always shows us things that we had never seen before. Gravity-wave astronomy is a window that is just opening, and LISA and its descendants are the instruments that will open it for us.
Space is often imagined as being empty. It is referred to as “the void,” and is called a vacuum. The implication is that the space between the heavenly bodies is a whole lot of nothing with a bit of dust and gas here and there.
Well, by now we know just how wrong this is. Space is absolutely teeming with bits of this and that, ranging from loose molecules to sizable chunks. Besides that, there are also dark matter and dark energy, which fill up much of what we once thought to be empty space. Obviously, the term “void” doesn’t even apply here.
But consider something else: what about space itself? If you could create a chamber and draw everything out of it- all the matter, energy and everything else- would it really be empty? This question is hard for us to grasp because we, being creatures of matter ourselves, are quite matter-centric in our outlook on the world. No, that’s not a real word, but you know what I mean. We tend to think that matter (and energy, which is just another form of the same thing, as Einstein said) is everything, and if there isn’t any matter or energy present in an area of space, we call it “empty.”
But Einstein had this idea that space itself is something. Even if you could get all of the matter and energy out of our hypothetical chamber, it still wouldn’t be empty. It would contain the space itself. Space, in this view of the universe, is a weird, rubbery substance, the shape of which is influenced by the presence of matter.
There’s a popular analogy that is sometimes used to illustrate this. Imagine a sheet of rubber that’s stretched out tight, like the head of a drum. That’s space. Now put something heavy on it- let’s say, a wet rock called Earth- and watch what happens to the rubber. Not surprisingly, it dips downward, and the wet rock ends up sitting in a depression in the rubber. Now, if you roll a smaller rock toward the big one, it will come to the edge of the depression, go over that edge, and roll into the big rock. (Whoa! The residents of Earth are having a very bad day! See our article on Near Earth Objects from a few weeks ago.) On the other hand, we can also imagine a state in which the smaller rock spins constantly around the edge of the depression, moving just fast enough to keep from falling in, but not fast enough to roll away. If it’s moving a little faster than that, it may just whiz right by, but its path will be bent by the edge of the depression.
The concept that is being illustrated here is this: gravity is not a form of radiant energy, like heat or light. Rather, it is an actual change in the configuration of space caused by the presence of matter. A chunk of matter literally warps the shape of the space around it, making a depression. More massive objects cause deeper depressions, which can only be escaped by a tremendous expenditure of energy. Objects that come too close, and are not moving fast enough to escape, fall into the depression. This is the force we think of as gravity.
Now, here’s the crucial point that makes gravity wave astronomy possible: when an extremely big event involving very massive objects occurs, it makes ripples in the rubber. For instance, when two black holes collide, it sets space rippling in a regular pattern of waves that may go on for a long time, and cover enormous distances.
Those ripples are gravity waves. If you have an instrument of sufficient sensitivity, you might be able to measure these waves, and perhaps learn something about the original event that caused them. If you studied them for a while, you might be able to compile a table of gravity wave signatures for different types of events. For instance, you might learn that the collision of two black holes causes this particular pattern of gravity waves, and that certain minute variations reveal specific facts about the source, such as the relative masses of the black holes and the angle at which they approached each other. If you got really good at it, and had extremely sensitive instruments, you might be able to learn quite a bit about events that happened far away in both space and time, just by measuring their gravity waves.
LISA is the beginning of that kind of study. It will be able to simultaneously measure the amplitude, direction and polarization of gravity waves. For the first time, scientists will be able to test theories by comparing them with actual measurements.
LISA is considered a single instrument, but that instrument will be composed of three separate spacecraft, spaced so that they form the points of an equilateral triangle five million km. on a side. This triangle will face the sun at an angle of 60 degrees to the plane of Earth’s orbit, moving with Earth around the sun. In effect, this will give us a space telescope that is five million km. wide.
That’s necessary because this rubbery stuff called space is quite stubborn, and even the most enormous events cause only slight movement in it. The waves may also be very long and slow, making them hard to detect. To get maximum sensitivity, you need a very big telescope- the bigger, the better.
Well, five million km. is pretty big, and should allow us to detect a lot of gravity waves. Here’s how it’s done:
Each of the three satellites will contain two telescopes accompanied by lasers and optical systems. Pointing in directions 60 degrees apart, the telescopes in each satellite will communicate with those in the other two satellites by laser beam. Inside each telescope is a four-centimeter-wide, free-floating cube of gold-platinum alloy, which is used as a reflector for the incoming laser beams. This provides a reference for measuring the distance between spacecraft. When a gravity wave moves through the observation field, it literally changes the shape of space so that there is a slight change in the distance between the satellites. By measuring this change, the strength, direction and polarization of the wave can be derived.
This method is so precise, even the pressure of sunlight on the satellites can alter their position in relation to each other, spoiling the measurement. Because of this, the LISA satellites will have to constantly monitor such extraneous forces and counteract them with their electric thrusters.
These thrusters will only be used after the spacecraft have reached their final orbits. The actual job of getting them there will be handled by other means. The three spacecraft will be launched together on one Atlas V launcher. Once they have left the launch vehicle, they will independently move to their respective positions around the sun using propulsion modules, which will be jettisoned before the science mission begins. The full journey, from launch until the three craft are in their working orbits, will take one year.
In some cases, space probes are able to start making observations before they actually reach their destinations, often sending back useful data months or even years before their mission officially begins. Unfortunately, that will not be possible here, since LISA is a single instrument, and will be completely non-functional until all the units are in place.
The frequency range that LISA can “see” is determined by the distance between the satellites. This frequency range has been chosen carefully to facilitate the study of the most interesting sources of gravity waves, massive black holes and binary stars.
While the LISA mission is still under consideration by both the ESA and NASA, we can already get a pretty good idea of the kinds of things it will be observing. Some of the best candidates are binary systems, in which two stars orbit each other. This spinning motion should generate a pattern of gravity waves that will be easily identifiable. At first, some of the objects LISA will study will probably be things that have already been observed by other means. Good candidates include X-ray binaries, neutron-star binaries, black-hole binaries and helium cataclysmic variables.
We have already mentioned black hole collisions, and the biggest of these are the super-massive black holes, whose collisions are the most powerful generators of gravity waves in the universe. Observation of these events will provide an opportunity to test general relativity and particularly black hole theory with an accuracy never possible before.
In general, the LISA mission could be regarded as just one part of a larger effort to look at the universe using means other than visible light. As we have seen in past articles, there is a big push going on right now, trying to expand our view of the events around us into wavelengths that have not been studied before. By expanding into gravity waves, astronomers are looking at the world of extremely low-frequency wavelengths. If past experience is any indication, this will reveal things we have never seen before, and teach us things about our universe that we would never have learned by any other method.
When that happens, you can read about it here. Stick with us, and we’ll tell you all about it.
Sources:
ESA Space Science: “LISA Factsheet- Detecting Gravitational Waves” at website of European Space Agency: esa.int/esaSC/SEM5TDWO4HD_index_0.html
ESA Space Science: LISA Overview at the website of European Space Agency: esa.int/esaSC/120376_index_0_m.html
ESA Space Science: “What is Gravity?” at the website of the European Space Agency: esa.int/esaSC/SEMDYI5V9ED_index_0.html
ESA Space Science: “Gravitational Waves- ‘Dents’ in Space-Time” at website of the European Space Agency: esa.int/esaSC/SEMLY2T1VED_index_0.html
