These are some of the most
frequently asked questions about LISA. Answers to questions of more technical nature can be found at the LISA
Project Office website.
True, we have not yet achieved the direct detection of gravitational waves, but we have
overwhelming evidence that they exist.
We observe that the orbits of binary pulsars lose orbital energy and shrink exactly at the rate predicted by general relativity because of emission of gravitational radiation. Thus, we are confident that general relativity describes gravitational waves correctly.
What LISA will do is use those waves to learn about the
astrophysics of dark objects that can be seen in no other way, and to
probe strong-field gravity, for which we have no other test.
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LISA's goal is not to discover low frequency gravitational waves, but to use them as a
new window into astrophysical and physical phenomena that cannot be studied in other ways. For instance, LISA will observe the lighter massive black holes that do not have enough matter around them to be visible as active galactic nuclei.
As for physics, we know that Einstein's general relativity is a very good description of
weak-field gravity (i.e., in the neighborhood of relatively light bodies, or far enough from more massive bodies); but we have only circumstantial evidence for its validity in the
strong-field regime (e.g., near the event horizon of a black hole). LISA will shed light into this physics, revealing whether the massive rotating objects at the center of galaxies are really described by the "vacuum Kerr solution" of general relativity, or are more exotic and unexpected.
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No.
LISA and LIGO are complementary because they look at entirely different sources with different astrophysics. Observations by LIGO at frequencies between 10 and 1000 Hertz (oscillations per second) will reveal the mergers of neutron star binaries and stellar-mass black holes.
Seismic activity and mass motions on the Earth prevent LIGO from looking at lower frequencies, and for heavier systems.
By contrast, the much
longer LISA baseline and the quietness of space allows LISA to work between 0.03 milliHertz and 1 Hertz, the band of gravitational-wave emission for the much more massive black holes found in galactic centers. Thus, LISA will give us new insight about the initial formation and growth of these black holes, as far as the edge of the observable Universe.
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From "conventional" electromagnetic astronomy we know of several compact compact binaries in the Galaxy that are already emitting gravitational waves in the LISA band, and only await for it to be launched. In fact, we expect LISA to observe
thousands more such binaries that we have not yet seen with telescopes. The rates of merger events that involve massive black holes are uncertain, but LISA can see these even if they are very far (at "redshifts" up to 20), so even the most pessimistic rate estimates allow LISA
many exciting discoveries.
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LISA is similar to a radiotelescope in that it measures both the strength and phase (i.e., the oscillation) of signals. Much like a radio receiver, LISA can be
tuned to zero in to a specific source, silencing all others; but the tuning happens at the time of data analysis, without disturbing the instrument. Thus, it will be possible to
separate many sources this way. It is true that there will be interference between sources that appear at the same frequency, such as some galactic binaries, but LISA will still be able to detect and characterize thousands of them without confusion.
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LISA's response is not very directional, but its orbital motion around the Sun results in a
modulation of long-lived signals in both
amplitude and phase as the LISA "antenna pattern" of sensitivity sweeps across the position of the source. This modulation can help
determine sky position, typically to about
one degree.
Another way to look at this is that during a year LISA establishes an observational baseline of two astronomical units (the Earth-to-Sun distance).
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The three spacecraft are stacked vertically in a
single launch vehicle, each attached to a propulsion module. Once launched, the spacecraft separate and follow
three independent trajectories to their final orbital injection points, taking 10-13 months to arrive "on station." Once there, each spacecraft separates from its propulsion module and begins the process of setting up laser links with the other two.
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