Ask ethan how can lisa, without fixed-length arms, ever detect gravitational waves – forbes universal consciousness inc.

An artist’s impression of the three LISA spacecraft shows that the ripples in space generated by longer-period gravitational wave sources should provide an interesting new window on the Universe. These waves can be viewed as ripples in the fabric of spacetime itself, but they are still energy-carrying entities that, in theory, are made up of particles. EADS Astrium

Since it began operating in 2015, advanced LIGO has ushered in an era of a new type of astronomy: using gravitational wave signals. The way we do it, however, is through a very special technique known as laser interferometry. By splitting a laser and sending each half of the beam down a perpendicular path, reflecting them back, and recombining them, we can create an interference pattern.

If the lengths of those paths change, the interference pattern changes, enabling us to detect those waves. And that leads to the best question I got about science during my recent Astrotour in Iceland, courtesy of Ben Turner, who asked:

LIGO works by having these exquisitely precise lasers, reflected down perfectly length-calibrated paths, to detect these tiny changes in distance (less than the width of a proton) induced by a passing gravitational wave. With LISA, we plan on having three independent, untethered spacecrafts freely-floating in space. They’ll be affected by all sorts of phenomena, from gravity to radiation to the solar wind. How can we possibly get a gravitational wave signal out of this?

Aerial view of the Virgo gravitational-wave detector, situated at Cascina, near Pisa (Italy). Virgo is a giant Michelson laser interferometer with arms that are 3 km long, and complements the twin 4 km LIGO detectors. With three detectors instead of two, we can better pinpoint the location of these mergers and also become sensitive to events that would otherwise be undetectable. Nicola Baldocchi / Virgo Collaboration

When a wave enters a detector, any two perpendicular directions will be compelled to contract and expand, alternately and in-phase, relative to one another. The amount that they contract or expand is related to the amplitude of the wave. The period of the expansion and contraction is determined by the frequency of the wave, which a detector of a specific arm length (or effective arm length, where there are multiple reflections down the arms, as in the case of LIGO) will be sensitive to.

With multiple such detectors in a variety of orientations to one another in three-dimensional space, the location, orientation, and even polarization of the original source can be reconstructed. By using the predictive power of Einstein’s General Relativity and the effects of gravitational waves on the matter-and-energy occupying the space they pass through, we can learn about events happening all across the Universe.

Sky localizations of gravitational-wave signals detected by LIGO beginning in 2015 (GW150914, LVT151012, GW151226, GW170104), and, more recently, by the LIGO-Virgo network (GW170814, GW170817). After Virgo came online in August 2017, scientists were better able to localize the gravitational-wave signals. LIGO/Virgo/NASA/Leo Singer (Milky Way image: Axel Mellinger)

But there’s a limit to what we can do with detectors like this. Seismic noise from being located on the Earth itself limits how sensitive a ground-based detector can be. Signals below a certain amplitude can never be detected. Additionally, when light signals are reflected between mirrors, the noise generated by the Earth accumulates cumulatively.

The fact that the Earth itself exists in the Solar System, even if there were no plate tectonics, ensures that the most common type of gravitational wave events — binary stars, supermassive black holes, and other low-frequency sources (taking 100 seconds or more to oscillate) — cannot be seen from the ground. Earth’s gravitational field, human activity, and natural geological processes means that these low-frequency signals cannot be practically seen from Earth. For that, we need to go to space.

The primary scientific goal of the Laser Interferometer Space Antenna (LISA) mission is to detect and observe gravitational waves from massive black holes and galactic binaries with periods in the range of a tens of seconds to a few hours. This low-frequency range is inaccessible to ground-based interferometers because of the unshieldable background of local gravitational noise arising from atmospheric effects and seismic activity. ESA-C. Vijoux

But even without the terrestrial effects of human activity, seismic noise, and being deep within Earth’s gravitational field, there are still sources of noise that LISA must contend with. The solar wind will strike the detectors, and the LISA spacecrafts must be able to compensate for that. The gravitational influence of other planets and solar radiation pressure will induce tiny orbital changes relative to one another. Quite simply, there is no way to hold the spacecract at a fixed, constant distance of exactly 5 million km, relative to one another, in space. No amount of rocket fuel or electric thrusters will be able to maintain that exactly.