Science Interviews


Tue, 8th Dec 2015

Launching e-LISA the gravity spotter

Dr Harry Ward, The University of Glasgow

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Last week marked 100 years since Einsteinís theory of general relativity was first e-LIsa Pathfinderannounced; this proposed that gravity occurs because massive objects - like stars and planets, deform the fabric of space - something called ďspacetimeĒ - and this deformation draws bodies together and explains why Newtonís apple fell on his head. One of the other predictions of Einsteinís theory is that gravity should ripple through the fabric of space - at the speed of light - producing ďgravitational wavesĒ. And this week the European Space Agency launched the first phase of  mission called e-LISA in which, ultimately, several space-craft out in deep space will bounce a light beam between themselves to spot any gravity waves that come through. Glasgow Universityís Harry Ward is one of the mission scientists and spoke to Chris SmithÖ

Harry - Gravitational waves are a bi-product of Einsteinís theory almost exactly 100 years ago when he explained that gravity is not a magical force; itís a manifestation that matter curves space time and the curvature can change and the changes can propagate away from a source, and thatís what we call gravitational waves, and we want to detect those because it will provide a new window on the universe, a new way of looking at astronomical events.  And the way the gravity waves affect you, or me, or anything is that they stretch and squeeze us, by fortunately an infinitesimal amount but, nonetheless, by an amount that we hope will be detectable.  And the way we plan to do it is to measure between separated items, typically mirrors because we are using laser light to do it, and we monitor their separation looking for tiny, tiny changes.

Chris - Bouncing between the mirrors is what -  a beam of light?Ö

Harry - A beam of laser light, yes.

Chris - And youíre able to detect, obviously, if a gravity wave comes through and it squeezes the space a little bit or stretches the space a little bit, then the light will take a bit longer or a bit less time to go between the two and, therefore, its colour,  its wavelength is going to change very subtly.

Harry - Thatís right.  Provided youíve got some reference against which to make that measurement.  And so in all of the experiments to date, we typically do that by having two arms, two paths along which we make measurements and we orient them at right angles to each other, because it turns out that a gravity wave affects two things at right angles to each other, and so the two arms gives us a natural way in which we compare one arm with the other arm.

Chris - Now this is the longer term project isnít it?  Your initial work is going to inform how that happens, so what are you actually putting into space now and whatís that going to do?

Harry - The longer term project is a thing called e-LISA which would have three spacecraft forming these two arms, or maybe even three arms if we if we complete the triangle, but the challenge is in the technology.  I mean itís a daring thing to try to do and so, itís a proof of principle; itís designed to allay risk and to demonstrate to all of us that we have the technology under control.

Chris - I sound like an on-line sales rep when I ask ďwell whatís in the box then?Ē

Harry - Essentially, two reflecting masses which float completely freely.  They look beautiful actually; theyíre gold platinum cubes and these, in principle, float freely within a spacecraft and the laser beams propagate to and from these proof masses and make the measurement.

Chris - Will the test device Ė pathfinder device Ė actually have a capacity to do science or will it just prove that you can stably mount this system in space?

Harry - Itís prime target is technology demonstration; itís not science.  It was never sold as being a science mission, and remember itís not just a technology demonstration for something like e-LISA. Thereís a growing interest in making these kind of sensitive measurements of interaction of gravitational fields with masses much more locally.  I mean we have missions that do climate research by monitoring the gravity perturbations of the Earth, and so the technology thatís demonstrated for this mission has wider applicability than just for pure science.

Chris - Now say it does come off Ė we hope it will.  What will it mean to science if we actually do detect these gravity waves going through space?

Harry - Well, I actually hate the word detect because that sounds as though itís a one thing and then itís over.   Genuinely this is new astronomy plus also, you know, test of some of the remaining Einsteinian predictions of general relativity.  It allows us to see right to the heart of some of the most extreme events that you could ever imagine in the universe.  The millions of solar mass black holes that we know that exist at the centres of galaxies.  Things that we couldnít realistically ever hope to see through the electromagnetic observations, so gravity waves allow, if we can observe them, is really a tool for seeing to the heart of, essentially, the force that shapes the evolution of our universe.


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