Launching e-LISA the gravity spotter

Gravitational waves were predicted by Einstein's theory of general relativity but they have never been detected... But that may soon change!
08 December 2015

Interview with 

Dr Harry Ward, The University of Glasgow


Glasgow University's e-LISA Pathfnder


Last week marked 100 years since Einstein's 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 "spacetime" - and this deformation draws bodies together and explains why Newton's apple fell on his head. One of the other predictions of Einstein's theory is that gravity should ripple through the fabric of space - at the speed of light - producing "gravitational waves". 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 University's Harry Ward is one of the mission scientists and spoke to Chris Smith...

Harry - Gravitational waves are a bi-product of Einstein's theory almost exactly 100 years ago when he explained that gravity is not a magical force; it's a manifestation that matter curves space time and the curvature can change and the changes can propagate away from a source, and that's 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?...

Harry - A beam of laser light, yes.

Chris - And you're 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 - That's right.  Provided you've 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 isn't it?  Your initial work is going to inform how that happens, so what are you actually putting into space now and what's 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 it's a daring thing to try to do and so, it's a proof of principle; it's 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 "well what's in the box then?"

Harry - Essentially, two reflecting masses which float completely freely.  They look beautiful actually; they're 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 - pathfinder device - actually have a capacity to do science or will it just prove that you can stably mount this system in space?

Harry - It's prime target is technology demonstration; it's not science.  It was never sold as being a science mission, and remember it's not just a technology demonstration for something like e-LISA. There's 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 that's demonstrated for this mission has wider applicability than just for pure science.

Chris - Now say it does come off - 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 it's a one thing and then it's 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 couldn't 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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