Does gravity or magnetism affect light?

Gravitational wave rider

In November 1915, Albert Einstein predicted the existence of so-called gravitational waves in his general theory of relativity. Since then, scientists all over the world have tried to find evidence of their existence. Yesterday the time had come: that Laser Interferometer Gravitational-Wave Observatory (LIGO) announced in a press conference that the empirical evidence had finally been successful. On September 14, 2015, the LIGO was able to intercept gravitational waves that arose from a collision of two black holes 1.3 billion years ago. A huge sensation, news goes around the world, Nobel Prizes are waving, everyone is happy. But what exactly does that mean? What are gravitational waves, and what can we do about them with this knowledge?

In 1915, Albert Einstein introduced a concept that would dramatically change our view of the universe: gravity. In 1918 he published another article in which he expanded his general theory of relativity, and in that again he predicted the existence of gravitational waves. Einstein describes gravity as the way in which matter interacts with the flexible "space-time" that is embedded in it. Particularly massive bodies, such as black holes or neutron stars, deform space-time and influence its curvature (and thus also the movements of objects such as suns). Whenever such objects are accelerated, they produce small fluctuations in the structure of space-time, the gravitational waves.

The following applies: the more massive the object, the stronger the waves. So when two neutron stars or two black holes circle or collide, it creates particularly strong gravitational waves. These waves deform the whole room and affect everything that is in it. But they don't move by the space such as light or electromagnetic waves, but they are waves directly in the Room. The change in curvature spreads at a certain speed. Newton assumed that gravity can be infinitely fast. Einstein, in turn, assumed that gravitational waves can "only" propagate at the speed of light. If our sun suddenly disappeared, the light would not go out for us on earth until eight minutes later. But also the change in the curvature of space-time, which would spread in all directions from the place where the sun was just a moment ago, would also take eight minutes, according to Einstein, before it would have reached earth.

When a gravitational wave reaches our earth, it simply runs through it, because unlike light, for example, matter does not affect the gravitational waves, or at least hardly at all. Put simply, a gravitational wave “stretches” and “compresses” space-time in a certain rhythm, a wave pattern. The problem is that any measuring devices are also stretched and compressed again, which makes a measurement extremely difficult. Only with extremely strong gravitational waves do we have a chance to measure them, which is why neutron stars or black holes play such a crucial role: only they are “heavy” enough to trigger sufficiently strong waves.

So far we have only been able to detect gravitational waves indirectly. The two astronomers Russell Hulse and Joseph Taylor received the 1993 Nobel Prize in Physics for their observation of two neutron stars orbiting each other. They found that the two stars were getting closer and closer, which means that they were losing energy "along the way". Hulse and Taylor calculated that the lost energy roughly corresponds to the value one would expect if both neutron stars "radiated" gravitational waves. But direct proof by means of a measurement could not be carried out.

Then the LIGO came into play. The Laser Interferometer Gravitational-Wave Observatory are two four kilometer long tubes that cross at right angles at one end. There is a laser at the intersection that sends a beam into both tubes. At the end of the tubes it is then sent back by mirrors, meets a mirror again at the opposite point, which throws it back again, and so on. The laser beam thus covers a much longer distance than just the four kilometers of the tubes. The LIGO is constructed in such a way that the two tubes are exactly the same length; the rays of light, coming from both tubes, should arrive exactly at the same time at the starting point. However, the mirrors are set so that the laser beams do not arrive at the detector at the same time. Instead, they arrive so delayed that they cancel each other out (thanks to the wave structure of the light) - so no light is measured. If a gravitational wave passes through the LIGO, the tubes are stretched and submerged, i.e. a little longer and then shorter again. Because the tubes are at right angles to each other, the gravitational wave does not deform them in the same way. The light rays that then hit the detector no longer cancel each other out.

Because the effects of the gravitational waves are so small, the LIGO has to work extremely precisely and measure very precisely. At the moment it is possible to determine differences in length in the tubes that are a thousand times smaller than the diameter of a proton. That is just enough for measuring the strongest gravitational waves. A number of factors can interfere with the experiment, even if it's just a passing train. In addition, the atoms vibrate in all objects whose temperature is above absolute zero, which does not make things any easier. That's why two plants were built about three thousand kilometers apart, one in Livingston, Louisiana, the other in Hanford, Washington. Because the gravitational waves are likely to move at the speed of light, one system will be captured by the wave a little later than the other. If both measure a change in length with a time delay that corresponds to the one measured in the other system, it is very likely that it is a gravitational wave.

The first generation of LIGO experiments was not yet successful due to insufficient technical development. Which is not surprising either. From 2010 to 2015, LIGO was equipped with new measurement technology, and in autumn 2015 it went back to the start. But nobody expected that a gravitational wave would be discovered immediately. However, LIGO sounded the alarm almost immediately, to be more precise: Four months before the official start, in September 2015. Since then, the research team has been looking for other possible causes - external faults, faulty measuring devices and so on - in order to be absolutely sure that you really were Discovered gravitational waves.

What do we do with this realization now? As I said, gravitational waves are hardly influenced by matter. They can spread unhindered through space. From the measured values, we can draw conclusions about the event that triggered them - and about the object (or objects) that were involved in this event, such as black holes or neutron stars.

Black holes (in the sense of perfect spheres of pure, empty, bent space-time) have only been observed indirectly so far; especially with the help of the behavior of stars or gas clouds in their vicinity. Einstein predicted their existence and also speculated that they could merge with each other. You can think of it as two soap bubbles moving towards each other, then sticking to each other, so to speak, and finally becoming a single soap bubble, which shortly afterwards takes on its spherical shape again. The gravitational waves that are triggered could confirm this theory if it can be shown that they propagate in a certain pattern.

Neutron stars are the remains of large stars that collapsed after their “fuel” ran out. They are very small - only a few hundred kilometers in diameter - but still as heavy as the suns they used to be. Due to the enormous forces that act during the compression of the “solar matter”, neutron stars should actually form into perfect spheres. However, we do not know whether this is really the case. It could well be that there are “mountains” on a neutron star that are only a few millimeters high. The gravitational waves would have to emanate from a spherical object exactly symmetrically. Deviations in the waves could mean that some neutron stars are not perfectly spherical - and that in turn could help us answer the question of how matter behaves under such extreme conditions as a star collapses. About an explosion, when the sun becomes a supernova. This also creates gravitational waves, from which we could infer how the mass was distributed in the sun shortly before the explosion.

On a larger scale, gravitational waves could tell us what the universe looked like shortly after it was born. Shortly after the Big Bang, popular theory suggests that it expanded fairly quickly, which continues to this day. Objects that are far away from us and that move farther away appear redder because the light waves are, so to speak, elongated as they move. Cosmologists can estimate how fast the universe is expanding by comparing the redshift of observed galaxies with their distance from Earth. The instruments are calibrated with the brightness of a very specific type of supernova explosion. If several gravitational wave detectors on earth and in space (such as the ESA's LISA Pathfinder missions) all recorded signals from a high-energy cosmic event, such as the merging of two black holes, scientists could deduce from this in which galaxy this event took place and then compare it with the redshift and the measured distance - and thus arrive at a much more precise estimate of the rate of expansion of the universe.

Basic physics could also be turned upside down by gravitational waves. A common conceptual model of string theory assumes that gravity is mediated by a particle, the so-called graviton, which is analogous to the massless photons that, in simple terms, make up light. If the graviton also has no mass, gravitational waves, as generally assumed, should travel at the speed of light. If it does have a tiny mass, it affects the gravitational waves - and with the advancement of technology we would be able to measure that.

Or, as suggested by British astronaut Tim Peake, who is currently aboard the ISS, we could just ride the wave:

You can find more background information on gravitational waves and what you could do with them at, among others. One of the best (scientific) novels about neutron stars, The dragon egg by Robert L. Forward, we have in our shop. Everything I know about these unique stars is thanks to this novel.

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