A Window to the Universe

In my last post Surfing Gravity’s Waves, I mentioned that the oscillations that LIGO detected were thousands of times less than the nucleus of an atom. How, then, could they have possibly known that there was an oscillation? Well, scientists are clever, and they can come up with all sorts of ingenious ideas.

A simple laser emits a focused beam of light at a beam splitter, which is at exactly 45°. The beam splitter allows 50% of the light through, so half is reflected at right angles and half passes through. The two beams then travel down ‘arms’ and are then reflected back by a mirror at the end of the arms. The two beams then recombine at the beam splitter before arriving at a detector.

Basic_michelson_labeled

This technique is called interferometry, first demonstrated by American physicist Albert A. Michelson in the 1880s in an attempt to detect the luminiferous aether, a theory to explain why light was able to travel without a medium. Although the theory proved negative, the accuracy of the techniques he used exceeded that of his time and earned him the Nobel Prize in 1907.

What can we tell from the detector? Light is a wave, and when two waves combine, they interfere with each other in a concept called superposition. When the peaks of one wave match the peaks of another wave, they constructively interfere to form an even larger wave. When the peaks of one wave match the troughs of another wave, they destructively interfere to form nothing – the waves essentially cancel each other.

Constructive and Destructive Interference

If the two beams of light in the interferometer travelled the exact same distance (as they are set up to do), then they should appear as how they were emitted. But if one wave travelled even a minutely longer distance, perhaps due to oscillations caused by gravitational waves, they will have interfered with each other by the time they reach the detector.

With more advanced technology, especially the invention of the laser, interferometers today are heavily upgraded versions of the original Michelson interferometer. While Michelson’s interferometer had arms 11 m long, LIGO’s interferometer extends for 4 km, allowing for much more precise observations.

Another modification is the inclusion of Fabry-Pérot cavities. Mirrors are placed that reflect light back and forth along the arms about 280 times before it reaches the beam splitter. This essentially increases the arm length to 1120 km.

Basic_michelson_with_FP_labeled

While length does increase the precision of the interferometer, higher laser power also increases resolution, allowing the detector to detect tiny changes. The power needed is so high that it is impractical to build a laser that powerful, so power recycling mirrors are added. Any light that would pass through the beam splitter to arrive back at laser is instead reflected back at the beam splitter, ‘recycling’ the light back into the interferometer. This power recycling mirror allows light to travel through in one way but not in the other way.

Basic_michelson_with_FP_and_PR_labeled

Other ‘upgrades’ include enhancing the signal that arrives at the detector for higher resolution, and damping that removes unwanted vibrations.

By modifying something as simple as a Michelson interferometer, LIGO is able to detect oscillations thousands of times less than the nucleus of an atom. When the idea was proposed forty years ago, people thought it was insane to attempt to build something even close to this degree of precision, but this just goes to show how far humanity has come.

The discovery of gravitational waves is amazing and all, but what implications does this have for the future of science?

For one, the LIGO interferometer represents the pinnacle of interferometry technology. The process of finding gravitational waves itself has resulted in many improvements in scientific apparatus.

In addition, what many people don’t realise is that scientists have not actually directly observed black holes. Because they emit no light, we can only observe the effect of black holes on nearby light. However, these gravitational waves have shown the physical energy released by the merging of two black holes, providing the most concrete evidence we have of the existence of black holes. Consequently we can advance our understanding in the field of astrophysics, and in particular stellar evolution.

The existence of gravitational waves also provides potential information on the properties of the graviton. Gravity is one of the four fundamental forces (a post about this will soon arrive), and is the only one whose exchange particle, named the graviton, has not been observed.

But the greatest impact of all is most certainly its impact on our understanding of the Universe as a whole. The extreme energy of the early Universe made it opaque to electromagnetic radiation such as light, preventing us from being able to use light as a key to describing the history of the Universe. Gravitational waves, on the other hand, would not be affected, thus opening a window to the wonders of the earliest cosmos.

The further accumulation of data from distant cosmological events can shape models of the history of the Universe, and also of the nature of dark matter, one of the most obscure fields of study in all of physics.

Our eyes were open to some of the wonders of the Universe, and now our ears open us up to some more.

Yanhao

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