The past, the present and the future: Einstein’s gravitational waves

 

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[Here, in The Beauty Contest, we plan not to write only about finance and macroeconomics, but about other topics too. We are lucky to start this endeavour having José de Arcos with us, who will write today about the significance of Einstein’s gravitational waves. José de Arcos holds a Ph.D. in Physics from the Illinois Institute of Technology and he is currently working as a Postdoctoral Research Fellow at the Harvard Medical School. He also participated in the Daya Bay reactor neutrino experiment in Shenzhen, China]

I recently had the great honor of attending a lecture by Dr. Rainer Weiss where he introduced the achievements of the Laser Interferometer Gravitational-Wave Observatory (LIGO) experiment [1] in an easy going and approachable tone. He has inspired me to spread the word and share with you one of the most remarkable accomplishments in human history in my own way.

In the past, no such long time ago in the timescale of the universe, the first human species looked up toward the stars for first time, wondering, as we do today, what secrets were hidden in those twinkling lights. The first astronomers deduced that there was some kind of pattern in the never-ending dance of stars, a periodicity that taught them when was optimal to harvest their crops, and as a result the first civilizations flourished. The same lights showed the way to all travelers, sailors and wanderers who pushed our own frontiers a bit further at a time, until we mapped the whole world. But even then, we kept looking up to the infinity of the darkness, still wondering what secrets were hidden in those sparkling dots. One of the first experimental physicists, Tycho Brahe, kept track of the position of the planets for decades, providing Johannes Kepler with enough data to recognize a mathematical pattern: the planets moved in an elliptical fashion. And along came Newton, who, in a staggering demonstration of intelligence, was able to unravel the laws of gravitation. The work of three men changed the history of all humanity in a remarkable way.

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Celestial navigation was one of the first practical applications of astronomy

The theory of gravitation can be summarized in a “simple” mathematical equation powerful enough to describe the attraction of all the bodies in the universe, which in turn is used to work out the motion of planets, stars, galaxies… you name it. In my humble opinion, in all of history of mankind nobody has been able to match such an intellectual accomplishment until Einstein, ironically the other father of gravity, realized that the theory of gravitation had to be reformulated. Indeed, Newton’s theory works fantastically well describing motions that aren’t close to the speed of light, but fails miserably otherwise. As humans, we are only aware of a reality very distinct to the one light experiences and, thus, we have been able to passively ignore this discrepancy until the discovery of electromagnetism. Radio, TV, satellite communications, cell phones… all of them work thanks to the physical understanding of light, but using them in far distance communications such as the ones between the Earth and space shuttles wouldn’t be possible without Einstein’s theories.

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Newton and Einstein’s equations describe gravitational interaction in a complete different fashion

A century ago, Einstein completed the framework of his General Theory of Relativity, which in simple words is a geometrical description of the gravitational interaction. Einstein found out that the movement of planets, stars, galaxies were in agreement with the warping of the fabric of spacetime, which is a fancy name for the dimensions (x,y,z,t)  in which our universe expands. Previously, in the Special Theory of Relativity, Einstein was able to relate the spatial coordinates with time in such a way that nowadays it doesn’t make sense to distinguish between them. It is an accepted fact that spacetime is the reality in which bodies exist, and the mere presence of matter (or energy) distorts its geometry in its entirety. One of the most interesting results of this description is the possibility of detecting perturbations in spacetime when matter or energy move. The size of these perturbations is so minimal that even Einstein thought it would be frankly hopeless to detect it [2] but, to be fair, he couldn’t predict the amazing technological advancements humanity has been able to produce in the former century, such as the invention of the transistor and the laser.

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Gravitational attraction represented by the warping of spacetime

In the present, with all those new tools in hand, a considerably large collaboration of scientists has built two incredibly precise interferometers able to measure the change in the Earth’s spacetime due to waves generated by the gravitational interaction of supermassive bodies several light years away. Again, somehow ironically, the counterintuitive results of another interferometer experiment [3] inspired Einstein to came up with his Special Theory of Relativity, which led to the General Theory of Relativity, and eventually, just a little bit more than a century after that, the loop closes with LIGO’s results confirming the existence of Einstein’s gravitational waves.

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Experimental setup of the LIGO experiment

The previous introduction was aimed to put this remarkable news in context: thanks to the existence of celestial bodies and to the innate curiosity of mankind, we have learnt to predict the future, to guide ourselves in the present world, and from now on we will also be able to look at the past, or should I say, to hear the past. Until now we have used the light to explore the universe, up to the extent that we have managed to take a peek at the density of matter a few thousands of years after the big bang, but thanks to LIGO the game has changed: now we have a radical new way to not only keep working out the secrets of those twinkling lights, but also of the darkness that surrounds it (black-holes, dark matter…). Furthermore, we can dare to look into the origin of the universe, overcoming the inherent “limitations”of light. And by limitations I mean properties: one of the most basic characteristics of light is its ability to interact with charged particles (protons, electrons…), which in the early stages of the universe were compacted in a very dense plasma. When the universe cooled down, these particles formed neutral atoms that didn’t interact with light so strongly, releasing the primal radiation of the universe, a snapshot of the configuration of spacetime a few thousand years after its conception. Thus, any information before that specific moment cannot be provided by light itself, but maybe by other signals originated at the very beginning of the universe, such as primordial gravitational waves or even neutrinos, but that’s another story for another time.

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Cosmic microwave background measured by Planck

The future belongs to those goals we believe unachievable today. Some theories seem untestable and thus useless, but some great personalities struggle to make things happen, to measure the immeasurable, to contradict the common sense and amaze humanity with their achievements. To name a few recent stunning results, we have cherished the discovery of gravitational waves (LIGO), the Higgs Boson (CERN) and neutrino oscillations (Super-Kamiokande, SNO, T2K, Daya Bay and many others). Thanks to these collaborations we can say it is an exciting time for science, and in my opinion we are very lucky to be witnesses of such great achievements, but future generations will be even luckier to enjoy the technologies all this knowledge will provide them.

References:

[1] B.P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration)
“Observation of Gravitational Waves from a Binary Black Hole Merger”,
Phys. Rev. Lett. 116, 061102 (2016)

[2] Einstein, A. “Über Gravitationswellen”, Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften Berlin, 154–167 (1918)

[3] Michelson, Albert A.; Morley, Edward W.,”On the Relative Motion of the Earth and the Luminiferous Ether”, American Journal of Science 34: 333–345 (1887)

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