“Spacetime Uncertainty” –LIGO Going Quantum

Binary Neutron Stars


Starting up in 2024, the enhanced Advanced LIGO Plus detector of gravitational waves will embed the Heisenberg uncertainty principle to improve the machine’s ability to detect ripples in spacetime. One of the foundations of quantum theory, the Heisenberg principle states that it’s impossible to precisely measure certain properties, such as the position and momentum of an object, at the same time.

Albert Einstein predicted the existence of gravitational waves as part of his general relativity theory. His equation taught us that the unbounded extensions of interstellar space ripple and sway like like the surface of the sea.

The effects of these “gravitational waves” are observed in the sky on binary stars and correspond to the predictions of the theory even to the astonishing precision of one part to one hundred billion. They are extremely faint and instruments with exquisite sensitivity are required to detect them. For example, two binary neutron stars, which are spiraling into each other, and located in the Virgo galaxy cluster will produce a signal no bigger than one thousandth of a proton radius.

Astronomers do not know how many binary neutron stars here are in the Milky Way Galaxy. A reasonable guess would be a hundred or so. If each had a lifetime of a hundred million years, then one would “die” every million years—such events are ten thousand times rarer than supernova explosions. A LIGO laser interferometer capable of detecting such a burst of gravitational radiation coming from several hundred million light-years away would, however, have more than a million galaxies like our own within range.

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A planned revamp of the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, reports Emily Conover for Science News, relies on finessing quantum techniques, LIGO scientists announced February 14. That $35 million upgrade could let scientists catch a gravitational wave every day, on average. LIGO’s current tally of 11 gravitational wave events could be surpassed in a single week, LIGO researchers said in a news conference at the annual meeting of the American Association for the Advancement of Science.

In LIGO’s detectors (one illustrated) light from a laser bounces back and forth through two arms, traveling as a wave. Those light waves recombine and are recorded by a light detector (bottom right). A new version of the detector will strive to minimize quantum fluctuations that affect this light.

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The Heisenberg uncertainty principle states that we can’t know both the position and the velocity of a quantum particle perfectly–the better we know the position, the worse we know the velocity, and vice versa. For light waves, the Heisenberg principle tells us that there are unavoidable uncertainties in amplitude and phase that are connected in a similar way.

One of the stranger consequences of quantum theory is that there must be fluctuating electric and magnetic fields, even in a total vacuum. In a normal vacuum state, these “zero-point” fluctuations are completely random and the total uncertainty is distributed equally between the amplitude and the phase.

However, by using a crystal with non-linear optical properties, it is possible to prepare a special state of light where most of the uncertainty is concentrated in only one of the two variables. Such a crystal can convert normal vacuum to “squeezed vacuum”, which has phase fluctuations smaller than normal vacuum! At the same time, the amplitude fluctuations are larger, but phase noise is what really matters for LIGO.

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“That’s exciting, but it comes with a penalty,” physicist Michael Zucker of Caltech and MIT LIGO Laboratory said in the news conference. Fluctuations in the power of the light are increased, which makes measuring lower frequency gravitational waves more difficult. “It doesn’t excuse you from Heisenberg’s uncertainty principle.”

But in Advanced LIGO Plus, scientists will use a system that will make the best of both worlds, squeezing the light one way for lower frequency ripples and another for higher frequency signals, to improve the machine’s performance overall. “That is another step in complexity,” says physicist Hartmut Grote of Cardiff University in Wales a pioneer of light squeezing techniques.

Recent progress in generating quantum states of squeezed vacuum has made it possible to enhance the sensitivity of the 4 km gravitational wave detector at the LIGO Hanford Observatory to an unprecedented level. LIGO, operates large Michelson interferometers with the goal of detecting gravitational waves from black holes, neutron stars, supernovae, and remnants of the Big Bang.

Elsewhere, in India, a detector called LIGO-India, is also expected to turn on at around the same time as Advanced LIGO Plus, employing the same quantum techniques.

The Daily Galaxy via Science News and LIGO.org 

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