See more grav­i­ta­tional waves by squeez­ing

Popular Science - - CONTENTS - by Matthew R. Fran­cis

IN 2015, SCI­EN­TISTS CAUGHT EV­I­DENCE OF A cos­mic throw­down that took place 1.3 bil­lion lightyears away. They spied this bi­nary black-hole col­li­sion by cap­tur­ing grav­i­ta­tional waves—rip­ples in space­time cre­ated when mas­sive ob­jects in­ter­act— for the first time. But now physi­cists want to see even far­ther. Do­ing so could help them ac­cu­rately mea­sure waves cast off by col­lid­ing neu­tron stars, im­pacts that might be the source of many Earthly el­e­ments, in­clud­ing gold. For that, they need the most sen­si­tive grav­i­ta­tional-wave de­tec­tors ever.

The de­vices that nab waves all rely on the same mech­a­nism. The U.S.-based Laser In­ter­fer­om­e­ter Grav­i­ta­tional-Wave Ob­ser­va­tory (LIGO) and its Euro­pean coun­ter­part, Virgo, fire lasers down two mile-plus-long arms with mir­rors at their ends. Pass­ing waves wig­gle the mir­rors less than the width of an atom, and sci­en­tists mea­sure the rip­ples based on when pho­tons in the laser light bounce off them and come back. Or­di­nar­ily, pho­tons exit the lasers at ran­dom in­ter­vals, so the sig­nals are fuzzy.

Imag­ine these pho­tons as rain­drops, and the time it takes for us to mea­sure the grav­i­ta­tional wave as a side­walk. Driz­zling drops hit the pave­ment in­de­pen­dent of one an­other. Some spots might stay dry longer, while oth­ers get soaked. The droplet na­ture of rain, like the pho­ton na­ture of light, leads to “grain­i­ness.”

Now physi­cists can even out those metaphor­i­cal rain­drops. In up­graded de­tec­tors, a prin­ci­ple of quan­tum physics called “squeezed light” is help­ing re­searchers paint a more pre­cise im­age. Squeez­ing light works by shin­ing a laser through a spe­cial crys­tal, which ad­justs the rate of the pho­tons’ flow so they’re spaced out more evenly.

In­stead of fall­ing ran­domly, each rain­drop is less likely to hit a spot that’s al­ready wet than a dry patch. The re­sult is a more even spritz­ing of the side­walk square. Or, in the case of grav­i­ta­tional waves, a nearly com­plete up-and-down wave pat­tern rather than an un­even and jumpy pic­ture.

Squish­ing in one di­men­sion means stretch­ing in an­other, but sci­en­tists can give up cer­tainty about the en­ergy of the light and not lose any cru­cial info. “We ma­nip­u­late light such that we put more of the un­cer­tainty in one [mea­sure­ment] in or­der to mea­sure the other one more ac­cu­rately,” says Kather­ine Doo­ley of Cardiff Univer­sity, a LIGO col­lab­o­ra­tor.

Squeezed-light tech has al­ready in­creased ac­cu­racy at the smaller GEO600 de­tec­tor in Ger­many four­fold. If LIGO and Virgo see sim­i­lar gains, we’ll squeeze out many more neu­tron-star col­li­sions from the cos­mos—the pot of gold at

the end of grav­ity’s rain­bow.

Not even close to scale. Our en­tire galaxy would be smaller than a pe­riod on this page.

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