One of the most profound open questions in modern physics is: \u201cIs gravity quantum?\u201d\u00a0<\/p>\n
The other fundamental forces \u2014 electromagnetic, weak, and strong \u2014 have all been successfully described, but no complete and consistent quantum theory of gravity yet exists. \u00a0<\/p>\n
\u201cTheoretical physicists have proposed many possible scenarios, from gravity being inherently classical to fully quantum, but the debate remains unresolved because we\u2019ve never had a clear way to test gravity\u2019s quantum nature in the lab,\u201d says Dongchel Shin, a PhD candidate in the MIT Department of Mechanical Engineering (MechE). \u201cThe key to answering this lies in preparing mechanical systems that are massive enough to feel gravity, yet quiet enough \u2014 quantum enough \u2014 to reveal how gravity interacts with them.\u201d<\/p>\n
Shin, who is also a MathWorks Fellow, researches quantum and precision metrology platforms that probe fundamental physics and are designed to pave the way for future industrial technology. He is the lead author of a new paper that demonstrates laser cooling of a centimeter-long torsional oscillator. The open-access paper, \u201cActive laser cooling of a centimeter-scale torsional oscillator<\/a>,\u201d was recently published in the journal Optica.\u00a0<\/p>\n Lasers have been routinely employed to cool down atomic gases since the 1980s, and have been used in the linear motion of nanoscale mechanical oscillators since around 2010. The new paper presents the first time this technique has been extended to torsional oscillators, which are key to a worldwide effort to study gravity using these systems.<\/p>\n \u201cTorsion pendulums have been classical tools for gravity research since [Henry] Cavendish\u2019s famous experiment in 1798. They\u2019ve been used to measure Newton\u2019s gravitational constant,\u00a0G, test the inverse-square law, and search for new gravitational phenomena,\u201d explains Shin.<\/p>\n By using lasers to remove nearly all thermal motion from atoms, in recent decades scientists have created ultracold atomic gases at micro- and nanokelvin temperatures. These systems now power the world\u2019s most precise clocks \u2014 optical lattice clocks \u2014 with timekeeping precision so high that they would gain or lose less than a second over the age of the universe.<\/p>\n \u201cHistorically, these two technologies developed separately \u2014 one in gravitational physics, the other in atomic and optical physics,\u201d says Shin. \u201cIn our work, we bring them together. By applying laser cooling techniques originally developed for atoms to a centimeter-scale torsional oscillator, we try to bridge the classical and quantum worlds. This hybrid platform enables a new class of experiments \u2014 ones that could finally let us test whether gravity needs to be described by quantum theory.\u201d<\/p>\n The new paper\u00a0demonstrates laser cooling of a centimeter-scale torsional oscillator from room temperature to a temperature of 10\u00a0millikelvins (1\/1,000th of a kelvin) using a mirrored optical lever.<\/p>\n \u201cAn optical lever is a simple but powerful measurement technique: You shine a laser onto a mirror, and even a tiny tilt of the mirror causes the reflected beam to shift noticeably on a detector. This magnifies small angular motions into easily measurable signals,\u201d explains Shin, noting that while the premise is simple, the team faced challenges in practice. \u201cThe laser beam itself can jitter slightly due to air currents, vibrations, or imperfections in the optics. These jitters can falsely appear as motion of the mirror, limiting our ability to measure true physical signals.\u201d<\/p>\n To overcome this, the team used the mirrored optical lever approach, which employs a second, mirrored version of the laser beam to cancel out the unwanted jitter.<\/p>\n \u201cOne beam interacts with the torsional oscillator, while the other reflects off a corner-cube mirror, reversing any jitter without picking up the oscillator\u2019s motion,\u201d Shin says. \u201cWhen the two beams are combined at the detector, the real signal from the oscillator is preserved, and the false motion from [the] laser jitter is canceled.\u201d<\/p>\n This approach reduced noise by a factor of a thousand, which allowed the researchers to detect motion with extreme precision, nearly 10 times better than the oscillator\u2019s own quantum zero-point fluctuations. \u201cThat level of sensitivity made it possible for us to cool the system down to just 10 milli-kelvins using laser light,\u201d Shin says.<\/p>\n Shin says this work is just the beginning. \u201cWhile we\u2019ve achieved quantum-limited precision below the zero-point motion of the oscillator, reaching the actual quantum ground state remains our next goal,\u201d he says. \u201cTo do that, we\u2019ll need to further strengthen the optical interaction \u2014 using an optical cavity that amplifies angular signals, or optical trapping strategies. These improvements could open the door to experiments where two such oscillators interact only through gravity, allowing us to directly test whether gravity is quantum or not.\u201d<\/p>\n The paper\u2019s other authors from the Department of Mechanical Engineering include Vivishek Sudhir, assistant professor of mechanical engineering and the Class of 1957 Career Development Professor, and PhD candidate Dylan Fife. Additional authors are Tina Heyward and Rajesh Menon of the Department of Electrical and Computer Engineering at the University of Utah. Shin and Fife are both members of Sudhir\u2019s lab, the\u00a0Quantum and Precision Measurements Group<\/a>.<\/p>\n Shin says one thing he\u2019s come to appreciate through this work is the breadth of the challenge the team is tackling. \u201cStudying quantum aspects of gravity experimentally doesn\u2019t just require deep understanding of physics \u2014 relativity, quantum mechanics \u2014 but also demands hands-on expertise in system design, nanofabrication, optics, control, and electronics,\u201d he says.<\/p>\n \u201cHaving a background in mechanical engineering, which spans both the theoretical and practical aspects of physical systems, gave me the right perspective to navigate and contribute meaningfully across these diverse domains,\u201d says Shin. \u201cIt\u2019s been incredibly rewarding to see how this broad training can help tackle one of the most fundamental questions in science.\u201d<\/p>\n","protected":false},"excerpt":{"rendered":"One of the most profound open questions in modern physics is: \u201cIs gravity quantum?\u201d\u00a0 The other fundamental forces…\n","protected":false},"author":2,"featured_media":132392,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3845],"tags":[58331,58333,58326,58327,58322,58323,58329,74,58325,58324,38197,70,58328,16,58330,15,58332],"class_list":{"0":"post-132391","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-physics","8":"tag-dongchel-shin","9":"tag-dylan-fife","10":"tag-laser-cooling","11":"tag-milli-kelvins","12":"tag-mit-department-of-mechanical-engineering","13":"tag-mit-meche","14":"tag-optical-lever","15":"tag-physics","16":"tag-precision-timing","17":"tag-quantum-amp-precision-measurements-group","18":"tag-quantum-gravity","19":"tag-science","20":"tag-torsional-oscillator","21":"tag-uk","22":"tag-unified-field-theory","23":"tag-united-kingdom","24":"tag-vivishek-sudhir"},"share_on_mastodon":{"url":"https:\/\/pubeurope.com\/@uk\/114572352838820331","error":""},"_links":{"self":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/posts\/132391","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/comments?post=132391"}],"version-history":[{"count":0,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/posts\/132391\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/media\/132392"}],"wp:attachment":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/media?parent=132391"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/categories?post=132391"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/tags?post=132391"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}