{"id":4731,"date":"2010-06-01T10:35:25","date_gmt":"2010-06-01T10:35:25","guid":{"rendered":"http:\/\/scienceblogs.com\/principles\/2010\/06\/01\/relativity-on-a-human-scale\/"},"modified":"2010-06-01T10:35:25","modified_gmt":"2010-06-01T10:35:25","slug":"relativity-on-a-human-scale","status":"publish","type":"post","link":"http:\/\/chadorzel.com\/principles\/2010\/06\/01\/relativity-on-a-human-scale\/","title":{"rendered":"Relativity on a Human Scale"},"content":{"rendered":"

While I mostly restricted myself to watching invited talks at DAMOP last week, I did check out a few ten-minute talks, one of which ended up being just about the coolest thing I saw at the meeting. Specifically, the Friday afternoon talk on observing relativity with atomic clocks<\/a> by Chin-Wen Chou of the Time and Frequency Division at NIST in Boulder<\/a>.<\/p>\n

The real technical advance is in a recent paper in Physical Review Letters<\/cite><\/a> (available for free via the Time and Frequency Publications Database<\/a>, because government research isn’t subject to copyright): they have made improvements to their atomic clock based on the spacing of two energy levels in a single aluminum ion (described in a Science<\/cite> paper<\/a>, also available free at the Time and Frequency Publications Database<\/a>), so it is now accurate to something like one part in 1017<\/sup>. They now have two “clocks” (you can argue about whether they really count as official clocks in the current configuration) based on the same ion that they can compare to unprecedented precision.<\/p>\n

This allows a couple of really cool tricks that haven’t been published anywhere (yet), but which were the subject of the DAMOP talk. These clocks are now good enough to observe relativistic effects on a human scale: they see time dilation from motion of the ion at speeds of 10m\/s or less, and the gravitational redshift caused by raising one of the two clocks a foot above the other.<\/p>\n

<\/p>\n

Einstein’s theory of relativity predicts a number of things about the behavior of clocks that strike most people as surprising. In particular, a stationary observer looking at a moving clock will see that clock “ticking” at a slower rate than an identical clock at rest (and a moving observer will see the same effect for the stationary clock). This has been demonstrated with big old jet airliners<\/a>, and is included in the time corrections to make GPS work, but the shift is minuscule for motion at ordinary human speeds– a clock moving at 10 m\/s would tick slower than a stationary clock by about 10-16<\/sup>s every second, which is way too small to see (about 10s difference over the age of the universe).<\/p>\n

The extraordinary stability of the aluminum ion clock, though, lets them measure this directly. The ions are held in small traps using radio-frequency fields to confine them, and the motion of the ions is very precisely controlled. They normally keep the ions at the trap center, where they move as little as possible (the small residual motion is one of the frequency shifts contributing to the uncertainty in the PRL paper), but for these experiments they offset one of the ions a little bit, so it sloshed back and forth in the trap at a well-known rate. This caused that clock to “tick” at a slightly lower rate (the “ticking” in this case is the oscillation of light connecting two energy levels, with a frequency of 1.21 1015<\/sup> Hz (a bit less than 250 nm in wavelength)), exactly as predicted by relativity. The shift is detectable down to speeds of less than 10 m\/s, thanks to the high quality of the clocks.<\/p>\n

The other big relativistic effect is the “gravitational redshift,” which says that a clock at higher elevation will run at a slightly faster rate than an identical clock at lower elevation (it’s a “redshift” because this manifests as a decrease in the frequency of light sent upwards in a gravitational field). This has been observed in a famous experiment at Harvard<\/a> which used Mössbauer spectroscopy to measure the tiny shift caused by shooting gamma rays up or down a 20-odd meter “tower.”<\/p>\n

In the talk at DAMOP, Chou described using their ridiculously stable ion clocks to measure the gravitational redshift over a much shorter distance– about 33 cm. They put hydraulic jacks under one of their laser tables, and ran for a while with the two clocks at the same height, to establish the difference between their operating frequencies, then jacked one table up by a bit more than a foot, and recorded some more data. Sure enough, the difference between the clocks changed by a part in 1016<\/sup> or so, exactly as predicted.<\/p>\n

These aren’t earth-shattering results, and won’t transform anybody’s understanding of how the universe works. They do, however, provide a beautiful demonstration that relativity is real even in situations where the speeds and distances involved are on a human scale– you don’t need to be moving at half the speed of light in the vicinity of a black hole, provided your clocks are good enough.<\/p>\n

Chou, C., Hume, D., Koelemeij, J., Wineland, D., & Rosenband, T. (2010). Frequency Comparison of Two High-Accuracy Al^{+} Optical Clocks Physical Review Letters, 104<\/span> (7) DOI: 10.1103\/PhysRevLett.104.070802<\/a><\/span><\/p>\n

Schmidt, P. (2005). Spectroscopy Using Quantum Logic Science, 309<\/span> (5735), 749-752 DOI: 10.1126\/science.1114375<\/a><\/span><\/p>\n","protected":false},"excerpt":{"rendered":"

While I mostly restricted myself to watching invited talks at DAMOP last week, I did check out a few ten-minute talks, one of which ended up being just about the coolest thing I saw at the meeting. Specifically, the Friday afternoon talk on observing relativity with atomic clocks by Chin-Wen Chou of the Time and… Continue reading Relativity on a Human Scale<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"1","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[78,19,169,36,238,7,23,141,225,11],"tags":[],"class_list":["post-4731","post","type-post","status-publish","format-standard","hentry","category-conferences","category-experiment","category-lasers","category-meetings","category-optics","category-physics","category-quantum_optics","category-relativity","category-researchblogging","category-science","entry"],"_links":{"self":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/4731","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/comments?post=4731"}],"version-history":[{"count":0,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/4731\/revisions"}],"wp:attachment":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/media?parent=4731"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/categories?post=4731"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/tags?post=4731"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}