![\"\"](\"https:\/\/i0.wp.com\/alexenders.net\/wp-content\/uploads\/2021\/10\/gyros.jpg?resize=494%2C378&ssl=1\")
In 1918, 2 years after Einstein unveiled his theory of General Relativity, two Austrian physicists (Josef Lense and Hans Thirring) noticed an intriguing side effect: Einstein’s equations predicted that large, spinning bodies (like Earth) should actually drag spacetime with them as they spun. It wasn\u2019t until GP-B came along, 86 years later and 44 years after the experiment was first proposed, that frame-dragging could actually be measured.<\/p>\n\n\n\n
The frame dragging effect, even after spending a year in Earth orbit 400 miles up, is tiny: it’s like measuring the width of a human hair from a quarter mile away. How do you do that? With four of the roundest objects ever made by mankind: the superconducting niobium coated fused quartz spheres. Second to neutron stars, these are probably the roundest objects in the universe.<\/p>\n\n\n\n
Spun at 4,000 RPMs, these things became the world’s best gyroscopes (a million times better than navigational systems) and Did. Not. Move. The satellite pointed itself at a faraway star, spun up the gyroscopes, and watched as spacetime stretched underneath it by the slightest of degrees as the gyroscopes stayed in their original orientation.<\/p>\n\n\n\n
There was a problem, however. There were “hot spots” (my term) where some charge would build up on the spheres and on the sphere housing. These hot spots would randomly cause a torque that would jerk the sphere left or right – only by a tiny amount, but even that tiny amount was a hundred times greater than the effect they were looking for. The team spent a frenzied 18 months trying to analytically account for these random jumps of the spheres. They could never reduce all of them.<\/p>\n\n\n\n
Why the “B” in Gravity Probe B? Because Gravity Probe A was launched in 1976, and measured how time goes slower close to a massive object like Earth. GP-A went about 6,200 miles straight up, and confirmed that up there, clocks run about 4 parts in 10 billion faster \u2013 just like Einstein said they would.<\/p>\n\n\n\n
If you want to dive waaaay down the rabbit hole of information, there is a technical journal called “Classical and Quantum Gravity” that devoted an entire issue to Gravity Probe B. You can find a great summary and all of the technical papers on their website<\/a>.<\/p>\n\n\n\n For a slightly more digestible summary, Stanford has a good writeup with lots of pictures<\/a>. Happy reading.<\/p>\n","protected":false},"excerpt":{"rendered":" The Gravity Probe B satellite \u2026 probably the most precise measurement you’ve never heard of. In 1918, 2 years after Einstein unveiled his theory of General Relativity, two Austrian physicists (Josef Lense and Hans Thirring) noticed an intriguing side effect: Einstein’s equations predicted that large, spinning bodies (like Earth) should actually drag spacetime with them … Continue reading Gravity Probe B<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"nf_dc_page":"","_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_post_was_ever_published":false},"categories":[1],"tags":[],"class_list":["post-118","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"jetpack_sharing_enabled":true,"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/alexenders.net\/index.php?rest_route=\/wp\/v2\/posts\/118","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/alexenders.net\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/alexenders.net\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/alexenders.net\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/alexenders.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=118"}],"version-history":[{"count":1,"href":"https:\/\/alexenders.net\/index.php?rest_route=\/wp\/v2\/posts\/118\/revisions"}],"predecessor-version":[{"id":120,"href":"https:\/\/alexenders.net\/index.php?rest_route=\/wp\/v2\/posts\/118\/revisions\/120"}],"wp:attachment":[{"href":"https:\/\/alexenders.net\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=118"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/alexenders.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=118"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/alexenders.net\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=118"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}