CINXE.COM
Just a moment...
<!DOCTYPE html><html lang="en-US"><head><title>Just a moment...</title><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=Edge"><meta name="robots" content="noindex,nofollow"><meta name="viewport" content="width=device-width,initial-scale=1"><style>*{box-sizing:border-box;margin:0;padding:0}html{line-height:1.15;-webkit-text-size-adjust:100%;color:#313131;font-family:system-ui,-apple-system,BlinkMacSystemFont,Segoe UI,Roboto,Helvetica Neue,Arial,Noto Sans,sans-serif,Apple Color Emoji,Segoe UI Emoji,Segoe UI Symbol,Noto Color Emoji}body{display:flex;flex-direction:column;height:100vh;min-height:100vh}.main-content{margin:8rem auto;max-width:60rem;padding-left:1.5rem}@media (width <= 720px){.main-content{margin-top:4rem}}.h2{font-size:1.5rem;font-weight:500;line-height:2.25rem}@media (width <= 720px){.h2{font-size:1.25rem;line-height:1.5rem}}#challenge-error-text{background-image:url(data:image/svg+xml;base64,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);background-repeat:no-repeat;background-size:contain;padding-left:34px}@media (prefers-color-scheme:dark){body{background-color:#222;color:#d9d9d9}}</style><meta http-equiv="refresh" content="390"></head><body class="no-js"><div class="main-wrapper" role="main"><div class="main-content"><noscript><div class="h2"><span id="challenge-error-text">Enable JavaScript and cookies to continue</span></div></noscript></div></div><script>(function(){window._cf_chl_opt={cvId: '3',cZone: "pubs.aip.org",cType: 'managed',cRay: '9293d6e08c404d87',cH: 'bYDEfOUlHRIPIyrYUsYbUyDndGI8eqZ95uodqXkYGpM-1743465973-1.2.1.1-eFQ.mfu_PnOZic4cIoqb.6oMleB20C4tQdB38cDZztUk9xpRfIEUv3Z11UhZARTp',cUPMDTk: "\/pte\/article\/44\/1\/40\/274174\/There-Is-No-Really-Good-Definition-of-Mass?__cf_chl_tk=x3XXjV8bES7xE3c6MwvH2XqYxMpc_5q1smO_po4SibA-1743465973-1.0.1.1-YgbOW1B5IclVTI6UVW2S12fiSr1IcYgTpQpK6WI0Evo",cFPWv: 'b',cITimeS: '1743465973',cTTimeMs: '1000',cMTimeMs: '390000',cTplC: 0,cTplV: 5,cTplB: 'cf',cK: "",fa: "\/pte\/article\/44\/1\/40\/274174\/There-Is-No-Really-Good-Definition-of-Mass?__cf_chl_f_tk=x3XXjV8bES7xE3c6MwvH2XqYxMpc_5q1smO_po4SibA-1743465973-1.0.1.1-YgbOW1B5IclVTI6UVW2S12fiSr1IcYgTpQpK6WI0Evo",md: "Ms1JoWqB6qfJ_gBnosjcCcZdsQRpYI2EkRvtuRVEx0w-1743465973-1.2.1.1-Bl95GkXK5Y1a2a94qTSJYjofwfrIfQ5ZF9r1KfpzhAQgVTvGVWqWlghdhIjACes3rnl2ou02j9S2TDMyeC1ajLixAH6wFZM8glnkNft0yw_NiWWB6P3TtQTaeWN76_7lxCyBixaFbXGBaIECmQYVw3fEmt0GJ0D8XCyg4CrppMdJnmADCRcZkhYqK8d1V2m2GOLw7HQiFlRPkOP8vPRF88eWuxDhIpK00L0PdcNX7p_d0s5f3aXjpqSWzpci7iMzgKt.4m3rz5l5SO2lnlZgvzPerHn8RucPQE7StV.SRXLJkaD5iTCYz0LeP.F8e_WbeNQ8hDQAEDjkZpyPxPIsTPDjf1ebAqL6_zqMEsHAjpKderUc3ktDgJAKjBntRwqN1ue2I6o8b1zhzMN3HYzHFKcir6OIvrE4y.ZFNQpjKhgy.8lRp0ESi4kwnilnZBouZJlO57Wh1t0SpoPJQKl4QfmkQjQ7oc2v2m8FJrp7lyE5aj3TEEEVvP6mDgqQD1AdIv84Mj9j4xDxrEdu6PBagJ4Huzu2MNSH3igK7gs0Hc4pHyQ8.CJHJm_L0nnxvfytXhxM78YzsAubpTqimrTThWh.pGKpysjvzDsWF6xoYcjOakUmbWdkd1JCkGHVDCNUuRHHguznJLrvG7DuFtrrF8DEH3IG831J6TBfdBhWE2Ym0CWsvOdZhlIUYXr_eKW.VAJvBm0wJSzvDR1AMBJHcFImTn11Lcz__KPxIElE7KoBIm6aPF_j372DtSiyy6ctbTmMKBhdZb4wzXK8frQ3a4h17auUs0hDHCGeQZIjg0CJsT71Ush36qgpQ2pA5_.wc1siz8TKe3CJfxOiK3aRLOc._z4lTg5seE8XK3mlPSxZlkX74fWbgOlW5p.2wBiawtDWMrhPSTHnu7HmMSruy_18dp7K.y.cW1_a2.fhgQ6Z5M5xYJ0XNw_HWcTvMvm52_rSsKDdLV0Q5yaY3G8pvS1hr.ADNuXe9BPQzZ_mCgbWaDnwe5OKE_34UnLT60YDH4Wp37MmO.HDMo_U_oroVizkaILXRdlzj9lMVgmdwrJupOR0AZoS3Br9gJwF6DrUhsBMC5ed8VPy828QiJ53eWDohRIiy4DuA2mgDhi4VeYvQliAwjHGSbdF5ArntQDH2YbS0nTThkZgmBk8wrhEhGvFDTilSU6jwpzFb6WhyEVOjKsQMv3LE6WJhrjUhJT6B9BeB_8t.78SDEKVJ.xwZj9K2JDE0.J6_9_TgEpN5EZ5EldNOHhp_aPrY0MfK3xX_AAY_G9JNZG9v_t626SZuGJ87th_Y6jFJHy0JIv0QOoSMuU82LdrSfIvUlwYb.5uuXUFt2Rtge6ugZ6aw3IyFO9pXkEQe1yrjpufot3i7KxDzNF0box9N8GGzrz7UoHT",mdrd: "fomwpZCnybQ69my4u4o2d_n9wr9tlGFaqZPTQ.tHi6E-1743465973-1.2.1.1-Bni6s4X3e0Fyje4ZVq9Hw9Kulci6lxFzTO2gwM8w7FYLgYeaxBnOXfaPBWJHbaBRJbyTf71ejqq80M42X0qQVtrdsRf9eeRNRpwIzXH.EWBwEBUo0lBxAlfgF27YnGJbh.mqhPLwlf0hvpRnJIBwur6V6SpdmMLFBPhTp1NbsJNEoxieBOI0idxWkmVjf2QgBgWIVZri.Vg0qRKZ7N9x.zI_Pl8HZnZwhXnQrl0I8oz3pQpl9cXZymAR9gTWa1sA8BvOIWuk9ztGYAiBdczSK6EWL8wYEcMhaOQFhB5OCqfh_OF6gGgk4V2bJraJZiTBQNlT5626P8_6xINiHo4z9bSTNtVF.O.xMwV7wyq8FtQGkktVbMBEXb_ZhUdcRCveFM7OzAg.431ys4XvsRFRFmSJc3Fm2SzdpOR_Twc.hUMhLk24ko5NHCDO5PYTHwzjliJxwa2LtGZjxaBjJ1QnNFD5XkpyjzN2XDvMg1atuKmh5ukwBDmG9.Yp516o8fhNHkiI6ueIjXsOoxoZQyGRtZ_0JnEsxPhr.rx5LCJis.n2j46d6xhJX8XGMHhua1MJpY3v993nBHM5KoaqkNY.fF4k9IBa2IDpq40A9cjoY5iE4mu8LSP6Lmgfes7TEG5UlQwyikpuMWNl3DNEL92FKe0EfSHdk_JtEde74wZStCy1sQ_ERHqhzUgQav9_ybK.ViQ6z7aZv6GEeFtc31p5.Rldwepu5zL9hkRUDwU8Twarbw5kLy3l0ip83smRfhvxmwGeXiloe5PNGI.dkJh9upHDd8vPPN8cvFhQhX_45eMe6DHcOyWgYnOmAo10_5iCKUfZxVsgmrQdHy542ZmKRgBSzd4eb9xamYvbE3gF0yBTW723zbHiBAyxfN_BPvC8kzT5vY6HpLh4VkgfTAhWDVmmrtOvxOz6QpucCgVP_kOcrjJbzUMmOWBaH5UQ01lfuCpXoj4wsyJ7Ug8LmOmf1d_JSwUhRO8WCksEknL5lY4y9tBH_yEcDftYXkTHCgy6uO8ICq8o6jjs5WswE_cJCZXclplrcrECbH49Bbuo4TiiewYo2oychjit7d7OxfA.fh4tAht3kMPljD4ZWLsMKmcykDmfbiRrKmsWPNNnS3JTtrSeyeAyV3FEPNpKFiCchuyyCG9gJTwroJ9hX3wh7CFy0Tqcv6AOsuwKtVvOuu6VI3SpTAYx0JHkxpsIvK4qLdZmWgrEZckaNmx8PryxYEddP486fK2eUXfV9WHN7.F3ATaj._Htt_F7nHMqemeys4Xw27jlmAlrzuk5zJW.Ax15d6tXmmffY519KIjMcUa5meVY8.NhttyeUcwkjYY98uSvfkYqcW5eaep0PGX5CC1MegF13SN0Au1OlDlUyPt.QoMDz1HskVH37UGfWuOewgF5ZMu9aa50Z2pbgF8YJKOEIpahJ3IZ7oCpRId3v58BPY2FS6twWEf27U1ZZSfJtBsEf6.fahlihnMFyPFESnrUDUr6oJ8veeCH2L8U_tM3S984eq7JaU0g6bM1KZxWHVMTE6zRhei74R6toH6aQ9ZHMgK_Jmjd_Tc5t5yVqx9UYrGfl26HiY_RwcG3OShJyhj3NKfEhCxoACWvBo3JHiH.44ezuCqIOXIV9JdDV5Kr1oJzVKIeUH1qan6piPll3oRAaYDOs2TERABlSPw9Oi8zQ3qYfij2RdL2qe6qisjdGqT7qZyQ5Lhi2ZvACslrL8LtdVXl9cGz9Lqw6ZxsOINU5XRNWrGM4FUVnF6Ejl_wTk5M37V4YqL1p.t903D4kvR34UYx8WPL8VlPwdUaqehQ7IVMvey8oc9EK1GZxdKuv0En_HPu5yc8z_2jgUpEm7rkbNUFXbPpuvDUG67MNebfS1NVQxvqjjCfEbOYUMYAPF6pNRR09NcNXKao34I1xoSUa9zpv31x3WGR7TxhDVgC2tbDg3W0o72n9Etqn3_y2Kg1EtH2u3S5T8ruTjHL_P3PQ2173xyL6.JtaezOsDfaSNItKC8JEC.iBjUnclpYme2hnk3nYZfAYpY7uWJNjSyPxjumpuLdjRcdJmrPCR_8sQKp1H.gx6_nKgqTZLo"};var cpo = document.createElement('script');cpo.src = '/cdn-cgi/challenge-platform/h/b/orchestrate/chl_page/v1?ray=9293d6e08c404d87';window._cf_chl_opt.cOgUHash = location.hash === '' && location.href.indexOf('#') !== -1 ? '#' : location.hash;window._cf_chl_opt.cOgUQuery = location.search === '' && location.href.slice(0, location.href.length - window._cf_chl_opt.cOgUHash.length).indexOf('?') !== -1 ? '?' : location.search;if (window.history && window.history.replaceState) {var ogU = location.pathname + window._cf_chl_opt.cOgUQuery + window._cf_chl_opt.cOgUHash;history.replaceState(null, null, "\/pte\/article\/44\/1\/40\/274174\/There-Is-No-Really-Good-Definition-of-Mass?__cf_chl_rt_tk=x3XXjV8bES7xE3c6MwvH2XqYxMpc_5q1smO_po4SibA-1743465973-1.0.1.1-YgbOW1B5IclVTI6UVW2S12fiSr1IcYgTpQpK6WI0Evo" + window._cf_chl_opt.cOgUHash);cpo.onload = function() {history.replaceState(null, null, ogU);}}document.getElementsByTagName('head')[0].appendChild(cpo);}());</script></body></html>