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The BBC News article Physicists observe 'negative mass' talks about a recent observation of negative effective mass in a BEC gas; in its description of the experiment the article writes

To create the conditions for negative mass, the researchers used lasers to trap the rubidium atoms and to kick them back and forth, changing the way they spin.

Is the 'spin' mentioned in that sentence the same thing as the spin generally known from particles?

Gerold Broser
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  • Please change the title of this question to one that actually describes your query. – Emilio Pisanty Apr 19 '17 at 17:10
  • @EmilioPisanty I'd be thankful if you could make an appropriate proposal. – Gerold Broser Apr 19 '17 at 17:10
  • I've edited in a proposal; obviously feel free to roll back if you're unhappy with it. – Emilio Pisanty Apr 19 '17 at 17:17
  • @EmilioPisanty Thank you. That's fine for me. Though it's a bit more than just the title. May I now also upvote since the question is more yours than mine now? ;) – Gerold Broser Apr 19 '17 at 17:25
  • https://en.wikipedia.org/wiki/Negative_mass this is not mainstream physics , " It is used in certain speculative theories," – anna v Apr 20 '17 at 11:14
  • @annav I wasn't aware of that. Does that mean BBC News, Science & Environment is not a reputable source and "Prof Peter Engels, from Washington State University (WSU), and colleagues" are, well, what then? On the other hand, WP isn't really quotable, AFAIK. – Gerold Broser Apr 20 '17 at 11:25
  • "non mainstream" is not synonymos with "not reputable". Mainstream physics means physics that is taught in university courses . the paper is peer reviewed so it has passed the first step but it is still in the realm of research and needs new, non standard, theoretical models. – anna v Apr 20 '17 at 11:34
  • see the answers to this question https://physics.stackexchange.com/questions/44934/does-matter-with-negative-mass-exist – anna v Apr 20 '17 at 11:38
  • @annav Dear Anna, can it be that there's something in my comment that can be misunderstood? I'm not a native English speaker, so that can happen on occasion. With "not a reputable source" I intended to refer to BBC News ... (and to WP, for that matter, though my wording was "not quotable" in this respect) not to "non mainstream" physics, physicists, research they perform, papers they produce and models they develop, may they be non-standard, theoretical or whatever. With kind regards, – Gerold Broser Apr 20 '17 at 11:47
  • Dear Anna, thank you very much for the link. Apparently the page it links to contains a lot of valuable information, even high-level knowledge. Apart from that the only circumstance worth mentioning is the fact that it doesn't contain a single occurrence of the word "spin". With kind regards, – Gerold Broser Apr 20 '17 at 11:51
  • @GeroldBroser Thought it might just be useful to give a link to another question on the same news: Negative mass, or just the 'appearance'? What's the acid test?. Although the answer there doesn't speak of spin in particular, it does indeed address the core issue: "Once you push, it accelerates backwards" (in the article, immediately after referencing "spin"). In that sense, I guess, spin shouldn't mean quantum spin – Dhruv Saxena Apr 21 '17 at 21:46
  • @DhruvSaxena Thank you for linking this other question here. Re "I guess, spin shouldn't mean quantum spin" – That's most probably a very good guess (BTW, it was and still is mine, too) but I'm a SW developer for a living since decades. I have to be exact there and this is one of the reasons why I like it. Hence I like to know things for sure, without having to guess about them. That's why I was asking about this particular term here. – Gerold Broser Apr 21 '17 at 22:59

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The short answer to this question is that the "spin" referred to is quantum spin.

A longer answer involves describing the superfluid Bose-Einstein Condensate experimental technique. One technical paper (from 2011) describing a form of the method is Spin-Orbit-Coupled-Bose-Einstein Condensates.

The relevant aspect to your question is that Rubidium atoms (in a Bose-Einstein Condensate) have two quantum spin states selected (by an experimental technique), say |up> and |down>. The momentum of the components is determined by the particle's mass and by a "spin-orbit coupling". This generates a "pseudo-momentum" and associated "effective mass" - in short the component's (pseudo-)momentum is determined not only by its mass and velocity but also by its spin state. This coupling strength is controllable by lasers, so that pseudo-momentum can vary by manipulating laser strengths.

An added complication is that the behaviour of the BEC is described not directly by the Schrodinger equation, but by a non-linear approximation called the Gross-Pitaevksii Equation.

This non-linear equation can have unusual solutions like solitons.

Overall a synthetic quantum environment has been created (in the superfluid) in which one can apparently simulate: negative mass, Galilean non-covariance, and other unusual dynamical pseudo-properties.

Roy Simpson
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