Friday, April 21, 2017

No, physicists have not created “negative mass”

Thanks to BBC, I will now for several years get emails from know-it-alls who think physicists are idiots not to realize the expansion of the universe is caused by negative mass. Because that negative mass, you must know, has actually been created in the lab:

The Independent declares this turns physics “completely upside down”

And if you think that was crappy science journalism, The Telegraph goes so far to insists it’s got something to do with black holes

Not that they offer so much as a hint of an explanation what black holes have to do with anything.

These disastrous news items purport to summarize a paper that recently got published in Physics Review Letters, one of the top journals in the field:
    Negative mass hydrodynamics in a Spin-Orbit--Coupled Bose-Einstein Condensate
    M. A. Khamehchi, Khalid Hossain, M. E. Mossman, Yongping Zhang, Th. Busch, Michael McNeil Forbes, P. Engels
    Phys. Rev. Lett. 118, 155301 (2017)
    arXiv:1612.04055 [cond-mat.quant-gas]

This paper reports the results of an experiment in which the physicists created a condensate that behaves as if it has a negative effective mass.

The little word “effective” does not appear in the paper’s title – and not in the screaming headlines – but it is important. Physicists use the preamble “effective” to indicate something that is not fundamental but emergent, and the exact definition of such a term is often a matter of convention.

The “effective radius” of a galaxy, for example, is not its radius. The “effective nuclear charge” is not the charge of the nucleus. And the “effective negative mass” – you guessed it – is not a negative mass.

The effective mass is merely a handy mathematical quantity to describe the condensate’s behavior.

The condensate in question here is a supercooled cloud of about ten thousand Rubidium atoms. To derive its effective mass, you look at the dispersion relation – ie the relation between energy and momentum – of the condensate’s constituents, and take the second derivative of the energy with respect to the momentum. That thing you call the inverse effective mass. And yes, it can take on negative values.
If you plot the energy against the momentum, you can read off the regions of negative mass from the curvature of the resulting curve. It’s clear to see in Fig 1 of the paper, see below. I added the red arrow to point to the region where the effective mass is negative.
Fig 1 from arXiv:1612.04055 [cond-mat.quant-gas]

As to why that thing is called effective mass, I had to consult a friend, David Abergel, who works with cold atom gases. His best explanation is that it’s a “historical artefact.” And it’s not deep: It’s called an effective mass because in the usual non-relativistic limit E=p2/m, so if you take two derivatives of E with respect to p, you get the inverse mass. Then, if you do the same for any other relation between E and p you call the result an inverse effective mass.

It's a nomenclature that makes sense in context, but it probably doesn’t sound very headline-worthy:
“Physicists created what’s by historical accident still called an effective negative mass.”
In any case, if you use this definition, you can rewrite the equations of motion of the fluid. They then resemble the usual hydrodynamic equations with a term that contains the inverse effective mass multiplied by a force.

What this “negative mass” hence means is that if you release the condensate from a trapping potential that holds it in place, it will first start to run apart. And then no longer run apart. That pretty much sums up the paper.

The remaining force which the fluid acts against, it must be emphasized, is then not even an external force. It’s a force that comes from the quantum pressure of the fluid itself.

So here’s another accurate headline:
“Physicists observe fluid not running apart.”
This is by no means to say that the result is uninteresting! Indeed, it’s pretty cool that this fluid self-limits its expansion thanks to long-range correlations which come from quantum effects. I’ll even admit that thinking of the behavior as if the fluid had a negative effective mass may be a useful interpretation. But that still doesn’t mean physicists have actually created negative mass.

And it has nothing to do with black holes, dark energy, wormholes, and the like. Trust me, physics is still upside up.


Shantanu said...

Thanks for the nice explanation. note that Luc Blanchet has published no of papers arguing that dark matter consists of negative mass particles

Amit Misra said...

Negative effective mass is appealing explanation of behavior as appeared in media. Thanks for detailed explanation as negative mass still illusive in lab.

J. e. L. said...

Thank you very much, very helpful.

Dionisiatis said...

So is the effective mass something like inertia? What i expected from reading all these headlines was that the m in F=ma was negative. After reading this article it is clear that the reality is more complicated than that but would it be correct to sum this up as the discovery of a case where the inertia of the mas is negative?

Roy said...

The "Effective Mass" phenomenon could be viewed as a quantum simulation of mass, which in this case is negative. By missing out "effective" the Papers are suggesting that it is real mass which has been found which is negative, which is indeed not correct, but there is still a simulation here. Also I believe that the expansion of the BEC was asymmetric confirming this phenomenon only on the right hand side of the expansion - the effective mass to the left was positive.

The nomenclature begins with solid state physics with dispersion relation approximation E = E0 + p^2/2m*. In a valence band E0 might be a maximum, more momentum reduces the energy, so m* is negative.

Uncle Al said...

Meta-materials offer negative refractive indices. A concave lens will focus light. Special relativity is not threatened. A material with negative (electric) permittivity and (magnetic) permeability is merely following the math. The same stunt obtains by immersing a lens in a medium with higher refractive index.

"Scientist creates grant funding out of vacuum!" Sure...but conservation laws demand somebody else got cheated.

yerpa58 said...

Thank you for adding clarity to this interesting topic.

notevenwrong said...

If you want to know where this nonsense comes from, the main problem is that of universities issuing misleading press releases. In this case see

"'Negative Mass' created at Washington State University'

which includes the nonsense about black holes.

PRL seems to have a policy of encouraging press releases of this kind, I really wish they would take action to stop this kind of thing. It doesn't help with the public understanding of science, quite the opposite.

jim_h said...

The "March For Science" should be followed by the "March For Journalism".

Xerxes said...

I think you forgot to note that according to the plot, the researchers appear to have created infinitely negative mass, which obviously resulted in the gravitational disruption of the entire Earth. I don't remember that happening, but I was busy that day.

Unknown said...

Semiconductor scientists & engineers have long been familiar with "effective masses" of charge carriers, although I've never heard of an effective mass being negative.
In fact, the _absence_ of an electron can behave as a _presence_ of a positively charged particle -- a "hole" -- with its own effective mass, mobility, etc.


George Herold said...

From my perspective, effective mass comes from band diagrams in solid state physics, If you wanted you could describe holes (electrons in the valance band) as having a negative mass. Instead we say that they have a positive charge. (You can put the negative sign where ever you like in the equations.)

Bee, I just discovered your blog, Love it!

Edouard Seban said...

Is it possible to conceive the negative mass as a bump of space-time, or a field of negative Higgs? What would confirm a supersymmetry between mass and electrical charge of the baryonic material ...

Michael Raymer said...

Thank you. As a physicists I was also bothered by the hypey title and new stories. Reminds me of the stories about faster-than-c light pulses" about 15 years ago. No - they were no faster than c.

APDunbrack said...

@Roy: I was going to ask something along that line.

My instinctual assumption was that, rather than some particle having a negative mass, some QUASIparticle had a negative mass. Could you also phrase it in this way (in a non-relativistic field theory), or something similar?

If so, is there some sort of relationship between the QFTs that leads to the "black hole nonsense"? Or at least some potential analogy, and that analogy is itself analogous to other analogies between condensed matter and high-energy in terms of QFTs?

Bruce Rout said...

This makes it more and more difficult to invoke the crackpot epithet.

akidbelle said...

British journalists prove the existence of negative IQ.

Anything new under The Sun?