http://www.bbc.com/news/science-environment-39642992

 

"Physicists have created a fluid with "negative mass", which accelerates backwards when pushed."

 

Seems like it might be a big science story. Is it?

 

Anyone care to comment?

Edited April 19, 20178 yr by geordief

There is a good (but short) discussion of this on another forum (https://forum.cosmoquest.org/showthread.php?164841-Negative-masses)

 

The key point is that is is negative effective mass, not really negative mass. Effective mass is shown by a number of things, including for example "holes" in semiconductors (where an electron is missing and so you get a virtual positive charge carrier).

A negative effective mass can be realized in quantum systems by engineering the dispersion relation. A powerful method is provided by spin-orbit coupling, which is currently at the center of intense research efforts. Here we measure an expanding spin-orbit coupled Bose-Einstein condensate whose dispersion features a region of negative effective mass. We observe a range of dynamical phenomena, including the breaking of parity and of Galilean covariance, dynamical instabilities, and self-trapping. The experimental findings are reproduced by a single-band Gross-Pitaevskii simulation, demonstrating that the emerging features—shock waves, soliton trains, self-trapping, etc.—originate from a modified dispersion. Our work also sheds new light on related phenomena in optical lattices, where the underlying periodic structure often complicates their interpretation.

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.155301

Say you have a helium-filled balloon in a car (I've also seen this done with a cork suspended in a jar of water). When the car accelerates, the balloon accelerates in the opposite direction, relative to the car. One might describe the balloon as having negative mass — but that's just a way of modeling the behavior, like (as Strange said) modeling a hole in a semiconductor.

Say you have a helium-filled balloon in a car (I've also seen this done with a cork suspended in a jar of water). When the car accelerates, the balloon accelerates in the opposite direction, relative to the car. One might describe the balloon as having negative mass — but that's just a way of modeling the behavior, like (as Strange said) modeling a hole in a semiconductor.

If the car accelerates wrt a stationary observer what does the helium balloon wrt the same observer? Accelerate less in the same direction?

If the car accelerates wrt a stationary observer what does the helium balloon wrt the same observer? Accelerate less in the same direction?

 

 

Yes, I believe so.

Say you have a helium-filled balloon in a car (I've also seen this done with a cork suspended in a jar of water). When the car accelerates, the balloon accelerates in the opposite direction, relative to the car. One might describe the balloon as having negative mass — but that's just a way of modeling the behavior, like (as Strange said) modeling a hole in a semiconductor.

I've seen this before, its a neat illusion.

If you tie the helium balloon to the floor of a van and accelerate foward, then the balloon leans forward instead of backwards making it behave like negative mass.

 

Why it happens is helium is lighter than air. So the heavier air in the car is what is falling towards the back of the car and the lihter balloon is floating forward over the air. The illusion is created by what we see (balloon) being lighter than what we don't see (air) which is intuitively not usually the case.

The effective mass of holes in conductors is generally positive.

 

It is electron behaviour that can require negative effective mass to explain classically.