
‘Negative Mass’ Created at Washington State University - stonlyb
https://news.wsu.edu/2017/04/10/negative-mass-created-at-wsu/
======
gilgoomesh
The headline is deeply misleading.

This article is _not_ about classical negative mass or exotic matter – which
would be major breakthroughs.

[https://en.wikipedia.org/wiki/Negative_mass](https://en.wikipedia.org/wiki/Negative_mass)

It is about "engineering" of the dispersion relation – wave-related phenomena
that can have counter-intuitive effects at small scales where multiple waves
interfere:

[https://en.wikipedia.org/wiki/Dispersion_relation](https://en.wikipedia.org/wiki/Dispersion_relation)

In this case, the "counter-intuitive" effect is that the particles appear to
move the wrong way when subjected to forces, resulting in an "effective mass"
with a negative sign:

[https://en.wikipedia.org/wiki/Effective_mass_(solid-
state_ph...](https://en.wikipedia.org/wiki/Effective_mass_\(solid-
state_physics\))

The particles still have exactly the same (positive) mass, they're just moving
the wrong way due to wave interference.

If you want to be _super_ misleading... why not call it a "tractor beam"?
They're applying a push force but the particle is moving towards the push.

As others have noted, the abstract for the paper correctly characterizes the
phenomena as "negative effective mass". That word "effective" makes all the
difference.

~~~
SubiculumCode
Thanks. Too bad it isn't real negative mass. How about imaginary number mass?

~~~
nograpes
I thought you were joking, but I came across this:

[https://en.wikipedia.org/wiki/Mass#Tachyonic_particles_and_i...](https://en.wikipedia.org/wiki/Mass#Tachyonic_particles_and_imaginary_.28complex.29_mass)

and was surprised that a plausible notion of imaginary mass exists.

~~~
godelski
Many times people will see an asymptote and say "Hey, what would happen if I
crossed that? What would the math and physics look like?"

Tachyons are actually a great example of this. Taking the relativistic mass
equation and asking "What would happen if I allowed for speeds larger than the
speed of light?" Science is more a process of elimination. What does the math
say? Can we observe what the math says should happen? Repeat.

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c517402
First sentence of abstract: "A negative effective mass ..."

Maybe change the title from clickbait.

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pdonis
As usual with pop science reporting, they fail to give a link to the actual
paper. Here it is:

[https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.11...](https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.155301)

And here is the preprint on arxiv.org, for those who don't have access to
academia paywalls:

[https://arxiv.org/pdf/1612.04055.pdf](https://arxiv.org/pdf/1612.04055.pdf)

TL/DR: The substance in question is a Bose-Einstein condensate, which is not
an ordinary "fluid" and should not be expected to be have like one. This is
simply one of the counterintuitive effects of quantum mechanics showing up in
an experimental setting.

~~~
rubidium
Thanks for the paper link. Fun to see it show up on HN. I did my PhD in this
area and am familiar with the Engels group (one of my papers is cited by
theirs).

You're correct that this is no ordinary fluid, but observing "negative mass"
type behavior is a nice find (a theory paper about this came out just
recently). There is a massive amount of work being done in this area. The
Engels group did a nice job of presenting a paper that captures some of the
progress in the field and the broader impacts of the work.

I must commend the article for doing a good with the PR side of things...
other physicists would have done the same work and buried it in an obscure
language that would have drawn zero interest.

------
charles-salvia
The legendary "Alcubierre drive" concept for faster than light travel relies
on the existence of negative mass to work. Up until now, I have always read
that it was generally assumed that negative mass could not exist, making the
Alcubierre drive little more than a fanciful equation. But... I guess not.

Edit: although, it's not clear to me how significant a breakthrough this
really is, compared with earlier attempts to create negative mass in a
laboratory setting.

~~~
pdonis
The term "negative mass" in the article does not mean the kind of stuff you
would need to make an Alcubierre drive. That is a classical kind of "negative
mass" (actually that term is somewhat misleading, the usual term is "exotic
matter"). The "negative mass" in this article is a quantum kind of "negative
mass", which is something different and does not have the properties of exotic
matter. I've posted a link to the actual paper upthread.

~~~
Filligree
The term 'exotic matter' means... exotic forms of matter. There are quite a
few concepts which fall under the category, and the only thing they share is
that they're all, well, exotic.

Negative-mass matter certainly fits in the category, but 'negative mass' is
still a correct description.

~~~
pdonis
_> The term 'exotic matter' means... exotic forms of matter._

In General Relativity, it has a specific meaning: a substance that violates
one of the energy conditions (usually the weak energy condition). That's how I
was using the term, since that's what's required to make an Alcubierre Drive.

 _> Negative-mass matter_

Meaning what? If you mean the Bose-Einstein condensate referred to in the
article, no, it is not "exotic matter" by the GR definition.

~~~
Filligree
General relativity isn't all there is to physics. If you check e.g. wikipedia
(..I know, but eh), you'll find that the term has been used in cosmology (dark
matter) and quantum mechanics as well. (Everything from strings to gravitons.)

Negative-mass matter is matter which could fit the 'exotic matter' definition
you're using, though. Not the stuff in this article.

~~~
pdonis
_> General relativity isn't all there is to physics._

It's what's relevant to a discussion of the Alcubierre drive, which is what
the post I originally responded to in this subthread was talking about. That's
why I was using the GR definition of "exotic matter", as I've already
explained.

 _> you'll find that the term has been used in cosmology (dark matter) and
quantum mechanics as well. (Everything from strings to gravitons.)_

Which term? "Exotic matter" or "negative-mass matter"?

I have not seen the term "exotic matter" used for any of these things in
actual science textbooks or papers. Pop science sources might use the term
colloquially to refer to all kinds of things, yes, but that just makes the
term useless. Science tries to use precise definitions for a reason.

I have not seen the term "negative-mass matter" used at all in science
textbooks or papers except when they explain why such a thing can't exist. So
I don't know what you mean by this term if you're using it in any sense other
than to refer to the Bose-Einstein condensate described in the WSU paper.

------
AlphaCentury
This is true that the mass of the individual atoms in the BEC is a positive
scalar and we only observe a collective behavior that acts like an object with
negative mass. But, that engineered object (the wavepacket) is behaving like a
negative mass. If we assume that everything that we measure in physics is
collective (a subsystem of another - there is no such thing as an isolated
system), we are not so wrong assigning that 1-D subsystem a negative mass.
Let's close with this question: How are we so sure that a "positive" mass of
an object is not a collective interaction between the object itself and the
rest of the universe?

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kowdermeister
I don't understand the negativity here (pun intended). There's an article and
an abstract, both for different audiences. Every tech / science article
targeted towards layman people will have oversimplifications and here it seems
good enough for me.

~~~
TheOtherHobbes
It's not real negative mass, which would imply antigravity and the possibility
of wormholes and time travel.

It's an experiment designed to highlight a quirky but much less interesting
quantum effect.

They're light years apart - metaphorically, if not quite literally.

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nickpsecurity
I'm looking at it from a lay scientist perspective with this. They're blasting
it with lasers, it's internally unstable, things are flowing out of it, and
evidence of negative mass is that it moved in direction different than
expected. There's a lot of variables here that might cause that movement that
have to be eliminated. Maybe they have but experiments like this leave me
uncertain about the outcome.

Note: Also, they should look for integer/floating-point errors in the
measurement code while they're at it. ;)

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marcosdumay
Ok, no wormholes this time. But very cool anyway (literally too).

~~~
djfm
And no hoverboards, damn!

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outworlder
Yay we got a Mass Effect Drive and... oh, clickbait. Oh come on, this came
from an university, we should expect better than that.

~~~
rspeer
Universities have clickbait departments to promote their research.

They wouldn't do it if it didn't work.

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jeffdavis
If there were really an object of negative mass, would its gravity
(antigravity?) attract or repel objects with positive mass?

~~~
thaumasiotes
As far as I know, there are two definitions of mass:

(1) Mass is defined by gravitational interaction with other mass, F = G m_1
m_2 / d^2 . This would mean that a negative mass, by definition, repels
positive masses and attracts negative masses.

(2) Mass measures resistance to acceleration, F = m a . By this definition, a
negative mass would, if you pushed on it, accelerate _toward_ you. That seems
like the start of a nasty feedback loop. But, according to gilgoomesh's
comment, that's what's happening here.

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dcl
I'm guessing it doesn't 'fall' up instead of down?

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milesf
Pics or it didn't happen.

