It is about "engineering" of the dispersion relation – wave-related phenomena that can have counter-intuitive effects at small scales where multiple waves interfere:
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:
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.
Even the first line of the article: "Washington State University physicists have created a fluid with negative mass, which is exactly what it sounds like."
"Exactly what it sounds like"?? Like...antigravity?? Reads further Ugh. Clickbait.
In classical physics there are 2 types of mass considered (and measured to be equal):
1. gravitational mass used in Newton's Law of Gravity
2. intertial mass used in Newton's Second Law
Measurements show that these two kinds of mass properties of an object are equal (are in linear proportion) to a very high precision. This was suspected and even Newton has considered and measured this, but the first really high precision measurements were done by baron Loránd Eötvös[1].
im sorry, but to me that comic doesn't explain it one bit. the university press in the comic doesn't (appear) to deliberately click-bait the headline, it seems to be a process in your example.
Washington State University appears to to have bypassed that process and gone straight for the misleading, click-bait title. This is disappointing because who better than the University Press themselves to try to provide as much clarity as possible. It's not like they don't have access to good sources...
The incentives of university's PR department aren't to provide accurate reporting on what's happening in said university to other scientists; it's to generate "buzz" around the university so that ultimately it gets more money. In this way, it's no better than regular news sources, and thus the quality is more-less the same.
But they also do have more of a reputation on the line. Universities are expected to be dignified institutions, moreso than your average news org I would think.
The bit I removed from the "Negative mass" article said:
In April 2017, researchers at Washington State University claimed to have created a fluid with negative mass inside a Bose-Einstein Condensate.
I expanded it ever so slightly to read:
In April 2017, researchers at Washington State University claimed to have created a fluid with negative effective mass inside a Bose–Einstein condensate, by engineering the dispersion relation.
Slightly awkward, but hopefully at least a slight improvement.
PS: also added a "disambiguation" thingy at the top ("For negative mass in theoretical physics, see Negative mass."), now realizing it may be of dubious utility.
Thanks. Every time I see a university press release on HN I jump to the comments first, because there's always someone like you who's helpfully debunked the PR spin (no pun intended) to explain what's actually happening.
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.
Well being a layman, I had no idea if I was joking or not. :)
Actually the idea of negative mass made me wonder if this would open the possibility for FTL space travel...Ran into a comment on the internets somewhere that said "no, because the mass is squared in the relation with energy, but the existence of imaginary masses might"
There should be some kind of voting mechanism on HN to automate what you did here (like a [hoax] flag) or something. I almost clicked on the bait but reading your comment clarified things. I was excited because this is a prerequisite of the Alcubierre (warp) drive: https://en.wikipedia.org/wiki/Alcubierre_drive
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.
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.
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.
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.
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.
> 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.
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.
> 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.
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?
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.
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. ;)
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.
This article is not about classical negative mass or exotic matter – which would be major breakthroughs.
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
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...
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.