Negative mass

Originally posted by DeepThought
Well, except in the fairly exotic circumstances described in the article referenced in the OP, we don't see negative mass particles so the problem doesn't arise. Since negative effective mass particles are exotic the term "repulsive force" can be read as short for "force that would normally be repulsive.".

I guess I would need to study the physics. I just thought that for all the equations with mass and force in, you wouldn't be able to tell whether the negative was on the mass or on the force.
I guess is a force is between two particles, then you can assign it either value and one particle will be 'negative mass' and the other 'positive mass'.

Originally posted by DeepThought
Unfortunately one needs to subscribe to Physical Review Letters to see the full text. But the abstract states that they engineered a negative effective mass. So as a result of the dynamics of the whole system some of the excitations behave as if they have negative mass. What they have not done is produce a fundamental particle with negative mass, unless anti-matter has this property which I doubt strongly, that will not happen.

The arXiv-version is here:
https://arxiv.org/abs/1612.04055

As you say, what they have demonstrated is just negative effective mass, a collective property of the system. Ultra-cold atoms allow for the extremely flexible tuning and modeling of condensed matter systems. There is no known fundamental particle with negative mass.

Originally posted by KazetNagorra
The arXiv-version is here:
https://arxiv.org/abs/1612.04055

As you say, what they have demonstrated is just negative effective mass, a collective property of the system. Ultra-cold atoms allow for the extremely flexible tuning and modeling of condensed matter systems. There is no known fundamental particle with negative mass.

Thanks for the link, I'll take a look at it.

Originally posted by moonbus
Thanks. Negative mass as such defies imagination, but I can accept a net negative effect as plausible.

Doesnt Einstein's equation E=mc^2 assume +ve mass and +ve energy?

As it comes from E^2 = (mc^2)^2

?

Or more generally:
https://en.wikipedia.org/wiki/Energy%E2%80%93momentum_relation

I believe that in quantum mechanics, the kinetic energy (mass x c^2 ) can be negative.

YouTube

Originally posted by twhitehead
The equations are the facts....

No, that claim merely increases the confusion. You know more than I do about quantum physics, but I do know that the physicists play with the math, looking for symmetry and simplicity, and then check their favorite equations against the mass of data available. The physical events which provided the data are the facts, the equations are not. The data itself can be considered as facts - but only within the context of the relevant events.

Originally posted by twhitehead
I believe that in quantum mechanics, the kinetic energy (mass x c^2 ) can be negative.

I believe that is an example of my point: the equations used are an attempt to force the data into a preconceived view. Quantum physics is weird and we struggle to get a handle on it, but the equations and their results are not the facts of the matter.

The particle/wave duality should highlight the issue enough. There is another and deeper reality than particles and waves, and trying to force that reality into our preferred equations is what causes the paradox.

Originally posted by twhitehead
[youtube]B4VSqAB_JcU[/youtube][/b]

"Surprising only for its beauty". That is what physicists look for. The lack of symmetry and simplicity within an atom knocked physical science upside the head. Of course we grab at straws. But that's not working either. I believe science will continue to grow past the need for for reductive and deterministic explanations.

Originally posted by apathist
The particle/wave duality should highlight the issue enough. There is another and deeper reality than particles and waves, and trying to force that reality into our preferred equations is what causes the paradox.

Although I am in agreement with you that it is more properly the observations rather than the equations that are the facts, you are incorrect here.
The Schrödinger equation provides a highly accurate description of reality. As far as I know there are no known violations of it.
The paradox's and confusions are not with the equations, they are with attempts at interpreting those equations in every day terms. It is the belief in a deeper reality and then guesses about what that deeper reality is that leads to contradictions and confusion. If you stick to the equations, there are no paradox's.

Originally posted by apathist
I believe science will continue to grow past the need for for reductive and deterministic explanations.

And I believe that your sentence needs a lot of expansion to have much meaning at all. Right now it looks like just wishy washy talk.
Science is by definition mostly reductive and deterministic explanations.

Originally posted by apathist
The lack of symmetry and simplicity within an atom knocked physical science upside the head.

No, it didn't.

Originally posted by twhitehead
Although I am in agreement with you that it is more properly the observations rather than the equations that are the facts, you are incorrect here.
The Schrödinger equation provides a highly accurate description of reality. As far as I know there are no known violations of it.
The paradox's and confusions are not with the equations, they are with attempts ...[text shortened]... s that leads to contradictions and confusion. If you stick to the equations, there are no paradox's.

But to claim interpretations must go beyond 'everyday terms' is exactly what a deeper reality means. Look, please, I did not invent this problem with equations and scientific study. Coincidentally, while exploring the current state of ai research, I found this article:
https://backchannel.com/our-machines-now-have-knowledge-well-never-understand-857a479dcc0e

It isn't short, but I found it fascinating. We are outgrowing the old paradigms in unexpected ways. Knowledge is not what we have thought it is. Our system of building a simple model with great explanatory power isn't getting us far enough.

I've been haunting pool halls lately, a little bit. The best expert can't control more than a few banks, and the article looks at that, so I was moved. Even within the deterministic view, we are getting new handles on events, even as we lose our grasp. This is good stuff. I don't want to be a dinosaur.

Originally posted by twhitehead
And I believe that your sentence needs a lot of expansion to have much meaning at all. Right now it looks like just wishy washy talk.
Science is by definition mostly reductive and deterministic explanations.

Quietly, the move past reductive and deterministic explanations is kinda everywhere in all science fields. Because, you know, they want to progress. Take any cutting edge research, and look for this. The clockwork universe never existed.

Originally posted by twhitehead
Or more generally:
https://en.wikipedia.org/wiki/Energy%E2%80%93momentum_relation

I believe that in quantum mechanics, the kinetic energy (mass x c^2 ) can be negative.

[youtube]B4VSqAB_JcU[/youtube]

No, kinetic energy cannot be negative. The energy is the timelike component of the four momentum. One finds this using a generalisation of Pythagoras' theorem. One the Euclidean plane the distance from the origin to some point with Cartesian coordinates x and y is given by:

d^2 = x^2 + y^2

So, given x and y, there are two values for d given by the positive and negative square roots. In normal geometry one would take the positive square root. But in the mass energy relationship and setting the speed of light to 1 we have:

m^2 = E^2 - p^2

Here m is the length of the vector, notice that we get a minus sign in front of the space like component of the four momentum. So there is an additional complication. If the four momentum is space like, corresponding to a faster than light particle, the mass is imaginary. Restricting ourselves to slower than light particles we can choose the comoving frame and set p = 0. This gives us:

E = +/- m

In classical physics one simply ignores the negative root. In quantum mechanics the situation is less simple. Using the naïve procedure one obtains something called the Klein-Gordon equation, but it predicts negative probability densities which meant it was rejected. This spurred Dirac to produce his equation, which predicts a four component wavefunction called a spinor, as the name suggests it naturally describes particles with spin and is the correct description for electrons. The probability densities are positive but the energy can still come out negative. His interpretation of this was to have the vacuum filled with negative energy particles, if one of the negative energy particles were excited into a positive energy state then it would leave a hole which would behave like a positive mass electron with positive charge. So in quantum theory these negative energy solutions correspond to positrons.

This isn't quite the end of the story because we have E^2 = m^2. So we could have both the mass and energy negative. This is being resolved at CERN in experiments on bunches of anti-atoms to see how they respond to gravitational fields. My feeling is that it's positive semi-definite and they won't see anything unusual, but physics is an empirical subject and it's experiment that will drive this.