Physicists Have Finally Seen Traces of the Long-Sought 'Axion' Particle (livescience.com) 40
fahrbot-bot shares a report from Live Science: Scientists have finally found traces of the axion, an elusive particle that rarely interacts with normal matter. The axion was first predicted over 40 years ago but has never been seen until now. Scientists have suggested that dark matter, the invisible matter that permeates our universe, may be made of axions. But rather than finding a dark matter axion deep in outer space, researchers have discovered mathematical signatures of an axion in an exotic material here on Earth. The newly discovered axion isn't quite a particle as we normally think of it: It acts as a wave of electrons in a supercooled material known as a semimetal. But the discovery could be the first step in addressing one of the major unsolved problems in particle physics.
The research team worked with a Weyl semimetal, a special and strange material in which electrons behave as if they have no mass, don't interact with each other and are split into two types: right-handed and left-handed. The property of being either right- or left-handed is called chirality; chirality in Weyl semimetals is conserved, meaning there are equal numbers of right- and left-handed electrons. Cooling the semimetal to 12 degrees Fahrenheit (minus 11 degrees Celsius) allowed the electrons to interact and to condense themselves into a crystal of their own. Waves of vibrations traveling through crystals are called phonons. Since the strange laws of quantum mechanics dictate that particles can also behave as waves, there are certain phonons that have the same properties as common quantum particles, such as electrons and photons. [Study co-author Johannes Gooth, a physicist at the Max Planck Institute for Chemical Physics of Solids in GermanyGooth] and his colleagues observed phonons in the electron crystal that responded to electric and magnetic fields exactly like axions are predicted to. These quasiparticles also did not have equal numbers of right- and left-handed particles. (Physicists also predicted that axions would break conservation of chirality.) "It's encouraging that these equations [describing the axion] are so natural and compelling that they are realized in nature in at least one circumstance," said MIT theoretical physicist and Nobel laureate Frank Wilczek, who originally named the axion in 1977. "If we know that there are some materials that host axions, well, maybe the material we call space also houses axions."
The research was published in the journal Nature.
The research team worked with a Weyl semimetal, a special and strange material in which electrons behave as if they have no mass, don't interact with each other and are split into two types: right-handed and left-handed. The property of being either right- or left-handed is called chirality; chirality in Weyl semimetals is conserved, meaning there are equal numbers of right- and left-handed electrons. Cooling the semimetal to 12 degrees Fahrenheit (minus 11 degrees Celsius) allowed the electrons to interact and to condense themselves into a crystal of their own. Waves of vibrations traveling through crystals are called phonons. Since the strange laws of quantum mechanics dictate that particles can also behave as waves, there are certain phonons that have the same properties as common quantum particles, such as electrons and photons. [Study co-author Johannes Gooth, a physicist at the Max Planck Institute for Chemical Physics of Solids in GermanyGooth] and his colleagues observed phonons in the electron crystal that responded to electric and magnetic fields exactly like axions are predicted to. These quasiparticles also did not have equal numbers of right- and left-handed particles. (Physicists also predicted that axions would break conservation of chirality.) "It's encouraging that these equations [describing the axion] are so natural and compelling that they are realized in nature in at least one circumstance," said MIT theoretical physicist and Nobel laureate Frank Wilczek, who originally named the axion in 1977. "If we know that there are some materials that host axions, well, maybe the material we call space also houses axions."
The research was published in the journal Nature.
Arxiv (Score:4, Informative)
Arxiv preprint of the Nature article: https://arxiv.org/abs/1906.045... [arxiv.org]
Proof by Analogy (Score:5, Informative)
If I'm remembering/understanding correctly, this isn't showing evidence of detection of actual axions. Rather it is comparing two physical phenomenon that happen to share the same equations (like say, resonance occurs repeatedly in different parts of physics). They detected behavior in one system that is mathematically analogous to the predicted behavior of axions in the other system. But just because our current best models of these two systems has identical math doesn't mean that the actual behavior is identical when we get down into details we haven't been able to measure before. So it is interesting, but not evidence of axions.
Re:Proof by Analogy (Score:5, Insightful)
The abstract, from the arXiv article linked by a poster above:
An axion insulator is a correlated topological phase, predicted to arise from the formation of a charge density wave in a Weyl semimetal. The accompanying sliding mode in the charge density wave phase, the phason, is an axion. It is expected to cause anomalous magneto-electric transport effects. However, this axionic charge density wave has so far eluded experimental detection. In this paper, we report the observation of a large, positive contribution to the magneto-conductance in the sliding mode of the charge density wave Weyl semimetal (TaSe4)2I for collinear electric and magnetic fields (E||B). The positive contribution to the magneto-conductance originates from the anomalous axionic contribution of the chiral anomaly to the phason current, and is locked to the parallel alignment of E and B. By rotating B, we show that the angular dependence of the magneto-conductance is consistent with the anomalous transport of an axionic charge density wave.
TL/DR: Axions are sliding modes in charge-density waves (phasons) in a Weyl semimetal. The authors experimented with a Weyl semimetal (TaSe4)2l and found behavior consistent with the presence of axions. Sounds to me like they have indeed found evidence of axions. Or to put it another way, if it looks, swims, and quacks like a duck, it's probably a duck.
Re:Proof by Analogy (Score:5, Informative)
Here's a better popular article that explains things better: https://gizmodo.com/physicists... [gizmodo.com]
It's a quasi-particle: a collection of known particles (electrons, in this case) inside a condensed matter system that act like a single particle with anomalous properties. From the arxiv prepring:
> However, it has been recently understood that axions can emerge as collective electronic excitations in certain crystals, so-called axion insulators.
This is similar to the quasi-particles with "negative (effective) mass" announced some time ago: http://www.bbc.co.uk/news/scie... [bbc.co.uk]
Re:Proof by Analogy (Score:5, Funny)
Don't you mean quarks like a duck? :-)
Re: (Score:3)
Don't you mean quarks like a duck? :-)
You're right. Dammit. /thread
Re:Proof by Analogy (Score:5, Informative)
The described phenomenon is a quasiparticle, not an actual particle. There is an important difference. So no, the authors did not discover axion particles; instead, they made axion quasiparticles.
The discovery of an actual axion would be big news; to me, this discovery is more "meh."
Re: Proof by Analogy (Score:2)
Per the abstract, and another post on this thread, an axion is a quasiparticle.
Re: Proof by Analogy (Score:2)
Oops, never mind. I just looked more closely at TFA that As_I_Please provided. An axion is a hypothetical, but actual particle. The researchers found an axion-like quasi-particle. I fold. I'll let myself out.
Not proof at all (Score:3, Interesting)
If I'm remembering/understanding correctly, this isn't showing evidence of detection of actual axions.
You are exactly correct. This is no different than saying that someone who ran a computer simulation of axions has discovered axions. It is certainly interesting that they have found a physical system that should mimic axions, if they exist, since if they observe behaviour different to prediction it might suggest that theorists missed something when predicting how axions behave although it might equally be due to the fact that their system does not perfectly simulate axions...so essentially just like a com
Re:Not proof at all (Score:5, Insightful)
This is no different than saying that someone who ran a computer simulation of axions has discovered axions.
This is quite a bit different from a computer simulation, which can be programmed with arbitrary laws. It shows that quantum mechanical systems in the real world behave in exactly the way as is postulated for axions. This is much stronger evidence of the validity of the theory than any digital computer simulation which is limited by precision, the possibility of unknown physics, unrecognized defects in the code, and so forth. Still not any sort of detection of the axion particle, true, but it is still a significant physical validation of the theory and must be considered to raise the likelihood that axions are real.
Same simulation, different hardware (Score:2)
This is much stronger evidence of the validity of the theory than any digital computer simulation which is limited by precision, the possibility of unknown physics, unrecognized defects in the code, and so forth.
Absolutely not true. This is exactly the same. The system is an artificially devised setup that is designed to behave the same way as a hypothetical axion. If the system showed deviations from expected axion behaviour the response would NOT be to start re-writing the laws of physics to explain this but it would be to investigate the system to see if the way that it _simulates_ axions leads to inaccuracies under certain conditions. This is _exactly_ the same response as you would have to a computer simulati
Re: (Score:1)
Yup, a much better title would have been "Scientists discover quantum system that expresses same math as long sought Axion particle".
Re:Proof by Analogy (Score:4, Interesting)
Re:Proof by Analogy (Score:5, Interesting)
Rather it is comparing two physical phenomenon that happen to share the same equations
So, like "entropy". Entropy is nothing but some math that happens to work for a variety of systems. Even so, it's a useful tool. Axions, or axion-like paricles, or axion-like pseudoparticles: any of these would be interesting.
But just because our current best models of these two systems has identical math doesn't mean that the actual behavior is identical when we get down into details we haven't been able to measure before.
All of science is based on faith in some unprovable assumptions that can be summed up as "induction works". Everything in all of science can be described as "a model that describes the current behavior but might not be right for details we can't yet measure". And sometimes induction doesn't work, and you get it wrong. Sometimes you spend centuries on the science of white swans, blissfully unaware that 85% of swans are dark. That's science.
So it is interesting, but not evidence of axions.
You seem to be confusing "evidence of" with "conclusive proof". This is "evidence of". The weight of evidence is not so heavy, as to compel belief, but that is built one bit of evidence after another.
Re: (Score:3)
The problem with that argument is that that's the only way we understand the "fundamental particles" as well. So if you say "Those aren't really axions." you have to explain why you accept electrons and photons. The only argument I can think of is utility, but it could turn out that these axions are also useful.
Quasi particles aren't elementary particles (Score:5, Insightful)
Please don't confuse discovering a quasi particle with certain properties with the existence of an elementary particles with said properties. Quasi particles are interesting in their own right, but their only value for any putative elementary particle is as a sanity check for the equations describing them and their interactions.
Re: (Score:3)
The difference isn't so sharp when it comes to condensed matter physics [illinois.edu]
Many of the particles dealt with in condensed matter are "pseudoparticles" in the sense of being collective phenomena exhibited by the fields representing the more fundamental electrons. We still use wave-functions for these.
In fact, since we don't really know the deepest nature of the fields underlying observed phenomena, it's quite likely that all the particles with which we are familiar (and which we represent by wavefunctions) are collective effects in some deeper more fundamental fields.
We don't even know for sure if there is a deepest level. Could be pseudoparticles all the way down.
That could be more general. Is there really a difference between an "elementary particle" and a pseudoparticle? There doesn't seem to be one in quantum field theory, so may just be how humans like to label things.
Cool (Score:2)
Well, actually not that cool. 12f isn't that cold, it has already got down below that a couple of times this season. (its officially still fall, even though the equinox was 2 months ago)
Now prepared for the new trolling (Score:1)
So, "axion friend" will probably become the new "invisible friend" epithet.
Good that Slashdot keeps me informed of these things.
Re: (Score:1)
And, here is proof that vibrating strings exist: (Score:2)
"If we know" (Score:5, Interesting)
Oh, dear. ""If we know that there are some materials that host axions, well, maybe the material we call space also houses axions.""
This kind of logic is the basis of sympathetic magic, and it is not reliable.
Re: (Score:2)
It's unknown if there's any life on Mars. If someone made a life form in a lab here on earth that could live on Mars, it would still be unknown. And you could say that trying to measure something's "unknowniness" is a meaningless exercise, but I think most of us would take that as evidence corroborating the possibility for life on Mars. This is somewhat similar, they haven't found the particle they're looking for. But they've proved that some other particle behaves in the same strange way it's hypothesized
All units are arbitrary. (Score:1)
I like Celsius better too, but it also uses very human-centric arbtrary points to define its scale.
I'm much more particular to Planck units.
Re: (Score:2)
For some reason your note is showing up as a reply to my note about sympathetic magic. I'm assuming that was unintentional.
While Planck units might be more "pure" in theory, they're awkward to measure precisely without some precise guidance in the tools and units used for making that measurement. The SI units give means to definite the units by measurement. Planck units are derived from physical constants that are not derivable, and which have to be measured to discover their values. So I'm sorry to say tha
Re: (Score:2)
I like Celsius better too, but it also uses very human-centric arbtrary points to define its scale.
Celsius is also better to a large majority of everyday humans. We need to stop valorizing the obsolescent.
They have NOT found the Axion! (Score:2)
TFS misrepresents this.
They have found an analog of the axion in a supposed analog of spacetime.
Phonons are not real particles, but sound waves in real particles. So it is merely an analogy.
It is still great, since that analogy goes suprisingly far and is surprisingly useful. So it does offer a hint that axions can be a real thing.
It is still only an analogy though. And that analogy can break any time. So take it with a large grain of salt.
Ok, so what? (Score:2)
Dark Matter Nonsense (Score:1)
The Dark Matter camp is so desperate they just don't stop.
1. The Dark Matter models have a material that doesn't interact with normal mass.
2. Axions are predicted to exist in Weyl semi-metals.
3. Axions are predicted to not interact with normal mass.
4. Dark Matter must be axions without the Weyl semi-metals.
Even assuming the models, #4 doesn't make any sense - the normal mass of the semi-metals /would/ interact with other normal mass and axions are a quantum phenomenon within matter, not stand-alone.
Dark Mat
Re: (Score:2, Informative)
But you make it look as if the horrible hack was a bad thing. Introducing Aether to explain light was also a horrible hack. but it got us the right formulas for Special Relativity. Until today, we are using the Lorentz factor and the Lorentz transformation introduced by Hendrik Antoon Lorentz' Aether Theory of 1893.
Expressing dislike for the solution does not make the p
Re: (Score:3)
Everyone knows Dark Matter looks like a horrible hack introduced to save some theories
Everybody knows no such thing. Dark matter is literally a direct observation, not even a "theory". It is the mass distribution that we can directly observe in the Universe by gravitational attraction, but that we know is not any sort of matter that we currently have detected in any other way, or have a theory of. It is a real thing that must be explained.
Thus far we have had success at ruling possible things it might be. Neutralino? Looked like a good candidate, but it has now been excluded. So too nearly a
Re: (Score:2)
Inventing an literarily invisible type of matter (it does not interact with photons) is a horrible hack. But apparently it is one we need, because there are so many independent observations which point to the fact that there has to be so much non-baryonic matter that it outnumbers visible (photon-interacting) matter by a factor of 5:1. Attempts at modification of existent theories (MOND, TeVeS et.al.) to avoid the introduction of a new type of matter proved t
Re: (Score:2)
Dark matter is real - we can detect and map its distribution on the scale of stellar clusters and larger. It does "interact with normal mass" by gravity.
The existence of dark matter is not speculative, nor inferred. There is no serious dispute that it is real. We simply don't know what it is. We are working on that, and have success at disproving a whole bunch of plausible theories, which is real progress. The idea that studying dark matter isn't "real research" is idiotic or ignorant, or both.
An axion is not a particle. (Score:1)
Do you want a Death Stranding? (Score:1)