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The first full characterization measurement of an accelerator beam in six dimensions will advance the understanding and performance of current and planned accelerators around the world.
A team of researchers led by the University of Tennessee, Knoxville conducted the measurement in a beam test facility at the Department of Energy's Oak Ridge National Laboratory using a replica of the Spallation Neutron Source's linear accelerator, or linac. The details are published in the journal Physical Review Letters.
"Our goal is to better understand the physics of the beam so that we can improve how accelerators operate," said Sarah Cousineau, group leader in ORNL's Research Accelerator Division and UT joint faculty professor. "Part of that is related to being able to fully characterize or measure a beam in 6D space -- and that's something that, until now, has never been done."
Six-dimensional space is like 3D space but includes three additional coordinates on the x, y, and z axes to track motion or velocity.
"Right away we saw the beam has this complex structure in 6D space that you can't see below 5D -- layers and layers of complexities that can't be detangled," Cousineau said. "The measurement also revealed the beam structure is directly related to the beam's intensity, which gets more complex as the intensity increases."
Previous attempts to fully characterize an accelerator beam fell victim to "the curse of dimensionality," in which measurements in low dimensions become exponentially more difficult in higher dimensions. Scientists have tried to circumvent the issue by adding three 2D measurements together to create a quasi-6D representation. The UT-ORNL team notes that approach is incomplete as a measurement of the beam's initial conditions entering the accelerator, which determine beam behavior farther down the linac.
Source: https://www.sciencedaily.com/releases/2018/08/180810132608.htm
(Score: 1, Interesting) by Anonymous Coward on Monday August 13 2018, @10:36AM (4 children)
Here's what's actually happening: particles in an accelerator beam have both positions and velocities. There are nontrivial correlations between positions and velocities, that only become apparent once you measure everything at once or, in other words, in "6D phase space". These guys managed to make the measurements, hence all the excitement.
The "6D" is misleading, to say the least, and everyone involved in the publication knows it.
I personally don't see why they didn't just say "7D", since they're doing these measurements over time as well (although it's true that they average over time however).
For instance in various field sports it would be trivial to discuss the 4D trajectory of players (and anyone who wants to avoid or cause collisions between players is implicitly thinking in "4D"), but nobody calls it that.
Another analogy, in case the previous one failed: if you're going to write a pool (billiard, the game with the sticks and balls on a green table) simulator, you need to program in 4D at a minimum in order to approximate the physics involved.
(Score: 3, Informative) by PiMuNu on Monday August 13 2018, @11:08AM (3 children)
Once mass is known, you need six variables to fully describe a particle's trajectory.
Typically in accelerator physics one defines a reference plane at which the measurement is made and measure in horizontal and vertical position and time, along with the conjugate momenta (e.g. velocities). It is equally valid to define a point in time and make a measurement in the 3 positions and conjugate momenta. The conjugate of time is energy, while the conjugate of z-position is z-momentum. It would be incorrect to claim a 7-dimensional measurement as the longitudinal position and time are necessarily correlated. One can define an 8-dimensional phase space, by defining the proper time to be an independent variable, in which case the momenta are related by E^2 = p^2+m^2 with E total energy, p momentum and m mass; speed of light is 1. However, the concept of measurement at a plane in proper time for me is not clearly defined.
A reference might be SY Lee, Accelerator physics, or Andy Wolski does good lectures e.g.:
https://www.cockcroft.ac.uk/wp-content/uploads/2014/12/wolski-1.pdf [cockcroft.ac.uk]
(Score: 3, Insightful) by shrewdsheep on Monday August 13 2018, @11:14AM (2 children)
In the context of the article, 6 dimensions mean 6 numbers measured on a single unit. This kind of speak is usual in science, but confusion arises when allowing the connotation with spacial dimensions as probably intended for by the journalist (for click-baityness).
(Score: 2) by PiMuNu on Monday August 13 2018, @11:50AM (1 child)
Sure, I just reject the GPs comment that what is done is wrong or ill-defined. It is well defined for anyone with knowledge of the field. I agree that it was intended by journalist as click-bait.
(Score: 0) by Anonymous Coward on Monday August 13 2018, @12:17PM
Sorry, I didn't mean to make it sound that they are technically wrong.
I just wanted to say that the 6D is misleading because anyone but a physicist would think of something other than the phase space.
And the explanations they give are convoluted as hell, like they want people to misunderstand.
As physicists, it's our job to give the simplest explanation possible for phenomena.
And, as someone somewhat involved in experiments with particles (tracers in fluids, no quantum stuff), I find this press release extremely confusing.
I had to go to the actual paper to understand what they're talking about, and it's very different from what I was thinking about, although I assume it's quite adequate for this field.
In any case, it looks to me like momenta are not measure directly, but deduced from multiple position measurements, so it's not literally a 6D measurement (which would mean, for me, something like "take a picture: location of photon gives location of particle, color of photon gives momenta because of some complicated Doppler effect).
For those interested, here's a link to a soft but correct version of the paper: https://physics.aps.org/articles/v11/80 [aps.org] (not sure whether it's free though).