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andars commented on Bizarre Particles Keep Flying Out of Antarctica's Ice   livescience.com/63692-sta... · Posted by u/daegloe
whatshisface · 7 years ago
That's not so much a crack in the foundation as it is a patch of dirt next to the foundation.

To get an idea of what happens to old physics when new physics is discovered, realize that Newton's laws are still correct, and can be derived from QM. That's what you get when you do a good job of actually checking the truth with experiments. All theories have implicit tolerances embedded within the known precision of the experiments used to confirm them, and with these tolerances you can say "Newton's laws are right" without denying other, finer details. Similarly, scientists 1000 years from now will agree with everything we presently know about the Standard Model, because all of our beliefs are tempered by how closely we know our experiments are looking.

>Quantum entanglement

I should add that entanglement isn't "shaky" at all, it was predicted from the start and has been observed in countless experiments to date.

andars · 7 years ago
Borrowing from an a old comment of mine:

On one hand, on some scales, Newtonian mechanics is correct "enough" to give results that work, and so in that sense it is just incomplete in that its domain is restricted. On the other, relativity and QM change everything. These new theories may reduce to Newtonian mechanics given certain assumptions, but Newtonian physics assumes things about the structure of spacetime that are fundamentally incorrect (e.g. velocity is not additive). In this sense, one can fairly say that Newton's mechanics are not just incomplete or missing some fine details, but wrong.

I think there is more to the foundations than just the best numbers we can come up with for a given experiment. Our numbers for the gravitational constant, for example, are pretty similar (if more precise) to the numbers in 1891, but the setting in which that number is completely changed. There is no aether, no absolute space, velocities don't add (even though it's "mostly" right on most scales we experience and measure, it is false), space and time get mixed up, etc. Those were all pretty foundational ideas just over a hundred years ago.

andars commented on Free Software Foundation Receives $1M Donation from Pineapple Fund   fsf.org/news/free-softwar... · Posted by u/madmax108
klez · 8 years ago
In the past, the GNU operating system tapes cost hefty money, for what is worth.

Also, I don't see anyone getting mad at people not paying. I only see one HN poster saying it's a shame that organization depending on GNU on their infrastructure don't support the work of the FSF, especially after what emerged in the wake of heartbleed (I know OpenSSL was not a GNU project, but the problem is similar).

andars · 8 years ago
"Pony up" must mean something different for me than it does for you. I don't know what it means to OP, in my comment I mistakenly assumed my understanding of the phrase was fairly universal.
andars commented on Free Software Foundation Receives $1M Donation from Pineapple Fund   fsf.org/news/free-softwar... · Posted by u/madmax108
belorn · 8 years ago
I get stuff all the time that is presented as free while still having the understanding that there are costs. The streets, parks, education, they are all free if I don't consider that the Swedish government takes about 1/3 of my income.

If a friend helps you there is usually a unspoken contract that you will help them in return. They are not a 1:1 trade but any social relationship is a give and take. Very few things in life has one party that only take and an other that only gives.

andars · 8 years ago
Realistically, if the government didn't tax and the benefits were available without payment, how many people would voluntarily pay? Many (most?) will accept a free ride when it is available.

The strength of that "unspoken contract" diminishes as you move from personal friend to distant organization.

I agree that this software has value and it would be nice for people to support it monetarily, but it seems unreasonable to get upset when people don't.

andars commented on Free Software Foundation Receives $1M Donation from Pineapple Fund   fsf.org/news/free-softwar... · Posted by u/madmax108
dmm · 8 years ago
Free Software means free as in speech not free as in beer.
andars · 8 years ago
But their software is also free as in beer? It seems strange to make something available without payment and then get mad when people don't pay.

I understand the other non-monetary freedoms that the FSF advocates, this is not about those.

Deleted Comment

andars commented on PID Without a PhD (2016) [pdf]   wescottdesign.com/article... · Posted by u/darshanrai
fest · 8 years ago
There was an "aha" moment for me when I realized that PID loop only works well for linear systems (and sometimes it's not obvious if a system is linear or not).

Real world example: imagine you have a single-axis motion stage driven by electric motor and you want to control position of a carriage. Usually your control output is motor voltage. Motor voltage approximately translates to current through motor windings, which in turn approximately translates to torque exerted by motor. Torque exerts a force (T = F * r) on carriage. Applying force to the carriage makes it accelerate (F = m * a). Acceleration linearly increases the carriage velocity (v = v0 + a * t). Carriage having some velocity finally causes the position change (s = v0 * t + a * t^2).

In the essence, the system turns out to be non-linear with the respect to parameter you are controlling.

To improve this system, one solution is to add velocity sensor (or differentiate the position sensor, if it's resolution is high enough) and introduce a cascading PID loop topology- the outer loop takes position error and outputs velocity error, which is fed to inner loop (input = velocity error, output = acceleration). The coefficients for the loops have to be tuned starting from innermost loop.

Another solution is to use different control algorithm which is suited for non-linear systems (e.g. LQR).

andars · 8 years ago
Could you clarify what is nonlinear about that system? From your description, it sounds like a high (4+) order (mostly) linear system. The motor probably has nonlinear dynamics, but so does everything and treating it as linear should work ok.

I suspect the difficulties you've experienced stem not from nonlinearity, but from trying to control a system poorly approximated by a second order system with a second order controller. By cascading, both PID controllers effectively "see" a plant that looks more or less second order, so the controllers are much more effective.

andars commented on Study produces 3D images that float   news.byu.edu/news/better-... · Posted by u/snr
klintcho · 8 years ago
Would it be possible to suspend these kinds of particles using a standing wave of ultrasound ? Perhaps something similar to https://www.researchgate.net/publication/265401333_Cell_and_... and then project the laser on them.
andars · 8 years ago
Sorry, I'm not qualified to give a good answer.

I would guess that the relatively long wavelength of ultrasound could potentially cause resolution issues.

andars commented on Study produces 3D images that float   news.byu.edu/news/better-... · Posted by u/snr
snr · 8 years ago
How does this compare with the plasma dots approach? https://www.popsci.com/secret-interactive-holograms-plasma-a...

(Thanks @iliis for posting about this research)

andars · 8 years ago
I'm not very familiar with the plasma dots approach, but from a cursory look it appears that it is not capable of RGB. On the other hand, there is nothing that can fall out of confinement so the display is probably more stable.

That page says the femtosecond plasma display is pretty safe. This display was - and I believe still is, though I could be wrong - using a UV laser for confinement, which is sorta scary.

andars commented on Study produces 3D images that float   news.byu.edu/news/better-... · Posted by u/snr
iliis · 8 years ago
Awesome achievement! This really looks like a viable way of true floating 3D-images.

Can you levitate multiple particles at the same time? How fast are they moving? How sensitivie is it to disturbances in the air?

andars · 8 years ago
> Can you levitate multiple particles at the same time?

Yes, but their device currently is only capable of levitating a single particle. With the current approach, the device would need one laser for confining each particle. The lasers used for RGB could potentially be multiplexed between particles, but this is less likely to work for the confinement due to the instability of the trap.

> How fast are they moving?

Pretty slow. All the "big" images (Leia, grad student, etc) are long-exposure. You can see the particle moving in real-time at about 0:50. In the videos, you can see the device can almost do real-time persistence of vision for volumes substantially smaller than a fingertip.

> How sensitive is it to disturbances in the air?

As I mentioned above, the confinement is weak and the trap is pretty unstable. Its possible that they have improved the longevity of the trap in the last few months, but when doing long-exposure the device is surrounded by a heavy cloth barrier to block both external light and air movement.

andars commented on Study produces 3D images that float   news.byu.edu/news/better-... · Posted by u/snr
starpilot · 8 years ago
Where does the cellulose come from? Is it just ambient particles or are they emitted from the machine? Where does it go?
andars · 8 years ago
The cellulose comes from the black liquor on a spoon (see 0:46 in the video). The put the spoon at the focus of the laser and with luck some particles get confined. After the trap is lost, the particle just sort of drifts away in the air, since they are only tens of microns.

u/andars

KarmaCake day1715November 3, 2013View Original