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Paper Centrifuge

gizmodo.com

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Re: Paper Centrifuge

#111

Post by taneq »

tbabb wrote: post #77
nneonneo wrote: post #41

As usual, what you really want to do is read the paper: http://www.nature.com/articles/s41551-016-0009 This is a well-written and very clear paper. It covers several aspects that went unmentioned in the Gizmodo article, and highlights that although building such a device may be trivial, evaluating and studying it is not. Some highlights: the paper covers how the device was built in detail, including information on th…

Holy crap. 125,000RPM for a 5mm disc means the edge is spinning at nearly 9,000mph. On top of that, the g-force would be a = v^2 / r v = 2 * pi * 5mm * 125000/60 s^-1` r = 5mm ...giving 87,000 gees?! I start to wonder if general relativity effects (frame-dragging?) start to become noticeable at that acceleration.

You used 5m instead of 5mm, putting you out by a factor of 1000:

    2 * Pi * 0.005m * 125000/60 s^-1 = 65.4m/s = 235km/h.

Re: Paper Centrifuge

#112

Post by taneq »

csours wrote: post #66
nneonneo wrote: post #41

As usual, what you really want to do is read the paper: http://www.nature.com/articles/s41551-016-0009 This is a well-written and very clear paper. It covers several aspects that went unmentioned in the Gizmodo article, and highlights that although building such a device may be trivial, evaluating and studying it is not. Some highlights: the paper covers how the device was built in detail, including information on th…

If I was using this thing, I would fix one end to a solid endpoint and pull the other end so that I would not be in the line of fire if the thing exploded.

The video shows them doing exactly this with one end tied to a door handle.

Re: Paper Centrifuge

#113

Post by function_seven »

dogma1138 wrote: post #97
hguant wrote: post #81

Earlier quoted context omitted.

Quick math and google says no: 9,000 MPH = 0.00001342 C Threshold for relativistic effects is .01 C according to this paper: http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/rellim.ht... EDIT: Rereading that paper, they're focused at the particle level, which may or may not make this irrelevant. I know anecdotally that GPS satellites have to take relativity into account. Geostationary satellites move at 1.9 miles p…

Relativity has almost no effect on GPS at it's worse it would have an accuracy degradation of less than 1cm, there are considerably bigger sources of errors in GPS than relativity and various corrections to those. Shadows, echoes, atmospheric effects and the doppler effect have considerable effect on GPS accuracy, relativity will not.

Relativity has a massive effect on GPS. If unaccounted for, the error would increase by 11km per day.

EDIT: I might be wrong on the “11km” part. See reply

http://physics.stackexchange.com/questions/1061/why-does-gps...

Re: Paper Centrifuge

#114

Post by titzer »

Part of me wonders, A.) if it's possible to enrich uranium using something like this, and B.) how many man-years of hand-spinning it would take to get enough weapons-grade material that way.

Back of the envelope calculation: U-235 is a little less than 1% of naturally occurring Uranium, so supposing that you could load up 1 gram of Uranium per spin session, you'd get a maximum of 10 milligrams per session, so you'd need about 2.2 million sessions to get the requisite 22kg of U-235. Of course, you'd get nowhere near 100% efficiency out of a hand centrifuge in the first spin session, so it would probably require a cascade of several stages. If each stage gets rid of half of the U-238, then the number of spins you would need to get to 100% would asymptotically approach 2x, so we are looking at 4.4 million spin sessions. If each were to take 1 minute, we'd need about 8 years.

To be honest that seems overly optimistic, and there are probably technical details like dealing with UF6 gas that are way harder than I thought.

(happy to be corrected by a Nuclear engineer if there is one around).

Re: Paper Centrifuge

#115

Post by dogma1138 »

function_seven wrote: post #113
dogma1138 wrote: post #97

Earlier quoted context omitted.

Relativity has almost no effect on GPS at it's worse it would have an accuracy degradation of less than 1cm, there are considerably bigger sources of errors in GPS than relativity and various corrections to those. Shadows, echoes, atmospheric effects and the doppler effect have considerable effect on GPS accuracy, relativity will not.

Relativity has a massive effect on GPS. If unaccounted for, the error would increase by 11km per day. EDIT: I might be wrong on the “11km” part. See reply http://physics.stackexchange.com/questions/1061/why-does-gps...

No it doesn't http://www.physicsmyths.org.uk/gps.htm This would only be an issue if you had substantial drift between different sattlites the 38 microsecond delay is irrelevant since all satellites in the constellation have a similar delay.

It's also less relevant when solving using the more common method which is time-tagging at the receiver, relativity might be a bigger pain if you are doing time-tagging at the transmitter but since the clocks of different satellites are not synchronized and can differ by as much as 1ms that delta would be more of an issue than relativity.

If you are doing normal GPS accuracy which is 15m you don't need to use relativistic correction in the receiver, is you are solving for almanac data you do not need to do relativistic correction in the receiver, you only really do relativistic corrections when you are using centimeter level accuracy GPS and even then you have considerably bigger corrections to do.

Yes relativity was accounted for when GPS was designed, but it's by no means would send you to China, not even to the wrong town.

What GPS needs to work is for the receivers to be able to calculate a proper geometric range delay or at least be constantly and consistently wrong when calculating the delay for different satellites in the cluster.

http://www.navipedia.net/index.php/GNSS_Basic_Observables

Discounting relativity would reduce accuracy, but it would be consistent and constant and so would not present that much of an issue, it's also by far not the biggest offender when it comes to corrections.

Re: Paper Centrifuge

#116

Post by sobani »

andrepd wrote: post #92
hguant wrote: post #81

Earlier quoted context omitted.

Quick math and google says no: 9,000 MPH = 0.00001342 C Threshold for relativistic effects is .01 C according to this paper: http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/rellim.ht... EDIT: Rereading that paper, they're focused at the particle level, which may or may not make this irrelevant. I know anecdotally that GPS satellites have to take relativity into account. Geostationary satellites move at 1.9 miles p…

I believe in GPS satellites relativistic effects are due in greater part to the difference in gravity between the surface of the earth and the satellite, more so than the special relativity effects.

According to a reference[1] from Wikipedia[2] the effect would be 45 μs for general relativity 7 μs for special relativity, combining into a 38 μs drift.

So although you are technically correct (45 > 7) they are both significant.

[1] http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.... [2] https://en.wikipedia.org/wiki/Global_Positioning_System#Hist...

Re: Paper Centrifuge

#117

Post by function_seven »

dogma1138 wrote: post #115
function_seven wrote: post #113

Earlier quoted context omitted.

Relativity has a massive effect on GPS. If unaccounted for, the error would increase by 11km per day. EDIT: I might be wrong on the “11km” part. See reply http://physics.stackexchange.com/questions/1061/why-does-gps...

No it doesn't http://www.physicsmyths.org.uk/gps.htm This would only be an issue if you had substantial drift between different sattlites the 38 microsecond delay is irrelevant since all satellites in the constellation have a similar delay. It's also less relevant when solving using the more common method which is time-tagging at the receiver, relativity might be a bigger pain if you are doing time-tagging at the tra…

Thank you. Is it fair to say that the 38µs correction done in the satellites is more for GPS’s other use? That is, as a source of accurate timekeeping to the receivers? It sounds like positioning is not affected like I thought, but the designers felt the need to engineer a solution to the relativity problem nonetheless.

Re: Paper Centrifuge

#118

Post by dogma1138 »

function_seven wrote: post #117
dogma1138 wrote: post #115

Earlier quoted context omitted.

No it doesn't http://www.physicsmyths.org.uk/gps.htm This would only be an issue if you had substantial drift between different sattlites the 38 microsecond delay is irrelevant since all satellites in the constellation have a similar delay. It's also less relevant when solving using the more common method which is time-tagging at the receiver, relativity might be a bigger pain if you are doing time-tagging at the tra…

Thank you. Is it fair to say that the 38µs correction done in the satellites is more for GPS’s other use? That is, as a source of accurate timekeeping to the receivers? It sounds like positioning is not affected like I thought, but the designers felt the need to engineer a solution to the relativity problem nonetheless.

There are different corrections for relativity, one is the "one time adjustment" that all clocks got which is on average closer to earth clock rates but it's not 100% constant since general and special relativity do not affect all satellites in the same manner and they all got the same clock correction.

The other "relativistic" correction is one done in the receiver and is needed for geometric range delay calculation, basically taking into account that light doesn't travel in a straight path, we've kinda known that since the early days of classical optics - Fermat's principle or the principle of least time.

Now while it's true that we are using the "relativistic" version of this in our calculation solving the classical or even ignoring it all correctly as long as we account for other effects would still allow you to find a starbucks.

Since GPS was initially intended for military use it had other uses such as time keeping that can be used to synchronize communication, encryption and other things these errors can be cumulative whilst navigation errors are often not since the drift would be more or less uniform with all satellites you see.

Re: Paper Centrifuge

#119

Post by pg314 »

dm3730 wrote: post #102
pg314 wrote: post #58

Earlier quoted context omitted.

Exactly. One of the things the OLPC (One Laptop Per Child) project initially ignored was the infrastructure needed, such as electricity and internet access.

> initially ignored Initially? Has that changed now? The founder, Nicholas Negroponte went on tear around the world claiming to have saved millions of children and being the father of tablet computing. He even talked about throwing OLPCs out of helicopters on to those brown and black masses. Even claimed that mothers would be better off getting a laptop than subsidized food. I recall he shat on some reporter who had…

I'm no longer really following what OLPC does, but as far as I know that hasn't changed. Other loosely associated organisations like e.g. OLE Nepal (olenepal.org) are addressing that, though.

I did not meet any Negroponte fans out in the field.

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