For Questions That Don't Merit Their Own Thread

[DerekL1963]

1 hour ago, wumpus said:

Any idea why Radon isn't used for ion engines?

Probably for the same reason mercury was abandoned as the propellant of choice....  Toxic propellants make life so much more difficult in so many ways.

[wumpus]

28 minutes ago, DerekL1963 said:

Probably for the same reason mercury was abandoned as the propellant of choice....  Toxic propellants make life so much more difficult in so many ways.

I missed why mercury would make a good propellant at all, but judging by the mN of force generated by throwing these substances at multiple times orbital velocity, I don't think Radon pollution would be a problem.  Depending on the location doing the research, it might not be measurable over the background Radon emitted from the ground.  Ion engines can't possibly emit all that much propellant.  I'm less sure about having a tank of radon around, but it certainly beats having LH2, LO, LCH4, etc. (not to mention hypergolics and many of the stars of Ignition!).

[DerekL1963]

17 minutes ago, wumpus said:

I missed why mercury would make a good propellant at all

Heavy (IIRC, compact tankage among other benefits), (IIRC) easily ionized, well understood (chemically), and (at the time) high purity mercury was easily commercially available.  It's only real drawback (aside from it's toxicity) was that it took power to vaporize.  Cesium (the other preferred alternative at the time) was even worse in that respect.
 

19 minutes ago, wumpus said:

I don't think Radon pollution would be a problem.

The potential for a radioactive release is always a problem.  Handling radioactive materials is always a problem.  Crapping up your test cell with radioactive material is a serious problem.

And radon's half life is only 3.8 days.  It's daughters are all solids, and plate out - and after an hour or so you start accumulating Pb²¹⁰...  Which is not only toxic in and of itself (and bioaccumulates), but is a beta emitter.

I shoulda looked up the half life first.  That alone stops your idea dead in it's tracks.

[wumpus]

23 hours ago, DerekL1963 said:

And radon's half life is only 3.8 days.  It's daughters are all solids, and plate out - and after an hour or so you start accumulating Pb²¹⁰...  Which is not only toxic in and of itself (and bioaccumulates), but is a beta emitter.

It's amazing that any gas with a half life of 3.8 days is detectable at all in someone's house (although anything so unstable it falls apart in 3.8 days is *nasty*.  I imagine venting the homes must be more serious than I thought and must do wonders for any insulation.

Presumably you could create a source from depleted uranium, but there's no way you have any sort of Isp carrying enough uranium to source radon.  There's always argon.  We can't run out of argon (we can't "run out" of xenon either.  Just run out of any xenon made as byproducts of making liquid oxygen and/or liquid nitrogen.  Liquifying the entire atmosphere is a bit expensive to squeeze out the last Xe).

[monophonic]

On 10.3.2018 at 10:57 PM, wumpus said:

It's amazing that any gas with a half life of 3.8 days is detectable at all in someone's house (although anything so unstable it falls apart in 3.8 days is *nasty*.  I imagine venting the homes must be more serious than I thought and must do wonders for any insulation.

Presumably you could create a source from depleted uranium, but there's no way you have any sort of Isp carrying enough uranium to source radon.  There's always argon.  We can't run out of argon (we can't "run out" of xenon either.  Just run out of any xenon made as byproducts of making liquid oxygen and/or liquid nitrogen.  Liquifying the entire atmosphere is a bit expensive to squeeze out the last Xe).

Amazing, well yeah... the standard method to conduct radon measurements here where I live is with small boxes that contain some kind of sensitive (to alpha radiation I believe) material. They are kept in the space being measured for two months before shipping them off to a laboratory that measures the actual results. Of course the boxes come in an airtight package to prevent accidental exposure before placing them. It is a slow but cheap and reliable method.

Best method to radon-proof homes is by having a well ventilated crawl space underneath an airtight floor. Basements and other underground spaces must be airtight and well ventilated too. The idea is to prevent the gas from seeping into the house from the ground and stop it from pooling in the lowest spaces.

[KG3]

On ‎3‎/‎9‎/‎2018 at 4:43 PM, DerekL1963 said:

 

The potential for a radioactive release is always a problem.  Handling radioactive materials is always a problem.  Crapping up your test cell with radioactive material is a serious problem.
And radon's half life is only 3.8 days.  It's daughters are all solids, and plate out - and after an hour or so you start accumulating Pb²¹⁰...  Which is not only toxic in and of itself (and bioaccumulates), but is a beta emitter.
I shoulda looked up the half life first.  That alone stops your idea dead in it's tracks.

Do you know anything about alpha emitter propulsion?  I think I came across it being mentioned somewhere. The idea was that a radioactive material could gently push a small (presumably unmanned) spacecraft without any moving parts for a very long time.  I guess it could also provide electricity as well, I'm not sure.  It sounded very simple but I don't know how effective it would be or what it would or what application it would be good for.  I don't even recall if they said what material would be used.  Is uranium hot enough, plutonium?  I can't seem to find the source of this article.

[DerekL1963]

3 minutes ago, KG3 said:

Do you know anything about alpha emitter propulsion?  I think I came across it being mentioned somewhere. The idea was that a radioactive material could gently push a small (presumably unmanned) spacecraft without any moving parts for a very long time.   It sounded very simple but I don't know how effective it would be or what it would or what application it would be good for. 

IIRC, the term you're searching for is "fission fragment propulsion".  tl;dr version - incredibly high ISP, absolutely lousy thrust.  And by lousy I mean "practically indistinguishable from zero".

[KG3]

 

15 minutes ago, DerekL1963 said:

IIRC, the term you're searching for is "fission fragment propulsion".  tl;dr version - incredibly high ISP, absolutely lousy thrust.  And by lousy I mean "practically indistinguishable from zero".

Ok, thanks.  I found it!  It does say it has a high ISP but doesn't say much about thrust.  I'll take your word for it.  I won't bother trying to build one at home. 

https://en.wikipedia.org/wiki/Fission-fragment_rocket

"The efficiency of the system is surprising; specific impulses of greater than 100,000s are possible using existing materials. This is high performance, although not that which the technically daunting antimatter rocket could achieve, and the weight of the reactor core and other elements would make the overall performance of the fission-fragment system lower. Nonetheless, the system provides the sort of performance levels that would make an interstellar precursor mission possible."

[NSEP]

Can a quantum particle with multiple positions change it position faster than light?

So lets say we have a quantum particle that is both right in front of me and has a twin particle one lightyear away. If i move that quantum particle, will it take one year for the twin-particle to move? Or does the twin particle move instantly?

Or is this not how quantum particles work at all?

[p1t1o]

32 minutes ago, NSEP said:

Can a quantum particle with multiple positions change it position faster than light?

So lets say we have a quantum particle that is both right in front of me and has a twin particle one lightyear away. If i move that quantum particle, will it take one year for the twin-particle to move? Or does the twin particle move instantly?

Or is this not how quantum particles work at all?

I've never heard of *motion* being part of the whole "spooky action" thing, but this sort of thing is approaching the edge of my knowledge.

With standard quantum entanglement, IIRC, the spooky action does take place at the same time (ie: "superluminal") but it is not possible to transmit information in this way. Or something.

The XKCD science forum is a good place for stuff like this, there's some folk there that are super into quantum stuff.

[Steel]

7 hours ago, NSEP said:

Can a quantum particle with multiple positions change it position faster than light?

So lets say we have a quantum particle that is both right in front of me and has a twin particle one lightyear away. If i move that quantum particle, will it take one year for the twin-particle to move? Or does the twin particle move instantly?

Or is this not how quantum particles work at all?

Let's break this down a bit. 

First, quantum particles do not have multiple positions. When measured they have one position (with some uncertainty). As @p1t1o mentioned above, you might be thinking about entangled particles, where two particles are "linked".

The properties of entangled particles do correlate superluminally (i.e when one changes, the other appears to change instantaneously). However, as also mentioned above, you cannot transfer information this way.

Edited March 16, 2018 by Steel

[kerbiloid]

On 16.03.2018 at 7:42 PM, Steel said:

The properties of entangled particles do correlate superluminally (i.e when one changes, the other appears to change instantaneously). However, as also mentioned above, you cannot transfer information this way.

The fact that you know that the counterpart particle is in opposite state to the near one, is itself a 1 bit of information.
You change this particle, you know that the counterpart particle changed its state.

[Steel]

3 hours ago, kerbiloid said:

The fact that you know that the counterpart particle is in opposite state to the near one, is itself a 1 bit of information.
You change this particle, you know that the counterpart particle changed its state.

Not quite. The problem here is that neither party can actually control what state they measure.

Let's say we have two entangled particles with Z spin equal to 0 - so if one has up spin the other has down spin.

Alice has particle A and Bob has particle B.

However, because these are quantum particles, each one has a non-zero probability when measured to have either up spin or down spin. So Alice measures the spin and gets up, so she knows that Bob will measure spin down if he measures it now. However when Bob goes to measure, there's no way for him to know whether he is measuring the opposite spin to the one Alice has already measured, or whether he has measured too early and now Alice is just measuring the opposite of what he has measured.

The only way to know for sure is to have another communication channel between the two.

Edited March 18, 2018 by Steel

[kerbiloid]

3 hours ago, Steel said:

The problem here is that neither party can actually control what state they measure.

Let's say we have two entangled particles with Z spin equal to 0 - so if one has up spin the other has down spin.

Alice has particle A and Bob has particle B.

However, because these are quantum particles, each one has a non-zero probability when measured to have either up spin or down spin. So Alice measures the spin and gets up, so she knows that Bob will measure spin down if he measures it now. However when Bob goes to measure, there's no way for him to know whether he is measuring the opposite spin to the one Alice has already measured, or whether he has measured too early and now Alice is just measuring the opposite of what he has measured.

The only way to know for sure is to have another communication channel between the two.

Brief version: without communication with the counter-universe, we don't know if there is an entangled particle.
If we could know that, we would get that 1 bit ("Is it?" "Yes, it is.").
But as we don't know exactly if it exists, we just know exactly which state would it get if we change the state of our particle, but can just presume this existence as fifty-fifty, so we get no information.

[YNM]

Does anyone here know about Thiessen polygons ? Is it possible to use one on an obtuse-angled triangle ? (let's say... that's the only option available from the station distribution.)

[Diche Bach]

Watched a very good movie on Amazon prime: Salyut 7: The True Story of the Soviet 'Apollo 13' 

Highly recommended, even though non-Russian speakers will have to read sub-titles.

[GoSlash27]

Trolley buses:

How do they keep the pantographs connected to the catenaries?

 

[monophonic]

3 hours ago, GoSlash27 said:

Trolley buses:

How do they keep the pantographs connected to the catenaries?

 

https://en.m.wikipedia.org/wiki/Trolley_pole

In short: a grooved shoe or wheel that rides the overhead line.

[KG3]

On ‎3‎/‎22‎/‎2018 at 6:58 AM, GoSlash27 said:

Trolley buses:

How do they keep the pantographs connected to the catenaries?

 

  There used to be such trolley busses around Boston years ago.  I seem to recall the pantographs slipping off (usually in the middle of a busy intersection) and the driver having to jump out and guiding it back onto the catenaries via a rope amid dozens of angry motorists.  

4 minutes ago, KG3 said:

  There used to be such trolley busses around Boston years ago.  I seem to recall the pantographs slipping off (usually in the middle of a busy intersection) and the driver having to jump out and guiding it back onto the catenaries via a rope amid dozens of angry motorists.  

  Ok, just checked.  There are still some trackless trolleys left in the Boston area.  

[GoSlash27]

2 hours ago, KG3 said:

I seem to recall the pantographs slipping off (usually in the middle of a busy intersection)

 I guess this is a better way of phrasing the question. With all the crazy angles these trolley poles operate at, I was wondering if there was some special design for the shoe/ wire that's not used on tracked trolleys to help them stay connected.

Best,
-Slashy

[KG3]

22 hours ago, GoSlash27 said:

 I guess this is a better way of phrasing the question. With all the crazy angles these trolley poles operate at, I was wondering if there was some special design for the shoe/ wire that's not used on tracked trolleys to help them stay connected.

Best,
-Slashy

Honestly I haven't look closely at how the system works.  The wires do cross at intersections and the busses are able to make turns.  The poles do slip off the wires somewhat frequently.  I would assume a driver over time gets to know where along the rout this might happen and how to drive to keep it from happening.  When they do slip off the bus just stops in the middle of the road and the driver jumps out and guides the poles back onto the wires via ropes (to the sound of blaring car horns).  They have replaced these busses with regular diesel busses in most places around Boston.  From what I can find it seems that there is a bus stop at Harvard that is in a tunnel and they use the trackless trolleys on that bus rout for reasons of air quality.    

[kerbiloid]

Spoiler

And

https://www.youtube.com/watch?v=y2SlEh3aFfY

(Audio is in Russian, but video is mostly self-explanatory)

[KG3]

On ‎3‎/‎28‎/‎2018 at 6:14 AM, kerbiloid said:

  Hide contents

 

(Audio is in Russian, but video is mostly self-explanatory)

Cool!

Ok, I can tell that wasn't Boston.  NOT because the guy was speaking Russian but because the traffic was being way too polite!  

[StrandedonEarth]

There are plenty of trolley bus routes in Vancouver. The poles are springloaded to keep the grooved shoe on the wire. The trolley buses in Vancouver, at least, have a small amount of battery power so they can drive out of intersections and pull over before the driver has to reseat the poles, or drive past sections where the wires are under repair.

[Wjolcz]

Why do big galaxies appear globular? Is it because they have their disc hidden and closer to the core?