Wikipedia:Reference desk/Archives/Science/2013 June 14

= June 14 =

Dense storage of positrons using superconducting coil?
Electrons can be "densely stored" in a superconducting wire coil and circulate as long as cyronic temperature is maintained. The Cooper Pairs (presumably) do "not knock" in to the coil. Could the same apply to positrons? If it did, positrons could be densly stored, allowing energy storage one hundred times more powerful than nuclear fusion, possibly allowing starship propulsion or anti-matter bombs. Switch off the cooling and the whole lot would annilhilate with total conversion of matter to energy. Some people have said I am irresponsible making this idea public, but making anti-matter is inefficient so only large states could do this. (Trevor Loughlin) — Preceding unsigned comment added by 91.224.27.227 (talk) 11:31, 14 June 2013 (UTC)
 * Wouldn't the superconductor also need to have antinuclei to conduct positrons? 139.193.214.10 (talk) 13:21, 14 June 2013 (UTC)


 * Funny post. The funniest part was "Some people have said I am irresponsible making this idea public". Dauto (talk) 18:21, 14 June 2013 (UTC)


 * In reality, positrons can be circulated in storage rings. The beams fly through a near-perfect vacuum, and are kept in position using magnetic fields.  If you have a lot of positrons, you can do almost anything you like with them; but if you direct a beam of positrons into a wire - supercooled or not - the positrons will rapidly annihilate - because the mean free path is very short and the collision frequency is very high in condensed matter.  Positrons are not easy to come by - so people who have access to them typically don't waste them - the particles are kept in ultra-high vacuums for as long as possible.  Nimur (talk) 19:14, 14 June 2013 (UTC)


 * The best method we have yet discovered for producing positrons is a machine at Lawrence Livermore labs that is the size of a house and produces 10 billion (1010) positrons every 30 minutes by zapping a gold brick with a petawatt laser(!). Each positron (and each electron that you're going to annihilate it with to produce energy) weighs in at around 10-30kg - so the results of 30 minutes production on this expensive machine is 10-20kg of antimatter.  E=mc2 and c2 is around 1017m/sec.  So the amount of energy you get from an antimatter "explosion" produced by 30 minutes of work by this massive machine is somewhere near 2x10-3 Joules - which is about 1/500th the energy produced if you drop an apple from a height of one meter.  The energy stored in a AA battery is about five million times greater.


 * The energy it takes to fire the "Titan" laser in the positron creation machine is hundreds of Joules. So having a great way to store positrons isn't going to lead to amazingly powerful positron batteries or anything. SteveBaker (talk) 20:11, 14 June 2013 (UTC)


 * From the linked source: "... the electrons each break apart into a lower-energy electron and its anti-matter opposite, a positron.". This would violate charge conservation, so I think the source is not reliable. 95.112.240.62 (talk) 21:38, 14 June 2013 (UTC)
 * The "linked source" is a brief and general-purpose news summary - it's bound to "gloss over" minor details (like the laws of physics) - for the purposes of brevity. If you require scientifically-accurate descriptions of the laser phenomena that Steve mentioned, you might read the publications section at the Jupiter Laser Facility at Lawrence Livermore.  It lists several published, peer-reviewed papers on the production and measurement of positrons in the laser laboratory.  Nimur (talk) 22:48, 14 June 2013 (UTC)

I like to hijack the original question and reformulate it:

95.112.240.62 (talk) 19:41, 14 June 2013 (UTC)
 * 1) Can electrons really be "densely stored" effectively anywhere? Superconducting coil or not, electrons repulse each other due to the electrical charge.
 * 2) Obviously, positrons would annihilate with electrons in normal matter. But if we would find some technical trick to strip the electrons off but still keep the nuclei in place and the positrons squeezed between them, wouldn't the positrons react with neutrons?


 * I guess it depends what you consider "dense." There are a lot of electrons densely packed in ordinary, regular, condensed solid matter!  I don't think you'll get denser packing of electrons-per-cubic-meter than what you'd find in a dense atomic lattice - like lead or uranium or a lump of any other other favorite heavy element.  There's a few dozen moles of electrons per mole of uranium, or about a septillion electrons per cubic centimeter (plus or minus a few orders of magnitude).
 * For your second question - everything I've ever learned about positrons concerns either of two scenarios: (1) positrons, carefully isolated in an almost perfect vacuum (i.e., not interacting with any matter), for the purposes of some particle experiment; or (2) positron emission followed almost immediately (i.e., after a very short duration of travel-time) by a positron-electron collision event. For example, in medical imaging (PET), the positron is emitted and it exists just long enough to bump in to the first atom it encounters, annihilating one electron and releasing a bright shiny pair of easily-detectable photons.
 * I'm not familiar with any other type of positron absorption event. Though part of me wants to say that positron emission might be reversible, thinking along a symmetry argument, I've never heard of positron absorption by a nucleus.  Nuclei absorb electrons, not positrons.  So: probably the math doesn't work out - and positron emission is not reversible even in theory; but even it it's possible, it's so exotic an interaction as to be completely un-discussed in most of the experimental physics community; or maybe somebody else has spent more time studying experimental particle physics has some additional insight.  Nimur (talk) 23:52, 14 June 2013 (UTC)


 * To add my own ignorant question to the mix: what controls the odds of electron-positron annihilation? I imagine that, given that an electron is "blurrier" than a proton, a positron ought to be localized to a region larger than a hydrogen atom, and likewise the electron it "orbits" just before annihilation.  They're not point particles heading toward each other, right?  Is there some period of time when they just sit there, one amidst the other, waiting for a probabilistic event to happen?  And does that event depend on their parameters of motion - spin, angular momentum, velocity relative to one another, what kind of orbital they're in?  Is it remotely conceivable to engineer an electron-positron material that somehow reduces the odds of annihilation by a measurable amount? Wnt (talk) 01:45, 15 June 2013 (UTC)
 * The electron and positron must collide - which is a well-defined event. The probability of the collision depends on the collisional cross-section of both particles, in turn which depends on the energy and momentum of the particles.
 * The way I would describe this: a collision between two particles is a purely deterministic thing. Particles have exact positions, and trajectories; we know when they hit each other.  The part that's "probabilistic" - and here is where quantum mechanical stuff gets weird and your intuition may fail - is whether you even have two particles in the first place - and where those particles would be.  In other words, if you have a given amount of energy, momentum, and a particular state of electric and magnetic fields, how probable is it that you observe those conditions in a particle-like way - that is, as a localization of the energy and the momentum and charge specifically in the form of an electron and a positron?  The energy is there, the charge is there, and the fields are all there, but whether those constitute a pair of high-energy photons or equivalently whether the same fields, charge, energy, and momentum are localized as a pair of particles - is the probabilistic piece.  Nimur (talk) 05:51, 15 June 2013 (UTC)
 * Hmmmm... I don't have the faintest sense how you calculate such a thing. But I suppose that simply classically, you could have some energy in an electron-positron pair, so that the two "orbit" each other (in a Bohr-ish model) and therefore don't collide.  I don't know if there's a comparable "orbital" in a QM sense where the two likewise would fail to interact.  What I'm wondering is... is there a chance that you could have a cloud of superconducting electrons in some simple material that are in a state such that a positron in some other specific state could be always near them, attracted by their charge, but never hit them, sort of like this hypothetical orbiting positron? Wnt (talk) 06:01, 15 June 2013 (UTC)
 * What you need is to make the transition between the intial state and a final state with one electron and one positron less (and two or more photons more) to be forbidden. But such a trnasition will always be possible. What you can have is that the annihilation between the electron and positron requiring 3 instead of 2 photons to be created. E.g. if you have positronium (bound state between elecxtron and positron) then the two spin 1/2's of the electron and positron can add up to a total spin 1 state (which can have a z-component of 1, 0 or -1) or it can have a total spin of zero. Then because the total spin of 2 photons will always be even the positronium with total spin 1 (which is called ortho-positronium) cannot decay into two photons, it has to decay into at least 3 photons. This causes the lifetime of ortho-positronium to be much larger than that of the spin 0 state (para-positronium). Count Iblis (talk) 12:46, 15 June 2013 (UTC)

Two different cicada species?
I asked this question on July 16, 2011, and currently my area is again covered with cicadas. Are the ones of 2011 and the ones presently here different species? It certainly hasn't been 13 years since 2011. Peter Michner (talk) 15:30, 14 June 2013 (UTC)


 * OK, it looks like 2011 was Brood XIX and the ones I'm seeing now are Brood II. Peter Michner (talk) 15:41, 14 June 2013 (UTC)


 * Yep, you got it. Technically, you may also see a few of the Tibicen mixed in as well, but they have probably not emerged yet this year. SemanticMantis (talk) 15:53, 14 June 2013 (UTC)


 * Each brood may be made of several different species though. Rmhermen (talk) 17:03, 14 June 2013 (UTC)


 * If members of each brood can only mate within the brood, then I'd expect them to drift into different species eventually, so there must be a mechanism for interbreeding. Overlapping breeding periods is the obvious mechanism.  I wonder, though, how the offspring of a cross-brood mating decide which brood they belong to.  Is it always the mother or father ? StuRat (talk) 20:20, 14 June 2013 (UTC)
 * These questions are addressed at magicicada, and refs therein. Magicicada is a genus, comprising ~7 spp, and 13-year dormancy is dominant over 17-year cycles. SemanticMantis (talk) 21:09, 14 June 2013 (UTC)

Who is right?
Newton says that Gravity is a force which accelerates objects toward each other. Let an apple is dropped from Pisa tower. T1 is the striking time of earth and apple.
 * Gravity force of earth on apple = F1 = m1ge = m1GM/R2 where ge=GM/ R2 = gravitational accelaration of earth (say 9.8 m/s/s)
 * Gravity force of apple on earth = F2 = Mga = MG m1/r12 where ga=Gm1/ r12 = gravitational accelaration of an apple

Where G= Gravitational constant, M= Mass of earth, m1= Mass of an apple, R= Radius of earth, r1=radius of apple

Although F1 = F2 but since earth can also be seen from apple and accelerate towards it with ga, therefore, an apple look a lot to drop to the earth as compared to the falling of earth toward apple which is so minuscule to be perceived as ge >>>> ga.

Let a steel ball of radius 1 meter is dropped from Pisa tower. T2 is the striking time of earth and ball.


 * Gravity force of earth on ball = F3 = m2 ge = m2GM/ R2 where ge =GM/ R2 = gravitational accelaration of earth (say 9.8 m/s/s)
 * Gravity force of ball on earth = F4 = Mgb = MGm2/r22 where gb=Gm2/ r22 = gravitational accelaration of apple

Where G= Gravitational constant, M= Mass of earth, m2= Mass of ball, R= Radius of earth, r2=radius of ball

Although F3 = F4 but since earth can also be seen from ball’s gravitational field and accelerate towards it with gb, therefore, a ball look a lot to drop to the earth as compared to the falling of earth toward the ball which is so minuscule to be perceived as ge >>>> gb.

Since F1 = F2 ≠ F3 = F4 as gb > ga therefore T2 < T1 by definition but Galileo was first to demonstrate that in the absence of air, all things would truly fall with the same acceleration and 300 years later demonstrated this by the crew of Apollo-15 on the lunar surface (which has gravity & also lacks air) by dropping a hammer and a feather. So what would u think who is right?

162.157.235.1 (talk) 23:02, 14 June 2013 (UTC)Eclectic Eccentric Kamikaze
 * Galileo
 * Newton or
 * If Both then how


 * My naive answer: Apollo_15 gives Galileo's theory as "all objects in a given gravity field fall at the same rate, regardless of mass". The ball, or apple, or feather, or hammer, contributes to the gravity field, although not much. Thus, both are right. (I think your fourth bullet point was meant to end with "ball", not "apple", by the way.) Card Zero  (talk) 00:24, 15 June 2013 (UTC)


 * You're talking about forces, but you should be talking about accelerations if you are interested in times. Someguy1221 (talk) 00:42, 15 June 2013 (UTC)

The Earth cannot be treated as a rigid object here. When you drop an object, a shock wave travels through your body to your feet and then it moves into the Earth. Then as the object approaches the Earth, the gravitational force it exerts on the Earth increases, but this doesn't affect distant parts of the Earth at the same time. Finally, when the object hits the ground, you get another shock wave that travels into the Earth.

One can still argue that the total momentum of the Earth should clearly increase as the object approaches its surface due to conservation of momentum, but even this statement is not without problems. This is because the Earth's total momentum is not a precisely defined quantity, according to quantum mechanics an object of mass M in a thermal bath at temperature T will have a typical spread of its momentum of order sqrt(M k T). For the Earth, taking T to be room temperature, this is of the same order as the momentum of the ball. This means that the probability that the Earth's center of mass state changes during the fall is close to zero (this is why you can observe interference phenomena, a photon taking one path or another, bouncing off different mirrors doesn't cause the state of the mirrors to be different in the two different paths despite conservation of momentum, due to the finite spread in the momentum of the mirrors and other macroscopic objects). Count Iblis (talk) 01:00, 15 June 2013 (UTC)
 * The shockwave Iblis mentions starts at the point of contact between the Earth and body, and moves upward from the feet to the head, while travelling downward from the sole of the foot into the earth. μηδείς (talk) 01:05, 15 June 2013 (UTC)

First off, your distinction between R and r1/r2 is wrong. The appropriate distance to use is the distance between the center of masses - which is the same for the apple acting on the earth and the earth acting on the apple. For the earth, a few dozen meters more or less doesn't make much of a distance in scale when compared to the radius of the earth, so we typically don't care about the distinction. However, the difference between the gravitational field (from the apple) on the surface of an apple versus an earth radius away is substantially different, so you need to be careful to use the center-of-mass to center-of-mass distance there. So gravity force of apple on earth = F2 = Mga = MG m1/R2 where ga=Gm1/R2 = gravitational acceleration of an apple at the distance of the radius of the earth. More to your point, you're trying to apply Newtonian mechanics to an non-inertial reference frame. Because the earth is so massive, the distinction between taking the earth as a reference frame and an inertial reference frame is negligible. This *isn't* valid when you're using the apple/ball as a reference frame. The deviations there become significant. To properly treat the system with the (accelerating) apple/ball as the fixed reference frame you need to add in fictitious forces, which makes your third law assumptions invalid. (You can't assume ga and gb keep a constant value once things start moving, changing your assumptions about how accelerations lead to times.) -- 205.175.124.30 (talk) 02:03, 15 June 2013 (UTC) Correction:

When earth is seen from a ball:

Also r1=r2=R (approximately) for on center distances.

Sorry about the mistake that I did in copy and past — Preceding unsigned comment added by 74.200.19.65 (talk) 06:28, 16 June 2013 (UTC)