This works. Just remember to tell the aliens to place the prism so that the fatter side is to their left.
Face the bow.
Left is now where port is.
(This assumes you’re asked the question on a ship.)
septimus beat me to it, and put it better than I could have.
I don’t think that matters. Read the description again. “Left” is defined as the side moving forward, implying clockwise motion. As long as the neutrino always spins in the same direction relative to its trajectory, that’s all you need.
Aren’t “front” and “back” just like “left” and “right” in this respect?
This. It’s not a particular problem with “left/right” but words in general. You can’t define anything without referring to other definitions.
We start to learn language not by learning definitions, but by making guesses from usage and then confirming those guesses through trial and error. It’s only after we’ve built up a critical mass of words that we can begin learning new ones by having them defined for us.
I know nothing at all about particle physics, but doesn’t it depend on a point of view.
Like, for instance, the Earth is spinning clockwise, when seen from above the North Pole, and anticlockwise when seen from above the South Pole. Would it be a similar situation describing a neutrino’s spin?
The first part of the proposed experiment gives us a definition of positive charge. Fine so far.
Okay, now we’ve established one direction (upwards) for our reference frame. Later references to directions like “forward” seem to tacitly assume the perspective of a biped accustomed to locomotion mostly orthogonal to the direction of perceived planetary gravity, but for the sake of argument let’s say we share that perspective with alien physicists.
Note that we’re introducing another object in addition to the neutrino, and the only property shared by both is their angular momentum. The macroscopic object only has to be rotating; its center of mass might very well be stationary in the designated reference frame.
I was under the impression that quantum mechanical “spin” is only a loose analog of classical rigid body rotation, although they both contribute to the total angular momentum of an object. Is it because the total angular momentum has to be conserved that the spin of an elementary particle is more than a loose analog of rigid body rotation, and the “left side” of the rotating macroscopic object is unambiguously defined?
Let’s say you’re communicating by tachyon telephone with aliens located in an unknown part of the universe. They want to create an invasion force that can slip in amongst the human population undetected, and for some reason you’ve decided to help them. You’ve given them an accurate description of human biology, complete with detailed images. They have decoded the images successfully, but there is no way to know whether they’ve oriented them correctly.
Using your directions, they created all the necessary parts, and are now trying to assemble them.
You tell them to attach a head to the torso along the torso’s longest axis. “From now on I’ll call the end with the head ‘top,’ and the other end ‘bottom’’” you tell them. “Now attach a nose to one side of the head at approximately right angles to the top-bottom axis and about halfway between the top and bottom of the head itself. Whichever side has the nose, I’ll call that ‘front,’ and the other side ‘back.’”
So far, so good. It doesn’t matter which end they put the head on or which side they stuck the nose on, because now that those two directions are established, you can have them locate the eyes, lungs, nipples, and toes accordingly. Of course, if they build their invaders upside down, they can turn them over when they get here, but it is vitally important for avoiding detection that the invaders all (or mostly) have hearts located on the left and slightly stronger right hands.
Of course, you could tell them to put the heart in first and define that as left, at which point it you’d run into problems with the nose (front and back). The point is, for any asymmetrical three-dimensional object, there are two ways of orienting the axes that are mirror images of one another. You can place the first two arbitrarily, but the third axis has to be defined according to a reference to avoid creating a mirror image of the object being plotted. By convention, we call the third axis left-right in everyday language because “up” and “forward” are defined biologically for us. But whatever directions the aliens use for “up” and “forward” doesn’t matter, because you can rotate the object to correct it. (Turn them over if the feet are pointing up.)
The neutrino is also traveling along a trajectory, which is defined as “up.” “Forward” is left undefined, but again, it can be described arbitrarily as above.
I have absolutely no idea. I was assuming from Chronos’s description that the spin of the neutrino has some physical directionality rather than being arbitrarily assigned. If you can’t make the macroscopic object spin in the “same” direction as the neutrino then it all breaks down and there’s no way to ensure that the alien invaders won’t be detected the first time they all sign something or get a physical.
Yes, angular momentum is angular momentum, whether it comes from masses moving in circles or from “spin”. The parallel only gets wobbly when you try to visualize it: a macroscopic object needs to have a non-zero size to have non-zero angular momentum, and yet particles like electrons or neutrinos or photons seem to have angular momentum while simultaneously behaving (so far as we can tell) like they’re points with zero physical size. So “spin” gives rise to real, measurable angular momentum, but you can’t actually think of the subatomic particles as “spinning” — it’s just an inherent property of the particle like its mass or its charge.
I tried doing this with only words, but I soon realized my definition of “only words” was a bit broad.
The first three words were: “Take a compass.”
I haven’t seen this addressed yet:
If you go to the Kaon page on Wikipedia, the neutral Kaon is a pair of quarks, a Down and an anti-Strange. This is the middle line in the table in the link. But this particle oscillates with its anti-particle. That is, if you start with the matter neutral Kaon, it changes into the anti-neutral Kaon, and back.
The additive and and subtractive combinations of the neutral Kaon and anti-neutral Kaon, however, don’t oscillate. These are the last two lines in the table. If you start with one of these combinations, it will stay that combination, and not oscillate into the other. The additive combination has a half-life about three orders of magnitude larger than the subtractive one. This is the neutral Kaon Chronos was referring to when he said “long-lived neutral kaon”.
That combination is (Down,Anti-Strange + anti-Down,Strange), and is its own anti-particle. So antimatter aliens will get the same result.
For most people “weak side” could be enough.
Not perfect, of course, but it might suffice for a certain percentage of reasonably intelligent people in an emergency (whatever kind of emergency might arise in the unique conditions applied). Better than standing there scratching your head.
Feynman had a discussion of this in his famous Messenger Lectures at Cornell about 50 years ago. He described how physics is almost completely symmetric with respect to left-right. The only violation is the type described by Chronos, which can only be seen by performing difficult particle physics experiments. He imagines a conversation with an alien in which you explain the experiment, so he can determine what we mean by left and right. Then Feynman goes on to explain that the symmetry is broken in exactly the opposite way for antimatter. He concludes by imagining the alien landing on our planet and extending his hand in greeting. Feynman says, if he puts out his left hand, don’t touch it! He’s made of antimatter.
This also raises the question of how a developing embryo knows how to break the symmetry to put the heart on the left side.
I wondered about this for many years and always assumed it had to do with the asymmetry of our biomolecules, which occurred by random selection early in our evolutionary history. In fascinating work done rather recently, the answer was finally determined and, indeed, it can be traced back directly to the handedness of biomolecules. Briefly, in embryogenesis, after the dorsal/ventral and anterior/posterior directions are chosen randomly, the left/right directions emanate from a center of assymetry created by the clockwise (or anticlockwise, I don’t remember) rotation of a cilium (or flagellum, I’m not sure of the difference). The cilium itself is handed because of the handedness of its microtubules, which are handed because of the handedness of their proteins, which are handed because of the handedness of their amino acids!
The beauty of science is unbounded.
I stand by my post!
Seriously, for anyone wondering how that jibes with what I posted above, mirror symmetry was expected to hold until 1956 when it was experimentally shown to be violated. In 1957 CP symmetry was proposed in its place. In 1964 CP symmetry was shown to be also violated, leading to proposed CPT symmetry (“They can’t do experiments with time reversed; let’s see them violate that one!”). So 50 years ago, in 1961, Feynman’s comment was in line with current thinking, but is now outdated.
See the CP symmetry page on Wikipedia.
To clarify: Neutrinos produced by beta decay processes do in fact always have their angular momentum in the same direction relative to their direction of motion (or the same chirality, if you prefer). Antineutrinos also all have the same chirality, the reverse of that of neutrinos (this may, in fact, be the only difference between neutrinos and antineutrinos, in which case there’s no real fundamental difference at all, since the chirality of a massive particle can be changed). This asymmetry is, so far as anyone can tell, absolute: Nobody has ever observed a neutrino to be produced the “wrong way”-- Just one more example of how the Weak Nuclear Force is weird.
Because the chirality is different for neutrinos and antineutrinos, to define “left” and “right” this way, you have to first agree on what’s matter and what’s antimatter. That’s where the decay of the long-lived neutral kaon comes in, which also shows an asymmetry, but rather than being a total asymmetry, it’s a very, very slight one. In some ways, a slight asymmetry like that is a lot harder to explain than a total one, and I don’t think anyone’s really all that sure about why it happens (it might involve something called axions, but nobody’s ever detected those).
My apologies, I didn’t read every post so someone may have already said this, but so many of the posts were so long and I feel I have a simpler way, if it id considered valid.
Due to the bilateral symmetry of the body I was thinking you could say left is the side which you can do this (make an L; link safe for work/anywhere: http://images.inmagine.com/168nwm/thinkstock/tstock_single5/tss0050054.jpg ) with your left hand, and thats a way to know it.
Another idea I just I thought of after mentioning bilateral symmetry is your heart is on the left side of your chest. As its possible to feel the difference in the heartbeat, and thus its location, in the left and right portions of the chest, this too seems a valid answer. Now, pick me apart!
You’ve just never seen the Grand Robert.
Yes, my apologies, I was not contesting it.
I should have been explicit about the fact that the Messenger lectures were given just before CP violation was proven. As far as I know CPT is still valid, which takes us back to Mangetout’s post #16 about time reversal. If the alien is traveling backwards in time and extends (or rather retracts) his left hand, watch out!
I saw those Messenger lectures in high school in the late 1960’s. I’d never heard of Feynman before, but I never forgot him after seeing them.