# measuring

by Honorable_Death
Tags: measuring
 P: 38 can you measure an objects spin or position, or polarization, without actually measuring it? like can you find out if they have any of those 3, without actually measuring it?
 P: 137 Yes, most famously in the Einstein-Podolsky-Rosen paradox in which we might have a particle which decays into two photons. If we measure the property of one photon we can know the corresponding property of the other photon without having measured it because the two photons' properties are correlated ("entangled"). The original EPR paper is about position and momentum, Bohm's later and more practical version is about spin.
P: 38
 Quote by caribou Yes, most famously in the Einstein-Podolsky-Rosen paradox in which we might have a particle which decays into two photons. If we measure the property of one photon we can know the corresponding property of the other photon without having measured it because the two photons' properties are correlated ("entangled"). The original EPR paper is about position and momentum, Bohm's later and more practical version is about spin.
right, but how can we check the other photons property......?

P: 137

## measuring

I'm not sure what you mean. Measuring some property of one photon allows you to know the value you'd find for the same property of the other entangled photon if you measured that one as well.

I'd add that a particle's properties can behave in ways as weird as the particle going through both slits at the same time in the two slit experiment.

We need measurement or some other physical interaction to actually allow us to say anything definite as particles like to be maddenly vague about everything.
 P: 38 well, when you measure one particle that is entangled you give that particle certain physical properties according to quantum mechanics, and when you do that the other entangled particle also gets that certan physical property, so is there anyway to see if the particle has that certain property without giving it that certain property?
 P: 137 You can do things like send a particle down a channel that splits into two channels like a "Y" shape. You make the reason the particle goes one way or the other its spin. If you put a detector in one channel and if you don't detect the particle at the time you'd expect it to have arrived at this detector, then the particle must be in the other channel at about the same distance along that channel. Without having detected and measured the particle, you know both its spin because of the channel its in and also its rough position in one channel from the time at which the detector wasn't triggered in the other channel but you have to be fairly certain at what time the particle started on its journey for the latter information. This is an "interaction-free measurement".
 P: 38 well, can you measure the polarization of a particle without giving it polarization?
 P: 137 You can measure properties of a particle which can be said to have been there before you measured them but the properties "exist" in a strange and non-classical way with some restrictive rules. The particle isn't given a measured property by the measurement but instead the measurement allows the measured property to go from being something without a clear meaning and value to something with a definite meaning and value. I'm no expert, my knowledge is mostly of the more recent attempts at clearing up issues in interpretation. Maybe others here feel they can answer better.
 P: 10 But if you use a detector in your "Y" experiment you are attempting to make a measurement, otherwise you wouldn't know which way it went. Take the interference experiment, for example (which is basically what your "Y" experiment is), if you put a cover over one slit, then you force the photon to make a choice of going through the covered slit or the open slit. Therefore you are making a measurement of position.
 P: 38 Well lets say that you measure a photon and give it a more definate position, then you put it in the double slit experiment, what would happen?
 P: 10 You can't do that. According to the Copenhagen Interpretation, you cannot know what the speed and position of a photon or particle at the same time. In the case of the double slit experiment, when you see the interference pattern you are making a velocity measurement, but if you put the block on one of the slits, you are making a position measurement, which in turn takes away the interference pattern. If you read "Where Does The Weirdness Go", it goes into great detail about that
 P: 38 well you can know its position on a certain axis, if the photon is going in a strait like you can measure its height without affecting its speed....right?
 P: 10 ok that i'm not sure of
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P: 8,100
 Quote by Honorable_Death well you can know its position on a certain axis, if the photon is going in a strait like you can measure its height without affecting its speed....right?

After you measure its height (position) it doesn't HAVE a well-defined speed. Its wave function is in a superposition of all possible speeds (actually momenta). You can THEN measure its momentum, but after you do that, guess what? Its position is now not well-defined; the wave function now is in a superposition of all possible position states.

It's maybe easiest to see this with Feynmann's example of spins around different axes as described by Feynman. You use a gadget called a Stern-Gerlach apparatus which accepts a stream of electons and depending on how it's set, filters out just those with particular spins around a given line. Spins around different axes are non-commuting, i.e. you can't measure them both at the same time. So you set up three of these gadgets in a row, the first one accepts a stream of electrons with spins mixed as "up", "down" and "zero" wrt the z-axis, and outputs a stream of electrons all with just spin "up" around the z-axis; the second takes that stream and produces an output stream of particles having just spins "left" wrt the y-axis. So apparently it produces a stream that are both "up-z" because they passed the first filter, and "left-y" from the second one. But when you measure the z-spin of this stream of particles with the third gadget you find it no longer has just up-z; it's now in a complete superposition of up, down and zero spins around the z-axis. By measuring the second spin as non-commuting with the first one you DESTROYED the results of the first one.

You really have to internalize this behavior of all quantum systems to appeciate the subtleties of things like entanglement.

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