physics question

Factual Questions
1

A while back I recall seeing a program on TV showing a physics demonstration where some particle was being ‘split’, but not into two halves, but into two identical particles. The interesting thing was that the particles still seemed contected on some level because when something happened to one, it happened to both. It was something like that.

Does anyone know which experiment I’m talking about? Do you remember what those particles were and how they split them. Better yet, do you know who the researchers were and if there are any articles (online or in print) about the experiment?

Thanks much!

Amanda

2

O[sub]2[/sub] --> O + O in the presence of a spark.

3

I’m pretty sure you’re referring to the EPR (Einstein-Podolsky-Rosen) experiment or its realization by Alain Aspect.

I am most definitely not the one to explain it. Until more knowledgeable types show up, check out this site.

4

Until someone with more quantum mechanics than my two semesters comes along, you can do a search on “quantum entanglement”, which should give you a little info.

5

It sounds like you are talking about the EPR experiment.

EPR stands for Einstein, Podolsky, and Rosen, three scientists who came up with a thought experiment that made QM seem absurd. The idea is that if two particles are part of a single quantum state, that no matter how far apart they get they are still related in a fundamental way. It turns out if you do the experiment, they are right - QM is far stranger than fiction.

For example, if you shoot a certain kind of photon at a certain type of crystal, there is a chance that the photon is split into two lower energy photons that are related in that their polarizations are the same. [sub]for pedants: or opposite. Doesn’t matter for my example.[/sub] Polarization is just a property a photon can have, and it is defined by a direction like ‘up-down’, or ‘left-right’, or at some angle.

When you measure the polarization of a photon, you can’t know its exact polarization before measuring, but you force the photon into a polarized state. This is kind of tricky, so I’ll try to explain carefully. What happens is that if a photon is polarized like this “/” (half way between up-down and left-right), and you measure for ‘left-right’ polarization, you have a 50/50 chance of the photon being polarized ‘left-right’ or ‘up-down’ when it comes out of the machine. But, you also have a 100 percent chance of measuring ‘left-right’ if it was ‘left-right’ to start with. You don’t know what it was before you measured just because you see ‘left-right’ on your polariz-o-meter, but you do know it is now definitely ‘left-right’.

Anyway, if you get these two photons and they travel apart for a while, they are still ‘entangled’ - meaning if they haven’t interacted with anything else their polarizations are still the same. So, after a while, you measure their polarizations at exactly the same time, and BANG they are the same. If one is left-right, they both are. If one is not, neither are. Well, how did that happen? If they were at some funky angle, and we’re measuring left-right, shouldn’t they sometimes be different? That is, say a photon has a 50-50 chance of getting through. Shouldn’t half the time the other photon ‘choose’ differently (or ‘lose the coin toss’)? Instead, they are always the same.

So, how DID that happen? Either the photons are talking to each other faster than the speed of light (since they were some distance apart and you measured them at the same time), or they are ‘entangled’ in some way that magically makes one related to the other no matter how far away you are.

If you do the math, involving something called Bell’s Theorem or Bell’s Inequality which I won’t get into here, it turns out that this means that the ‘local realistic’ view of the world is disproven. ‘local realistic’ assumes
[ul][li]There are real things that exist whether or not we observe them.[/li][li]It is legitimate to draw conclusions from consistent experiments.[/li][li]No influence (or information) travels faster than the speed of light.[/li][/ul]
Since we’ve just disproven this, what does that mean? All kinds of interesting philosophical and scientific questions arise from this.

I really can’t do this justice here, I recommend you find a book called ‘In Search of Schrodinger’s Cat’ by John Gribbin. (there is an umlaut over the o in Schrodinger, so just search for ‘Gribbin’.) It is out of print, but
should be available at any self respecting library. I just searched at my library and the 4 communities within 5 miles of me all have it.

6

ok ok ok my brain hurts now.

where was that bottle of gin…

7

Correct me of I’m wrong, but isn’t what ** douglips ** mentioned the basis for transportation (i.e., on Star Trek?)
I read an article in Scientific American about transportation, and it mentioned something about linked particles, or something.

8

I just want to clarify a few things in douglips’ otherwise excellent explanation (I stole some of it from one of my posts in the schroedinger’s cat thread):

Simplified description of the experiment:

The Aspect experiment: an atom with zero angular momentum decays, producing two photons. By conservation of momentum, the photons have exactly opposite polarization. Before you make a measurement, the two photon system is in a superposition of two quantum states: the state where photon 1 is up and photon 2 is down and the state where photon 1 is down and photon 2 is up. Then you send one of the photons through a polarizer, to see if it was up or down. The polarizer localizes the photon into one of the two polarization states. If it is (e.g.) up, then it can go through. If it is down, it is absorbed.

Why the experiment rules:

When one photon is localized, they both collapse instantaneously into their pure states, no matter how far apart they are. This proved that quantum mechanics is a non-local theory, which was very non-intuitive. At first glance, it seems to violate relativity. However, information can not be transmitted faster than the speed of light, using this wavefunction collapse, so it’s all still consistent. Weird, but consistent.

9

Wow! You folks are amazing!

So, is it possible for groups of those photons to accomplish some kind of work together while in that state of division? Like affect magnetic fields for example?

I guess my bigger (wilder) question is, could this theory explain telekinesis, telepathy or other ‘paranormal’ and rare events?

10

Well we could ask David Bohm about the implicate order. He knows a lot about quantum mechanics.

11

It’s not completely impossible, but it seems highly unlikely.

First, it is not easy to get entangled particles. Well, OK, it’s easy to get them but it’s not easy to keep them. So, saying something like “My brain is entangled with yours, that’s how I read your mind” is pretty ridiculous since there are so many particles in our brains and they have been separated for so long.

Also, remember that measurement destroys the entanglement. So, if in the above experiment some mischeivous elf comes along and sticks in a polariz-o-meter and measures one of the photons, it will destroy the entanglement and then the photons may not be the same when they get to our detectors.

Since measurement can be done by some accidental interaction (a particle of dust, for example) and our brains are in a dirty environment, I don’t see how we could still have any entangled stuff.

But, some people do claim that it is the basis for these phenomenon, which frankly I don’t think exist. See for example, Victor Stenger’s excellent Quantum Quackery (in CSICOP’s Skeptical Inquirer).

Sort of. This doesn’t take away from the fundamental coolness of the experiment. In order to do quantum teleportation, you need entangled matter - you need a whole bunch of it, enough so that at both ends of the teleport you have at least as much mass as the thing you want to teleport. How you keep this stuff entangled for so long is left as an exercise for the reader. Remember, if it bumps into something it will lose it’s entanglement.

Anyway, once you have created the entangled matter and put some of it at each end point of your journey, you then put the thing to be teleported (let’s call it the “victim”) into a jar and mix them thoroughly with the entangled matter you have. You then measure the state of the resulting stuff and record all the data in a computer (more computer storage than you can imagine for something as complex as a human.) You then send all this data to the other teleportation station and the technicians there use your information to shoot a whole bunch of energy into their entangled matter, rebuilding the victim. In theory, it will recreate the exact same quantum state at the other end of the process.

Limitations: You must destroy the original in order to perform this operation. You must not contaminate your entangled matter. You must not lose any data in the transmission.

IIRC, people have successfully teleported photons using this mechanism. Teleporting something as complex as a bacterium is a ways into the future, and may never be possible.

This is a simplified explanation that cuts out a lot of details, but should at least be correct in the concept. Scientific American had a great article on this about a year ago, IIRC.