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Originally Posted by lindagarrette
My question is how does one deliverately alter the state of a particle? It's my understanding that so far we have been able to observe various states (spin or charge) of a particle but that's about it.
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We've been able to change spin on particles for sometime. I would suggest reading up on how entanglement works for an answer to that. We can do more than observe.
Spin is the internal angular momentum of the particle. It is assumed that the total angular momentum before the emission process is 0. Because of law of conservation of angular momentum it has to remain 0 if there is no exchange of angular momentum with environment. Lets assume that we conduct a spin measurement on particle A at any axis of choice. Spin is quantized. It can have only discrete values. The only values we can obtain for a spin ˝ particle at a given axis are two possible orientations of spin opposite to each other along the choosen axis . Lets call them spin up and spin down. Prior to the measurement the spins of both particles are unknown. Lets assume that we find particle A in a spin up state. What is the consequence Of this measurement on spin of the particle B? If the angular momentum should be conserved , then the particle B should immediately undergo to a spin down state so that the vector sum of the spins remains 0, If we hadn’t done the measurement on particle A we would have both possibilities of spin up and spin down open also for particle B . But since we have measured spin of the particle A we know without doing the measurement on particle B that particle B must be in a spin down state. Thus the measurement on particle A immediately influences the outcome of the measurement that is conducted on particle B however far apart the particles A and B are from each other. May we consider the wave function itself as something physically real field or a probability wave, the situations after both measurements is certainly physically real since macroscopic measurement devices have changed their states. Thus correlation between the outcomes of measurements carried out at particle A and particle B shows that one cannot avoid but to accept that physical event at some location influences immediately the physical reality at some very distant location.
This consequence of the entanglement between two distant particles was until seventies only a theoretical issue. It was by Aspects experiments when the world realized that entanglement of distant photons is a fact(1). This was confirmed in later experiments(2).
The outcomes of two distant measurements are correlated , the correlation becomes apparent only if one can compare BOTH results. Each measurement itself is random. That is if one carries out 1000 measurements one sees a random distribution of outcomes for both particle A and particle B if evaluated independently. So no information is transferred from A to B and special relativity is not violated. The information can be obtained only if the outcomes are compared later by normal means. This has always been the assumption as far as proving something out. However, Relativity doesn’t concern itself with something so subtle like "information" in its terminology. In truth it is the energy/matter transport velocity or the velocity for influencing distant events each other what relativity limits. This misconception dates back to the times when it was discovered that the phase velocity of electromagnetic waves is exceeds light velocity c in a dispersive medium. Calculation and experience showed however that the group velocity namely the velocity with which a electromagnetic wave packet propagates is always less then c . Since electromagnetic waves are mainly used in signal transfer by modulations, this fact was expressed in the following way “although phase velocity can exceed light velocity the signal/information transfer velocity is always less then c”.
The Copenhagen interpretation, assumes the wave function is not a real physical entity but only a mathematical tool to predict the probability of a particular outcome in a measurement , nothing is physically transportat faster then light(3). But that is only one actual interpretation. If one assumes the wave function is real then one is left with a certain variety of information being transfered FTL. Roger Penrose has made the statement,
Although EPR-type experiments do not , in the ordinary sense of sending messages, conflict with causality of relativity , there is a definite conflict with the spirit of relativity in our picture of physical reality(4).
I happen to be one of those who agree and also tend to accept that the wavefunction is far more that a mathematical tool. I suspect there is a way to transfer information via this means. But at the current time our ability is simply too limited to do such. I also suspect that nature uses this type of path all the time and because of our inability to measure such paths we only get part of the picture. I think the mechanism of such involves an external frame of reference where the local velocity of light in that medium is different from ours. Such an external frame of reference would not actually violate relativity at all. And interesting enough this type of dual frames of reference has been showing up in articles related to brane theory, DSR, etc for sometime now. I have a couple of articles on one such model myself on the Cern server.
References
1.) Aspect 1982
2.) Chiao 1993 on the last page
3.) Stenger 1995 p.140
4.) Penrose 1989, p.370
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