| From: Michael | 5/02/99
18:41:27 |
| Subject: Faster than light | post id:
476 |
| You answered the question of if
anything can travel faster than light by saying that mass increases as
speed does. But I've heard that physicists have discovered that particles
can be brought into contact in such a way that once separated, even by
huge distances, if an event occurs to one, the other particle instantly
shows a response. Somehow the particles are communicating. Isn't this
faster than light communication? | |
| From: James Richmond (Avatar) | 8/02/99
1:53:40 |
| Subject: re: Faster than light | post id:
561 |
| Apparent faster-than-light
communication between particles is generally referred to as the
Einstein-Podolsky-Rosen paradox (EPR for short), after the authors of the
paper which first brought it to the attention of the scientific community.
The EPR paradox was put forward to point out an apparent conflict between
the theories of quantum mechanics and relativity. Einstein, in particular,
never really accepted the reality of quantum mechanics (as evidenced by
his often quoted statement that "God does not play dice"), even though he
was instrumental in the development of the theory. Here's a brief explanation of one formulation of the EPR paradox: Take a particle, say an electron, which has two fundamental quantum states (spin up and spin down). We can arrange to produce two electrons in such a way that their states are correlated or entangled, so that if one has spin up, the other necessarily has spin down. The EPR effect arises from a particular property of quantum mechanics, which says that a system can exist in a superposition of states. In this example, we treat the two electrons as a single system. Therefore, we expect two possibilities: electron 1 has spin up and electron 2 has spin down OR electron 1 has spin down and electron 2 has spin up. But quantum mechanics also says that before we measure the spin of either electron, the system can, in a sense, exist in both of the possible states at the same time (i.e. in a superposition of the two possibilities). Thus, each electron will be found to have a definite spin value only when we actually measure it. Now, suppose we separate the two electrons by a long distance, then measure the spin of electron 1. We will find it to be either up or down, with say a 50% chance of each outcome. Suppose it has spin up. Then, it turns out that no matter how far away electron 2 is, it will instantaneously be found to have spin down. But the result for electron 1 was not determined until we measured it, so how did electron 2 know to be the opposite? It seems that somehow electron 1 must have communicated its spin result to electron 2 at faster than the speed of light (contrary to relativity). The key to resolving the "paradox" is that even though the electrons are widely separated, they must be still treated as a single quantum system due to their entanglement. In a sense, all the information needed by both electrons is already contained in the system (though the measurement result is NOT predetermined - there are no "hidden variables"). It seems that this type of faster-than-light "communication" is permitted by relativity since it cannot lead to causality violations. The important feature of such communication as far as relativity is concerned is that it would be impossible to send a message using the process. Einstein, of course, was not comfortable with quantum mechanics leading to EPR effects. He called the whole process "spooky action at a distance". However, it appears that such effects do occur. Recently, this feature of quamtum mechanics has been used to "teleport" photons experimentally. | |