Skip to comments.Quantum causal relations: A causes B causes A
Posted on 10/03/2012 4:33:24 PM PDT by LibWhacker
One of the most deeply rooted concepts in science and in our everyday life is causality; the idea that events in the present are caused by events in the past and, in turn, act as causes for what happens in the future. If an event A is a cause of an effect B, then B cannot be a cause of A. Now theoretical physicists from the University of Vienna and the Université Libre de Bruxelles have shown that in quantum mechanics it is possible to conceive situations in which a single event can be both, a cause and an effect of another one. The findings will be published this week in "Nature Communications".
Although it is still not known if such situations can be actually found in nature, the sheer possibility that they could exist may have far-reaching implications for the foundations of quantum mechanics, quantum gravity and quantum computing.
Causal relations: who influences whom
In everyday life and in classical physics, events are ordered in time: a cause can only influence an effect in its future not in its past. As a simple example, imagine a person, Alice, walking into a room and finding there a piece of paper. After reading what is written on the paper Alice erases the message and leaves her own message on the piece of paper. Another person, Bob, walks into the same room at some other time and does the same: he reads, erases and re-writes some message on the paper. If Bob enters the room after Alice, he will be able to read what she wrote; however Alice will not have a chance to know Bob's message. In this case, Alice's writing is the "cause" and what Bob reads the "effect". Each time the two repeat the procedure, only one will be able to read what the other wrote. Even if they don't have watches and don't know who enters the room first, they can deduce it by what they write and read on the paper. For example, Alice might write "Alice was here today", such that if Bob reads the message, he will know that he came to the room after her.
Quantum violation of causal order
As long as only the laws of classical physics are allowed, the order of events is fixed: either Bob or Alice is first to enter the room and leave a message for the other person. When quantum mechanics enters into play, however, the picture may change drastically. According to quantum mechanics, objects can lose their well-defined classical properties, such as e.g. a particle that can be at two different locations at the same time. In quantum physics this is called a "superposition". Now an international team of physicists led by Caslav Brukner from the University of Vienna have shown that even the causal order of events could be in such a superposition. If - in our example - Alice and Bob have a quantum system instead of an ordinary piece of paper to write their messages on, they can end up in a situation where each of them can read a part of the message written by the other. Effectively, one has a superposition of two situations: "Alice enters the room first and leaves a message before Bob" and "Bob enters the room first and leaves a message before Alice".
"Such a superposition, however, has not been considered in the standard formulation of quantum mechanics since the theory always assumes a definite causal order between events", says Ognyan Oreshkov from the Université Libre de Bruxelles (formerly University of Vienna). "But if we believe that quantum mechanics governs all phenomena, it is natural to expect that the order of events could also be indefinite, similarly to the location of a particle or its velocity", adds Fabio Costa from the University of Vienna.
The work provides an important step towards understanding that definite causal order might not be a mandatory property of nature. "The real challenge is finding out where in nature we should look for superpositions of causal orders", explains Caslav Brukner from the Quantum Optics, Quantum Nanophysics, Quantum Information group of the University of Vienna.
Unintented consequences are a fact of life.
“How can you be in 2 places at once when not anywhere at all?”
How many frames of reference were you using? An outcome is still a singular event; the WTC is still gone, for example.
If a video game programmer can code superposition into a game what would it look like? And of course our Creator can easily “code” superposition into our existence. (If you haven’t guessed already.. I know nothing about physics and very little about science in general.)
“But if we believe that quantum mechanics governs all phenomena...”
The key phrase. Quantum mechanics is an attempt to explain the behavior of subatomic particles and the forces that act upon them. If the theory works, it’s provisionally true. But this doesn’t mean we understand quantum behavior as it really is, just that we have an explanation of the quantum behavior that we have observed in experiments, which is an infinitesimally small proportion of all quantum behavior.
So it’s “Bob & Carol & Ted & Alice” or “Alice & Carol & Ted & Bob” depending on your “perspective”...
Nah; that was Schrödinger’s brain cells.
That’s easy when you work for the Department of Redundancy Department.
One is used to moving a video game character through space; now imagine that the character can move just as easily through time. Time is just another “direction” in which the character can move. So, from the character’s perspective, what is a cause, and what is an effect? If the character moves backward through time, the “effects” come before the “causes”. Now consider the subjective philosophical perspective of the game character...it would likely see a cause-effect relationship as being a duality, not a fixed, one-way relationship.
Hidden in Plain Sight Kindle Edition is 99 cents. The author is Andrew Thomas.
It helps, I think, if you've been exposed to quantum and relativity for awhile, and are either in awe or puzzled that the two regimes don't reconcile.
The other stuff I've been reading is heavy on string theory, which is sort of interesting and aims to put some more meat on what Mr. Thomas is getting at; and books about "dark matter" and "dark energy." Very strange physical world we live in, in many many ways.
This reminds me of Borges’ masterful “Tloen, Uqbar, Orbis Tertius”:
So should I get a Obama phone or not?
You seem to be reading material similar to what my older brother reads. He’s a virtual wealth of conversation when it comes to the deeper things of science and discussing them in layman’s terms.
and if things are in parallel universes, they aren't the same thing anyway
Strange? As opposed to what, familiar? Physical? As opposed to what, mental? Relax, everything will be all right.
How would you know?
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