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The article "A Quantum Threat to Special Relativity" by David Z Albert and Rivka Galchen discusses how the quantum phenomenon of entanglement overturns our intuition that the world is "local". That is, we can directly affect only objects we can touch, and indirect effects must be transmitted by means of a chain of events that each act locally. The great physicists Niels Bohr and Albert Einstein clashed over the implications of apparent nonlocalities in quantum mechanics, but neither imagined the universe could actually be nonlocal. Yet work by theorist John S. Bell in the 1960s and by experimenters beginning in the 1980s has conclusively confirmed the nonlocal quantum nature of the world.
As Galchen and Albert describe here, scientists were haunted by apparent nonlocalities centuries ago, and thought they had successfully banished them from physics.
—The Editors
Nonlocality from Newton to Maxwell
Three hundred years ago, when Isaac Newton produced the first modern scientific description of it, gravity appeared to be a nonlocal force, and thus be ludicrous, and so it earned the doubt-drenched moniker, "action at a distance". Newton himself—after repeated attempts at producing a local account of gravitation, including one in which all of what appears to us to be empty space is in fact filled with tiny invisible jiggling particles—came to regard his own theory as a merely phenomenological account of what happened in gravitating systems, an account that failed to get at the fundamentals. Newton came to regard the apparent nonlocality of his theory (that is) as a symptom of its limitations. And so things stood, for a good, long while.
The study of electromagnetism during the 19th century initially brought with it the same apparent shortcoming, again seemed to show forces acting at a distance, and not through intermediary matter, and not through intermediary anything—a perennially unwelcome magic.
But as the century progressed, what began merely as a notational convenience—the notation of fields—grew fashionable, and field notation allowed physicists, at least formally, to describe electromagnetism in a way that superficially conformed with the requirements of locality. Electrically charged particles (for example) were treated in this notation as giving rise to an electric field, which extended out to infinity, and penetrated into every point in space. And the capacities of such particles to push and pull on one another from a distance was treated in this notation as arising out of a perfectly local interaction between each one of the particles in question and the electric fields associated with the others. Nobody, however, thought of fields, early on, as anything more than a matter of bookkeeping. They were not considered a part of the fundamental ontology of the world, not something you might, say, stub your toe on, not (most certainly) a real thing.
But then, under the influence of Michael Faraday and James Clerk Maxwell, there was a great sea change, precipitated by a number of transformative observations.
First: by the latter part of the 19th century, the laws of conservation of energy and momentum were deeply entrenched in the scientific conception of the world—and it was noticed that the total energies and momenta of isolated collections of material particles were frequently not conserved when electric or magnetic interactions were involved. Maxwell then realized that a very simple formula could assign energy and momentum to fields, and that after doing so, conservation of energy—of particles and fields together—could be restored. (The formula for assigning energy and momentum to fields was remarkably elegant, and therefore compelling.) So there was a kind of invitation: Treat fields as real, and you can hang onto the laws of conservation.
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18 Comments
Add CommentI still don't understand how magnetic and gravitational forces are "local." When I hit a based ball I am touching one object against another. That's clearly local. But whether I am dealing with entangled particles or an electron and a proton, whatever forces them to change their spin, in the former or move toward each other, in the latter, it is not any sort of contact. To say that one is local because we can detect or measure a field and one is not because we can't see any connection between the two isn't very helpful to me. Or to put it another way, if physicists aren't perplexed by spooky action close by I don't understand why they are so agitated by spooky action at a distance.
Reply | Report Abuse | Link to thisThe term "local" is used to characterize causally connected events, meaning they are close enough together in space and time to have a cause and effect relationship that happens no faster than it would take light to travel between them. Hitting a ball with a bat is actually an electromagnetic interaction between atoms in the bat and atoms in the ball. At the microscopic level the term "contact" loses meaning. The electric and magnetic fields around atoms interact with each others' charges to deflect one from another when they come close.
Reply | Report Abuse | Link to thisThe above is not considered "spooky" since it complies with standard physics which asserts no influence can propagate faster then the speed of light. The Bell's Theorem experiments show that for quantum entangled particles, something seems to be passing between particles faster than the speed of light - even instantaneously. That's spooky.
I have always "felt" that everything big and small is all connected. Everything is actually a solid to a degree. I like to think that our perception of the three dementions covers up the reality that there is no distance from point A to point B. It's the same.
Reply | Report Abuse | Link to thisI have difficulty in understanding why entanglement is considered non-local. If two particules are entangled in such a manner that a property A is complimentary between the two, isn't the setup of entanglement the "local" connection? Thus the process of entanglement could confer the requirement to evolve in a complimentary manner. Thus it does not matter what value the entangled property has for each particle (it is essentially unknown until someone chooses take a measurement on one of them) and I preume Quantum Mechanics would indicate complete ambiguity until then. At this point we know the value of the other instantly, however there was no need for the one to communicate this to the other, and therefore no violation of special relativity seems in order. Now I do not know how stable entanglement is, however I would have assumed it would be easily broken. If some experiment could be performed on one of the entangled particles that forced a change in the property that was entangled without breaking it and this nonetheless forced the other particle to change, that would be something else. Have experiments of this nature been performed?
Reply | Report Abuse | Link to thisRon you ask good questions and have good intuition about the quantum world. Your main conceptual misunderstanding is that members of the entangled pair, at creation, do not have definite states, and wont until at least one is measured. While collectively they must conserve linear and angular momentum, that can be satisfied by an infinity of different states. For example, A could spin up and B could spin down, or A could spin at a small angle relative to up and B would necessarily have to spin with a small angle relative to down, ad infinitum. The constraint is that their spin (vector) sum must equal zero if they were created from a zero spin state.
Reply | Report Abuse | Link to thisSo if you let them propagate away from each other for, say a year, and then arrange to measure A with some apparatus that forces it into a 45 degree angle spin state then its partner, now a lightyear away, will instantaneously collapse to a an exactly opposing spin state. Had you instead chosen to measure A with an apparatus forcing it into a spin up state, its partner would instantaneously collapse (from the ambiguous entangled state) to a definite spin down state. This influence propagates instantaneously over a light year this is why it is called nonlocal influence.
You are very correct in characterizing the entangled state as fragile. Any unintentional measurement (noise) made on either particle will collapse the states and unentangle them - both will be in definite states. There is a lot of research underway to learn how to avoid this issue - called decoherence.
I am confused about "non-communication" regarding quantum teleportation. With being able to see whether the state has been determined, how is it impossible to transmit information in such a fashion? For example, say I have 4 particles entangled with 4 corresponding ones some incredible distance apart. In preparation, someone on the other side detects the states of some of the particles, say 1, 3, and 4. Then when I go to "read" the particles, I just check which ones have been determined, which would be yes, no, yes, yes (1011 = 11 in binary) thus transmitting data.
Reply | Report Abuse | Link to thisI know my understanding of the measurement process is horribly lacking, but it seems that if we can know that transmission occurs, that knowledge itself is a viable medium for information.
Blindwatchman: The problem is that you can't just "check" whether particles' partners have been read or not at the other end in the sense I think you mean. The only thing you can do at the far end is perform measurements on the particles and see what state allowed by your measurment apparatus the particles take. To see the effects of entanglement you have to compare your results with those at the other end after a statistically significant number of trials and then see if there is an unusually high correlation. This, of course, dashes the idea of using it as an FTL communication scheme.
Reply | Report Abuse | Link to thisYou are right to point at the measurement process as the non-intuitive aspect of the problem. At the far end you need to make a choice about, say, the angle to set your apparatus at to test the unknown state of the partner. Once set the particle has no choice but to collapse to an eigenstate of the apparatus, regardless of what state is distant partner was meaured with. If A was forced into a +-45 degree state and B is measured with an apparatus set for up/down, you have a 50:50 chance of B collapsing to an up or down state. This is where statistics come in, and where use to communicate is frustrated.
Thinking of fields as "things" which can have an existence separate from particles is a fascinating concept. But why do fields have such different properties? Maxwell united electricity and magnetism, but electric fields and magnetic fields have very different properties. Why can magnetic fields be wound up as if they are organized in lines of force? Why can the lines of force be broken and reconnected as if they are real things? Yet, electric fields and gravitattional fields seem to have no such properties.
Reply | Report Abuse | Link to thisElectric and gravitational fields (can) come from so-called monopoles of charge and mass, respectively. A particle like an electron has both charge and mass and is the source of both an electric field and a (small) gravitational field. It would take the existance of the hypothesized magnetic monopole to perform the same service for a magnetic field. No one has yet, unambiguously seen a magnetic monopole, though many have searched for them. In some unified field theories the magnetic monopole is thought to exist, but it is very massive, unstable and decays quickly.
Reply | Report Abuse | Link to thisMagetic fields, therefore, are always closed lines of force, since magnetic monopoles don't seem to exist in normal conditions. They do not start or stop on any particles. While electric fields can be generated by electric charges (monopoles) they may also be created as closed lines of force, like magnetic fields, according to Faraday's law (one of Maxwell's equations) in the vicinity of a changing magnetic field. Another of Maxwell's equations (Ampere's law) says that a changing electric field (E) will generate a magnetic field (B). If you connect these two ideas you have sense of how light (electromagntic radiation) propages: E generating B regenerating E, etc.
Gravitational fields can also be generated as closed loops of force through time changing mass-energy distributions, like a spinning neutron star. Gravity waves are such a phenomenon, in which the field has been liberated from the mass orginally generating it.
All of the above are classical physics concepts. Quantum mechanically, fields are also particles. The quantum of electromagnetic force (field) that mediates interactions between charged particles is the (virtual) photon (particle of light). So in a sense fields and particles are just two sides of the same coin.
Thank you for your effort in replying to our questions.
Reply | Report Abuse | Link to thisI am puzzled in your reply to my original comment as to the difference between "unknown" and not having a definite state. The latter sounds like a mathematical result implying the circumstances do not confer any obvious choice of value, only the fact that the two entangled particles will progress in a complimentary manner. Yet unknown (until measured) would seem to be synonymous in this context.
Given that it is not clear how we could at present conduct experiments at light year distances, I am wondering how scientists have been able to demonstrate that the property of the other entangled particle does in fact
change at the same time as the first is forced into a specific state. I tried a simple thought experiment of my own and I can see that even if it does change at the same time, we would not be able to take advantage of that fact to communicate information at a speed faster than light, so in that regard it does not cause issues with special relativity (as had been clarified by John von Neumann and stated in the original article).
I assume these quantum computations on the evolution of the entangled state are being done with a form of quantum mechanics that is compliant with special relativitiy (Silly question really but I do need to confirm this).
Einstein said that nothing could exceed the speed of light, gravity included. So how is it that the early universe expanded faster than that speed limit? It has also been said that the universe did not exceed that limit, but only space-time did, which must surely be a field of some form or another, as it eventually created the cosmos as we can detect it. The limits of our perception would appear to defy rational logic.
Reply | Report Abuse | Link to thisRon - regarding quantum states, I would agree "unknown" and "indefinite" are pretty much the same thing in this context.
Reply | Report Abuse | Link to thisSaying measurements are lightyears apart is dramatic but not necessary to demonstrate the effect. Actual experiments have been done many kilometers apart. For example, if A and B are 3km apart, any signal propagating at the speed of light would take 10 microseconds to traverse this distance. Using atomic clocks experiments can be synchronized and measurements can be made much faster and with greater precision than 10 microseconds, so A and B can perform measurements independently and after the fact compare results. The statistics demonstrate that Bs result was influenced by As settings faster than a signal communicating As setting could have been sent to B. The statistics are often summarized in something called Bells Inequality condition.
Recently an experiment was done with an 18km separation, removing many of the so-called loopholes in previous experiments. Interesting read at http://www.physorg.com/news132830327.html
On your last point you are touching on a very tricky aspect of QM. While the entangled state as it evolves can be described as a wavefunction that obeys the appropriate QM equation for the kind of particles created (Schroedinger, Dirac, QED - all consistent with Special Relativity), the measurement process itself is not described by QM. It is referred to as collapse of the wavefunction into a definite state of the measurement apparatus, but the collapse process itself has no analytic equation to describe it, as far as I know. The process is treated as an instantaneous one triggered by the action of an observer, according to the Copenhagen Interpretation of quantum theory. There are other schools of thought, like Bohms guide wave theory, but the mystery of the collapse is at the heart of the debate about non-local influences and violations of Special Relativity.
Hope this helps.
The first thing that came to mind when reading A Quantum Threat to Special Relativity (March 2009) was Rupert Sheldrake's research on dog behaviour (see Dogs That Know When Their Owners Are Coming Home--And Other Unexplained Powers of Animals). Sheldrake's research appears quite sound methodologically but completely lacking a physical explanation, and maybe this is it. Dogs have somehow tapped into quantum nonlocality out of love of people--or dog biscuits. Perhaps what have been traditionally considered supernatural phenomena are natural after all, if nonlocality is real.
Reply | Report Abuse | Link to thisDavid Murchie
The first thing that came to mind when reading A Quantum Threat to Special Relativity (March 2009) was Rupert Sheldrake's research on dog behaviour (see Dogs That Know When Their Owners Are Coming Home--And Other Unexplained Powers of Animals). Sheldrake's research appears quite sound methodologically but completely lacking a physical explanation, and maybe this is it. Dogs have somehow tapped into quantum nonlocality out of love of people--or dog biscuits. Perhaps what have been traditionally considered supernatural phenomena are natural after all, if nonlocality is real.
Reply | Report Abuse | Link to thisDavid Murchie
" Entanglement violates some of our deepest intuitions about the world.Entanglement may undermine Einstein's special theory of relativity. "
Reply | Report Abuse | Link to thisSocrates warns against "Sophistry, words losing meaning". Nonlocality/entanglement, does not undermine relativity because entanglement is a special relation-ship/relative-ity. - a sharing of information by energy particles.
Entanglement, does not violate our intuitions about the world, it enhances them!. "The really valuable factor is intuition. There is only the way of intuition, which is helped by a feeling for the order lying behind the appearance." Einstein
Intuition is a vital part of mental health:I was told in medical school that in dire situations I may have to rely on intuition - valuable advice!
" Entanglement violates some of our deepest intuitions about the world.Entanglement may undermine Einstein's special theory of relativity. "
Reply | Report Abuse | Link to thisSocrates warns against "Sophistry, words losing meaning". Nonlocality/entanglement, does not undermine relativity because entanglement is a special relation-ship/relative-ity. - a sharing of information by energy particles.
Entanglement, does not violate our intuitions about the world, it enhances them!. "The really valuable factor is intuition. There is only the way of intuition, which is helped by a feeling for the order lying behind the appearance." Einstein
Intuition is a vital part of mental health:I was told in medical school that in dire situations I may have to rely on intuition - valuable advice!
Hello Woops72,
Reply | Report Abuse | Link to thisGreat questions, three more:
Q1: Please confirm that QM is deterministic in the sense that given a specific experiment, the outcome is consistent, repeatable and it does not depend of the peculiarities of the observer (race, age, religious belief ...) conducting the experiment. In other words, "the pot does not take longer to boil when observed". (Some writers insinuate otherwise)
Q2: experiments to create non-locality are not trivial to do. Have we found phenomena where non-locality occurs naturally?
Q3: it is nice to know that we can use intuition to further physics, that it is not just limited to the ones working with the math equations. Can geometry be "pinched" in cases of non-local events, so that the distance function is just an "optical illusion"?
Thanks!
Assuming that I can establish by some means a set of entangled objects at some arbitrary distance apart, and I have the means to force the set at one end into a series of states (which I am told you can do) that corresponds to a message such as 1001. Then the set at the other end would instantly become the bit inversed message 0110. It would seem that the only sacrifice that is necessary for this to be FTL communication, is that both sender and receiver must agree on a time at which this transformation will be made. As long as the receiver starts taking his measurements at the right time, that time can be arbitrarily close to the agreed upon time, making the message FTL.
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