One of the most puzzling problems facing modern quantum physics theories is the central paradox of how elementary particles like photons can simultaneously exist in multiple states until the point of being observed.
In 1935, Austrian physicist Dr. Erwin Schrödinger illustrated the improbability of the situation with a famous thought experiment: a cat is placed into a box with a radioactive particle, a Geiger counter (which measures particles shed by decaying, unstable subatomic particles), and a vial of poisonous gas. If the particle decays, shedding radioactive energy, the Geiger counter triggers a release of poison gas, killing the cat. If there is little or no nuclear decay, the poison-gas vial remains intact, and the cat survives. The question is, when the box is opened, do you find a live cat or a dead one?
According to accepted theories of physics, you could have both - at the same time. This absurd paradox illustrates the central problem with quantum mechanics - forcing us to either conclude that A) matter works much differently on the subatomic scale than it does in the observable world; or B) something is wrong with the theory itself, even though it has consistently held up to mathematical and experimental scrutiny and led to many scientific advances over the last century.
Most physicists resolve the paradox by saying that observation collapses particles into a single state, but this still raises troubling questions, such as: if there is more than one observer, which one is entitled to cause the collapse? and where does the border lie between the laws of the subatomic world, in which these paradoxical states (superpositions) can exist, and the observable world, in which they cannot?
In other words, at the quantum level, it seems that objects are not really definable until they're observed - it seems particles don't actually resolve themselves into a single, measurable state unless they're being observed, and return to a multiple set of potential states when they're not.
In 1996, Dr. Christopher Monroe and colleagues Dawn Meekhof, Brian King and David Wineland at the National Institute of Standards and Technology in Boulder, Colorado cooled a single beryllium ion to near absolute zero, trapping it in a magnetic field and bringing it to a near motionless state. Stripped of two outer electrons, the ion's single remaining outer electron could be in two quantum states, having either an up or down spin. Using lasers, the team applied a tiny force one way to induce an up state in the electron, then in the opposite direction to induce rapid oscillation between both the up and down spins.
The electron was eventually induced to spin in both orientations simultaneously - in what physicists call a superposition. The team then used lasers to gently nudge the two states apart physically, without collapsing them to a single entity, so that the two states of the single electron were separated in space by 83 nanometers, 11 times the size of the original ion.
In other words, while the old proverb says You can't have your cake and eat it, too, it appears that, at least on the quantum scale, you certainly can.
Schrodinger's cat is alive and well... dead. Simultaneously.
We Can't Be Certain
In 2010, in a darkened laboratory at the University of California Santa Barbara, a tiny metal paddle the width of a human hair was refrigerated, and a vacuum created in a special bell jar. The paddle was then plucked like a tuning fork and observed, as it simultaneously moved and stood still. Superposition was being directly observed – objects in the visible world existing in multiple places and states simultaneously.
In quantum mechanics, there is an inherent uncertainty to reality; the location of an electron can never be precisely pinpointed at any moment in its orbit. Instead, its position can only be predicted in terms of probability. According to some physicists, if there are a thousand possibilities, eventually all thousand possibilities will occur, and so, at the quantum level, the outcome of experiments cannot be 100% predicted. All things are only based on probabilities.
A great deal of modern technology works upon the paradoxical principles of quantum physics, but this seems to contradict everyday common sense - when we observe objects in the real world, they always exist in only one place and one state at a time.
Quantum physicists call this paradox the measurement problem, and have long resolved it by saying that particles collapse into single states at the time of observation. But in the 1950s, some physicists first began to hypothesize that in fact all possible states exist, separating into different realities at the moment of change.
Wavefunction collapse is the point at which a particle existing in several potential states resolves into a single state. Some physicists now believe that at this moment, reality branches off into every one of these potential states, and our path of reality simply continues along the path we've observed. And this Many-Worlds theory means that an infinite number of alternate realities exist.
Quantum physicist Hugh Everett says that this is because a quantum particle doesn't collapse into a measurable state, but actually causes a split in reality, with a universe existing for every possible state of the object. Superposition, he asserts, actually means parallel worlds exist - one arising anew out of each of a particle's states.
For example, photons beamed through double-slits strike an opposing surface in both single streams and in spread-out wave patterns, depending upon whether or not they're being observed. When the particle is observed, it functions as a particle; but when it's not observed, it acts like a wave. At the moment of observation, according to Dr. Everett's Many-Worlds theory, the universe splits, and both results occur, but in different realities.
Objects you can observe, he hypothesizes, can thus exist simultaneously in parallel universes - the reality in which you continue, and those alternate ones in which the other yous exist. This implies that anytime alternative actions can be taken, the universe splits into alternate realities, where each decision was taken.
According to Berkeley's Dr. Raphael Bousso and Stanford's Dr. Leonard Susskind (among others), this means an infinitely expanding range of multiple universes exists, some with distinctly different laws of physics than the ones governing ours. Our reality is one "causal patch" among an infinite number of others.
The theory suggests that every conceivable possibility eventually comes to pass in one of these infinite realities and perhaps information from separate causal patches of reality can leak across between universes. Although parallel universes are still much under debate, recent cosmological models support their existence, reality splitting into an infinite number of paths for every event occurring in space-time.
More, if you're interested, can be seen in this pair of MIT lectures:
Peeping into the Box
Cutting-edge technology developed in 2011 at the National Research Council of Canada is allowing researchers to quickly measure the position and momentum of photons - a complex, 27-dimensional quantum state - in a single step. This is a huge leap in efficiency over former quantum tomography, which required multiple measurement stages and a significant amount of time, analogous to photographing a series of 2D images from different angles and assembling them into a 3D image.
Such a precise, rapid, accurate and efficient means of measuring states with multiple dimensions is likely to be critical for advancing our knowledge of quantum mechanics, not to mention the development of advanced-security quantum communications technology, which will in theory be impossible for interceptors to decode.
Image: This diagram illustrates the setup for the experiment, which incorporates a HeNe laser, beam-splitters, lenses, a special "fan-out hologram", wave-plates, and additional equipment. One can conceive of light as a spiral and the degree of "twist" to that spiral is called the orbital-angular-momentum quantum number. The spiral is "untwisted" before being measured. Illustration: M. Malik, Nature Communications.
According to Dr. Robert Boyd, Optics and Physics Professor at the University of Rochester and Canada Excellence Research Chair at the University of Ottawa, this new type of direct particle measurement is likely to play an increasingly important role in future quantum communications technology.
A cooperative effort between the Universities of Rochester and Ottawa and Glasgow demonstrated how direct measurement can be used to circumvent Heisenberg's uncertainty principle, simultaneously, accurately measuring two aspects of a quantum state without sacrificing accuracy in either measurement. This is because the act of measuring a quantum state alters either the particle's position or its motion, "collapsing the wave function".
But direct measurement performs two different measurements one after the other: an initial "weak" measurement followed by a "strong" one. The initial measurement is gentle enough to only slightly disturb the particle's state, avoiding wave function collapse.
According to the study's main author, the University of Vienna's Dr. Mehul Malik, this experiment allows physicists to peek into Schrödinger's box, without fully opening it. He says that the weak measurement is faulty, leaving uncertainty about the state of particles, but repetitions of it lead to near certainty of the particle's state, since it doesn't destroy the particle's state, it allows for a subsequent "strong" measurement of the second variable. The alternating sequence of weak and strong measurements can then be repeated for several identically-prepared particles, until a measurement of the wave function at the required precision is derived.
Sources: The Path Book I: Origins, Eric A. Smith, Polyglot Studios, KK
"Peeking into Schrodinger's Box", press release, January 20, 2014, Leonor Sierra, University of Rochester