If the cat in Erwin Schrödinger's famous thought-experiment behaved according to quantum theory, it would be able to exist in multiple states at once: both dead and alive. Physicists' common explanation for why we don’t see such quantum superpositions — in cats or any other aspect of the everyday world — is interference from the environment. As soon as a quantum object interacts with a stray particle or a passing field, it picks just one state, collapsing into our classical, everyday view.

But even if physicists could completely isolate a large object in a quantum superposition, according to researchers at the University of Vienna, it would still collapse into one state — on Earth's surface, at least. “Somewhere in interstellar space it could be that the cat has a chance to preserve quantum coherence, but on Earth, or near any planet, there's little hope of that,” says Igor Pikovski. The reason, he asserts, is gravity.

Pikovski and his colleagues’ idea, laid out in a paper published in Nature Physics on 15 June¹, is at present only a mathematical argument. But experimenters hope to test whether gravity really does collapse quantum superpositions, says Hendrik Ulbricht, an experimental physicist at the University of Southampton, UK. “This is a cool, new idea, and I’m up for trying to see it in experiments,” he says. Assembling the technology to do so, however, may take as long as a decade, he says.

How gravity collapses the cat

Cinema-goers who saw the film Interstellar are already familiar with the basic principle behind the Vienna team’s work. Einstein’s theory of general relativity states that an extremely massive object causes clocks near it to run more slowly because its strong gravitational field stretches the fabric of space-time (which is why a character in the film aged only an hour near a black hole, while seven years passed on Earth). On a subtler scale, a molecule placed nearer the Earth’s surface experiences a slightly slower clock than one placed slightly further away.

Because of gravity’s effect on space-time, Pikovski’s team realised that variance in a molecule’s position will also influence its internal energy — the vibrations of particles within the molecule, which evolve over time. If a molecule were put in a quantum superposition of two places, the correlation between position and internal energy would soon cause the duality to 'decohere' to the molecule taking just one path, they suggest. “In most situations decoherence is due to something external; here it’s as though the internal jiggling is interacting with the motion of the molecule itself,” adds Pikovski.

A practical limit

No one has yet seen this effect because other sources of decoherence — such as magnetic fields, thermal radiation and vibrations — are typically much stronger and cause quantum systems to collapse long before gravity becomes an issue. But experimenters are keen to try.

Markus Arndt, an experimental physicist also at the University of Vienna, already tests whether quantum superpositions can be observed for large, though not cat-sized, objects (see Fattening up Schrödinger's cat). He sends large molecules through a matter-wave interferometer, a system that gives each molecule a choice of two different paths. In the classical view, a molecule travels down just one path; a quantum molecule effectively passes through both routes at once and interferes with itself to create a characteristic wave-like pattern (see image, below).

Ref. 3

A quantum interference pattern built up from complex molecules known as phthalocyanines.

A similar set up could be used to test for gravity's ability to destroy quantum behaviour: by comparing a vertical interferometer, in which the superposition should soon decohere due to the stretching of time in one path versus the other, with a horizontal set up, where the superposition could remain. Arndt, who has tested the effect for molecules of up to 810 atoms², points out that large molecules would be good for testing the gravitational effect because they contain lots of particles contributing to internal energy. But not only would the researchers have to further suppress the external environment to reduce other decoherence effects, they would also need to increase the separation of the two paths from micrometres to metres, or else use molecules perhaps 1 million times more massive. "It's certainly very challenging," says Arndt.

If gravity’s effect does limit quantum behaviour on Earth, tests of quantum reality for large objects may eventually have to move into space, says Angelo Bassi, a physicist at the University of Trieste in Italy. "But from a deep, fundamental point of view, this is nothing new," he says. A gravitational field is merely another environment to interact with, so invoking it does not explain whether quantum behaviour might lead to classical reality if gravity’s influence were mitigated — for example, by doing the experiment in gravity-free space.

Nor does the effect described by Pikovski and colleagues say anything about quantum gravity: a theory that would unite gravity and quantum mechanics in a single description, which many researchers are working towards. “It’s an interesting effect, but it’s still quantum physics applied to classical general relativity. In that way it doesn’t change our picture of the world,” adds Bassi.

Journal name:

Nature

DOI:

doi:10.1038/nature.2015.17773

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1. 
   Paul Creswick • 2015-06-26 02:53 AM

   There's at least one Nobel laureate who has described a special case where this would not apply: "Macavity, Macavity, there's no one like Macavity, He's broken every human law, he breaks the law of gravity." Old Possum's Book of Practical Cats: T. S. Eliot (1939)

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2. 
   Stefan Wasilewski • 2015-06-23 03:55 PM

   Coming from a completely different perspective , economic, this proposes a good deal of ground work to a simple concept, in my forthcoming thesis, that recursive physical structures (networks) throttle agent (quantum) behaviour by increasingly embedding them in deterministic processes and that Time therefore becomes a function of those processes. The Observer has a powerful role in this universe (IMHO all Universes) because the act of observation collapses, or reinforces, the existing local structure and possibly by dint of collective recursive action the neighbourhood or, in exceptional circumstances (i.e. the 2008 Credit Crisis), the whole framework. I'm not a theoretical physicist but would enjoy thinking this through further as it relates entirely to the liquidity and capital crunches that exist today in our global economy.

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3. 
   Vladica Velickovic • 2015-06-21 11:22 PM

   There are many aspects of this article that would make Platon very proud. Existence of abstract mathematical objects in physical reality and our effort to confirm that would make him smile. But let's go back for the moment from metaphysics to physics (even quantum one) and think about what wave-function is it actually? Wave-function is OUR description of a probability, and the value of the wave function is related to the likelihood of the particle’s being in a certain place at a certain time. That's the way we quantify OUR uncertainty about particle position. But when WE observe one particle we don not need any more probability theory to describe particle position and then OUR description of a probability (called wave-function) is no longer needed. This irrelevance of OUR description of a probability after particle being observed you can name as "collapse of the wave-function" or you can also call that: break, breakdown, decay, debacle.. of the wave-function. But our way of quantifying uncertainty is not a physical object and nothing will really collapse, decay and so on,only certainty will remove uncertainty in OUR minds. When you are looking for suitable place to publish your article, description of a probability to publish in a certain journal at a certain time can be named "article function", and in line with probability theory article in YOUR mind "would be able to exist in multiple states at once: both publish or unpublish". But when you publish you article on 17 June 2015 in Nature, "article function" collapse and we can finally read it. The reason, I assert, is gravity. Somewhere in interstellar space it could be that the article has a chance to preserve coherence, but on Earth, there's little hope of that. Let's have a thought-experiment with flipping fair coin only once to prove the effects of gravity on the level of our uncertainty. Before I toss a coin there is a "coin function" describing probability of "heads" and "tails" occurring. When I finally throw the coin, "coin function" collapse and it is collapse due to gravity. Somewhere in interstellar space it could be that the coin has a chance to preserve coherence, but on Earth, there's little hope of that.

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4. 
   George Kanev • 2015-06-19 09:45 PM

   About quantum mechanics connection through the USM www.kanevuniverse.com and is it flowing towards the general relativity? First of all congratulation to Brian about very interesting point about quantum mechanics and general relativity compatibility! First of all let me begin with this: Now let talk a little about the last episodes (there is talking about “Cosmos” series), where was explained the properties of the light and properties of the neutrino. There I have the following objection: Indeed the dark lines in the spectrum analysis of elements are result of jumping of the electrons on the electronic cloud from lower to the higher orbit and there was said that the reverse process it isn’t clear yet, which is correct in the point of view of quantum mechanics, but not according to USM www.kanevuniverse.com Again quantum mechanics give the answer about quantum action as a preservation of the phase in the diffraction wave in the different energy orbits of the atom’s electron (in concrete case hydrogen atom), which means that there the period of wave must to change whole number times to be possible the particle to be the same. This explanation despite it gives thorough quantity coincidence it isn’t essence of the quantum action. That is why the quantum mechanics can give only probability of electron jumping from higher to lower orbit. According to USM pages 85 to 98 (about atom’s energy stimulation) and pages 153 to 174 (about the nuclei’s energy stimulation) www.kanevuniverse.com the essence of the quantum action in the atoms and nuclei actually is the belated inertial reaction of the energy stimulation (see also part I and II) and the disputed dark lines in the spectrum of atoms in fact are the times which is necessary that the field reaches to the closest orbital system to communicate about the has come changing in the equilibrium of the stimulated atom. Now about well formulated by Brian problem: “the probability to find out the electron it isn’t pointed to one particular spot but several such spots, but we can measure experimentally that the electron actually is in some concrete spot, not in several spots simultaneously”…well that is the problem…isn’t it? What is the resolution and whether either the quantum mechanics is incorrect or the general relativity is incorrect? The answer is neither one nor another! Let return towards USM www.kanevuniverse.com part I pages: 8 and 9, where is given the essentially explanation of field creation and there is said that the inter movement between the objects can to be event almost segment (straight line). On the fig 4 and 5 page 11 (USM) is given the essence of electrical charge creation and you can see that the hypothetical electron (can to be the muon and other particles which use the resonance radius of electron, see further in theory), this electron there moves in some harmonic curvet line because of the inertial action and contraction with the nearest orbital systems. So we can make a conclusion: the harmonic oscillation of some particle (through which it creates its own centripetal field) can become almost straight line because there the space continuously expands or contract around this line. Here must to remind that according to USM in the universe doesn’t exist absolutely space, the spaces are only the distances between the objects and if these distances absent, there wouldn’t be orbital systems and therefore no fields no masses, simply nothing, which means there it isn’t something to be described which to shows existence of absolute space itself. Now using the conception that actually the electron’s curvet harmonic line not expand or contract the space on the balanced middle line, follows that we can find out the electron exactly there on some measurement spot, where we presumably quietly accepted that this spot is on our mind creation, called absolutely space! So if we refuse to accept the existence of absolutely space the problem disappear! G.Kanev

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5. 
   George Kanev • 2015-06-19 09:44 PM

   Gravity cannot affect differently upon different light waves, because of the nature of gravitation acceleration which has nothing to do with the general relativity explanation which is wrong certainly…See this: First of all what is the essence of the field according to USM (Undivided Structure of the Matter) www.kanevuniverse.com ? On the part I is shown how the gravitational field is creates and summarizing its looks like this (see part I USM): When the new appearing object (for example the Moon around the Earth) springs up, there this object (the Moon) is births with tangential velocity of birth (V) and the body is moves tangentially and freely until the light ray has “communicated” to the attracting body (the Earth) about the has coming changing in the equilibrium between centripetal and centrifugal forces (appearance of new object). Then appears attracting reaction from the Earth and the Moon bends its trajectory until the centripetal acceleration is directed at the center of the trajectory the Earth. So in the beginning of springing up of the Moon the velocity and the centripetal acceleration are perpendicular each other, but when the centripetal acceleration begin to run (because this reaction belated) the two the velocity and acceleration are already parallel each other. So actually the equatorial plane of the Moon is revolves into 90 degree. Thus is creates circular symmetry in the plane of moon’s orbit and in the same way is creates the spherical symmetry but there is necessary to take into account the thickening and expansion of the space (see USM part I). And because of the expansion of the Universe (in fact our galaxy only) the micro cosmos is thickening and the macro cosmos is expands which increases the density of the atoms (nuclei) and decreases the density of the galaxies and between galaxy distances. So this process exactly coincides with the differences between the gravitational, electrical and nuclear forces (see part II USM), it means that there exists only one field, which appears like a gravitational field in between galaxy’s space, like a electrostatic field in between atom’s space and as a nuclear field in between nuclear space. Thus is explains the quantity difference between the three field, which shows that these three fields actually is the same only field. So the belated interaction between Earth and Moon and attraction force of Earth created the own centripetal acceleration of Moon and the belated interaction between the Sun and Earth and attraction force of the Sun creates the own centripetal acceleration of Earth and so on. But how is creates the centripetal acceleration of galaxies, see part I USM. Finally indeed the distances in the solar system gradually and very slowly about our clock ticing is increases. G.Kanev

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6. 
   Rodney Brooks • 2015-06-19 08:46 PM

   The first paragraph says that according to quantum theory the cat can be in two states. However two sentences later we read that a quantum object collapses into just one state whenever it "interacts with a stray particle or a passing field". I believe the truth is somewhere in between, at least according to Quantum Field Theory. We don't know exactly when field collapse occurs, but it surely happens long before the cat dies - indeed, before any macroscopic change is evident. That was the point of Schrödinger's now-famous cat experiment, although today it is taken the other way. In fact, Einstein had proposed a similar thought-experiment in which a bomb was triggered by a quantum instability. As he said, "a blend of not-yet and already-exploded systems... cannot be a real state of affairs, for in reality there is just no intermediary between exploded and not-exploded”. In my book "Fields of Color" (see quantum-field-theory.net), which explains QFT to a lay audience without maths, I wrote the following: QFT supplies a simple answer for Schrödinger’s cat, and also for Einstein’s bomb. There is no role of the observer. The bomb explodes (or not) and the cat dies (or not), regardless of whether anyone looks. Field collapse does not depend on an observer. The fields evolve according to field equations and then collapse, but neither process requires that someone be there to observe it. In Schrödinger‘s hypothetical cat experiment, the radioactive nuclei do not emit particles; they emit a field that slowly spreads through space. At some point in time that cannot be determined from the theory this field collapses into the Geiger counter and starts the chain of events that kills the cat. Until that time the cat is alive. After that time the cat is dead. In short, it is the possibility of macroscopic effects, not gravity, that causes field collapse, although the definition of "macroscopic" is at this time uncertain. .

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