Special relativity suppresses quantum effects…
The two pillars on which modern science is based are the quantum mechanics operating at sub-atomic scales and general relativity which operates on cosmic scales. Although both have been experimentally proven to be immensely successful, they remain incompatible.
A new study, however, has shown that gravity, at the very center of Einstein’s general relativity, also affects sub-atomic phenomena. Its findings are expected to open a new door for implementation of the mechanisms of the quantum world in our familiar, everyday world, which physicists call “classical” because it was described by pre-quantum physics. An immediate beneficiary may be the so-far-elusive quantum computers hoped to transform number crunching with strange paraphernalia of the sub-atomic realm.
Quantum phenomena defy our common sense conditioned to our perceptions and experiences in our “classical” world. One of these is the possibility of an object’s (e.g. a photon, electron or an atom) simultaneously being in different quantum states. In other words, its ability to simultaneously exist in different places. This counter-intuitive property is called quantum superposition.
“And” vs. “or”
Austrian physicist Erwin Schrödinger, one of the founding fathers of quantum mechanics, illustrated this phenomenon with a thought experiment. The experiment, which would later acquire fame as the “Schrödinger’s cat,” is based on a hypothetical cat in a sealed box, which could be alive or dead dependent on the random decay of an atom. When the box is left to itself, the cat is both alive AND dead in conformity with quantum superposition. Only when the box is opened the uncertainty dissolves and the superposed quantum states collapse to one of the classical “alive OR dead” states.
In the thought experiment Schrödinger proposed in 1935, a cat is enclosed in a steel box. A trace amount of radioactive material is placed in a geiger counter in the box. One of the atoms in the material may undergo a random decay, or it may not. In the event of decay, a suspended hammer will fall and smash a vial fiilled with lethal hydrocyanic acid, and the released gas will poison and kill the cat. If decay doesn’t happen, the cat stays alive. Since the interior of the box cannot be seen from outside, it is not known whether the cat is alive or dead. So, it is both alive and dead at the same time. Only when the box is opened and observation made the quantum superposition is disrupted and cat collapses into either one of the alive or dead states in the classical world.
Schrödinger’s aim in devising this experiment, also called “observation paradox”, was showing the impossibility of implementing the fuzzy dynamics of the sub-atomic realm described by the more popular “Copenhagen interpretation” of quantum mechanics to our “classical” everyday world. Contrary to the quantum fuzziness, precision is the passing currency in this world. Here, the quantum superposition of states disappear as the scales grow, with superposed particle losing what is called “coherence” or the entanglement of quantum states, due to interaction with surrounding particles or due to effects of light, heat or vibrations and only one of the states passing the gauntlet.
When the cat gets fat…
Now, however, physicists from Vienna, Harvard and Queensland universities have shown that general relativity, or more precisely , gravity that it describes, also plays a significant role in the prevention of quantum phenomena from applying to larger scales.
General relativity’s conformity to our large-scale everyday world doesn’t mean that it is without its own set of counter-intuitive phenomena. According to this theory, gravity is an effect of the curvature of space-time. Mass curves the space and the curvature cause gravity. In astronomical scales, our world is not as hefty as we think. If we can imagine the spacetime as a taut rubber sheet the depression the Earth causes does not exceed a mere centimeter. Hence, the gravity it exerts does not amount to much. You can defeat the gravitational pull of the whole planet on a pin on the table by lifting it with a school magnet. General relativity says mass (and hence gravity) also affects the flow of time. Large masses slow down time. In physics lore, this is called “gravitational time dilation.” And since according to Newton’s Law of Universal Gravitation – which holds in local scales – the gravitational force is “proportional to the product their masses and inversely proportional to the square of the separation between the two objects,” an object closer to earth feels stronger gravity. And since, according to Einstein’s general relativity gravity slows time’s flow, someone living in the ground floor ages slower than the tenant one floor above – by 10 nanoseconds a year!
Due to the same effect, researchers led by Caslav Brukner of the Vienna University and the Institute of Quantum Optics and Quantum Information, showed that as the building blocks of matter form larger structures like molecules or constructs made with their combination, time dilation on Earth suppresses their quantum mechanical behaviors.
As the building blocks make larger structures, they undergo miniscule vibrations. And the frequencies of these vibrations, subject to time dilation, slow down at the base and speed up as the structures gain height, that is, as they gain in mass and complexity and head for the classical realm. This effect disrupts the quantum superposition and forces larger structures to behave in the familiar way we are accustomed to see in the everyday world.
In the end, thanks to Einstein’s theory of general relativity, there is no risk for a boxed “classical” cat to die due to the random decay of a single atom. When you open the box, the observation you’ll make will be that of a scared cat.
Tracing cat’s footprints
The impossibility of knowing the outcome as long as the steel box enclosing Schrödinger’s fictitious cat remains shut , with its fate becoming clear only when the box is opened is the central assumption of the thought experiment. While the fate was a drawn-out uncertainty in the closed box, its revelation with its opening was an instantaneous event with the quantum superposition collapsing to either of the classical outcome – or, it was so until recently.
Physicists from University of California – Berkeley and Washington University have shown that the passage from quantum superposition represented by the imagined cat to the classical world is not an instantaneous event, but an extended process, during which information can be extracted from the box even though it remains closed.
In the experiment directed by a team led by Irfan Siddiqi, associate professor of physics at Berkeley, and assistant professor Kater Murch of Washington University, whose findings were published in the July 31, 2014 edition of Nature, the role of Schrödinger’s cat was assumed by a superconducting circuit chilled to the edge of absolute zero (-273.15°C). In the circuit, also called an “artificial atom” because it can be in discrete energy levels, the quantum system was represented by the ground level and the excited level just above it. Innumerable combinations of superposed ground and excited states exist in between.
While the immediate collapse of the system the moment an observation was made precluded the observation of these superposed states before, the Berkeley-Washington tea m succeeded in plotting the path the system chose between the ground and excited states through a technique called weak or gentle measurement. The method works like this: The superconducting circuit (cat) is placed into a box. Then a small number of microwave photons are beamed at the box and their quantum states interact with the superconducting circuit. But as the quantum states of the microwave photons are much different than those of the circuit, they cannot force it into the range between the ground and the excited states. Hence, instead of being absorbed, they exit the box bearing information about the quantum state of the circuit at that moment, in the form of phase shifts that is changes in the shape of peaks and troughs of its quantum wave function. But since the information is limited, the measurement has to be repeated – a million times – to determine the route from one quantum state to the other.
When the quantum cat becomes classical?
So, where’s the border between the quantum realm and the classical world? In other words, how much an object has to grow in order to cross into the world as we know it? In still other words, what is the maximum size the object can take without losing its superposed state?
Physicists Stefan Nimmrichter and Klaus Hornberger of Duisburg University (Germany) developed a mathematical model to calculate how “macroscopic” an object has to be in order to transit from the microscopic world ruled by quantum mechanics into the classical world where general relativity reigns supreme.
To do that, they determined the minimum alteration that has to be made in the dynamics of the Schrödinger’s (wave function) equation to disrupt the quantum state of an object (or to untangle the superposed states). The “macroscopicity” of an experimental result is shown with the number of alterations banned by that result. The more macroscopic an object is, larger is the number of banned altered states. This mechanism rests on the duration of coherence which shows how long the state of superposition lasts. Since a heavier molecule bans more changes compared to a lighter one, the mass of the object is also important. These parameters, coupled with another one showing the scale of the superposition, form a value shown as inthe logarithmic scale.
Nimmrichter and Hornberger obtained highest macroscopicity with an experiment at theVienna University in 2010, in which a 356-atom object yielded a value of 12. They foresee a 23 macroscopity in the future with the use of half a million gold atoms.Two researchers then calculated how long a “real” cat can remain in superposition. For that, they modeled the “cat” as a 4-kg, water-filled sphere. For the cat to be simultaneously at two places 10 cm apart, a value of 57 was found to be required. That corresponds to an electron being in superposition for 1057 seconds – that is, 1039 times the age of our 13.8 billion-year-old-universe. ”One should never say “never”, but it so seems that we won’t be able to put a cat into quantum superposition,” Nimmrichter says.
REFERENCES
- 1. “Einstein saves the quantum cat”, University of Vienna, 16 June 2015
- 2. “Quantum measurements leave Schrödinger’s cat alive”, http://www.newscientist.com/article/dn22336-quantum-measurements-leave-schrodingers-cat-alive.html#.VYeybUYXWUk
- 3. “Finding quantum lines of desire”, Washington University in St. Louis, 30 July 2014
- 4. “Watching Schrödinger’s cat die (or come to life)”, University of California – Berkeley, 30 July 2014
- 5. “How fat is Schrödinger’s cat?”, Physics World, 25 April 2013