We always thought that reality existed, but maybe not anymore. It is like the old question about the tree falling in the forest with no one around to hear it. Now quantum physics would say that if it makes a sound, there might not be a tree at all. You can’t be sure.
Seems an odd thought, but it is part of the territory of quantum mechanics. We expect the field to push boundaries when it comes to defining reality. After all, physicists are busy conducting experiment after experiment to show that particles spread out like waves. In fact, they are shown to be in more than one place at any given moment.
Now we accept this as a likelihood. We believe that particles assume a definite position only at one moment in time. As Einstein was noted as saying, “I like to think that the moon is there even if I am not looking at it,” he said.
More experiments are in the works in the world of quarks, atoms and qubits. Researchers are looking at the ordinary world of objects like tables, chairs, and even moons. Jonathan Halliwell at Imperial College London asks, “If you can go from one atom to two atoms to three to four to five to a thousand, is there any reason why it stops?”
It is a rather large question. In fact, it has to do with our conception of reality and its true nature – whatever it is. We have to face the possibility of a reversal of our previous conceptions about the existence of things apart from our visual perception of them.
We have to credit Einstein who in 1935 designed a thought experiment involving quantum mechanics. He postulated that it was not an accurate theory of reality and had to go to the wayside. He was working with Boris Podolsky and Nathan Rosen at the time. They were envisioning a pair of particles that while entangled were having an effect – one on the other. They imagined measuring one particle’s position or velocity. It would reveal the other’s position or velocity. Then they speculated about placing these particles at opposite ends of the cosmos. What if they measured them at this point? Well, it would yield information transmitted faster than the speed of light.
Pretty heady stuff, even for us in the 21st century! Einstein called it “spooky action at a distance”. It seems to be an apt phrase for entanglement experiments. He felt that the outcome of such experiments must be predetermined since the idea is so absurd.
The ill-defined nature of quantum mechanics puzzled John Bell, a physicist, who in 1964 put this paradox to the test in a mathematical equation, called Bell’s inequality. It was meant to see if Einstein had been right. If so, the inequality would hold true. More experiments followed, but Bell’s inequality was violated. Now we have two schools of thought, classical and quantum physics. Which one holds sway for entanglement? It depends on whether things happen faster than the speed of light according to Vlatko Vedral from the University of Oxford. If you go with Einstein, quantum theory breaks the universal cosmic speed limit.
Bell’s inequality may be violated but it isn’t done for. It addresses locality or the space between objects. While it may not answer questions about reality existing apart from perception, it does deal with particles in terms of position, speed, energy, and other properties. The position of realism accepts that these properties are rather well defined and can be measured without affecting the future action of an object. Quantum physics adds superpositions and uncertainty to this view such that multiple identities mix and collapse into one value when measured.
If you are considering macroscopic objects, realism becomes macrorealism. Take measuring the distance of the moon with a laser. Its distance will not change per our common sense perspective. Turning to Halliwell, we find that “macrorealism is the fullest expression of classical reality.” In fact, there is a test for it.
How is that possible? Anthony Leggett and Anupam Garg took a stab at it in 1985 with their Leggett-Gard inequality theory. The pair sought correlations between measurements in the hope of finding out which rules to follow: quantum or classical. Per Bell’s inequality, two particles are separated in space, but for Leggett and Garg, it had to do with one object over a period of time. The “quantumness” of large objects could be tested in theory such that their inequality would reveal the reliability of the concept of realism for our everyday lives.
More Leggett-Garg experiments have been going on recently on simple quantum systems. For example, systems like superconducting fluids and photons to atomic nuclei and tiny crystals. We now know that the microscopic world is “non-real”. The experiments were telling. Nonetheless, it was necessary to make sure they were non-invasive, meaning measuring a particle without disturbing it.
It was possible to do, and researchers discovered that the system was in a superposition of states for every non-invasive measurement made. Urbasi Sinha of India’s Raman Research says that it is time to test something bigger. “It all boils down to seeing how far we can push this…we don’t really know.”
Speaking of big things, Markus Arndt along with his colleagues at the University of Vienna was able to observe the largest things yet known to behave in a quantum way. They were conducting a unique experiment in 2020 with a double-slit setup. They would pass objects through one slit (one at a time) to watch their wavelike behavior. The objects formed interference patterns that resulted in the conclusion that proteins obey quantum rules.
As good as it sounds, there were inherent problems in the experiment because the objects being large were also complex. As a result, their quantumness disappeared rapidly during their interaction with the environment surrounding them. It is called “decoherence”. It means that quantum states are fragile and break when gas molecules bombard them along with stray photons of light and delicate electric and magnetic fields.
If not treated properly, any quantum object can behave classically per Chiara Marletto, from the University of Oxford. This is why the double-slit experiment was troublesome. It took quite some time to build up the double-slit interference pattern. This means that decoherence can “run riot”.
The Leggett-Garg experiments have their own sources of decoherence, however, but it still plagues researchers on how to measure a system without disturbing it. It has to be done nonetheless to be sure whether an object is in a quantum superposition or not. Per Sinha, you have to do the measurement in a clever way. “You’re trying to measure something, but on the other hand, you want to ensure that the act of measurement doesn’t leave any invasive mark.”
Moving on, you cannot drag most quantum systems into the classical world since they move in discrete steps. In the classical world, movement is continuous. It is therefore difficult to examine quantum objects and classical ones within the same experiment. In this regard, Sougato Bose, a theorist at University College London, came up with a plan to use an experimental setup to transcend both the classical and quantum worlds.
His setup was an object trapped inside a well moving like a swinging pendulum. In fact, it was a mere harmonic oscillator. He wanted to see whether it obeyed classical or quantum rules. With his collaborators, Bose expected to take a leap into the macroscopic world. Theoretically, there is no limit to how big a simple harmonic oscillator can be. So he used a nanocrystal 100,000 times more massive than those objects tested by Arndt’s team.
Bose and his group expected the swinging nanocrystal to be in the middle of the oscillator – positioned exactly on the border between left and right. He proclaimed, “we don’t observe, and then we suddenly take a snapshot observation.” It has to do with the detector and what is detected. The detector in effect sees only half the oscillator. If it is the nanocrystal, the researchers know on which side it resides. Otherwise, it must be on the other side!
The team wanted to know if the crystal was behaving in the “classical way”. If so, it would be there half the time during the first measurement. After it completed its swing and returned to the center of the system, it would again be measured. The group expected to see it half the time. But wait! If the particle is quantum and not classical, not seeing it in one half of the oscillator would result in the collapse of its so-called wave function. This entails a mathematical description of the quantum state.
It is clear that we don’t see the nanocrystal at all; however, we do know its position. Due to quantum uncertainty, the particle has momentum which changes its oscillation. The measurements could be repeated at intervals so the researchers could build correlations. The goal was to determine whether the nanocrystal was behaving in a quantum way – or classically.
Some measurements had to be thrown out – those in which the nanocrystal is seen. This would ensure that the measurements were non-invasive. The experiment set the tone for future advancements in the trapping and cooling of nanocrystals to avoid decoherence. New precision lasers have been devised since Bose’s time. In fact, he has now joined hands with Hendrik Ulbricht, an experimentalist at the University of Southampton in the UK, to conduct a test on a nanocrystal composed of a billion atoms. “It’s a big jump,” reveals Ulbricht.
Lasers had not been yet sharp enough to determine on which side of the trap the nanocrystal was oscillating. The laser has to work with widths the size of a mere water molecule since the bigger particles are described by smaller waves. Now we have the requisite technology. The results from Bose and Ulbricht are forthcoming. Then we will see if their work violates Leggett-Garg inequalities. If so, it will break the realism of macroscopic objects. We want to know how far quantum physics can extend. Are there limits?
Meanwhile, the Leggett-Garg experiments test whether a system behaves classically or quantum mechanically. It has to be one or the other. But does it? There are loopholes that muddy the once still waters. Leggett-Garg inequalities could be violated even from the classical perspective. It doesn’t even matter that the measurements are non-invasive. In Halliwell’s opinion, any stubborn macrorealist could cynically say that the measurement disturbed the system, nonetheless. He is busy devising ways to avoid such problems.
What are the loopholes? One is called the collusion loophole. Particles outside an experiment seem to violate macrorealism, while it is not actually the case. Another is the detection loophole in which the observer cannot register every particle, which changes the results of the detection. Sinha and his ilk are trying to close these loophole in Leggett-Garg experiments. Recently, she performed a watertight test in a system composed of protons. She avows closing the loopholes for the moment, but it is not a sure thing.
Some loophole-free tests of Bell’s inequality date back to 2015, long after it was even conceived. New cracks keep appearing in on-going experimental designs. Ulbricht attests to this fact. “There will be a very healthy and long debate, I’m sure.” Even so, an experiment has yet to contradict quantum mechanics. Its “weirdness” applies to objects the size of the moon, even larger, if you can isolate the tested system from the decoherence of the environment. Per Marletto, “from a theoretical point of view, quantum theory doesn’t put any limitation on how large an object you can put in a quantum superposition.”
Ulbricht still hopes that his experiments might reveal a point beyond which no quantum system can go. Call it a brick wall. It is between quantum and classical physics, and it could save reality by offering quantum weirdness to describe our common-sense world. Marletto goes on to say that there could be a universal mechanism, turning all quantum systems into classical ones as soon as they hit this wall.
In 1987, Lajos Diósi at the Wigner Research Centre for Physics in Hungary and Roger Penrose at the University of Oxford agreed that what we know of classical reality appears through instabilities in the structure of space-time. It takes testing bigger and even bigger objects to see if quantum mechanics still applies. In Ulbricht’s view, we can rule out some models of the objective collapse theory. This is an extension of quantum mechanics that predicts the location of the brick wall.
Is there such a wall? And what if there isn’t? Decoherence could collapse things, or as Bose maintains, sometimes the environment may be very conspiratorial. Wall or not, science must find alternatives to quantum theory predictions. Marletto postulates a new theory entirely since people are “frustrated that it is really good at being confirmed experimentally.”
So let’s return to the moon and the analogy of the tree in the forests. Is it there after all? Using tests of Leggett-Garg inequalities, the answer is no. Per Halliwell, “if macroscopic realism is violated, you can’t assume the moon is there.” So is reality “real” in the end? Maybe not. Both the classical and quantum rules (seemingly unbreakable) are in question. Vedral remarks, “this would give you a glimpse into some kind of post-quantum world. It’s hard to imagine what this could be, but I think we’re going to find something even weirder.”
Reality is now under scrutiny not only in the world of physics, but philosophy and theology. Simulation Theory and Simulation Creationism have a lot to say about what is likely real and what is not in answer to Vedral. That we live in a simulation is not a new idea, but Nir Ziso, founder of The Global Architect Institute gives it an additional explanation. The world expert on the subject has devised a model. A divine deity has a purpose in creating the universe. It is to study creation and the processes of life. How is this done? Information consisting of human thoughts and actions, along with the results of the five senses is sent from a relay station to an observer. He watches the data as if before a “movie”. He has consciousness as he records his own emotional response to what has been sent.
There is no one accepted theory of reality although physics has had a lot to say. We now can look at alternative perspectives to resolve the big questions asked by the venerable science.