What is reality?

Reality is merely an illusion, albeit a very persistent one – Albert Einstein

For several years I have pondered what Einstein meant when he made this statement. First, what is reality? A dictionary might define reality as the state of something (e.g. am object like a shoe or house or an event like a tornado) as they actually exist, rather than how they might appear or are perceived. So the crux of the biscuit is how valid is our perception of reality? This question of metaphysics has been discussed for centuries by many many others. For my part, I do not plan to describe any novel ideas or concepts. Instead, I hope this will be a short introduction to my view of reality.

A simple plate of homemade lasagna

Consider a plate of lasagna (one of my favorite meals). Lasagna is a dish made with alternating layers of pasta, a red tomato sauce, and other ingredients such as cheese, meat, or vegetables. The essence that lasagna possesses exists in our mindsbecause of our perception of lasagna makes it real. Our brain constructs this "essence" by integrating a series of electrical signalsfrom receptors and nerves to create what we define as "a plate of lasagna". For example, photons have no intrinsic color, only a wavelength and frequency. Your brain creates colors from signals sent from receptors in your eyes. The aroma and savory taste of your meal doesn’t exist when viewed as a complex mixture of volatile and water-soluble molecules. Using signals sent from your nose and taste buds, your brain creates the flavor and smell.

But exactly what reality is becomes murkier – there is no clear link between “what I see” and “this is real”. No philosopher or researcher has proved that our perception of the world matches reality. For example, over time I have lost the ability to hear high sound frequencies. This doesn’t mean they don’t exist, I do not perceive them. What is transmitted to the brain is limited range of light, sounds, and smells. Furthermore, animals clearly differ from humans in their limitations for perceiving external stimuli detected by their receptors. For example, bats and dogs can hear different sound frequencies; bees and spiders can see in the ultraviolet. Who knows what microbes perceive, but they certainly can move towards food. Our reality exists because of the way our brain evolved to present it. It also raises a point discussed by many that if we cannot perceive how other animals perceive reality, then their world is not conceivable. Likewise, our world is inconceivable to many animals.

The brain to me must exist as an integrator of stimuli, but I do not know how the brain acquired its ability to create reality. That tomato sauce in the lasagna had no color until my visual cortex processed electrical signals from photons hitting my retina. Likewise, a taste bud on my tongue can’t taste the lasagna, yet I can taste it.

But I do not think in hia quote Einstein was thinking of how we perceive reality through our senses, but more as to what the nature of reality as defined by quantum mechanics.

Reality and Quantum Mechanics

The desk in front me of is solid. I can feel it and hitting it with my fist can give me a bruise. It exists. However, this desk is an illusion, and as shown by quantum mechanics is really a cloud of particles. According to the laws of quantum mechanics, these exist in an undefined state lacking properties like a definite location and instead existing everywhere and nowhere at once. Only when a particle is measured or observed does it suddenly materialize. The bothersome conclusion is that nature is inherently probabilistic; reality has no hard properties, only likelihoods, until it is observed.

The two slit experiment

This classic experiment in quantum mechanics demonstrates the probabilistic nature of reality. In this experiment, a beam of electrons (it can really be any particle) is shot towards a pair of slits. If no one keeps track of the electron’s path, they seem to pass through both slits simultaneously creating a interference pattern on the other side of the slits. However, once you place an electron detector in front of one slit, the wavelike nature of the electron collapses into a definite state and move towards one slit or the other.

The implications of this experiment are pretty profound. First, matter can act as both a wave and a particle depending on whether or not it is being observed. Second, merely observing an event can change its outcome!

Schrödinger’s Cat

Proposed by Erwin Schrödinger in 1935, this gedankenexperiment illustrates quantum indeterminacy. Here a cat is placed in a box along with a radioactive sample, a Geiger counter and a bottle of poison. When the radioactive sample has decayed (randomly), it will be detected by the Geiger counter and trigger the release of the poison. The cat then dies. The “Copenhagen interpretation” (named after the city where Niels Bohr lived) states that a particle exists in all states at once until observed. The system within the box is represented by a “probability wave” weighting these various possibilities, and the wave collapses to a definite state once the box is opened. The equations of quantum mechanics do not address how a particle’s properties solidify at the moment of measurement, or how, at such moments, reality picks which form to take. The sample of radioactivity would have simultaneously decayed and not decayed in the sealed environment and the cat is both alive and dead until the box is opened. The act of opening the box and observing determines the state of the cat: dead or alive (a third case, the cat being really furious, will be ignored here). Another interpretation (the “many worlds interpretation”) argues that when the box is opened, the observer and dead-and-alive cat split into two realities, in one of which the observer sees a dead cat and the other an alive one.

Bell's inequality theorem

Einstein was never happy with quantum mechanics and the lack of a physical reality before observation (e.g. "I like to think the moon is there even if I am not looking at it." – Albert Einstein). In 1935, Einstein, Boris Podolsky and Nathan Rosen (EPR) proposed a gedankenexperiment (the EPR paradox) which essentially states that if an object can be known without it being observed, then that property could not have been created by observation. If it wasn't created by observation, it must have existed as a physical reality before its observation. Imagine two entangled photons sent to two different detectors Entangled photons have an inherent property that they must have orthogonal polarizations, so when the polarization of one photon is measured the other will be orthogonal ( e.g. vertical at one detector, horizontal at the other) even though the other photon has not been measured. The standard Copenhagen interpretation would say that the unmeasured photon would have no defined value until its measurement of the first photon has collapsed the wavefunction of both photons (nonlocality of the measuring process). The paradox arises because the communication of polarization values between the photons was forbidden by Einstein’s theory of Special Relativity and the way out of the paradox was to assume that all particles inherently had fixed properties (i.e. hidden variables). Hence, particles would possess more information than quantum mechanics says they should have and is thus incorrect or incomplete.

In 1964, Bell started with the same two assumptions used in the EPR paradox (i.e. reality (that microscopic objects have real properties determining the outcomes of quantum mechanical measurements), and locality (that reality in one location is not influenced by measurements performed simultaneously at a distant location). From these two assumptions he derived Bell's inequality, which implied that at least one of these assumptions must be false.

Experimentally, pairs of entangled polarized photons are shot towards two polarizers. Quantum mechanics says the photons instantaneously "decide" together which polarization to assume at the moment of measurement, even though they are separated in space. Hidden variable theory says that such instantaneous decisions are not need, because the same strong correlation could be achieved if the photons were somehow informed of the orientation of the polarizers beforehand. Experimental tests using entangled photons have repeatedly found agreement with quantum mechanics and rule out a large class of hidden variable theories known as (surprisingly) local hidden variable theories. In these theories, the hidden variables don’t travel faster than light. These experiments do not rule out theories where hidden variables travel faster than light.

So where are we?

“Reality” is just a word in our language. What we observe as reality is an illusion of our mind created by quantum processes, rather than a precise description of what we really observe. If we create reality what is it being made of? Quantum mechanics tells us that our mind and intuition are not capable of understanding the true nature of reality. Within this illusion of physical reality it is not possible using internal methods and concepts to fully understand it. A full understanding of reality would require the ability to observe it outside the boundaries of our illusions which frankly is not possible (I doubt it will ever be possible). Where observation ends and reality begins is up to each of us.

Reality is that which, when you stop believing in it, doesn't go away - Philip K. Dick
Reality leaves a lot to the imagination - John Lennon
Life is not a problem to be solved, but a reality to be experienced - Søren Kierkegaard