What is the ultimate nature of reality? This the core questions of physics, as well as of classical, rationalist philosophy. We now know that this question relates to interpretations of quantum mechanics.

“Those are the kind of questions in play when a physicist tackles the dry-sounding issue of, “what is the correct interpretation of quantum mechanics?” About 80 years after the original flowering of quantum theory, physicists still don’t agree on an answer. Although quantum mechanics is primarily the physics of the very small – of atoms, electrons, photons and other such particles – the world is made up of those particles. If their individual reality is radically different from what we imagine then surely so too is the reality of the pebbles, people and planets that they make up.”

The Many Interpretations of Quantum Mechanics, Graham P. Collins, Scientific American, November 19, 2007

To what can we compare our knowledge of the universe?

The allegory of Plato’s cave

The Allegory of the Cave was presented by the Greek philosopher Plato the Republic (380 BCE) He retells an analogy created by Socrates, about people who think that they know the true nature of reality – however, as the analogy progresses, we find that they have no idea what the real world is like at all.

The idea is that most people don’t actually understand our own real world – and that we never will without philosophical and scientific inquiry.

Socrates says to imagine a cave where people have been imprisoned from childhood. They are chained so that their legs and necks are fixed, forcing them to gaze at the wall in front of them, and not look around at the cave, each other, or themselves

Behind the prisoners is a fire, and between the fire and the prisoners is a raised walkway with a low wall, behind which people walk carrying objects or puppets “of men and other living things”

The masters walk behind the wall – so their bodies do not cast shadows for the prisoners to see. But the objects they carry cast shadows. The prisoners can’t see anything behind them : they only able see the shadows cast on the cave wall in front of them. The sounds of people talking echo off the wall, so the prisoners falsely believe these sounds come from the shadows.

The shadows constitute reality for the prisoners – because they have never seen anything else. They do not realize that what they see are shadows of objects in front of a fire, much less that these objects are inspired by real living things outside the cave

The philosopher (or scientist) is like a prisoner who is freed from the cave and comes to understand that the shadows on the wall do not make up reality at all, for he can perceive the true form of reality – rather than the mere shadows seen by the prisoners.

Plato then supposes that one prisoner is freed: he turns to see the fire. The light would hurt his eyes and make it hard for him to see the objects that are casting the shadows. If he is told that what he saw before was not real but instead that the objects he is now struggling to see are, he would not believe it. In his pain the freed prisoner would turn away and run back to what he is accustomed to, the shadows of the carried objects. 

Plato continues: “suppose…that someone should drag him…by force, up the rough ascent, the steep way up, and never stop until he could drag him out into the light of the sun.”  The prisoner would be angry and in pain, and this would only worsen when the light of the sun overwhelms his eyes and blinds him.” The sunlight represents the new knowledge that the freed prisoner is experiencing.

Slowly, his eyes adjust to the light of the sun. First he can only see shadows. Gradually he can see reflections of people and things in water, and then later see the people and things themselves. Eventually he is able to look at the stars and moon at night until finally he can look upon the sun itself (516a). Only after he can look straight at the sun “is he able to reason about it” and what it is.

• adapted from “Allegory of the Cave.” Wikipedia, The Free Encyclopedia. 29 May. 2016. Web. 3 Jun. 2016

http://bacchanalinthelibrary.blogspot.com/

Another illustration of Plato’s cave.

http://nickkahler.tumblr.com/post/10910319953

Are the laws of physics really absolute?

One of the major goals of physics is to emerge from the relative ignorance of the cave, and venture out into an understanding of the real world – how our universe really works.

We have made remarkable progress in doing so – everything we have learned in classical physics over the last two millennia is part of the human adventure.

What we have learned is, in an important sense, “real.” Physics lets us ask specific questions and then use math to make specific answers. We then compare our predictions to the way that universe really works.

Yet we need to be careful – we could make the mistake of using physics equations as if they are absolutely true. Yes, they certainly are true in the sense that they work. But are these math equations the absolute truth themselves – or are they really emerging from a deeper phenomenon? See The laws of physics are emergent phenomenon.

Is nature a simulation?

The simulation hypothesis proposes that our reality is actually some kind of super detailed computer simulation. This hypothesis relies on the development of a simulated reality, a proposed technology that would seem realistic enough to convince its inhabitants. The hypothesis has been a central plot device of many science fiction stories and films.

Simulation hypothesis (Wikipedia)

Video Why Elon Musk says we’re living in a simulation: YouTube, Vox

Elon Musk thinks we’re characters in a computer simulation. He might be right. 

Is the Universe a Simulation? Scientists Debate

Nick Bostrom: Are you living in a computer simulation?

Is the universe a hologram?

The holographic principle is a principle of string theories and a supposed property of quantum gravity that states that the description of a volume of space can be thought of as encoded on a lower-dimensional boundary to the region—preferably a light-like boundary like a gravitational horizon.

First proposed by Gerard ‘t Hooft, it was given a precise string-theory interpretation by Leonard Susskind who combined his ideas with previous ones of ‘t Hooft and Charles Thorn.

As pointed out by Raphael Bousso, Thorn observed in 1978 that string theory admits a lower-dimensional description in which gravity emerges from it in what would now be called a holographic way. In a larger sense, the theory suggests that the entire universe can be seen as two-dimensional information on the cosmological horizon. – Wikipedia

Our Universe May Be a Giant Hologram

Study reveals substantial evidence of holographic universe

Space’The Holy Grail for Physicists’: First Evidence Universe is a Hologram Uncovered

To learn more about quantum mechanics

The Cosmic Code: Quantum Physics as the Language of Nature, Heinz R. Pagels

One of the best books on quantum mechanics for general readers. Heinz Pagels, an eminent physicist and science writer, discusses the core concepts without resorting to complicated mathematics. He covers the development of quantum physics. And although this is an intellectually challenging topics, he is one of the few popular physics writers to discuss the development and meaning of Bell’s theorem.

Quantum Reality: Beyond the New Physics, Nick Herbert

Herbert brings us from the “we’ve almost solved all of physics!” era of the early 1900s through the unexpected experiments which forced us to develop a new and bizarre model of the universe, quantum mechanics. He starts with unexpected results, such as the “ultraviolet catastrophe,” and then brings us on a tour of the various ways that modern physicists developed quantum mechanics.

And note that there isn’t just one QM theory – there are several! Werner Heisenberg initially developed QM using a type of math called matrix mechanics, while Erwin Schrödinger created an entirely different way of explaining things using wave mechanics. Yet despite their totally different math languages – we soon discovered that both ways of looking at the world were logically equivalent, and made the same predictions. Herbert discussed the ways that Paul Dirac and Richard Feynman saw QM, and he describes eight very different interpretations of quantum mechanics, all of which nonetheless are consistent with observation…

In Search of Schrödinger’s Cat: Quantum Physics and Reality, John Gribbon

“John Gribbin takes us step by step into an ever more bizarre and fascinating place, requiring only that we approach it with an open mind. He introduces the scientists who developed quantum theory. He investigates the atom, radiation, time travel, the birth of the universe, superconductors and life itself. And in a world full of its own delights, mysteries and surprises, he searches for Schrodinger’s Cat – a search for quantum reality – as he brings every reader to a clear understanding of the most important area of scientific study today – quantum physics.”

External links

The Many Interpretations of Quantum Mechanics, Scientific American

Tom’s Top 10 interpretations of quantum mechanics

Learning Standards

SAT Subject Test: Physics

Quantum phenomena, such as photons and photoelectric effect – Atomic, such as the Rutherford and Bohr models, atomic energy levels, and atomic spectra. Nuclear and particle physics, such as radioactivity, nuclear reactions, and fundamental particles.

AP Physics Curriculum Framework
Essential Knowledge 1.D.1: Objects classically thought of as particles can exhibit properties of waves.
a. This wavelike behavior of particles has been observed, e.g., in a double-slit experiment using elementary particles.
b. The classical models of objects do not describe their wave nature. These models break down when observing objects in small dimensions.

Learning Objective 1.D.1.1:
The student is able to explain why classical mechanics cannot describe all properties of objects by articulating the reasons that classical mechanics must be refined and an alternative explanation developed when classical particles display wave properties.

Essential Knowledge 1.D.2: Certain phenomena classically thought of as waves can exhibit properties of particles.
a. The classical models of waves do not describe the nature of a photon.
b. Momentum and energy of a photon can be related to its frequency and wavelength.

Content Connection: This essential knowledge does not produce a specific learning objective but serves as a foundation for other learning objectives in the course.

A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas (2012)

Electromagnetic radiation can be modeled as a wave of changing electric and magnetic fields or as particles called photons. The wave model is useful for explaining many features of electromagnetic radiation, and the particle model explains other features. Quantum theory relates the two models…. Knowledge of quantum physics enabled the development of semiconductors, computer chips, and lasers, all of which are now essential components of modern imaging, communications, and information technologies