Science Seen Time One author Colin Gillespie helps you understand the physics of your world.

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In Search of the Physics of Causation

Does the world work on cause and effect or is this a world of random chaos? This question needs exploration at the universe’s smallest scale, far smaller than that of the atom. We are coming to see the answer is: neither and both!

Long ago, physicists as well as philosophers wrestled with this question. Albert Einstein, whose physics was rooted in philosophy, was wedded to the idea of causation. But he won a Nobel Prize for kick-starting quantum theory with his concept of the photon as the quantum of light. He became unhappy that the theory seemed to say that for a photon there is no causation. Famously, he said, ‘God does not play dice with the universe.’ He insisted there must be a definite reality behind the randomness of quantum theory. Years after he died, multiple experiments showed him to be wrong.

This leaves us to wonder about the reality behind quantum theory. If there is one, it must be a non-definite reality. For philosophy and physics this is the new frontier.

Let’s think of this in terms of size. Readers will recognize there is a smallest size. Physics has known of it for more than a hundred years. It is called the Planck size after Max Planck. Quantum theory deals with vastly larger sizes. For example a proton, the subatomic particle that the Large Hadron Collider smashes, is about a hundred-billion-billion times larger than Planck size. Thus it is not surprising that quantum theory cannot tell us about physics at Planck scale just as a theory of galaxies is not likely to depict the details of atoms.

There are no particles at Planck scale. The far larger particles of matter and energy that make up all we see are made of pure relationships—relationships between quanta of space. Papers by topologist Sundance Bilson-Thompson compare these relationships to half-twists in a ribbon. At that scale these twists do not move, they jump (which resolves long-standing philosophic paradoxes about motion). They can jump from one space quantum to its next-door neighbor. Each jump ‘takes’ one Planck time, which is another very small-scale unit. For example, one ‘tick’ of the clock on the chip that runs the iPhone 6 takes 0.9 billionths of a second. The Planck time is sixteen million-trillion-quadrillion times less than this.

It seems likely that those space-quanta relationships jump randomly. We don’t yet have a theory of how they hang together or of how they move around. Even a slight tendency to jump in one direction could amount to high speed at our scale. One Planck size per Planck time (or 1 fleck/tock) is the speed of light, the speed, that is, of photons.

Bilson-Thompson says the photon is made of the simplest set of relationships. Maybe when we have a Planck-scale theory we will find this is the reason why those photons motor on. What causes a photon’s motion?

Let’s return to large scale and revisit a famous thought experiment by Einstein. He took a trip along with photons to see Einstein-Thought-Experimentthings from their point of view. This led him to special relativity and the length contraction of a relatively moving object. From a photon’s point of view the whole universe is a moving object. When you step outside and see a distant star special relativity allows you to compute from its photons’ point of view the distance that they travel to your eye. Einstein’s equations tell you that it is exactly zero. So is the time it takes in transit.

Thus the photon’s own story gives itself no extended existence. It is an instant energy transfer from one electron to another at the same location. That transfer of energy can occur only because that electron in an atom in your eye was there to receive it. That is, from the photon’s point of view, both you and the star cause it. Who are we to disagree?

Source: Sundance Bilson-Thompson, “A topological model of composite preons”, http://arxiv.org/pdf/hep-ph/0503213v2.pdf

Other reading:

Colin Gillespie (2013), Time One: Discover How the Universe Began, New York: RosettaBooks, p. 357, http://www.rosettabooks.com/book/time-one/; “Twist N Shout”, http://www.timeone.ca/chapters/twist-n-shout.pdf

John Norton (2005), “Chasing a Beam of Light: Einstein’s Most Famous Thought Experiment”, http://www.pitt.edu/~jdnorton/Goodies/Chasing_the_light/

Image credit:

Nick Wolff (2016), http://valuesandinfluence.com/wp-content/uploads/2014/01/Einstein-Thought-Experiment.png

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