Five Questions Physics Asks Philosophy ✛ ✎ ✦
At night, an astronomer points a telescope at the edge of the visible universe and sees not stars but ripples — the imprint of the first moments. He knows these ripples should obey statistics, like the heights of people in a crowd. But he has only one crowd, and nothing to compare it with.
This is cosmic variance. The theory predicts what the 'average' universe looks like, but we live in one specific realization. The ΛCDM model with its six parameters cannot be fully tested, just as you cannot know the average height of humanity by measuring one person.
Next to this problem sits the Multiverse. Inflation expands space so rapidly that some regions recede beyond the horizon, while others remain with us. If there are infinitely many such regions, they exist, but their existence cannot be verified. It is like a library containing every possible book, but a reader can open only one shelf.
Quantum mechanics adds its own question. A dust grain in a beam of light is a quantum object, but it instantly loses its quantum properties when it collides with air molecules. Preparing two identical states of a dust grain is impossible, so an experiment with an ensemble of identical systems cannot be performed. The operational definability of macrostates slips away.
Finally, string theory. It speaks of ten or eleven dimensions, but these dimensions are properties of the background space in which strings move. At Planck scales, spacetime itself ceases to be smooth, like paper crumpled into a ball. The criticism is simple: if the background is not physical, then the dimensions are not physical either.
All five questions converge on one point: what does it mean for a mathematical structure to 'exist'? The author offers a map of this uncharted territory. Philosophers get a list of places worth visiting, and physicists get a reminder that their theories run into questions that no new collider can solve.
The night astronomer looks through the telescope again. Now he sees not only ripples but also the contours of a map marked with five trails. Each leads from an equation to a question for which there is no formula. It is there, at the edge of the knowable, that physics hands the baton to philosophy.
The author uses the example of the trillionth digit of the decimal expansion of √4711: this digit has not been computed or written down by anyone, but anyone who computes it will get the same result — an argument for the objective existence of mathematical reality. The decoherence time for a dust grain of 10 micrometers at room temperature and atmospheric pressure is 10⁻³¹ seconds — many orders of magnitude shorter than the Planck time (10⁻⁴³ s).
The idea that we might be mathematical objects within some fundamental mathematical structure resonates with the film 'The Matrix', where reality turns out to be a simulation, and with stories by Jorge Luis Borges, such as 'The Library of Babel', where all possible texts exist in an abstract space.