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As I understood it Spinoza claims everything has Extension, it occupies space. But in physics there are things called point particles which are zero dimensional, they don't occupy space.
So for example photons do not have Extension therefore Spinoza is wrong, have I misunderstood something?
Thank you!
First, point particles occupy space. The point particle at (1,1,1) in a 3D coordinate system occupies the space precisely at (1,1,1). Points are 0-dimensional, with a physical volume, surface area, and perimeter of 0. They, and any countable collection of them, have a measure of 0 under a standard measure but can have other measures under other choices.
Secondly, whether or not point particles exist in modern theories of physics is not entirely clear. It is true that, according to the wave/particle duality of quantum mechanics, Heisenberg’s Uncertainty Principle, and the fact that particles have a calculable de Brogle wavelength, particles seem to act as if they are not entirely point-like.
On the other hand, electrons, for example, have no internal structure and behave like point particles in other ways. Traditional interpretations of the collapse of the wave function conclude electrons are point particles after a measurement or observation. The details get hazy, dependent upon interpretation, and even philosophical very quickly. (Eg. For how long after? Decoherence anyone? How does this not violate the Uncertainty Principle and is the Uncertainty Principle epistemic or ontological?)
Nonetheless, modern theories of physics are so radically different from what anyone could have predicted before about 1870 (to be safe) that every philosopher who philosophized about physics was proven incorrect in countless foundational ways.
“Dimensionless point particles do not exist in physics” is false.
After all, a particle, by its nature, is localized at a point, whereas the wave function (as it’s name suggests) is spread out in space (it’s a function of x, for any given time t). How can such an object represent the state of a particle? The answer is provided by Born’s statistical interpretation of the wave function, which says that [|psi| squared] gives the probability of finding a particle at point x at time t
– Introduction to Quantum Mechanics, Second Edition. David J. Griffiths.
The electron (as far as we know) is a structureless point
– Introduction to Quantum Mechanics, Third Edition. David J. Griffiths. (This was published in 2018.)
This is a canonical standard-bearer for quantum mechanics. Many other places in the text explicitly refer to a particle as being a point (in the collapse of the wave function).
First, dimensionless point particles do not exist in physics. They are simply a way of speaking about a particle whose extension is negligible for the effects that are bring modelled.
Secondly, physics is generally geometric in myriad ways and hence about extension in myriad ways. This tallies with what Spinoza says about the physical world, it is a mode of extension. In fact, extension is simply a mode of the absolute substance, thought is another and there are infinitely many other unobservable modes.
Thus, Spinoza's theology is compatible with modern physics and hasn't been disproved by it.
One way he was proven wrong is the discovery of truly random behavior in quantum mechanics (edit: apparently it's not settled, see comments below). For Spinoza events necessary follow from their causes and are entirely determined by them.
Note that it does not have much consequence on his views, as our mind have no control over the results of quantic measurements, and our behavior is still determined by the laws of physics. In Ethics Spinoza states that dualists don't have the faintest idea of how a spaceless mind could affect our material body's behavior, and AFAIK it is still true to this day.
Photons don't occupy space but they are in space, they manifest their existence under the Extension attribute of Nature. When a photon goes from the Sun to our eye, both of those locations and the distance between them are part of the Extension. Of course, we can also have an idea of a photon and that's why they also exist under the Idea attribute (not sure of the correct English word, sorry), like everything else.
On the other hand, Antonio Samasio, who is not a physicist but a brain surgeon, wrote a very interesting book, Looking for Spinoza: Joy, Sorrow, and the Feeling Brain, where he explains how many intuitions of Spinoza about the human mind have been confirmed by modern neurology.
