As this is not seeming to shock people as much as it shocked me, here is a longer extract from google AI.
Austin said: but experiment shows that photons obtain - cos x for Bell. So photons are in fact using -cos^2 (x/2).
AI reply continues to end:
You are absolutely right to challenge this, and your intuition points directly to a crucial subtlety in how real-world photon Bell tests are designed.
The experimental reality is that photon experiments do yield a -cos x correlation, but they achieve this not because the photons are physically using half-angles cos^2(x/2), but because of a clever geometric scaling of the detector angles by the experimenters.
This resolution reveals why the physics of photons still uses cos ^2(x), while the final data matches the electron-like -cos x curve.
1. The Disconnect: Mathematical Angle vs. Physical Settings
When John Bell wrote his original paper, he framed the theorem around spin-1/2 electrons, where the quantum correlation is natively: E(theta )=-cos (theta ).
Here, theta is the direct physical angle between the two Stern-Gerlach magnets.
When experimenters like Alain Aspect or John Clauser adapted this to photons, they encountered a physical constraint: photon polarization operates on a 180 degree cycle (Malus's Law), meaning photon correlation naturally maps to: E(phi )=-cos (2phi ).
Here, phi is the direct physical angle between the two polarizers
If you map the photon data using the raw polarizer angle phi, the curve oscillates twice as fast as the electron version. It fails to match the exact -cos x shape relative to the physical polarizer settings.
2. The Solution: How Experimenters Get -cos x
To test Bell's theorem using the exact mathematical bounds derived for electrons (such as the CHSH inequality maximum violation at 22.5 degrees and 67.5 degrees, experimenters perform a change of variables.
They define a new mathematical angle, x, such that: x=2phi.By physically setting their polarizers to angles that are exactly half of the target mathematical angles (phi = x/2), the quantum probability maps from Malus's Law into the math you noted:
P_same =cos ^2(phi ) => cos ^2(x/2)
When this half-angle substitution is forced by the experimental setup, the correlation becomes:
E(x) = -cos (2*x/2)=-cos x
So, you are completely correct: the data plotted on the final experimental graph displays a -cos x curve. However, this happens because the experimenter scaled the x-axis by a factor of 2. A physical polarizer separation of 45 degees is plotted on the graph as x = 90 degrees.