I am a quantum physicist and I can confirm that’s not how probability amplitudes work . The amplitudes are just complex numbers. Remember that complex numbers describe oscillations. A number r * exp(i phi) is an oscillation with amplitude r and current phase phi. Quantum states are wave functions, hence the complex numbers describing the oscillations. Amplitude -1 is just the bottom of the oscillation. Then on top of that you have that probabilities are the squares of the amplitude (for which I’m not sure if have a concise explanation).