In fact there are measurable physical correlates of conscious perception; see https://www.cs.helsinki.fi/u/ahyvarin/teaching/niseminar4/Dehaene_GlobalNeuronalWorkspace.pdf and in particular chapter 7:
Our own brain imaging work relied primarily on
two techniques: retrograde masking, where the stimulus is flashed for a perceptible
duration but is made invisible by the subsequent presentation, at the same location,
of another shape, called the “mask;” and the attentional blink (AB), where a brief
target, presented for a duration that would be perceivable in isolation, becomes
invisible once the participants are temporarily distracted by a concurrent task. In
both cases, functional magnetic resonance imaging (fMRI), magneto-encephalog-
raphy (MEG), electro-encephalography (EEG) and intracranial recordings can be
used to record the progression of activation in the cortical hierarchy under
conditions of conscious versus non-conscious perception.
Scientists have started to unravel the mechanisms behind conscious perception, see https://doi.org/10.1126/science.aah6066 and further discussed at L5tt cells are attentional (paper summaries).