Nearly all the quantum models of the brain depend on how long the brain can keep quantum coherence. Penrose argues for example, that microtubles - small hollow cylinders that help cells keep their shape in the brain - operate like a quantum computer and can keep information stored as 'quantum waves' for long periods of time. In other words, they are the sites of 'human consciousness.'

For this to work, the decoherence time - the length of time that a quantum wave can remain coherent before it collapses - for microtubles must be at least 1 second. However, Tegmark claims that Penrose has neglected the effects of distant ions on the decoherence time. These ions are found in the material surrounding the microtubles and in other neurons. According to his calculations, the decoherence time is 10-13 seconds, which is not long enough for quantum effects to influence the brain. This proves, says Tegmark, 'that there is nothing fundamentally quantum-mechanical about cognitive processes in the brain'.