By Michael Clive Price
Many-Worlds predicts that the Everett-worlds do not interact
with each other because of the presumed linearity of the
However worlds do interfere with each other, and this enables
the theory to be tested. (Interfere and interact mean different
in quantum mechanics. Pictorially: Interactions occur at
the vertices within Feynman diagrams. Interference occurs when
you add together
different Feynman diagrams with the same external lines.)
According to many-worlds model worlds split with the operation of every thermodynamically irreversible process. The operation of our minds are irreversible, carried along for the ride, so to speak, and divide with the division of worlds. Normally this splitting is undetectable to us. To detect the splitting we need to set an up experiment where a mind is split but the world isn't. We need a reversible mind.
The general consensus in the literature ,  is that the experiment to detect other worlds, with reversible minds, will be doable by, perhaps, about mid-21st century. That date is predicted from two trendlines, both of which are widely accepted in their own respective fields. To detect the other worlds you need a reversible machine intelligence. This requires two things: reversible nanotechnology and AI.
1) Reversible nanoelectronics. This is an straight-line extrapolation based upon the log(energy) / logic operation figures, which are projected to drop below kT in about 2020. This trend has held good for 50 years. An operation that thermally dissipates much less than kT of energy is reversible. (This implies that frictive or dissipative forces are insignificant by comparison with other processes.) If more than kT of energy is released then, ultimately, new degrees of freedom are activated in the environment and the change becomes irreversible.
2) AI. Complexity of human brain = approx 10^17 bits/sec, based on the number of neurons (approx 10^10) per human brain, average number of synapses per neuron (approx 10^4) and the average firing rate (approx 10^3 Hz). Straight line projection of log(cost) / logic operation says that human level, self-aware machine intelligences will be commercially available by about 2030-2040. Uncertainty due to present human-level complexity, but the trend has held good for 40 years.
Assuming that we have a reversible machine intelligence to hand then the experiment consists of the machine making three reversible measurements of the spin of an electron (or polarisation of a photon). (1) First it measures the spin along the z-axis. It records either spin "up" or spin "down" and notes this in its memory. This measurement acts just to prepare the electron in a definite state. (2) Second it measures the spin along the x-axis and records either spin "left" or spin "right" and notes this in its memory. The machine now reverses the entire x-axis measurement - which must be possible, since physics is effectively reversible, if we can describe the measuring process physically - including reversibly erasing its memory of the second measurement. (3) Third the machine takes a spin measurement along the z-axis. Again the machine makes a note of the result.
According to the Copenhagen interpretation the original (1) and final (3) z-axis spin measurements have only a 50% chance of agreeing because the intervention of the x-axis measurement by the conscious observer (the machine) caused the collapse of the electron's wavefunction. According to many-worlds the first and third measurements willalways agree, because there was no intermediate wavefunction collapse. The machine was split into two states or different worlds, by the second measurement; one where it observed the electron with spin "left"; one where it observed the electron with spin "right". Hence when the machine reversed the second measurement these two worlds merged back together, restoring the original state of the electron 100% of the time.
Only by accepting the existence of the other Everett-worlds is this 100% restoration explicable.