What is the probability of your brain appearing briefly in space – just long enough to have a thought or two? We may not know the exact answer but the important fact is that it isn’t zero. Absurd as this may sound all that is required is for the universe to be infinite in extent or duration: in these conditions popping in and out of existence is what Boltzmann brains do best.
Boltzmann brains are named after the late nineteenth-century physicist Ludwig Boltzmann who combined statistics with Newtonian mechanics and introduced the concept of fluctuation. At first scientists rejected this notion as it allowed non-zero probabilities to be assigned to events that were considered impossible, such as air in a room momentarily squeezing into a tiny corner near the ceiling, a broken glass jumping back into shape without the slightest crack on its surface, or – to borrow an idea from Fred Hoyle – a hurricane in a scrapyard whipping together a perfect Boeing 747. Reluctantly, physicists came to accept fluctuation as a necessary feature of the universe, in fact quantum physics would never have been born without it.
A century later and the theoretical effects of fluctuation are still troubling scientists, this time in the form of the Boltzmann brain. The concern is due to the requirement that a correct theory of the universe must concur with the experiences of typical observers. If the universe is infinite in space or time then either the timespan of human existence will become vanishingly small in the chronology of an ageing void, or all human knowledge about it will be restricted to a relatively infinitesimal region. Either way, given enough time or space the population of Boltzmann brains will exceed the number of humans who have ever lived and will therefore lay greater claim to the status of ‘typical observer’.
This unfortunate situation would mean that Boltzmann brains would have the definitive take on what the universe is really like, and we can only imagine how different from ours their experiences would be.
To remedy this problem scientists have created models that either restrict the proliferation of brains or – more cunningly – deny them their status as typical observers. An intriguing solution is offered by Matthew Davenport and Ken D. Olum at the Institute of Cosmology in Tufts University, Massachusetts. In their article Are there Boltzmann brains in the vacuum? they focus on the process of quantum fluctuation in the vacuum of an infinitely old and empty universe; a reasonable fate in light of accepted cosmological theory. Such fluctuations would produce observing brains in the same manner as virtual particles which, according to quantum theory, pop in and out of existence so briefly that evidence of their presence can never be detected directly.
According to Davenport and Olum this fundamental undetectability of virtual entities is what gives us the advantage as it ensures that a brain appearing in the vacuum will have no means to announce its presence to other passing brains, and this means it cannot communicate its perception to another consciousness, which is what valid model building requires. Fully formed theories are never arrived at immediately, they depend upon the progressive interaction of minds over time. Boltzmann brains would have no say in any matter, and therefore no final word. In addition to this obstacle, as Boltzmann brains are insubstantial, the ‘unreal’ nature of their thoughts would invalidate their perceptions.
Davenport and Olum acknowledge they do not wholly succeed in denying the space brains a seat at the theorizing table. They imagine the results of a measurement taken precisely where a brain appears by chance. This would ‘solidify’ it (a process called decoherence), its thoughts would become real, its observations concrete, and the question would then have to be asked: how can we deny that any Boltzmann brain has meaningful thoughts on the basis of its unreality when there is always a possibility it can materialize mid-thought and validate itself?
Recent ideas in string theory have succeeded in banishing Boltzmann brains from the cosmos but in the absence of a means to confirm them experimentally they cannot yet count as proof. If string theory fails to win out as the ‘no-brainer’ of all time we may be lost forever in the murky metaphysics of consciousness and its role in creating and perceiving reality. Of course if we never solve this conundrum a Boltzmann brain may one day have the answer. But then who would know?