This is a fascinating look at some of the core issues in science using apparent paradoxes to illustrate them, without using any diagrams or formulae
The subtitle is rather misleading, as few physicists would argue that the physical and occasional mathematical paradoxes discussed here are the current greatest enigmas in science, although some of these are listed in the last chapter. What is important about the enigmas and paradoxes Al-Kahlili chooses is that their resolution helped push forward thinking. He starts with the paradoxes of probability, looks at the classical arguments of Zeno such as Achilles and the Tortoise, before going on to discuss the apparent paradoxes involved in relativity, such as the twins paradox, which was still a subject of heated debate as recently as the 1960s. He then looks at the infamous Schrodinger’s cat, which he settles by invoking decoherence, (if you define 'observation' to mean any interaction with the outside environment the paradox goes away). He also discusses time travel, and comes down on the side of the multiverse/many worlds interpretation to avoid things like the grandfather paradox. In the discussion on time travel he makes the important point on pages 139-40, that the idea of a passive observer, one who can acquire information from the environment but not interact with it, makes no sense, as he says “if you can see it, you can touch it”. This puts paid to literalistic interpretations of out-of-the-body experiences and the like.
The final 'paradox' he discusses is the infamous Fermi Paradox of why don’t we see obvious evidence of alien activity (no alien artefacts, no signals from space, no signs of stellar engineering etc.), and does this mean that we are alone in the galaxy? Here Al-Khalili rather goes astray into arguments as to whether life on earth is the exception. The trouble with this so called paradox is that it tells us much less than is often assumed; it says nothing about the number of biospheres, it doesn’t tell us anything about the presence or otherwise of 'intelligent' (whatever that might be mean in a alien context) life, it merely suggests the absence of certain technologies which have been imagined either in reality or fantasy by late 20th century members of the North Atlantic culture, several of which clearly inspired by the manic technocratic fantasies of the likes of H. G. Wells and Joseph Stalin.
Actually there may even be a real paradox, the absence of evidence of entities with humanlike dreams and skills, might mean that life is widespread in the galaxy, because if life or lifelike systems and structures or more generally, "zones of high complexity” are widespread, it would imply that 'life' is very unconstrained in its nature and form, and therefore the chances of any one biosphere being closely reprised by another are slim. Human life would be just one of myriads of unique lifeways. -- Peter Rogerson.