David Peat

Quantum Uncertainty 13change in pitch of the siren as an ambulance or police car approachesand then speeds off into the distance.) Because the radar radiation hasbounced off the rocket this means that an exchange of energy has takenplace. Of course in this case the amount of energy is entirely negligiblewhen compared with the energy of the traveling rocket.No matter what example you think of, whenever a measurement ismade some exchange of energy takes place—the rise or fall of mercuryin a thermometer, a Geiger counter’s clicks, the swing of a meter, electricalsignals from a probe that write onto a computer’s memory, themovement of a pen on a chart. In our large-scale world we don’t botherabout the size of the energy exchange. The amount of heat that isneeded to push mercury up a thermometer is too small to be concernedwith when compared to the energy of a pan of boiling water.Moreover it is always possible for measurements to be refined and anyperturbing effects calculated and compensated for.Things are quite different in the quantum world. To make a quantumobservation or to register a measurement in any way, at least onequantum of energy must be exchanged between apparatus and quantumobject. But because a quantum is indivisible, it cannot be split ordivided. At the moment of observation we cannot know if that quantumcame from the measuring apparatus or from the quantum object.During the measurement, object and apparatus are irreducibly linked.As a measurement is being made and registered the quantum objectand measuring apparatus form an indissoluble whole. The observerand the observed are one. The only way they could be separatedis if we could cut a quantum into two parts—one part remaining withthe measuring apparatus and the other with the quantum object. Butthis cannot be done. So the measuring apparatus and quantum systemare wedded together by at least one quantum. What’s more, the energyof this quantum is not negligible when compared with the energy ofthe quantum system.This means that every time scientists try to observe the quantumworld they disturb it. And because at least one quantum of energy mustalways be involved, there is no way in which the size of this disturbancecan be reduced. Our acts of observing the universe, our attempts togather knowledge, are no longer strictly objective because in seeking to