Inferences from relativity

The pictures on page 524 show that an idealized clock based on bouncing light between mirrors will exhibit relativistic time dilation. And from such derivations it is often assumed that the same result must hold for any possible clock system. But as a practical matter it does not. And indeed for example the clocks in GPS satellites are specifically set up so as to remove the effects of time dilation. And in the twin paradox one can certainly imagine that each twin could have an accelerometer whose readings they use to correct their clocks. Indeed, even when it comes to individual particles there are subtle effects associated with acceleration and radiation (see page 1062)—so that in the end it is not entirely clear that something like a biological system would actually in practice exhibit just standard time dilation.

One feature of relativity is that it implies that only relative motion is ultimately ever detectable. (This was also implied by Newtonian mechanics for purely mechanical systems.) And from this it is often concluded that there can be nothing like an ether that one can consider as defining an absolute state of rest in the universe. But in fact the cosmic microwave background in effect does exactly this. For in standard cosmological models it fills the universe, but is everywhere at rest relative to the global center of mass of the universe. And from the anisotropies we have observed in the microwave background it is thus possible to conclude that the Earth is moving at an absolute speed of about 10^-3 c relative to the center of mass of the universe. In particle physics standard models also in effect introduce things that are assumed to be at rest relative to the center of mass of the universe. One example is the Higgs condensate discussed in connection with particle masses (see page 1047). Other possible examples include zero-point fluctuations in quantum fields.

Outside of science, relativity theory is sometimes given as evidence for various general ideas of cultural relativism (compare page 1131)—which have existed since well before relativity theory in physics, and seem in the end to have no meaningful connection to it.