‘What else is possible if space and time can change’ By Sean Carroll

(Article on 100 anniversary of General Theory of Relativity)

Nicolaus Copernicus is famous for having suggested that the Earth moves around the sun, rather than the other way around. That’s a big deal, as it displaces the Earth from its presumed position at the center of the universe. But it’s easy for us to forget something equally amazing: the idea that the Earth can actually move at all. If anything seems like a solid foundation, it’s the Earth itself. But in our post-Copernican world, we know better.

Albert Einstein, with his general theory of relativity, took this conceptual revolution one step forward. Not only is the Earth not a fixed fulcrum around which the rest of the universe revolves, space and time themselves are not fixed and unchanging. In Einstein’s universe, space and time are absorbed into a single, four-dimensional “spacetime,” and spacetime is not solid. It twists and turns and bends in response to the motion of matter and energy. We perceive that stretching and distortion of the fabric of spacetime as the force of gravity.


“In Einstein’s universe, space and time are absorbed into a single, four-dimensional “spacetime,” and spacetime is not solid. It twists and turns and bends in response to the motion of matter and energy.”

The idea that space and time themselves are not immutable, but are dynamical quantities that can evolve through the history of the universe, is one of Einstein’s most dramatic legacies. It was so profound that Einstein himself had trouble accepting all the implications of the idea. When he investigated the universe as a whole in general relativity, he found that it should be expanding or contracting, not staying at a fixed size. That went contrary to his intuition, as well as to what astronomers of the time actually thought the universe was doing. When Edwin Hubble discovered the expansion of the universe in the 1920’s, Einstein realized that he had missed the opportunity to make one of the great predictions in the history of science.

Once space and time themselves are flexible rather than fixed, what else is left that we can say is truly constant? There is one obvious candidate, again from Einstein himself: the speed of light. According to relativity, the speed of light is an immutable feature of the universe, an unbreakable speed limit that places strict constraints on what matter and information can do.

Scientists have thought about “variable speed of light” theories, because theoretical physicists love nothing more than thinking about crazy new ideas. But letting the speed of light vary over time or space turns out, upon closer inspection, not to be very well defined. If you accept that space and time are unified into a single, four-dimensional spacetime, there needs to be a way to translate between “distances in space” and “intervals in time.” That’s inevitable, and in our world that role is filled by the speed of light. The fact that light actually travels at that speed is not the important point; what matters is that there is some way of converting length into time, and vice-versa.


‘If space and time can change, little else is sacred. Modern cosmologists like to contemplate an extreme version of this idea: a multiverse in which the very laws of physics themselves can change from place to place and time to time.’

But other so-called “constants” of nature are fair game. If Einstein’s lesson is that purportedly foundational aspects of reality like space and time are actually dynamical and evolving, it’s natural to wonder whether the numerical parameters that specify the laws of physics are similarly flexible. Could Newton’s constant, which sets the strength of gravity, or quantities like the mass of the electron, actually change with time?


posted by f.sheikh