The Edge of Space-Time summary

Light is the basis for life on earth, but it is so much more than that. Light is a central metaphor in human cultures for wisdom, divinity, goodness, and understanding. But when you look very, very closely, light is also quite strange. One of the most interesting facts about our universe is that it has a speed limit: nothing material can travel faster than light. There is no particular reason we can determine as to why a universe should have a speed limit, nonetheless it does. 

To break down why this is so weird, think about driving down the highway with a passenger. Since you and your passenger are moving at the same speed, you appear stationary to one another. Someone on the side of the road, however, would perceive you as traveling very fast as you fly past. A car traveling on the same highway, however, would perceive you as moving much more slowly than a pedestrian on the side of the road. 

The theory of relativity overturned centuries of assumptions: earlier laws of motion had assumed a fixed, universal frame of reference. Relativity showed that even space and time shift depending on the observer.

With space and time linked, a new model emerged. Space-time was woven like a fabric, dubbed Minkowski space after the German mathematician who first developed the model that explained the mathematics of special relativity. Minkowski space described the flat, four-dimensional spacetime of special relativity — but explaining gravity would require going further still.

The paradigm shifts we’ve encountered in previous chapters didn’t just revolutionize physics at the level of galaxies and solar systems, but began to shift the way we think about all matter right down to the subatomic level. The mechanistic models used for centuries to explain matter were being reconsidered in nuclear physics, too. 

As scientists probed deeper into the subatomic level, they were confronted with results that defied our classical laws of motion. Consider the Stern-Gerlach experiment, done way back in the 1920s, that fired silver atoms at a detector through a magnetic field. Traditional laws of motion would predict that atoms from the beam would disperse in random directions, but the experiment showed something quite different. Some atoms directed themselves upwards, the others downwards, forming a distinct pattern.

At the quantum level, particles sometimes behave like matter and sometimes like waves  depending on how you look at them. Physicists had long described energy using wave models, so extending wave-like behaviour to matter at the quantum level was a natural, if startling, next step. What it revealed was that matter and energy are not as distinct as they appear: at the quantum level, the boundary between them blurs.

To wrap your brain around this, think of quantum particles like beats in music. If you imagine a drummer playing Bebop, for instance, the four beats of each measure are experienced as fixed. At the same time, individual hits on the snare, toms, or cymbals can be a part of multiple rhythmic streams at once. Each beat influences the ones around it, too, so triplets create ripples on the surface of a quarter note beat, but they don’t dislodge it. Even when the beats are silent, that silence carries the potential for the pattern to re-emerge.