Space consists of three dimensions. Time, we are told, is also a dimension. So how come it is so different?
Answer: it isn’t. “Space and time are not concepts that can be considered independently of one another,” says physicist . In Einstein’s special theory of relativity, they dissolve into one entity. Two objects that to one person seem separated only in space can to another seem divided by space and time. Similarly, two events that seem to be separated only in time might from another perspective occur in different places as well.
That does not chime with our everyday experience, but only because we are not up to speed. Discrepancies between two observers’ views only become obvious when their relative speed is close to the speed of light – the cosmic speed limit.
Advertisement
Einstein’s physics reveals a deep truth: space and time are just different threads of a single seamless fabric called space-time. Yet there is still an obvious difference between the two. We can, in principle, travel in any direction in the three dimensions of space, but we can plod in one direction only in time – forwards from the past to the future. How do we explain that anomaly?
Ultimately, says physicist of Clarkson University in New York, that too is down to the cosmic speed limit. Imagine, for example, pulling back your curtains at 7 am on a bright, sunny morning. The sun, though, already doesn’t exist. It exploded at 6.55 am; we just don’t know it yet, because light from it takes about 8 ½ minutes to travel to Earth.
In this picture, any event – the exploding sun, us standing at the window – is a point in space-time with two associated “light cones”. One represents light racing towards the event through space and time, and one represents light racing away (see diagram). To see the sun explode as it happens would mean stepping outside our light cone and moving faster than the speed of light – something that our universe does not allow.
“It is the cosmic speed limit that makes some parts of space-time inaccessible,” says Schulman. It breaks the symmetry between time and space – and means we see information flowing smoothly in what we call time from the past to the future.
Read more: Beyond space and time