What happened? It's a question that gets asked a zillion times every day and the answer is always descriptive: I spilled tea over my keyboard, there was an accident, I had a birthday party. Whether or not something counts as a "happening" depends on your point of view, which is why no newspaper would report on my tea spilling event: what matters to one person might not matter to another.

Defining what constitutes an "event" in a general sense, without referring to particular ones as examples, is no easy task. Wikipedia gives eight different definitions referring to gatherings of people and another twelve referring to science, technology and maths, and those probably don't even cover it.

A birthday party definitely counts as an event.

Why does this matter? For most of us the slippery nature of events doesn't make a difference: we always know what we mean. But for physicists it goes right down to knotty questions arising from the foundations of the science. One thing we can probably all agree on is that an event is anchored in space and time. Whether it's an accident or a birthday party, every event has a location and a time at which it happens.

An unchanging stage

And this is where the problem lies — what exactly are time and space, and what does it mean to be located at one particular point in either of them? Isaac Newton, champion of what is today called classical physics, dodged the question by simply proclaiming what they are. According to Newton, space and time are unchanging, immutable, and provide the stage on which physics unfolds. As he wrote in his famous Principia Mathematica,

Absolute space, in its own nature, without relation to anything external, remains always similar and immovable.

And

Absolute, true, and mathematical time, of itself, and from its own nature, flows equably without relation to anything external.

Relational space and time

But even at Newton's time, long before strange modern theories like Einstein's general relativity and quantum mechanics came along, not everyone was happy with Newton's ideas. One dissenter was the famous mathematician Gottfried Wilhelm von Leibniz, who was a fond of something called the principle of sufficient reason. The principle says that everything must have a reason or a cause. God built the Universe according to this principle, so Leibniz thought, in order to make it possible for us to understand it.

Isaac Newton

And it's a powerful principle too. As an example, take Newton's law of inertia, which says that a moving object will continue to move in the same direction at the same speed unless external forces are applied to it. Why should this be true? Well, if you believe in the principle of sufficient reason, you have an explanation. The objects continues its motion without change because it has no reason to do otherwise.

But now think of Newton's notion of space. The Universe is located and oriented in this backdrop in a certain way. There isn't a reason, however, why god should treat some points in space differently from others — if space were absolute, consisting of essentially indistinguishable points, then god would have had to make an arbitrary choice in placing the Universe, so the principle of sufficient reason would be violated. A similar argument goes for time, which is why Leibniz rejected Newton's notion of both space and time. Instead, Leibniz thought of them, not as things in themselves, but as systems of relations between things or events. He quoted the analogy of a family tree: it's not really anything in itself, but something that represents relations between people. Without the people, the tree is meaningless.

The arrow of time

Even if you do agree with Newton's notion, time still presents a puzzle. We notice its passage because we observe processes in the world around us — and ourselves — that can't be turned back. A person gets old and wrinkly, and the leaves of a tree turn yellow and drop. These things are irreversible. And while they are also complex, they ultimately arise from particles — atoms and molecules — interacting in a way that's described by the fundamental laws of nature.

Nobody really understands what time is

But here's the thing. The laws of physics are time reversible. Newton's laws of motion, for example, tell you how objects behave when a force is applied to them. They also tell you that if you can make an object move in a certain direction by applying a certain force, you can also make it move in the opposite direction by applying the opposite force. Everything that can be done can also be undone. There therefore isn't a reason why some of the seemingly irreversible processes we observe, like growing old, couldn't be turned back. Yet, we never see them turned back. For some reason nobody quite understands, irreversible processes emerge from reversible ones.

This problem of the arrow of time has puzzled philosophers and physicists for some time. There are explanations of how it might emerge (see here) but not everybody is happy with them (See The future is time to find out more). The embarrassing fact is that nobody really understands what time is (find out more about this question here).

Relative spacetime

This troubling aspect of time hasn't deterred physicists from formulating their theories about the world. One of them was Albert Einstein, who published his general theory of relativity in 1915. Einstein's theory doesn't explain why time has a direction but it led to a fundamental shift in our understanding of time and its sibling, space, that echoes the ideas of Leibniz.

Gravity is the manifestation of the curvature of space and time. Image courtesy NASA.

General relativity describes the force of gravity. Einstein's great insight was that gravity isn't a mysterious force that whafts across the ether, but a warping of spacetime. If an apple falls to the ground then that's because the massive body of the Earth warps space, and, metaphorically speaking, the apple "rolls" into the "dip" created by it (see here for more). Time too is affected by massive objects. It passes slower near a neutron star than it does near Earth. It even passes slower in your basement (which is nearer the Earth) than it does in your attic. It's a strange concept, but it has been tested in many experiments.

The idea that space and time don't provide a fixed background, but are involved in the action, finds an echo in modern attempts to find a much-sought-after theory of quantum gravity, which would describe the whole Universe in one unified framework. Some physicists believe that such a theory should be defined, not with a fixed spacetime background in mind, but independently of space and time. Finding such a unified theory involves combining general relativity with the other great pillar of twentieth century physics: quantum mechanics. And in the context of events that strange theory opens a whole new can of worms. Find out more in the next article.

About this article

This article is part of our Stuff happens: the physics of events project, run in collaboration with FQXi. Click here to see more articles and videos about the difficulty in defining events.

Marianne Freiberger is Editor of Plus.