The concept of time can both be obvious and confusing. Consider, for example, one aspect of time that is particularly vexing: Our perception that it moves only in one direction. Quantum physics, however, suggests that the flow of time could be in both directions — forward and backward — with equal plausibility. Einstein's famous quote, "The only purpose of time is to prevent everything from happening at once," sheds some light on the problem. Is time just a framework that prevents all events from happening simultaneously? Or is time a thing, like a particle, in which the past, present, and future are all equally real? Often, the answers given by science can seem contradictory. One thing is clear, however: Contemplating the nature of time and related logical consequences can lead to headaches. So here are two ways to unpack the unsettling truths about time.

 

Source: Clocks image by Michal Jarmoluk from Pixabay. Blocks image by Matthias Böckel from Pixabay

Zeno’s Arrow

Time passes quickly for most of us, leaving us feeling as though our existence is fleeting and rapidly shifting. Right now, as you read this article, the present exists but it becomes the past in an instant. In the 5th century BC, the Greek philosopher Zeno devised a way to think about the nature of time. He explored this paradox of an ever-moving present by describing two identical arrows, one launched from a powerful bow while the other was released horizontally from above. When they meet, they freeze this moment in time, locking into place are two identical, arrows lying one atop another. That is what we think of as “now." But if this is a good simple description, what mechanism moves us from one moment to the next in time? For example, how does an arrow know it should move toward its target, and how fast is it going?

Time travels at the speed of light

To try to understand that, you need to consider whether time travels (like the arrow moving once it has been shot) or whether you travel through time (like the stationary arrow). And if you do travel through time, at what rate are you moving? The concept of how fast, or “speed," is about how far we move in a given period of time. For instance, when you drive your car at 20 miles per hour, you are moving, which results in your speed over time. But is that how fast time moves? To answer this question took a realization from Einstein that space and time cannot be separated. They form the fabric of what he called space-time. Before then, Newtonian science saw the world as having three dimensions. This meant that people would always agree on the timing of an event, such as the precise moment when something happened. But Einstein’s Theory of General Relativity proved this was wrong. Under the general relativity theory, you might see event 1 happen before event 2, while I might see the exact opposite, depending on our velocities and positions. This is because the faster something moves through space, the slower it moves through time, and vice versa. And if you are traveling at the speed of light, time would stop altogether. So while objects move through space and time at different rates, they all move through space-time at exactly the same rate: the speed of light. This means we are all moving at the speed of light through space-time. When you are not moving at all in space, you are moving at the speed of light. And when you are moving at the speed of light through space, you stop moving through time, which stops completely. This means that your speed through space combined with your speed through time always adds up to the speed of light.

The universe may be like children’s blocks

What this theory says about the nature of time is that that just as all space is laid out all at once, all time is also. This is because, if space is a thing that can be known and identified then it must follow the same logic as time. This is known as the Block Universe Theory, which contends that everything has already happened and will happen, so no "now" can be more important than another. Imagine that you are in your current location and your friend is located in Sydney. Both places seem equally real to you. Both locations exist, and neither can claim to be more real than the other. According to this theory, time is no different. This moment Now has no more validity than any other instant in time, whether it be one you experienced years ago or one you will experience years from now. The entirety of time has already been laid out, even though the concept may be beyond our comprehension. This also suggests that the distinction between the past, the present, and the future is only an illusion. From the beginning of the universe to its eventual death, every moment exists with the same importance as every other—like looking down from above at building blocks for children. Every block of moment in time is equally real.

The majority of physicists believe in the block-universe view because it is predicted by general relativity. However, this also results in a troubling conclusion: If the Block Universe Theory holds true, then everything is predetermined. The entire timeline of existence has already been laid out: the past, the present Now, and the future, which suggests the universe “knows” or somehow controls everything you do. Unsettling, isn’t it? If you feel that way, you’re not alone. The essence of time remains among the biggest problems in physics today. On the other hand, you can tell yourself, if you’re anxious about an event not happening, don’t worry about it. Either events happen or they do not. The outcome is either positive or negative. Since they may have already occurred, worrying is futile, as you can't change them.