Why is it that as we look into the future, it feels like there's no option but for it to continue on infinitely, while looking into the past, it only makes sense that it had a beginning? Wouldn’t it make sense that it would either be infinite in both directions, or finite on either end?
A similar thing happens when we think about the creation of the universe - it's hard to imagine the universe having existed infinitely into the past, yet we can't imagine it disappearing in the future. If it does, what will be left behind?
Let's start by looking at the definition of time.
According to Wikipedia:
Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future.
This is more or less how most of us would have described time, though maybe not as articulately. That still leaves a lot of questions: Where did time come from? When did it start, and when will it end?
The work of Carlo Rovello-an Italian theoretical physicist, whose focus is on quantum gravity -goes a long way in helping us wrap our heads around what time is, and its origins.
We think of time as a sequence of events occurring along a linear axis of time, common to the universe as a whole. We wonder what people across the world are doing at this moment, and what's happening on Mars right now, or in far away galaxies. Yet, this notion of time, as something the universe has in common, is dispelled, not with ideas or theories, but with facts that simultaneously can make us feel like we have a better understanding of how the world works, yet make it look ever
Our notion of time works in our limited view of the world, and in the limited geography we inhabit, but not on a large scale. If we truly want to understand time, its meaning and its consequences, we need to look at the universe both at a macro and micro level.
There are four facts that break show that the way we commonly view time is not, in fact, how it actually works:
1 ) You may have heard that astronauts that have been to space come back slightly younger than someone born at the same moment as them but never left earth. This is because time progresses at different speeds depending on its position in the universe. The higher the gravitational force, the slower time moves.
We now have clocks good enough to notice the difference in time as close as 32 inches apart. In other words, given two watches of this quality, if I held one over my head and put one in my pocket, there would be a registered difference in time in a relatively short period of time.
Obviously this effect is small on earth, and serves our purposes just fine, but at a larger scale completely breaks down.
If one were to be able to travel near a black hole for a couple years, then come back to earth, they'd discover that hundreds of years on earth had passed in that time. Time travel, anyone?
2 ) Time between two clocks is affected by how they move. Similar to time slowing as gravity increases, time also slows as velocity increases. The closer you get to the speed of light, the more time slows to a crawl.
So while time is in one sense speeding up for that astronaut traveling away from earth, once in orbit, they are zooming around at high enough speeds to slow time back down. In the end, velocity wins in this case, and the astronaut will come back to earth some milliseconds younger than his counterpart that never left earth.
The consequence of this, is that it's impossible to say the time between any two given points.
Armed with the understanding of time being affected by both location and velocity, we see that time is a local notion. Time can be local to a particular point in space, as well as to a given object with respect to its location and movement. Therefore, a single time variable no longer makes sense. Time is on a different trajectory for every object and every location in the universe.
If you did decide to travel back to that black hole, there would be no way for you to set a common meeting time with somebody still on earth.
3 ) Laws of physics have a hard time distinguishing the past from the future. In most laws of physics, anything that can happen in one direction can happen in the other. But in our world there is a difference between the past vs future. So where is the difference? How can we differentiate, with physics, the past from the future?
Turns out the one principle that helps us differentiate between the past and the future is heat and temperature, as defined by the second law of thermodynamics. Entropy, or, the gradual transition from order to disorder, only grows. Entropy does not decrease.
Let's look at an example. If you look at a video of a pendulum played forwards and backwards, you won't be able to tell the difference. It isn't until the object gradually slows down that it will be visibly obvious which video represents the actual sequence of events, versus the reverse. If you tie an object to string, and let it swing, eventually it will slow, due to friction, or heat. And this is the defining ingredient for differentiating between past and future.
At a micro level, it's the ordered motion of molecules moving, in turn causing the surrounding molecules to become less orderly. This is the only thing that distinguishes the past from the future - that things were ordered and caused disorder.
But who ordered them? Well, we don't know.
4 ) As discussed above, the structure of time is governed by gravity. That is, gravity slows down time.
Given this, we can see that time is relative. Similarly, and more obviously, the concept of up and down is uncontroversial, yet there is no up and down in the universe. If I'm standing in Paris, we can all agree that the sky is up, and our feet are down. But it's the opposite in Australia. It's not that the notions of up and down are wrong, it's just that it's relative. The universe does not have an up or a down.
How to think about the world without time
If we move further, and think about the world without time, we tend to think of things as "frozen" in time. But this is not the case, rather, it's the opposite. Events occur, but not in time. No object or location exists on a synced time scale. We can have multiple clocks that have different times.
"There is no preferred time all over the universe." -Rovelli
The consequence of this then, is that on a grand scale, we cannot identify events happening at the same time. This concept doesn't exist. And beyond this, everything we see, think, and comprehend is something from the past. This is easiest to understand what we think about looking at the moon, the distance for which light takes about 1.3 seconds to travel. By the time we see it, we're seeing it in the past.
We should not think of the world as a set of objects, but rather we should think of the world as a sequence of events happening with respect to one another.
"The difference between the past and the future has to do with the way we interact with the world. It's not just in the world, it's in the perspective between us and the world."
Most of what we think about the world is just in our perspective. Stars moving at night, sun rising. The universe isn't turning, we are.
The same happens in time.
"Time isn't the clock that goes around, it's exactly the memories of the past." -Rovelli
We can remember the past and intuit the future.
Music only exists because we can remember what came before any given note. Additionally we can anticipate what's coming. At any given moment we're only hearing one note or moment of silence, which we are not likely to find moves us emotionally on its own. It's our ability to recall what happened in the past and carry it forward as a memory that makes music have the effect on us that it does. This occurs in our brain, and has nothing to do with physics. Music is not something that
exists outside of our heads.
Time is not a neutral notion for us. Time is our identity. But also our good and bad emotions. We suffer because we lose things. The emotional component is largely what we're talking about when we talk about time. Time is what takes away everything from us. So let's not forget about it, or ignore it, but take the time to understand what it is outside of our perceptions to better understand and appreciate this universe.