The nature of time

By Marcus Loane

17^th April 2012

Time is strange

Time is strange. It is so familiar we take it for granted and yet if asked what it is we get into difficulties and what we say starts to sound like a circular definition. We can talk about time in different ways and we can view it in different ways.

Time for change

Time seems to be needed for there to be change or motion. There can be change over distance in a static universe so time is only needed for change over time, which of course sounds like a circular definition. Change over distance means things can be different here than they are there. Change over time means things can be different now from when they were before or will be after. Words like now, before and after when talking about time are analogous to the words here and there when talking about space. If there were no change over time then everything would be static. If there were no change over space then everywhere would be the same – there would be no structure, no galaxies, planets or life.

Time flowing

Time seems to flow. We seem to inhabit a present moment, a now, and we feel like the now becomes the past, and the future has a series of nows which we will experience later. It is almost as if we are moving along a line of nows.

Physics

In physics our concepts of time have changed dramatically in the last 100 years or so. In Newton’s day time was thought to be universal, that is, it changed at the same rate everywhere and it was thought that accurate identical clocks would run at the same rate no matter where they were in the universe. This is also the everyday view of time held by most people have who have not studied physics.

Then Einstein came along and showed that time can run at different rates in different places. This was a prediction from his theory of relativity and it has been supported by experimental evidence. Relativity predicts that time will run more slowly for objects travelling at high speed and for objects near strong sources of gravity. These predictions have been shown to be correct by comparing atomic clocks on the earth’s surface with those on airplanes or satellites. At these speeds the differences in the rate of time are small, amounting to fractions of a second. However at speeds close to the speed of light the differences can be much greater so for example, astronauts sent out on a space mission in a spaceship which continues to accelerate could return home after 20 years on their clocks and find that 1000 years had passed on earth. The universe is a strange place. Another result of the relativity of time is that no two observers can agree on whether two events happened at the same time ie. there is relativity of simultaneity. If one observer is close to a gravity source such as a planet or a black hole or is moving fast relative to the other observer then observer one will insist that two events happened at the same time while observer two will insist that one event happened before the other. Both observers will be correct as long as they say that relative to them the events were simultaneous or not. The passing of time is relative to an observer or as physicists say relative to a frame of reference.

The block universe with Newton

Let us go back to the simpler Newtonian view of time for now so I can introduce the concept of a block universe. In Newton’s time, before relativity and quantum physics, the universe was visualised as containing objects which moved around according to rules so that everything worked like clockwork. It was thought that if the position and velocity of every particle in the universe was known at a particular moment (a 3d “snapshot” of the universe) then in principle we could calculate the positions and velocities of particles the next moment, and also the previous moment. We would only need one snapshot of the universe at one time to work out all the other snapshots at other times. In other words we could represent everything that ever happened and that ever would happen. We could have all the snapshots for every moment. This is not that easy to visualise but we can make it easier by discarding one of the spatial dimensions and imagining each snapshot of the universe to be in two dimensions like a picture in a film strip. Then imagine arranging all these universe snapshots in a sideways pile, in order, with earlier moments to the left and later moments to the right.

This would be like those little books which if you flick quickly through their pages you see an animation. What we end up with is a block universe, a static block which contains everything that happens, past, present and future. In this block the earth’s orbit would be a like a coil or helix proceeding from left to right. Your own life would be embedded in the coil, starting with your birth, then winding about within the coil as we cast our gaze from left to right (from our privileged viewpoint outside the block) until your death. There would be one section of this block universe containing your entire life.

The block universe is static and eternal. It does not change. It is only from the point of view of someone within the block that change seems to happen and that time seems to flow. If you can imagine zooming into the block to a particular location in it, that location will be a particular place at a particular time. One location could be empty space just outside of Venus, 5000 years ago. Another location could be you waking up this morning in your bed. Another location could be you at ten years old at the local zoo and another location could be you seeing your fifth grandchild at your daughter’s house in 40 years time. Another location would be what we have called the “Big Bang”. You might insist that the future has not happened yet which would be true from your internal point of view inside the block universe, but we, with our God-like outside view can see the future is not qualitatively different from the past. Of course some locations in the block universe contain configurations of matter forming observers such as humans with brains and consciousness and each of them will experience what they call “now”. Every single conscious brain at all locations (and hence times) in the block universe will be experiencing a now because all experience feels like a now. Each now will contain memories of other nows and plans and anticipations of future nows. From our outside view there is no now that can be singled out as the real now.

There is nothing sweeping across the block universe making any snapshot the “real” now because that makes no sense. No now, no present moment is more special than any other. Every now, every present moment, while experienced, feels like the “real” now so they are all equally real. Some equations of physics use time as a parameter but they do not describe a now that moves along a time line – there is simply no need for it. This is better explained here - further reading on time.

The block universe with Einstein

So what does Einstein’s relativity do to this picture? We can still keep the block universe. Due to relativity you will remember that time runs at different rates and events may be simultaneous or not depending on which observer in the universe is doing the observing. It turns out that all we have to do to incorporate relativity into the block universe is to slice it up at a different angle! In the Newtonian view we imagined the block universe being made of slices (snapshots) with each snapshot being a present moment containing everything that happened at that moment. With relativity if an observer is moving close to the speed of light their snapshot will sit at a different angle from other observers so their view of which events are simultaneous will be different. Think of slicing up a loaf in typical fashion at right angles. These slices would be the nows of observers at rest. Then imagine slicing up the loaf at a 45 degree angle. Those slices would be the nows for an astronaut travelling close to the speed of light. This neatly fits the mathematics of relativity and accommodates the concept of no universal time and no universal now. The rate of time flowing and the idea of a present moment only makes sense if it is relative to a particular observer. Note our two observers with differing views of when simultaneous events are and of what rate time is flowing are both right from their point of view and are both describing the same block universe – they are just considering differently angled snapshots through it to be their frame of reference. This is very neat and it means that time can flow at different rates in different places and that two events can be simultaneous for some observers and not others without any illogical consequences. The consequences while not illogical can be surprising and unfamiliar such as your brother ageing more slowly than you because he is involved in space travel (perhaps in the future we will have craft able to reach nearer to light speeds).

Does time have a direction?

I have compared time to space earlier but time does seem to have an important difference. It appears to have a direction. The past seems fixed and the future seems open. We have memories of the past but not of the future. There is an asymmetry to our experience of time. If we play a movie backwards there would often be clues which make it obvious such as eggs un-breaking or milk and coffee being un-stirred. What is interesting though is that at the sub-atomic level the direction of time seems to make no difference. If we played a movie of what is going on, zoomed right in to the scale of atoms then we would not be able to tell if the movie was being played in reverse. The explanation for this uses the concept of entropy which can be roughly thought of as disorder. Entropy (disorder) tends to increase over time in a closed system. The usual example of this is a box containing a gas with all the gas molecules in one half of the box. This is an unusually ordered state of affairs and the molecules will rapidly spread out and fill the whole box thus increasing entropy. However, again at the molecular level the molecules are bouncing around following rules which take no notice of the direction of time. It turns out that the solution to this asymmetry is that there are many more ways that a system can be disordered than it can be ordered so it is overwhelmingly more likely to become disordered in the future. It also means that it is more likely to have been more disordered in the past but this is not what we observe – the solution to that puzzle is that the universe started out in a highly ordered state and has been increasing in total entropy ever since.

Note 1

The universe started out in a very uniform state and you might think that that is high entropy (disorder) as it has no structure. However at those times gravity was much stronger with everything in the universe close together and in situations of high gravity, lumpiness is the natural state of affairs and lumpiness would be considered higher entropy than a smooth uniform state. In the presence of strong gravity the smooth uniform state of our early universe was very low entropy.

Note 2

Some people think the tendency for entropy (disorder) to increase contradicts the theory of evolution but they are wrong because the earth is not a closed system and is being fed energy from the sun. There can be local decreases in entropy (disorder) in the universe while its total entropy increases.

If we return to our block universe then we see a low entropy (high gravity uniform state) on the left of our snapshot pile and then as we move to the right the snapshots have more total entropy even as little pockets within them become more ordered with the evolution of life for example.

More thoughts

I hope this little tour has given you some useful concepts and metaphors for thinking about time. I have not touched on free will which people fear is negated by a fixed future. I think this stems from confusion about what free will really is. I have not delved into how multi-verse ideas or quantum physics can further enrich our view of what time is and isn’t but that can be a project for another day.

The block universe idea is useful for understanding concepts about time in physics. It can also be a psychologically attractive view of time in that it shows us that the passage of time is partly illusory and in some sense we always exist. Our lives are “forever” within this static block universe with each subjective present moment or “now” being experienced. The past is as real as the present and so is the future.

Age-old questions

Viewing the universe (or multiverse) as a static eternal block also makes some questions less relevant. Talking about what happened before the big bang becomes nonsensical. The big bang is just a location within the four (or more) dimensional block. Talking about what “caused” the universe to come from nothing also doesn’t make much sense as the block universe always is. It either can be seen as existing eternally or existing with all times at once in an instant, either way makes no difference to our internal experience of it.

There never was nothing.

Defining a time means that you are defining something to exist. Time is not absolute, it is an internal property of the system defined by observers within it. For an object to be at a place is only meaningful if the place is relative to some other object or objects in the universe from which it can be measured. Similarly for an event to occur at a time only has meaning if its occurrence is relative to some other event or events in the universe. Time and place are properties of events relative to other events, rather than some absolute standard. If the universe consisted of one object which never changes then time and place would have no meaning. The object cannot be a distance from another object and there are no events to define time with or construct clocks with.

We define time to be the direction in which entropy decreases, that is, we define the backwards time direction towards a space-time location we have called the big bang. Time only seems absolute to us because we evolved embedded within the space-time block and our memories can only work in the backwards direction because that direction is more predictable than the future direction ie. there is more order/less entropy in the past. There may be other space-time bubbles (parallel universes) and they also will have their internal property of time which is separate from ours. Therefore asking,

“How did something come from nothing?”

is making a wrong assumption that there could have been a time when nothing existed. However such a time cannot be, if time is a property of something which already exists such as the block universe. The age old question suddenly becomes meaningless and superseded by another question -

“Why does the block universe have the structure that it does?”

and its structure includes its entire history along its time dimension. The question becomes why does space-time have this structure and not some other structure and the answer should not make references to a time before or a cause that precedes. The answer will have to be one that does not assume time always exists. Space-time must be constructed from some more fundamental ingredients which are timeless and space-less and these, I believe, will be mathematical objects as mathematical objects need neither time nor space to exist. This would also explain why so much of the universe is governed by mathematical rules – it is built from mathematical objects, which must exist. The universe has no choice but to exist. It is a goal of physics to find out exactly what these most basic ingredients are. Some physicists have proposed that they may be informational bits, the ones and zeroes used in computer systems and you cannot get much more fundamental than that. The final answer might be in my lifetime or it might be in millennia to come, from the human species or some other. At this point we do not know if the structure of the block universe (which includes the future of course) contains its own explanation encoded in the brain of some being within it.

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Marcus Loane

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