Our Changing Conceptions Of Time

The first to break from the ancient view of space and time dating back to the Greeks were Newton and Galileo in the 17th century. Newton, using Galileo's data, formulated his three laws of motion and also his universal law of gravitation. In the process, he discovered to his surprise and dismay that all motion is relative. A modern way to illustrate this is to consider two astronauts approaching each other in space. Call them Alex and Karen. Alex thinks that he is stationary and Karen is approaching him but then realises that it could be that she is stationary and he is moving towards her. Karen has the same dilemma and neither can say for sure because there is no absolute frame of reference. In other words Newton discovered that space is not absolute.
                 
Over two centuries later Einstein showed how two observers can describe the same event differently. Consider the well-known example of a man who is bouncing a ball in a train as it leaves the station and a woman looking on from the platform. When asked to describe the path of the ball they give different answers. The man says 'Straight up and down' but the woman says it is a slight 'V' because of the lateral motion of the train and both are right. Likewise Einstein showed that time is not absolute but is instead relative to the individual who in effect measures time personally. The rate at which time passes for any individual depends on his or her motion and/or their proximity to a gravitational field such as that caused by a massive body. Also, what are two simultaneous events for one observer may not be for another observer. Einstein united these two new notions of space and time into a single entity, spacetime which is seen as a ( 4D ) fabric or surface that is warped by the presence of mass or energy with the curvature thus caused being what we call gravity and this is still our best picture of this force.
                 
We can describe quite well the evolution of the universe according to known laws but we don't yet have a theory of everything and related to this we don't know the nature of the big bang and what was happening for a fraction of a second after – the Planck era. Instead we can only go back so far and then our laws break down to be replaced by others probably but we don't know the boundary conditions. If we did we could plug in our laws and see how they evolve and compare them with the real world. But maybe the concept of imaginary time provides a clue. In the light cone model space is perpendicular to real time which is the central vertical axis. Imaginary time is also perpendicular to real time so it has the same sense as space and though temporal it ‘behaves’ as a fourth spatial dimension. This imaginary idea has interesting consequences for the real universe and it is a valid, informative perspective.
                 
You can model cosmic evolution using both versions of time. In the case of real time you get a cone that starts as a point, the big bang, and expands as a disc ( 3D space ) as you move forward in time or upwards in this model i.e. like a vertical cone. The second model is different; it’s a sphere. It takes the 2D expanding disc that represents 3D space and adds the fourth dimension of time which is just an extra dimension to the disc making it into a 3D ‘sphere’ since it is like an extra spatial dimension. The resulting ‘sphere’ represents 4D spacetime or the universe. Looking at these models there is one important difference – the cone has a special point; the pointy tip at the origin – this is the big bang singularity that is so difficult to describe in current theory. Variations of the real time model all have the common feature that the world lines converge at the big bang singularity which is a point of infinite spacetime curvature and therefore beyond current theory's ability to describe much less define. This point is different to every other point on the cone which are relatively smooth. Using imaginary time on the other hand you get a sphere and on a sphere all points are the same including the bottom one which is just another point the way the south pole on earth is just another place. In other words there is no boundary condition, the big bang is a normal point and the universe just exists. This is the No Boundary Proposal of Jim Hartle and Stephen Hawking. It is not clear which model is correct or indeed if both are wrong. This proposal conveniently eliminates the problematic boundary/singularity but posits a universe without a Creator or real origin.
                 
Then there is the problem of defining the 'present'. To use an example from fiction and assuming that clocks on Earth and Vulcan are synchronised we look at Captain Kirk who is idly wondering what Spock is up to one morning. He looks at his watch and sees it is 7am so he thinks that Spock is meditating before his breakfast. Just then Sulu flies past in the Excelsior (towards Vulcan) and looks at his watch and concludes it is 7:30 on Vulcan so, wondering the same thing, thinks that Spock is having his breakfast. A moment later Chekov flying the other way in the Regent (away from Vulcan) and looking at his clock concludes it is 6:30 on Vulcan and therefore assumes that Spock is still sleeping. ( Flying away from or towards a distant location at a high velocity moves you into that location’s past or future respectively ). This highlights a problem with ‘now’ and that concept may only be useful in local, slow settings even if this is mostly our experience of things.
                   
The next question is about the direction of time which is always forwards. More precisely it seems to move in the direction of increasing entropy or disorder – the second law of thermodynamics. It is not impossible to go from a disordered state to a more ordered one naturally but this is very improbable and in some cases impossible such as a broken cup reassembling itself into a cup on the table. This is why we know the future is different to the past and why when we look at a film of certain irreversible processes like burning or painting we all know when the tape is being run backwards. But this is not evidence of the flow of time but rather of time's asymmetry between earlier and later events.
                   
Another aspect of time is that the laws of physics are time-symmetric, that is they don't change or lose validity if you reverse time. This is because, generally, the laws of science don't change under the CPT symmetry, that is, if you reverse the charge of every particle, take the mirror-image of every particle (parity) and reverse the motion of every particle (time).So, a universe that was the CPT image of this universe would behave consistently and obey the laws of physics.
                   
A more philosophical question is 'Is time out there in the world or is it just inside my head, a construct?' Immanuel Kant had some interesting ideas about that very question. He said that time – and space – were modes of perception and not attributes of the physical world at all. They are part of the human make-up, how we perceive things and along with causality were how humans experienced reality. There is, however, a sense in which time is a part of us in a more ordinary way – our bodies can measure it whether innate or external. Modern research shows that the brain has an internal 'stopwatch' that can track seconds, minutes and hours. There is also a clock that synchronises bodily functions with day and night. Finally there is a counter that governs how many times a cell can divide thereby putting a limit on longevity.
                   
Next, there is the vexed question beloved of sci-fi fans: Is time travel possible? Well, travelling to the future will be a lot more feasible in the coming centuries. According to relativity if you go off exploring in a fast spaceship for a few years when you return much time will have passed on earth, you will have effectively travelled to your home planet's future, say the 25th century. Travelling to the past is much more difficult and was once thought to be impossible because of paradoxes that would arise if you went back in time and killed you grandfather as a young man for example. To get past these Stephen Hawking has proposed the 'Consistent Histories Approach' which says if you can travel back in time your actions must be consistent with the laws of physics and yield no paradoxes. How this would work in practise is unclear – if you pulled a gun on your grandfather, what would stop you pulling the trigger? Even if time travel to the past is possible it would be difficult because we would have to find or create a wormhole which is a special spacetime configuration that theoretically permits time travel but finding, creating or navigating one is thought to be extremely difficult.
                   
Two modern contenders for the theory of everything have something to say about time too. String theory, though interesting and promising in many ways has a deep problem with time called 'background dependence'. Basically, in this theory, vibrating strings (or branes) correspond to fundamental particles as they vibrate in different ways. One of these must produce the graviton, the fundamental particle of gravity and many strings executing this vibration must give us a gravitational field or just gravity. But we know that gravity is curved spacetime so the strings are effectively producing or generating space and time with their vibrations. But that begs the question 'What are the strings themselves vibrating in or through – what is their background?' This is a serious problem that has yet to be resolved by theorists. Loop Quantum Gravity on the other hand faces no paradoxes but just makes the startling claim that time is discrete not continuous coming in ‘ticks’. Space too is quantised in this theory into tiny ‘bits’. Lee Smolin and his colleagues by carefully combining General Relativity and Quantum Theory and dropping the assumption that spacetime is continuous were able to confirm that time 'ticks' at intervals of 10^-43 seconds. This is called the Planck unit of time. Future experiments may confirm the theory.
                 
Some physicists have decided to make both past and future fixed and real including obviously the present. This is the 'block' view and in it time does not pass or flow it just is – all times are real. After all, if time flows what does it flow in reference to and how fast does it flow? These questions show a problem with the idea of flowing time. In the Star Trek example earlier there are three real 'presents' and each is real. This then is a mini-block of time an hour ‘thick’, i.e. 6.30 – 7.30. Could block time be built up this way?  This is just my speculation on one way the block view could work and I could be wrong.
                     
Because time is such a tricky idea some experts have said we should construct time from the physical properties of the things around us. Smolin wants to discard the idea that the laws of physics have always been true and will be true forever. He asks 'How does an eternally true law apply to a universe that began only a relatively short time ago?' Instead, he believes, space and the laws of physics must emerge as the universe unfolds. These may be emergent but time itself is fundamental, it is thought, in this approach. Or, is time emergent too, based on the behaviour of particles and forces after the big bang? Can time only exist in a non-empty universe? We don’t know.
                     
What about the quantum side of time? Quantum experiments do not measure how particles mark the passage of time. Particles such as electrons and photons are not bound by the same arrow of time that we are. Instead the quantum state that describes them evolves both forwards and backwards in time. Researchers are performing experiments with photons and electrons where the observations made affect the nature of their past. This is called 'post-selection'. Why then when particles have it both ways do we perceive time as running only forwards? The key to understanding time and its direction might lie in a 40-yeard old anomaly that centres on the time-travelling properties of particles called neutral kaons. These long-lived particles are peculiar because two of their properties known as charge and parity violate an otherwise well-respected conservation law known as CP-Invariance. Physicists have found that neutral kaons decay in ways that are only possible if they flout CP-Invariance. This has wider implications. It means that things do not run the same for neutral kaons if you also reverse the direction of time. They are not time-symmetric. That's why I said earlier that CPT Symmetry (or Invariance) holds generally but not always. Researchers have found that the present state of a neutral kaon particle is much more likely to evolve into a future state that a past state. The overall effect is like a random walk with a strong bias in one direction. It's possible that this effect yields a universe that as a whole violates CP-Invariance because the neutral kaons it contains do and that time thus evolves forwards as has been shown elsewhere when discussing the second law of thermodynamics and which is our own experience anyway.

A question many physicists are now asking is: ‘Does time exist at all, is it an illusion?’ It’s thought a deeper understanding of time is needed to reach a unified theory. But a timeless theory faces hurdles. It goes against one of our deepest intuitions and comes up against the problem of explaining how we perceive change around us if the world isn’t really changing. Time, as said earlier though, could be an emergent property of the world like other things and based on something more fundamental.

Quantum mechanics says objects have a much broader range of possible behaviours that can be captured classically. The full description for a system is contained in the ‘quantum state’ which evolves in time and yields the probabilities for any experimental outcome. Because quantum theory is probabilistic two identical quantum states may give different outcomes like throwing the same di twice. But time in quantum theory makes contradictions possible and plays a very strong role unlike in general relativity. Some physicists are trying to rewrite quantum mechanics in a more timeless way like relativity but it’s difficult and this kind of effort has caused problems in the past even when done the other way. When physicists in the 60’s applied the ideas for electromagnetism to gravity they discovered an equation that had no time variable at all which was very surprising!

Relativity though essentially timeless manages to incorporate some idea of time by relating physical systems to one another rather referring them to some notion of a global time. Time is left out because it’s not fundamental really just like money for example. Money was invented as a medium of exchange, it’s not valuable in and of itself and is just a placeholder that replaced barter because that was too awkward but barter was just as real. Perhaps events too could be measured in terms of each other without a need for the common ‘currency’ of time. A full moon, the half life of a radioactive isotope or the lifetime of a star might not each be so many units of time long but could be multiples of each other with no need to refer to a unit of time. So is time necessary?

Even if the world is fundamentally timeless we still do seem to experience time. So where does it come from? An old idea called semi-classical time says that time could be a function of space or part of it so even if the system as a whole is timeless, pieces of it are not. We sense time because we are as humans one of those pieces. But there are still situations that will require quantum gravity to explain. Historically, physicists began with the absolute time of a fixed past, present and open future but have, over the years, gradually dismantled this structure. Our tendency to believe time flows happens because we don’t put ourselves in the picture, we see ourselves as separate. But we are not watching the river flow from the bank, we are in the water. In the end time may have no independent existence but instead may arise simply as a way to describe the relations among objects.
               
This concludes my article on time which has been largely collated from several sources including some speculation by me. I hope I got the balance right and that it was somewhat interesting.