Exactly what is it that we are measuring?
We can begin to answer the question with the basic description that we are measuring the interval between events, using units that we have chosen for the purpose. We may say, for example, that the next train will be due in 5 minutes. While this information may be very useful for telling us how late the train is when it eventually arrives, it does nothing to describe just what it is that we are measuring. We want to know exactly what the 'interval' is.
In order to investigate the nature of time it may help to break it down into four main questions.
1) How does time flow?
2) Does time flow in only one direction?
3) Is there a constant 'Universal' time?
4) Is time a 'real' dimension?
1) How does time flow?
We tend to perceive time as 'flowing', as though it were in smooth and perpetual continuous motion, but is this view correct? We have learned that at the quantum level energy is not released continuously - there is a limit to how small a change in energy an atom can experience - it is released in discrete quanta by the emission of a single photon. Could there also be a limit to the change in time? This would mean that time would advance in small discrete steps and not move continuously, in other words it would move in a similar way to watching the progress of a story on a film or video; the individual 'frames' of time may be so small that it only gives the appearance of being continuous. This can be tested experimentally by using sophisticated equipment to observe chemical changes taking place at very small fractions of a second. If time does move in small steps, then by probing ever smaller segments of time it may be possible to reach a limit at which these steps can be observed to take place.
Equipment has been constructed that can 'slice' moments in time small enough to capture a chemical reaction take place, rather like a 'freeze frame' picture. This requires an extremely small fraction of a second to observe the process taking place, and is called a Femtosecond. The method of observing such small moments of time is achieved using pulsating laser beams. How small is a Femtosecond? It is one thousandth of one trillionth of a second, and can be expressed as 1/1,000,000,000,000,000th of a second. To try and put this very small fraction of a second into perspective we can use a comparison. According to my calculations, a Femtosecond is to a second, as a second is to 32 million years! So dumbfounded was I by this comparison I checked it out three more times! Even when dividing time up into this incredibly small unit there is no indication of time passing in discreet steps, it still appears to flow smoothly.
What conclusions may be drawn from these observations of time at the level of a Femtosecond? All we can say is that either time does flow smoothly and continuously, or if it moves in discrete steps we have not yet reached a level small enough to observe it.
In terms of pure research, scientists refer to an even smaller unit of time, called the Attosecond, which is one quintillionth (10-18) of a second. This is a decimal point followed by 18 zeros and then 1 (0.0000000000000000001) and is a term used in photon research. The smallest measurement of time that can have any meaning within the framework of the laws of physics as understood today, is known as 'Planck Time', and is equal to 10-43 seconds. We can only describe the universe as coming into existence when it already had an age of 10-43 seconds. It may be that we still have some work to do in order to observe time moving from one 'frame' to the next, if that is indeed what happens.
The Hubble Space Telescope has been used to try and determine if time is continuous or not. Dr Richard Lieu and Dr Lloyd Hillman observed a number of galaxies at a distance of more than 4 billion light years from the Earth. They were looking for light patterns that shouldn't be present if the standard ideas from quantum theory apply to time. According to quantum theory the inherent uncertainty means that time (and hence speed) cannot be measured to infinite accuracy, but that it flows 'fuzzily' on the quantum scale. What they found was the images of the galaxies exhibited a sharp 'Airy' diffraction ring. This implies that the speed of light didn't change by more than 1 part in 1032 as it travelled to through space to reach us. If quantum theory of times are correct then it should not be possible to measure to this degree of accuracy. We may have to accept that time does flow smoothly and not in discreet steps.
In light of these new findings it would appear that the conventional solution of arguing that the fuzziness of time smears out the singularity, keeping density finite, now seems impossible.
2) Does time flow in only one direction?
We perceive time as flowing from the past through the present and into the future. We have memory of past events, but of course no memory of future events. Time provides us with a base line reference point in which events can be placed in order of occurrence, and in this manner we are able to establish that one event occurred before or after another, and this provides us with the so called 'arrow of time'. Interestingly, there is nothing in the laws of physics to suggest that time actually flows from the past through the present and into the future. So what is it that gives time a definite direction, the arrow of time? To seek the answer we need to examine the laws of thermodynamics.
At a subatomic level there is no distinction between the past and the future. In a typical interaction involving subatomic particles, two particles may come together and interact in some way to produce two different particles, which then separate. According to the laws of physics there is no reason why these two new particles could not then interact and revert to their initial condition. By studying these particles it would be impossible to determine the order of events that took place, or indeed if any event had taken place. At this level there is no way to distinguish the past from the future simply by looking at each pair of particles.
In the macroscopic world - at the level detectable by our own senses - we are clearly able to discern the arrow of time. If we see a picture of a tumbler of water on a table, and another of a broken glass on the floor lying in a puddle of water, we know the order of events that took place. We know that broken tumblers never reassemble themselves and that spilled water will not gather together and place itself back in the glass. But why not? According to the known laws of physics every interaction involving the atoms of the tumbler as it smashes is reversible, as is the spilled water. But there is an inbuilt arrow of time, pointing from the past to the future, when we are dealing with complex systems which contain many particles. This distinction between past and future events can be expressed mathematically by the science of thermodynamics, which is based on analysis of the way things change as we 'move' from the past into the future.
The second law of thermodynamics, the most fundamental law of physics, states that the entropy of a closed system always increases, entropy being the measure of disorder. In other words, in a closed system - and the universe is a closed system - disorder will always increase, things will never arrange themselves to a degree of higher order. If for example you put together a jigsaw puzzle in a box to form the completed picture, then close the lid and shake the box, you would not expect to see the pieces rearrange themselves back to the complete picture again, no matter how long you shook the box. The explanation is simple statistics, there is only one possible correct solution, but many wrong ones, so we would expect to keep getting wrong ones. If we could keep trying for long enough then it is statistically possible that the puzzle may by chance put itself correctly back together again, but this is very unlikely, and the universe may not have enough time for this highly unlikely event to occur. Thus left to its own devices, a system will run to disorder, and not order, and this gives us an arrow of time.
Another arrow of time is the Big Bang, and this may be described as the ultimate arrow of time. No matter at what place or time you are in the universe, the Big Bang always lies in the past direction of time. We see the same arrow of time in the expansion of the universe. As the universe ages and expands galaxies are moving further apart. Where galaxies are closer together points in the past direction of time, and where they are further apart points to the future direction of time.
The first law of thermodynamics states that the amount of energy in a closed system cannot change. The total energy of the universe was determined at its creation, but what the second law tells us is that the total amount of 'useful' energy decreases. If and when all the stars and other sources of energy in the universe have given up their heat, the entire universe will be in a state of uniform temperature in which nothing ever changes, it will have suffered 'heat death'.
What have we learned from the study of thermodynamics in relation to the arrow of time? We have learned that the reason why events are reversible on the microscopic scale but irreversible on the macroscopic scale (why the arrow of time points only one way) is that the law of increasing entropy is a statistical law; a decrease in entropy is not so much forbidden as extraordinarily unlikely. Sounds similar to the quantum probability wave doesn't it? See What is Quantum Mechanics? So the answer is that time does appear to flow in only one direction, on the macroscopic scale.
3) Is there a constant 'Universal' time?
If we were to take two atomic clocks and synchronise them to read the same time, we know that we could leave them to 'tick' away for years and they would still read the same time. But if we separated the two clocks and took one of them on an 'plane journey around the world, then when compared to the other clock on return it would be seen to be running a fraction of a second slower. It will only be a very small fraction of a second, but the difference would be real, our globe-trotting clock will have run slower than our stay-at-home clock, travelling the world really does keep you younger! This is not just a theoretical concept, many different experiments, including the 'globe trotting clock', have been carried out and have proved the theory to be correct. So what's going on? The answer can be found in Einstein's theory of relativity, because that tells us that the faster an object moves the slower time runs, until at the speed of light time comes to a stop. This effect is known as 'time dilation', and is very small when travelling at day to day speeds, but becomes significant at relativistic speeds - speeds that are approaching the speed of light. For an example of time dilation at speeds that we are familiar with, an astronaut having spent a year aboard the space station will have aged 0.0085 seconds less than the rest of us that stayed on Earth - hardly eternal youth is it?
It is important to understand that relativistic speed, or any other speed come to that, does not effect the speed at which clocks run, it is time itself that slows down.
We have seen that the speed at which time passes varies in direct proportion to the speed of the observer, but there is another factor that needs to be accounted for, and that is gravity. Powerful gravitational fields, such as those found at the event horizon of black holes, also slow down time. Similarly, a clock at sea level will record time running slower than a clock at high altitude on a mountain top.
To answer the original question, 'Is there a constant Universal time?', the answer is clearly no. We all experience time as passing at different speeds, relative to our speed in relation to one another and the strength of any gravitational field that we may happen to be in.
4) Is time a 'real' dimension?
The Big Bang theory describes how the universe was created from the Big Bang singularity, where all matter and space is contained in a single point of infinite density. At the moment of creation of the universe - the Big Bang - all matter, space and time came into existence, before that time did not exist. Our universe could not exist without time, and time could not exist without the universe, they are different components of the one entity.
According to Einstein's theory of relativity, time is regarded as a fourth dimension, on an equal footing with the familiar three dimensions of space. Einstein says that you can imagine all of space and time represented as a four dimensional space-time map, on which all of history, the present and the future of the universe can be represented. The four dimensions of space and time are collectively referred to as the space-time continuum, which by the way, is not just an invention from the script writers of Star Trek. The problem we have is in trying to visualise these four dimensions because we can only see the three dimensions of space, we cannot 'see' time. However, even though we cannot see it, it is necessary to include time if we are required to define a precise location. We can, for example, define an object's position in a room by three simple measurements, such as how far forward, how far to the left and height above floor level. These co-ordinates will define where the object is, but only where it is now, it may be somewhere else tomorrow.
Let's take a closer look look at the implications of time existing as a dimension. Imagine an object, not a quantum particle this time, or a massless photon moving at the speed of light, but an ordinary day to day object; let's use a one ton boulder. Now this boulder is behaving itself, as only boulders can, and isn't doing anything, its just sitting there existing. It's real, solid and big, there is no doubt at all in our minds that it is actually there, we can walk up to it and kick the thing. The question is, how long is it there for? Now at this point I should mention that this is neither the time nor place for a Smart Alec geologist to come along clutching his Ph.D and inform us that it is so many million years old and will remain in that condition for so many million more years. There is an even chance he would end up under it! We want to know how long in time it is there, in that spot, in front of us. We already have a pretty good idea, we know it is less than a Femtosecond, which to remind you is one thousandth of one trillionth of a second. So after this very brief moment of existence in our time frame, what happens to it? Does it simply disappear when it slips from the present into the past? Equally where does it come from? Does it just spring into existence into our time frame? As time flows past us and the 'now' becomes history, has that moment gone forever, or does the past, and the future, actually exist independently of our own personal time frame?
Again, according to relativity, the past and the future, as well as the present, all exist equally. As I said earlier, 'you can imagine all of space and time represented as a four dimensional space-time map, on which all of history, the present and the future of the universe can be represented.' This would mean that our boulder isn't only existing in front of us in our personal time frame, it would be existing in every time frame, along with everything else of course. We can try to visualise this the following way. Imagine our boulder is resting on a glacier that over a period of 5 thousand years is slowly moving, and we want to show this movement over time. We could make a jelly cube to represent time (I bet scientists have a lot of fun). The bottom would represent the past, at the beginning of the period we are interested in, and the top would be the finish, or the present. The length of our cube could represent a north/south movement at an appropriate scale, and likewise the width could represent an east/west movement, and as stated, time is represented by height. We could slice through the cube at different heights to represent different slices of time.
To show the movement of the boulder over time we simply have to poke a rod through the jelly cube. If the boulder moved at an uneven rate we would have to go high tech and use a bendy wire instead.
We can now slice up our cube into five slices to show us where the boulder was at 1000 year intervals. We could of course keep slicing our cube into ever small fractions showing ever more precise movements over ever smaller periods of time. Viewed this way, we can see that the existence of our boulder is continuous, it exists like a 'worm' through time, it only looks like a boulder at each 'slice' of time. We can make these slices as small as we like, but we will always only see the boulder as it is at that slice, or more accurately, at that moment in time. In this manner we can see that time does not 'flow' at all, it is a fixed dimension just as like the three dimensions of space, it is us that is moving through it.
The direction that we move through the dimension of time would appear to be governed by the arrow of time, as already described. As for the speed at which we travel through time, that is governed by the speed we are moving through the dimensions of space, as described by relativity.
What does this all mean?
If we put all this together, the picture we get is that there is only one speed we are able to travel at, and that is the speed of light, and it is a combination of our speed through space and through time. The faster we travel through the dimensions of space, the slower we travel through the dimension of time, and vice versa. Thus an astronaut zooming along at light speed has used up all their speed 'allocation' in the space dimensions, and as a consequence does not travel through time. This would seem to suggest that the speed of light really is the limiting speed within the universe, and if we had no motion at all through space then we would be travelling at light speed through time. In terms of our diagram, the more movement we make across the cube (space) the less we make in the direction of height (time).
If we use the same analogy of moving through our cube to describe moving through time, we can look again at the consequences of the globe-trotting clock. We will repeat the experiment using not only clocks this time, but twins as well, and we shall call them Bill and Ben. We send Bill on an exciting journey into space with an atomic clock, travelling at high speed. Ben stays at home, polishes his clock every day, and sorts his CD's into alphabetical order. When Bill returns home his clock of course will be running behind the time as shown on Ben's clock, and also will not be as shiny. We understand why this is so now, (especially the shiny bit) but what are the implications for the twins regarding their ages? Before Bill's journey they were the same age, but now Bill is a little younger than stay-at-home Ben. How can this be when they are still both sharing the same time frame? This is where we return to our jelly cube to make the situation easier to understand. Both twins will have their own time lines represented by their holes through the jelly cube. Due to Bill's travels he has moved across the space dimension a great deal more than Ben, and so has made less progress up the cube, which translates to less progress through time. In order for them to be still sharing the same time line, (slice) and have a difference in age, all we have to do is instead of cutting our slice of time parallel to the cube, as we have been doing, is cut it an an angle so that they both appear on the same exposed plane. To put this a little more scientifically, we could say that our movement through the space dimensions alters the angle at which we intersect the time dimension. I should clarify at this point that this is merely a representation to show the relationship between speed and time, and that the correct interpretation of relativity states that movement through space reduces our movement through time.
Another point that may need clarifying is defining movement through the space dimensions. In our example with Bill and Ben, although Ben stayed at home, he is of course still moving through the universe at high speed. The Earth is spinning on its axis while it orbits the Sun, which in turn is wheeling around the galaxy, which itself is flying through space, and so on. The movement we are referring to is the relative movement between objects that we are using to compare time.
The diagram of the jelly cube that we have used is only a schematic of course, as it is not possible to draw a representation of four dimensions. For the purpose of a visual representation I have had to 'steal' the dimension of height and use it to represent time. In reality the dimension of time is described as being at right angles to the dimensions of space, a concept that is impossible for us to imagine. Hopefully though, our jelly cube has served a useful purpose.
However...
Although the model we have used describing time as another dimension agrees with relativity, it does have some unfortunate consequences, it says that the future is already determined! I dislike the idea that the future may be 'already there' just waiting for us to pass over it, because if true, where does that leave destiny and freewill? If on the other hand the future is not already there, then where does that leave the special theory of relativity ? If time is 'real' in the same way that the other three dimensions are real, this would indicate that it does exists and is there waiting for our consciousness 'to move over it'. How much flexibility that allows for the future I do not know, but I must confess I don't like the sound of it.
I prefer to imagine time as being like our jelly cube, a fixed dimension that can be sliced up at any angle, but an 'empty' dimension, a blank canvas waiting for us to put our mark on it.
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