What makes time fast,what makes it slow,what is past ,present ,future, how it is related to gravity,how it influence us, How does time flow? Does time flow in only one direction? Is there a constant ‘Universal’ time? what makes it completely stop? and why it is so bizarre?
Imagine time running backwards. People would grow younger instead of older and, after a long life of gradual rejuvenation – unlearning everything they know – they would end as a twinkle in their parents’ eyes. That’s time as represented in a novel by science fiction writer Philip K Dick but, surprisingly, time’s direction is also an issue that cosmologists are grappling with.
While we take for granted that time has a given direction, physicists don’t: most natural laws are “time reversible” which means they would work just as well if time was defined as running backwards. So why does time always move forward? And will it always do so?
What is spacetime
In physics, spacetime is any mathematical model that combines space and time into a single interwoven continuum. Since 300 BCE, the spacetime of our universe has historically been interpreted from a Euclidean space perspective, which regards space as consisting of three dimensions, and time as consisting of one dimension, the “fourth dimension”. By combining space and time into a single manifold called Minkowski space in 1905, physicists have significantly simplified a large number of physical theories, as well as described in a more uniform way the workings of the universe at both the supergalactic and subatomic levels.
Space and gravity
Also known as Gravitational time dilation.Gravitational time dilation is a form of time dilation, an actual difference of elapsed time between two events as measured by observers situated at varying distances from a gravitating mass. The weaker the gravitational potential (the farther the clock is from the source of gravitation), the faster time passes. Albert Einstein originally predicted this effect in his theory of relativity and it has since been confirmed by tests of general relativity.
This has been demonstrated by noting that atomic clocks at differing altitudes (and thus different gravitational potential) will eventually show different times. The effects detected in such Earth-bound experiments are extremely small, with differences being measured in nanoseconds. Demonstrating greater effects would require greater distances from the Earth or a larger gravitational source.
Gravitational time dilation was first described by Albert Einstein in 1907, as a consequence of special relativity in accelerated frames of reference. In general relativity, it is considered to be a difference in the passage of proper time at different positions as described by a metric tensor of spacetime. The existence of gravitational time dilation was first confirmed directly by the Pound–Rebka experiment in 1959.
Does time have a beginning?
Any universal concept of time must ultimately be based on the evolution of the cosmos itself. When you look up at the universe you’re seeing events that happened in the past – it takes light time to reach us. In fact, even the simplest observation can help us understand cosmological time: for example the fact that the night sky is dark. If the universe had an infinite past and was infinite in extent, the night sky would be completely bright – filled with the light from an infinite number of stars in a cosmos that had always existed.
For a long time scientists, including Albert Einstein, thought that the universe was static and infinite. Observations have since shown that it is in fact expanding, and at an accelerating rate. This means that it must have originated from a more compact state that we call the Big Bang, implying that time does have a beginning. In fact, if we look for light that is old enough we can even see the relic radiation from Big Bang – the cosmic microwave background. Realising this was a first step in determining the age of the universe (see below).
But there is a snag, Einstein’s special theory of relativity, shows that time is … relative: the faster you move relative to me, the slower time will pass for you relative to my perception of time. So in our universe of expanding galaxies, spinning stars and swirling planets, experiences of time vary: everything’s past, present and future is relative.
It turns out that because the universe is on average the same everywhere, and on average looks the same in every direction, there does exist a “cosmic time”. To measure it, all we have to do is measure the properties of the cosmic microwave background. Cosmologists have used this to determine the age of the universe; its cosmic age. It turns out that the universe is 13.799 billion years old.
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.
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 the 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. So the answer is that time does appear to flow in only one direction, on the macroscopic scale.
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.
Will time end?
Time had a beginning but whether it will have an end depends on the nature of the dark energy that is causing it to expand at an accelerating rate. The rate of this expansion may eventually tear the universe apart, forcing it to end in a Big Rip; alternatively, dark energy may decay, reversing the Big Bang and ending the Universe in a Big Crunch; or the Universe may simply expand forever.
But would any of these future scenarios end time? Well, according to the strange rules of quantum mechanics, tiny random particles can momentarily pop out of a vacuum – something seen constantly in particle physics experiments. Some have argued that dark energy could cause such “quantum fluctuations” giving rise to a new Big Bang, ending our timeline and starting a new one. While this is extremely speculative and highly unlikely, what we do know is that only when we understand dark energy will we know the fate of the universe.
So what is the most likely outcome? Only time will tell.
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