If one day, someone were to approach you and ask - "What is time?" (in a qualitative sense). You would soon realize that the answer is not as easy as you thought. "Time is something that flows?" or "Time is an inevitable entity which keeps passing?" or "Perhaps the constant ticking of the second hand of your watch.". But, these answers seem abstract and possess no rigid meaning. All these answers might be described as characteristics of time but does not really define its meaning. It is as if you are trying to find out the identity of a person by saying - "He has green eyes. dark hair and muscular built." However, the main question of who the person actually is remains unanswered. For decades Physicists' views about the nature of Time has changed drastically. From old Newton's view of absolute time, to Einstein's view of relative time. till the present in which some theories actually disprove the existence of time. The aim of this fairly lengthy post is to summarize all these definitions (or characteristics) of time and finally present my views about what it might mean.
Let us go back to the possible answers stated in first paragraph. We all experience passage of time. The rising of Sun in the east and then with time setting in the west, people getting older with time, the changing of years, etc. It is something that flows. But what are some of the physical aspects which truly define passage of time or the "arrow of time". The answer lies in one important branch of Physics - "Thermodynamics". This subject defines a physical quantity called as - "Entropy". It is a measure of disorder or randomness of physical system. There is a law in thermodynamics which states that the entropy of a system always increases and never decreases. For example - take a glass full of water and add a few drops of ink to it. With time, you will notice that the ink spreads into water and after sometime the water gets colored. Prior to adding ink to water, the ink was in a more ordered state. It was confined in the container and its particles (molecules) were close enough. But once it was added to the water, it spread out into the water and got less confined i.e. more disordered. There are loads of other examples where it can be shown that entropy always increase. How does this explain the nature of time? - Cosmologists have known since the early 20th century that our Universe is expanding. They concluded this when galaxies were observed to move away from each other by an astronomer - Edwin Hubble. It was then the famous astrophysicist - Stephen Hawking realized that as Universe is expanding with time, if one reverses the direction of time then it should contract. Until, one reaches an instant when all the matter of the Universe is contracted into an infinitesimal point -called "Singularity". This led to - "The Big Bang Theory", which said that the beginning of our Universe was from such infinitesimal point. All the concentrated matter then underwent a rapid phase of expansion and now it is still expanding. In other words, our Universe was in a ordered state at the time of Big Bang and as it started expanding, its disorder increased i.e. its Entropy increased. The Entropy of our Universe now is constantly increasing as it expands and that is what defines the Arrow of Time. In the future, there is a chance that this expansion will be stopped by the force of Gravity and everything will start contracting back into a Singularity. During this phase of contraction, the Universe will start going back into an ordered state, Entropy will decrease. This can mean that the Arrow of Time will get reversed. Since increasing Entropy provides us with our current "Arrow of Time", a decreasing entropy shall simply reverse its direction. The consequences will be far more bizarre. For example, in our present arrow of time associated with increasing entropy, a vase falling off the table will simply break. But during the contraction phase of our Universe, when the arrow can be reversed, a vase will be broken first and then re-arrange into a more ordered state on the table. In our current arrow of time, we can remember our past but cant remember the future. This may sound absurd now. but it can be a consequence of our current arrow of time. When this arrow is reversed, it might happen that we will remember the future and not the past. Hence, we have our answer. The flow of time is characterized physically by increasing Entropy of our Universe. When we experience the passage of time, we are actually experiencing an increase in entropy.
But there are many other theories, which treat time much differently. After all the notion of "time" is an important part of many Scientific as well as Non-Scientific theories. The "time" variable is used in Science to calculate rate of change of specified quantities. "Speed" of an object, rate of population increase, measure of electric current, etc are some examples in which the concept of "time" is a basic necessity. How is time defined in such theories then? - Majority of these theories make use of a variable - t. For example - If someone wants to measure the rate of increase of population in a city, then the variable t would be used exclusively in the Mathematical model. This variable would be zero at the start of measurement and then its value would increase periodically. The person can then measure the population at each value of t and thus obtain the rate of increase of population. Many physicists along with Sir Issac Newton referred to time as being "absolute". This means that passage of "time" is same for every individual regardless of his/her state of motion. There is a "Universal Clock" and time passes the same for everyone in accordance with that clock. Of course, this notion is hard wired in the brains of most people. We really think that time passes the same for everyone. I am not referring here to the time as we measure using our clocks or calendars. That time is always different in different regions, giving rise to time zones. The time, I am talking about here is the variable - t, which I used just earlier. What this means is that if there were two people living in a city. One located in his apartment which somehow provides him the view of entire city, while another guy is cruising around in his car. Let us go back to our experiment where we saw how to measure rate of increase of population. They both start counting simultaneously, then according to Newton's view of "Absolute time", the rates they calculate should be the same. It would not make any difference, even if one of them was moving while other was stationary in his apartment. Time would pass the same for both of them.
This view was neat and convenient. It comforted our intuitions about the nature of time and many Physicists remained in its favor. Until the game changer of Physics published four papers in 1905 that shattered this view of time. The game changer was no one but - "Albert Einstein". In his paper, he introduced - "The Special Theory of Relativity" which radically altered our views of space, time, mass and energy. The subject itself is vast and deserves another blog post. In brief, what this theory did was it proved that Space and Time are not absolute but are "relative". The idea of absolute time has no meaning in Special Relativity and that how one experiences time depends on his/her state of motion "relative" to another observer. Considering our previous example, the guy moving in car would get a different rate of growth of population than the guy located in apartment. This effect is called time dilation. In this effect, when one observer is moving relative to another one at rest, then on comparing his time with the stationary observer, he/she would find out that his/her time is actually running slower than the time of stationary observer. Initially, the theory was hard to digest. However, there was nothing that could be done to disprove it. Einstein further extended this idea to General Theory of Relativity, explaining the origin of gravity. In this theory, "Gravity" is an illusion. The presence of mass and energy bends space and time, and objects only follow this curved path through space and time. This theory treats time as a fourth dimension along with three dimension of space. We can move forward-backward, up-down, and left-right in our three dimensional space. However, we always move forward in the time dimension (We get older). This serves as a great idea for science fiction, where there might be some superior living beings able to control their motion through the time dimension as well. The General Theory of Relativity was elegantly represented by Einstein's Field Equations which used geometry of curved surfaces (Differential Geometry). The theory was proved to be consistent over and over. Even after 100 years of its proposal, we are still observing some of its consequences for the first time ever.
So, the Theory of Relativity turned the boring time variable into something dynamic. It can slow down, it can bend, stretch and warp. But, the story does not end here. Remember when I said, some new theories are aiming to disprove the existence of time. Why disprove something which is so dynamic and whose effects are noticed in everyday life? The answer lies in something much more deeper - The Theory of Everything. As the name suggests, Physicists are aiming to formulate a theory explaining our entire Universe in one single, elegant equation. Of course, we are way too far from achieving such theory. But everyday we are getting closer than before. The key in formulating such theory involves unifying all the forces of Universe i.e. Electromagnetic Force, Strong and Weak Nuclear Forces and ultimately Gravity. It turns out that the first three forces can be unified, but Gravity is the hardest to unify with other forces. One essential step required to do so, is to obtain a Quantum Theory of Gravity. Whenever the word Quantum pops up, you should know that things get tiny. Much, much tinier than your eye or even any artificial microscope can peer into. Whenever this word is put in front of any physical entity (field), then that entity is said to be "quantized". Put it in front of an electric field, it gets quantized and we get - "electrons" (particles of electric current). Put it in front of an electromagnetic field and we get - "photons" (particles of light). As one can guess, putting it in front of a gravitational field would yield us with "gravitons" (particles of gravity).The process of quantization can be thought of as using a "cheese grater". Here, the big cube of cheese is a field and on grating it or "quantizing" it, we get smaller chunks of cheese or the field We saw earlier that Gravity is nothing but curvature of space and time (more appropriately spacetime). In other words, space and time is the gravitational field. Thus, we need to quantize space and time. Modern physicists have came up with many brilliant ways to quantize space. The two leading theories are - the famous String Theory and Loop Quantum Gravity. Only problem facing the way is quantizing time. The mathematics for this process gets utterly complex. This is the reason why some physicists aim to completely eliminate time itself from the equations. I shall present one final argument here. In second paragraph, when I was describing the time variable - t, I described how this variable would take the value of 0 at the start of measurement and then increase periodically. The question arises - "How do we know that the value of this variable is increasing periodically?". We would need a reference object to make sure that the value of t is increasing periodically or uniformly. This reference object should be periodic itself, for e.g. the pendulum of wall clocks. But then we have no firm reason to believe that the pendulum itself is periodic. We would need another reference object to confirm its periodicity. It seems like we are caught in a loop here. This truly makes us take a step back and think about "time". It can then be said that "Time" is nothing but an interwoven connection of variables. The t variable, or the pendulum of clock, the rotation of Earth etc. But none of these variables have a separate meaning, as we would have no means to confirm that they are periodic. We can only refer to them in terms of connection with other variables. How can one then believe in Time? Something which possess no true meaning when viewed without connection to other. This is another reason to disprove its existence.
Summing it all up, we have many theories viewing time in different ways. I believe that "Time" is our own construct. Something which we use to make things convenient. It would be absurd if I invited you to a party and gave you only the location of the place where it is held. When I supplement the invitation with appropriate time, then it will be convenient for you to find the "event" i.e. The Party. Nevertheless, it is completely appropriate to keep using it in its different form. Until, we reach a dead end only then we have to find a new way of expressing it or else completely abandon it.
- Thank You.
Let us go back to the possible answers stated in first paragraph. We all experience passage of time. The rising of Sun in the east and then with time setting in the west, people getting older with time, the changing of years, etc. It is something that flows. But what are some of the physical aspects which truly define passage of time or the "arrow of time". The answer lies in one important branch of Physics - "Thermodynamics". This subject defines a physical quantity called as - "Entropy". It is a measure of disorder or randomness of physical system. There is a law in thermodynamics which states that the entropy of a system always increases and never decreases. For example - take a glass full of water and add a few drops of ink to it. With time, you will notice that the ink spreads into water and after sometime the water gets colored. Prior to adding ink to water, the ink was in a more ordered state. It was confined in the container and its particles (molecules) were close enough. But once it was added to the water, it spread out into the water and got less confined i.e. more disordered. There are loads of other examples where it can be shown that entropy always increase. How does this explain the nature of time? - Cosmologists have known since the early 20th century that our Universe is expanding. They concluded this when galaxies were observed to move away from each other by an astronomer - Edwin Hubble. It was then the famous astrophysicist - Stephen Hawking realized that as Universe is expanding with time, if one reverses the direction of time then it should contract. Until, one reaches an instant when all the matter of the Universe is contracted into an infinitesimal point -called "Singularity". This led to - "The Big Bang Theory", which said that the beginning of our Universe was from such infinitesimal point. All the concentrated matter then underwent a rapid phase of expansion and now it is still expanding. In other words, our Universe was in a ordered state at the time of Big Bang and as it started expanding, its disorder increased i.e. its Entropy increased. The Entropy of our Universe now is constantly increasing as it expands and that is what defines the Arrow of Time. In the future, there is a chance that this expansion will be stopped by the force of Gravity and everything will start contracting back into a Singularity. During this phase of contraction, the Universe will start going back into an ordered state, Entropy will decrease. This can mean that the Arrow of Time will get reversed. Since increasing Entropy provides us with our current "Arrow of Time", a decreasing entropy shall simply reverse its direction. The consequences will be far more bizarre. For example, in our present arrow of time associated with increasing entropy, a vase falling off the table will simply break. But during the contraction phase of our Universe, when the arrow can be reversed, a vase will be broken first and then re-arrange into a more ordered state on the table. In our current arrow of time, we can remember our past but cant remember the future. This may sound absurd now. but it can be a consequence of our current arrow of time. When this arrow is reversed, it might happen that we will remember the future and not the past. Hence, we have our answer. The flow of time is characterized physically by increasing Entropy of our Universe. When we experience the passage of time, we are actually experiencing an increase in entropy.
But there are many other theories, which treat time much differently. After all the notion of "time" is an important part of many Scientific as well as Non-Scientific theories. The "time" variable is used in Science to calculate rate of change of specified quantities. "Speed" of an object, rate of population increase, measure of electric current, etc are some examples in which the concept of "time" is a basic necessity. How is time defined in such theories then? - Majority of these theories make use of a variable - t. For example - If someone wants to measure the rate of increase of population in a city, then the variable t would be used exclusively in the Mathematical model. This variable would be zero at the start of measurement and then its value would increase periodically. The person can then measure the population at each value of t and thus obtain the rate of increase of population. Many physicists along with Sir Issac Newton referred to time as being "absolute". This means that passage of "time" is same for every individual regardless of his/her state of motion. There is a "Universal Clock" and time passes the same for everyone in accordance with that clock. Of course, this notion is hard wired in the brains of most people. We really think that time passes the same for everyone. I am not referring here to the time as we measure using our clocks or calendars. That time is always different in different regions, giving rise to time zones. The time, I am talking about here is the variable - t, which I used just earlier. What this means is that if there were two people living in a city. One located in his apartment which somehow provides him the view of entire city, while another guy is cruising around in his car. Let us go back to our experiment where we saw how to measure rate of increase of population. They both start counting simultaneously, then according to Newton's view of "Absolute time", the rates they calculate should be the same. It would not make any difference, even if one of them was moving while other was stationary in his apartment. Time would pass the same for both of them.
This view was neat and convenient. It comforted our intuitions about the nature of time and many Physicists remained in its favor. Until the game changer of Physics published four papers in 1905 that shattered this view of time. The game changer was no one but - "Albert Einstein". In his paper, he introduced - "The Special Theory of Relativity" which radically altered our views of space, time, mass and energy. The subject itself is vast and deserves another blog post. In brief, what this theory did was it proved that Space and Time are not absolute but are "relative". The idea of absolute time has no meaning in Special Relativity and that how one experiences time depends on his/her state of motion "relative" to another observer. Considering our previous example, the guy moving in car would get a different rate of growth of population than the guy located in apartment. This effect is called time dilation. In this effect, when one observer is moving relative to another one at rest, then on comparing his time with the stationary observer, he/she would find out that his/her time is actually running slower than the time of stationary observer. Initially, the theory was hard to digest. However, there was nothing that could be done to disprove it. Einstein further extended this idea to General Theory of Relativity, explaining the origin of gravity. In this theory, "Gravity" is an illusion. The presence of mass and energy bends space and time, and objects only follow this curved path through space and time. This theory treats time as a fourth dimension along with three dimension of space. We can move forward-backward, up-down, and left-right in our three dimensional space. However, we always move forward in the time dimension (We get older). This serves as a great idea for science fiction, where there might be some superior living beings able to control their motion through the time dimension as well. The General Theory of Relativity was elegantly represented by Einstein's Field Equations which used geometry of curved surfaces (Differential Geometry). The theory was proved to be consistent over and over. Even after 100 years of its proposal, we are still observing some of its consequences for the first time ever.
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| A Computerized representation of bending of 4-D Spacetime by Earth |
So, the Theory of Relativity turned the boring time variable into something dynamic. It can slow down, it can bend, stretch and warp. But, the story does not end here. Remember when I said, some new theories are aiming to disprove the existence of time. Why disprove something which is so dynamic and whose effects are noticed in everyday life? The answer lies in something much more deeper - The Theory of Everything. As the name suggests, Physicists are aiming to formulate a theory explaining our entire Universe in one single, elegant equation. Of course, we are way too far from achieving such theory. But everyday we are getting closer than before. The key in formulating such theory involves unifying all the forces of Universe i.e. Electromagnetic Force, Strong and Weak Nuclear Forces and ultimately Gravity. It turns out that the first three forces can be unified, but Gravity is the hardest to unify with other forces. One essential step required to do so, is to obtain a Quantum Theory of Gravity. Whenever the word Quantum pops up, you should know that things get tiny. Much, much tinier than your eye or even any artificial microscope can peer into. Whenever this word is put in front of any physical entity (field), then that entity is said to be "quantized". Put it in front of an electric field, it gets quantized and we get - "electrons" (particles of electric current). Put it in front of an electromagnetic field and we get - "photons" (particles of light). As one can guess, putting it in front of a gravitational field would yield us with "gravitons" (particles of gravity).The process of quantization can be thought of as using a "cheese grater". Here, the big cube of cheese is a field and on grating it or "quantizing" it, we get smaller chunks of cheese or the field We saw earlier that Gravity is nothing but curvature of space and time (more appropriately spacetime). In other words, space and time is the gravitational field. Thus, we need to quantize space and time. Modern physicists have came up with many brilliant ways to quantize space. The two leading theories are - the famous String Theory and Loop Quantum Gravity. Only problem facing the way is quantizing time. The mathematics for this process gets utterly complex. This is the reason why some physicists aim to completely eliminate time itself from the equations. I shall present one final argument here. In second paragraph, when I was describing the time variable - t, I described how this variable would take the value of 0 at the start of measurement and then increase periodically. The question arises - "How do we know that the value of this variable is increasing periodically?". We would need a reference object to make sure that the value of t is increasing periodically or uniformly. This reference object should be periodic itself, for e.g. the pendulum of wall clocks. But then we have no firm reason to believe that the pendulum itself is periodic. We would need another reference object to confirm its periodicity. It seems like we are caught in a loop here. This truly makes us take a step back and think about "time". It can then be said that "Time" is nothing but an interwoven connection of variables. The t variable, or the pendulum of clock, the rotation of Earth etc. But none of these variables have a separate meaning, as we would have no means to confirm that they are periodic. We can only refer to them in terms of connection with other variables. How can one then believe in Time? Something which possess no true meaning when viewed without connection to other. This is another reason to disprove its existence.
Summing it all up, we have many theories viewing time in different ways. I believe that "Time" is our own construct. Something which we use to make things convenient. It would be absurd if I invited you to a party and gave you only the location of the place where it is held. When I supplement the invitation with appropriate time, then it will be convenient for you to find the "event" i.e. The Party. Nevertheless, it is completely appropriate to keep using it in its different form. Until, we reach a dead end only then we have to find a new way of expressing it or else completely abandon it.
- Thank You.
Another mind blowing article on very interesting subject. Nicely explained all phases of inventions, "Graviton" is an interesting term.
ReplyDeleteVery Good Dear Ajinkya ! Small advice :- Whenever you are referring somebody it is good to follow standard referencing >
ReplyDelete