Today’s topic deals with one of the simplest topics which is deeply rooted in our very understanding of almost everything — Time. In this episode I shall discuss different mind blowing questions and try to unveil some secrets of time and time-travel. What is time after all?
So let’s begin!
What is TIME? Is it real or just a concept?
But what time actually is? Is it a physical thing like space and matter, or an abstract concept which we have created through our experiences?
If you’re thinking that physicists have an answer, then you are wrong. Time is still one of the greatest mysteries in physics which questions the very definition of physics itself. This is the simplest puzzling question asked by a five-year old and also by the greatest physicists. But unfortunately none have the answer.
There is nothing new in the pursuit of the definition of time. Humans are brainstorming about it since time immemorial. Early humans used to calculate time by looking at the position of the sun in the sky.
Ancient philosopher, Plato in the Timaeus, considered
“Time as the period of motion of the heavenly bodies” 
Later his student Aristotle, in Book IV of his Physics, defined
“Time as the number of changes with respect to before and after.” 
So we see, time was and is still a head scratching topic.
Later on with the development of science and technology we got to know more and more about different aspects of time and its effects. But still we could not find its real cause.
Knowing the complexity of this simple question “What is Time?”, Einstein, when he was asked, simply said:
“Time is that which a clock measures”
This is what is called an operational definition. But it should better be called an escape.
This type of problem raises the possibility that we could be looking at things all wrong, as we have done in the past (e.g. “The Earth is flat” or “Hey, let me put some leeches is on you to cure your disease”). Maybe aliens can understand time more easily through their alienatic way of thinking and experiencing life.
There is difficulty in defining time probably because it is very much ingrained in our experiences and our way of thinking. Time is how we relate the ‘now’ we have now with the ‘now’ that we had before. But time is also about the future and how we relate it with the past and our present experiences.
This is how physics thinks about time. In fact, time is embedded in the very definition of Physics, and maybe this is the reason because of which we are facing difficulties in defining time. This is how Wikipedia defines Physics:
“Physics is the natural science that involves the study of matter and its motion and behaviour through space and time.”
So in the definition of Physics we have ‘time’ as an element. Even the word ‘motion’ assumes the concept of time. Actually the basic job of physics is to use the past to understand what futures are possible and how we can affect them. Physics is meaningless without time.
So what exactly is Time? Is it a dimension?
But fortunately, our universe is much more interesting; there is movement and change in it and many strange events are taking place at every moment. So those snapshots don’t exist independently in our universe. Time relates them to each other in two important ways.
First, it connects the snapshots together in a chain, putting them in a particular order. This is similar to what happens when different still images are connected and animated to create a movie.
Secondly, time arranges these snapshots in such a way that the next snapshot depends on the previous one. That means each moment in the universe depends on what happened just before it. This is nothing more than cause and effect. For example, you can’t be sitting on sofa watching TV at one moment and then be halfway through a marathon in the next.
As time bears some striking similarity to another fundamental part of the universe: space, it is possible that both are part of a greater continuum. The same logic of slicing our journey through time into static snapshots can also be applied to space (like slicing a solid we get a plane, slicing a plane we get a line and slicing a line we get a point). This leads us to consider the possibility that time and space are closely related. Indeed, modern physics, considers it better to use them in combined form: space-time, because of their similarities. Just like space, time may also be considered as another direction in which we can move.
Don’t get too much excited. This connection between time and space doesn’t mean that you can regard time as a dimension of space with all of the implications that come with it. You can imagine time and space as mango and orange respectively, both are fruits but their tastes are different. There are many more mysteries yet to be unveiled, just keep reading.
Why does time move forward? Will it ever stop or reverse?
We can remember things that happened in the past but not the things that happen in the future. There are irreversible processes. It seems that time has a preferred direction. The basic question – “Why does time move forward?”, has puzzled physicists for a long time. In fact, what does “forward in time” even mean? In some universes where time moves the other way, the scientists make call that direction forward. Really interesting huh!
So, entropy is one physical law which cares about how time flows. But many of the processes like the laws of kinematics that affect how gas molecules bounce off one another, could work perfectly even if time flows backward. But, in aggregate, they follow a law that requires the amount of order to decrease with time. So, time and entropy have definitely some connection. But we cannot say whether entropy causes time to flow or the arrow of time causes entropy to increase.
It is really difficult to believe that the papers were neatly stacked at the beginning of the universe. How that extreme order achieved in that universe?
It’s really weird, isn’t it? But physicist Sean Carroll thinks differently. He believes that our universe is a part of a bigger multiverse. Like, there can be neatly stacked papers (low entropy) on the desk. But that desk is a part of a room and that room is a part of a house and that house is situated in a city. So there can be a low entropy condition on the desk but there are high entropy conditions throughout the city as a whole. Going through this line of thought, Sean Carroll hints that the Big Bang was not the beginning. And if that’s correct, then the question becomes “Why did a part of the universe go through a phase of low entropy condition?” And this might be easier to answer.
It’s tempting to dismiss the notion of time stopping right out of the gate. We have never seen time doing anything else except to going forward. But if the arrow of time is the function of the second law of thermodynamics i.e. entropy increase, then some speculations can be made. What will happen when the universe reaches maximum entropy? That universe will be in highest disorder, and an equilibrium will be maintained where no order could be created. At that moment time will either stop or it will have no meaning.
Some philosophers even speculate that, at that moment the arrow of time will reverse itself and entropy will start decreasing, leading the universe to shrink back to a tiny singularity. But this is more a speculation than a scientific prediction. What will actually happen to the universe and its laws, we don’t know at all.
Do we all feel time the same way?
Newton also believed in the concept of absolute time and space which provided a theoretical foundation to Newtonian mechanics. In Principia Mathematica, he writes:
“Absolute, true and mathematical time, of itself, and from its own nature flows equably without regard to anything external…”
According to Newton, absolute time exists independently of any observer and progresses at a consistent pace throughout the universe.  He believed that humans are only capable of perceiving relative time, motion of objects and heavenly bodies relative to other.
But later scientist like Gottfried Leibniz, George Berkeley etc. did not agree with his views. Gottfried Leibniz was of the opinion that time made no sense without the relative movement of bodies. George Berkeley argued that without a point of reference, a sphere cannot be conceived to rotate in an otherwise empty universe.
Einstein famously predicted that moving clocks run more slowly. If an astronaut takes a trip to a nearby star by traveling close to the speed of light, he will experience less time than those felt back on Earth. Let’s assume that the astronaut feels just half the rate of time as experienced on Earth. So at that speed everything in the spacecraft will be slowed by a factor of ‘2’, when observed from Earth. But the astronaut won’t feel that slow motion effect in his spacecraft. This is because, not only the clocks and movement of things will slow down but also all the biological processes like heartbeat, breathing etc. of the astronaut will slow down. Even his thinking process will slow down. So for him everything happening in the spacecraft will appear perfectly normal. This process of variation of speed of time for two observers is called ‘time dilation’.
So, the observer on Earth will say that time is running slow for the astronaut while the astronaut will argue that, it’s not the time but the distance that has been squashed up. It cannot be denied that both are true from their perspectives. This is the weirdness of time and relativity.
These are also experimentally proved by the use of highly accurate atomic clocks. Researchers of the US National Institute of Standards and Technology in Boulder, Colorado, monitored two atomic clocks placed a foot apart from each other vertically. One at sea level and another one foot higher. They found that both the clocks read different time, although the difference was very minute, but it was there. Time really ran faster for the higher clock.  It testifies Einstein’s prediction that mass (or maybe gravity) affects time.
Einstein also accepted the idea of Maxwell that light travelled at a constant speed in vacuum(it may change with change in medium). It won’t change with the change of frame of reference or observer. It’s impossible for anything in the universe to go faster than the speed of light (3.00 × 108 m/s). Einstein took the speed of light as a constant entity and with respect to that he tried to observe the universe and to predict its behavior. But why do we have the speed limit in this universe? Let’s find out.
Why we cannot go faster than light?
Let’s do a thought experiment which will make the thing clearer. Suppose you are sitting on your couch and you turn on a flashlight. To you, the light from the flashlight is zooming away from you at the speed of light.
The answer you get is “No”. In that situation also, light moves at its actual constant speed not only for you, but also for any other observer watching you from the outside of the rocket. For this to happen something has to be different. In Einstein’s special relativity, space and time become stretchy and variable to keep the speed of light constant at all times and for all observers.
To make sense of all this, we have to go back to the idea of time as the fourth-dimension of space-time. It helps us to imagine that the speed limit of the universe applies to your total speed through both time and space.
But if you are on the rocket moving close to the speed of light relative to earth, your speed through space is very high. So, in order to keep you within the max. speed limit of the universe your speed through time has to decrease. So your clock will measure less time relative to the clocks on Earth.
There is yet another interesting result that we get from Einstein’s special relativity. When something speeds up, its mass increases relative to its mass at rest. Some part of the energy used in speeding up the object is converted into mass. So you might think, “So my car will be more heavy when I drive it at high speed?” Not really. This is because the mass increase at normal speed is negligible, you will lose more of your petrol (or battery) than any mass gain.
Can we travel back in time?
We know a little about the universe and much is left to know. Many things that were once regarded as impossible, are now possible with the development of scientific knowledge. And many things which are now considered to be impossible, may be possible in future with further development in science and technology.
But in the case of time-travel, modern physics is a certain as it can be that this is not possible. Any method through which we can travel backwards in time quickly leads to paradoxes that violate our deep and basic assumptions about the workings of the universe.
First, moving backwards in time can break causality. If that happens universe will make no sense. ‘Effect’ will happen before the ‘Cause’. Your credit card will be billed before you buy anything. You will have your food before you prepare it. So it’s a really a big deal for a normal human being and animals.
However, it seems to be possible to travel into the future (actually at every moment we are travelling towards the future). As Albert Einstein once suggested that to travel into the future we must approach the speed of light and to travel into the past we must surpass it. The current record holder is Sergei Krikalev, he has reached a grand total of 0.02 seconds in future by travelling about 337 miles in orbit at some 28,083 kmph. So, he is now ahead of us in time by 0.02 secs.
Although, the present physics negates the possibility of travelling back in time, but some physicists suggest that, person travelling backwards in time will actually travel to a parallel universe. Thus his activity in that universe will not affect the present of the universe, from where he left off. This parallel universe theory may solve the inconsistencies in time travel due to numerous paradoxes.
But what is the actual nature of the Universe we never know. Does parallel universe ever exist? Even if we travel to a parallel universe, will we be able to return back to our original universe?
Time to Conclude
But still there is quest for the knowledge, as human curiosity demands for it. The younger researchers and more enthusiastic, and are willing to wade into such risky territory.
Perhaps we will make progress by working directly on the difficult topic, or perhaps we will stumble upon a crucial insight when working on a different problem. Only time will tell.
Phew! Finally, it’s over!
So, what’s your thinking about time? Have you got any new idea about time which can help our physicists, who are breaking their heads for this timeless question?
Do let us know through your comments. 😉
 – Timaeus : https://en.wikipedia.org/wiki/Timaeus_(dialogue)
 – Physics, Book IV : http://classics.mit.edu/Aristotle/physics.4.iv.html
 – Philip K. Dick : Counter-Clock World
 – Sean Carroll, Jan 2010 : From Eternity to Here: The Quest for the Ultimate Theory of Time
 – https://en.wikipedia.org/wiki/Absolute_space_and_time
 – http://www.independent.co.uk/news/science/einsteins-theory-is-proved-and-it-is-bad-news-if-you-own-a-penthouse-2088195.html
 – https://en.wikipedia.org/wiki/Speed_of_light
 – https://en.wikipedia.org/wiki/Grandfather_paradox
[•] – Jorge Cham and Daniel Whiteson (2017) : We Have No Idea