What is Time? Is Time-Travel Possible?

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?

What Is Time
Time throughout the Universe | World Science Festival

We spent a lot of time talking about time but almost no time talking about what time actually is! We check time all the time. We talk about having good times, bad times, old times and even crazy times. We save time, make time, cut time, pass time and what not. It waits for no man or woman. Sometimes it flies away and sometimes it creeps on you. Most of the time, we simply run out of time.

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” [1]

Later his student Aristotle, in Book IV of his Physics, defined

“Time as the number of changes with respect to before and after.” [2]

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.

Einstein’s clock 😀 | Pexels

So why are we not able to explain time? Well, this is not the only case. There are many other basic things staring us right in the face, we don’t have any clearer explanations for. Like biologists have been arguing for decades about the definition of life and cosmologists have been arguing about the extent of the universe.

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?

Time : 4th-dimension
Snapshots of Time Dimension | Forbes

Time is the factor due to which our present moment fades away into past and we move towards future. If we consider our every single moment as a snapshot, we will find each is frozen, static, just like a photograph. If there were no concept like time, then the universe would have been one of those frozen snapshots, incapable of change or motion.

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.

Big-Bang inflation
The Big-Bang inflation : Cause – Effect? | The Conversation

This is how laws of Physics work, after observing the present status they tell us how the universe can change and how it cannot. Physics also considers time as the fourth dimension.

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.

Motion in space-time plane
Describing motion on Space-Time Plane | We Have No Idea

In fact, in every type of motion time is a fundamental entity. Physics better uses the space-time plane to describe a motion. It becomes mathematically simpler and clearer if we treat time as the fourth-dimension (assuming we have only three spatial dimensions).

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?

Inexorable march of Time in reverse – Counter-Clock | Pexels

Imagine time moving backwards, you will grow younger instead of becoming older, forgetting your experiences and unlearning everything you know and you would finally end as a twinkle in your parents’ eyes. This is how the science fiction writer Philip K. Dick represents time in his novel Counter-Clock World.[3] But unfortunately this is not possible in our universe. Time’s direction is another issue that cosmologists are grappling with.

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!

Time entropy
Final result of entropy increase | We Have No Idea

Now let’s come to the point, why does time move in the direction it does? This idea of the arrow of time goes back to the age of Ludwig Boltzmann (1870) who figured out the thing called ‘entropy’. As I have discussed in couple of my previous episodes, entropy is the measure of disorder and it tends to grow. This is the second law of thermodynamics: entropy increases with time in this universe and things become more disorderly. For example, if you neatly stack papers on your desk and walk away, you won’t be much surprised to find them in a mess when you come back. But, you definitely get surprised when papers in mess stack neatly all by themselves. This is entropy, the arrow of time.

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.

time possibilities
Time – Entropy Possibilities | We Have No Idea

Even if we accept the idea that time flows due to entropy increase, but why entropy was so low before Big Bang? Was there no time dimension before Big Bang?

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.

timeless multiverse
Sean Carroll’s Multiverse Model | Wired

So, Sean Carroll proposes a different model of the universe. He suggests a static universe (or better say a multiverse) where the arrow of time doesn’t exist. There’s no future versus past, everything is equal to each other. From that static universe, different smaller universes pop-off and travel in different directions of time. How can anything occur in that static universe without the presence of time? Regarding this Sean Carroll argues that events that happen in that universe don’t have causality, don’t have progress, don’t have memory and don’t have aging or metabolism or anything like that. There are just random fluctuations. As in quantum mechanics, things can happen occasionally. Things can fluctuate into existence. There is a probability for a change to occur. [4]

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?

time inconsistency
The inconsistency of Time

Before twentieth century, science considered time to be absolute and pretty universal: everyone and everything in the universe felt time the same way. It was believed that if we put two identical clocks at different parts of the universe they would continue to agree with each other forever.

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 Newtonabsolute time exists independently of any observer and progresses at a consistent pace throughout the universe. [5] 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 LeibnizGeorge 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.

Albert Einstein

Later on Albert Einstein made a more descriptive observation of the flow of time through his theory of relativity. Einstein’s theory of relativity suggested that time and space and not constant as we observe in our day to day life. They tend to change according to the position and velocity of the object and of course our frame of reference. Einstein’s special theory of relativity connected both space and time and showed that both are dependent on the motion and position of the frame of reference.

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’.

Nonsymmetric velocity - time dilation
Time Dilation : With respect to the blue clock, the moving red clock is perceived as ticking slower | Wikipedia

But you might think, if the speed of time is half for the astronaut, then he may know that his time is running slow because he will reach his destination in only half the expected time. But that won’t happen, because there is a second effect of the relativity theory – when the astronaut will be travelling at a speed closer to the speed of light, he will observe that the distance between Earth and the star have squashed up, so he has to cover only the half of the predetermined distance. Sounds really weird, huh! But this is the effect of relativity. It affects not only time but also distance. This squashing of distance is called ‘length contraction’.

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. [6] 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?

Max. speedlimit
Maximum speed limit of the universe | We Have No Idea

Physicist James Clark Maxwell suggested that light was vibration or an electromagnetic wave which travelled at a constant speed in vacuum. [7] But more than 100 years earlier Newton had proposed his Laws of Motion which showed that speed of an object would differ depending on who is measuring it and the relative motion of the observer. But there was a problem in applying Newton’s law to light. In Maxwell’s equations speed of light is constant for given medium. There is nothing that can allow the speed of light to be different for different people even if they are moving at different speeds relative to light. It is really bizarre, if we think about it.

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.

flashlight from rocket
Flashlight fired from the top of a fast moving rocket | We Have No Idea

But what if we strapped your couch to the top of a rocket and the rocket blasted away and started to move really fast? What happens now when you turn on the flashlight pointing towards the front? Does the light move at “the speed of light” plus “the speed of the rocket”?

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 relativityspace 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.

Slower movement through space, faster movement through time | We Have No Idea

If you are sitting on your couch on Earth, you have no speed through space relative to Earth, so your speed through time can be high.

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.

Faster movement through space, slower movement through time | We Have No Idea

Thus, the universe will never let us cross the speed of light. Even if we try to go faster, it will slow down our time to keep our speed inside the limit.

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.

What you may think. 😀

This result is only effective when an object moves close to the speed of light. At that speed, any extra energy you put into the object does not make it faster but just increases its mass. This explains why nothing can travel faster than light. Basically mass and energy are the same thing but that is another story.

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.

Poster of the 1960’s film, The Time Machine | Wikipedia

Many science fictions like H. G. Well’s “The Time Machine”, show that aliens or advanced humans are able to perceive time as a spatial dimension and move back and forth over it, just like we move on the ground or in the space. They are really fun stories, but physics, which is ruining fun since ancient times, find serious problems in them from physics perspective.

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.

Grandfather Paradox
Grandfather Paradox

Without causality, nothing really makes sense. For example, if anyone makes a time machine, travels back to his past when his grandfather was a child and kills him. Then there will be no chance of the birth of his father and then he himself should not exist, so no time machine was built and no journey to past could have happened. This is known as ‘grandfather paradox’. [8] There are also many other paradoxes like bootstrap paradox, predestination paradox which create issues with time travel. So the moral of the story is that time-travel to past is not possible because it violates causality.

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.

Sergei Krikalev
Sergei Krikalev | Wikipedia

So we see time travel to the future is relatively possible with scientific development. But even if we travel to the future we’ll not be able to return back to the past. That will be a one-way journey without any possibility of return.

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

We are feeling time right now 😀 | We Have No Idea

Time is something which is an integral part of not only our lives but also the whole universe. These questions about the nature of time are very deep, and the answers have the potential to shake the very foundations of modern physics. But while these questions about time are exciting to think upon, it also makes them very much difficult to tackle. We can’t stop time to study it and we can’t make repeated time measurements of the same event.

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!

Futureland – an imagination | Pexels

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. 😉


[1] – Timaeus : https://en.wikipedia.org/wiki/Timaeus_(dialogue)
[2] – Physics, Book IV : http://classics.mit.edu/Aristotle/physics.4.iv.html
[3] – Philip K. Dick : Counter-Clock World
[4] – Sean Carroll, Jan 2010 : From Eternity to Here: The Quest for the Ultimate Theory of Time
[5] – https://en.wikipedia.org/wiki/Absolute_space_and_time
[6] – http://www.independent.co.uk/news/science/einsteins-theory-is-proved-and-it-is-bad-news-if-you-own-a-penthouse-2088195.html
[7] – https://en.wikipedia.org/wiki/Speed_of_light
[8] – https://en.wikipedia.org/wiki/Grandfather_paradox

[•] – Jorge Cham and Daniel Whiteson (2017) : We Have No Idea

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