From the classic “Back to the future” to “TENET,” time travel is probably one of the most overused concepts in modern fiction, guess what’s the other most popular? (something more than a hint: quantum).
Even though Einstein’s theory of relativity turns on the possibility of time travel, the popular media tends to over-fantasize the subject and, in the process, drift away from the practical realms of Science.
This is evident through the paradoxes that usually arise out of the storylines.
We will go through some of the wildest inconsistencies and storylines and dive deep into the paradoxical world of time travel.
Time Dilation
As a High School student, you might already be familiar with some basic physics of frame-of-references, accelerations, etcetera.
These already not-so-innocent concepts become more terrible at big space scales.
First of all, let us understand what Einstein’s theory of relativity has for us in the domain of time travel.
Kinematic Time Dilation
According to Einstein’s Special theory of relativity(SR), time is relative to the speed one is traveling at.
As you travel faster and faster, time travels slower and slower for you, this phenomenon is known as time dilation.
It happens to compensate for the fact that nothing in this Universe can travel faster than the speed of light.
For example, if an object travels at the velocity of 0.5 times the speed of light, then thinking very normally – light emitted by this speeding object should travel at the rate of 1.5 times the speed of light.
But we know it’s against the law of nature (though we still don’t understand why it happens, we know it does or consider it as an assumption that works very well).
Time runs at the regular rate for the speeding object, but it seems to run slower for an observer at rest.
However, how come time runs slower?
You can understand it in this way – A second is defined as the amount of time in which light travels a distance of 299 792 458 m.
Consider a light clock on Earth and another similar clock on a speeding spaceship. There are two points at a certain distance in the light clock, and a light beam is bouncing between the two points.
In the light clock, let’s say 1 second = one successful bounce – the beam starts from a point, hits the other point, and comes back.
Assume that the distance between these points is x.
The fact that the speeding spaceship is itself traveling through space, so it is certain that the beam will cover more distance than x, as the horizontal path between the points will not be the shortest distance anymore.
But light travels at the same speed regardless of what reference you use, due to which time dilation takes place
The light beam has to travel more than just the distance x in the spaceship, i.e., to perform a successful bounce. The following art illustrates this.
One cycle of light pulses between two green ships takes two seconds of “green time”, one second for each leg. As seen from the perspective of the reds, the transit time of the light pulses they exchange among each other is one second of “red time” for each leg. As seen from the perspective of the greens, the red ships’ cycle of exchanging light pulses travels a diagonal path that is two light seconds long. (As seen from the green perspective the reds travel 1.73 light-seconds of distance for every two seconds of green time.) The animation cycles between the green perspective and the red perspective, to emphasize the symmetry. By Cleonis – Own work, CC BY-SA 3.0
But the light beam in the light clock, which is on Earth, has to travel just the distance x.
This is why an observer on Earth would vouch that time elapsed for someone in the spaceship is relatively slower.
In this sense, the faster an object travels through space, the slower time runs for it relative to a stationary observer. Well, the above explanation might be a little vague.
Let’s do time dilation properly in some other articles, and meanwhile, you can watch these brilliant videos by Fermilab below to have a better grip on this subject – Einstein Clocks, How people get time dilation wrong.
Gravitational Time Dilation
Another type of time dilation, called gravitational time dilation, is explained by the General theory of relativity(GR).
According to GR, gravity bends space-time, which leads us to another type of time dilation due to the difference in the strength of gravity.
Einstein described the Gravitational field as a curvature or warping in the space-time fabric.
The more an object warps space-time, the slower time would run on it, concerning observers on bodies that possess relatively weak gravity.
The best way to visualize gravitational time dilation is in the movie Interstellar.
In the movie, as a result of being near the black hole, the man barely aged while his daughter was much older than him. From a theoretical standpoint, this is certainly possible and valid.
This also means that the surface of the earth may be more than a year older than the earth’s center, assuming that both were formed at the same time. Although the resulting errors in satellite timing are measured in nanoseconds, lightspeed is 30 centimeters (1 foot) per nanosecond so the combined effects can result in GPS errors as large as 15 meters if not taken into account. By Orbit_times.png: P. FraundorfDerivative work: Spotsaurian (talk) – Orbit_times.png, CC BY-SA 3.0
For example, a person standing still on Earth can say that his legs are younger than his head as the gravitational field is a bit stronger at the bottom.
Also, time would run slower for someone on Jupiter (more massive than Earth and hence has a stronger gravity) relative to the time on Earth.
NASA had once sent one of the twin brothers, Astronaut Scott Kelly, into space for 520 days.
The following is what he said upon returning to earth, “So, whereas I used to be just 6 minutes older, now I am 6 minutes and 5 milliseconds older” and that makes sense now.
You can watch him saying this here.
So don’t worry about your head aging faster than your legs, as the Earth’s gravitational field is not strong enough to produce enough to result in a significant time dilation.
It’s only drastic when you’re near objects like a black hole.
Time dilation may let us travel into the future in a limited sense.
For example, you may leave Earth in a speeding spacecraft and go so fast that when you return to Earth, you have aged merely days or weeks, but on Earth, it’s been 3(randomly inserted) years since you left.
So voila! Just a few days in a spacecraft, and now you have traveled into the future Earth.
But this example we just told you is practically impossible.
For instance, to make a time difference of mere 2 seconds, you will have to accelerate up to precisely the speed of light, which is not possible at least now.
Are Wormholes Time Portals?
Einstein’s general theory of relativity provides solutions for objects called wormholes.
These may act as time portals, but wormholes are claimed to be highly unstable, and to date, we have never come across a wormhole.
Although some Physicists like Roger Penrose consider objects like wormholes purely fictional, wormholes require negative energy and ‘exotic matter‘ to exist.
Not much about the exotic matter, but we know that negative energy does exist such as negative gravitational potential energy.
So there might be a possibility that the universe has a way figured out for a wormhole to exist.
Traversable wormholes are certain kinds of wormholes, if exist, could be stabilized and allow time travel, while scientists say microscopic wormholes have more chances to exist but even if they do, it would be very hard to detect them, and again it would not be suitable for any kind of interstellar travel.
Want to race with light? Not so easy..
The hitch in this matter is that as you reach closer and closer to this magic number, that is, the speed of light, your length would be contracted to nothing, and your mass(relativistic mass) would be increasing infinitely.
This is absurd because it would require an infinite amount of energy to accelerate to speeds more remarkable than that of light further.
We can’t even reach near the speed of light, so there should not be a question about surpassing that limit.
So whether we are concerned about time-traveling into the future or the past, this limit of the speed we may possess through space-time creates a very big obstruction.
Law of Causality
Now, why is the speed of light even a limit in this Universe? It’s not the speed of light actually, but it’s the speed of something called causality, a more fundamental property of the Universe.
Defining causality, It appears that the speed of any object in our universe is not as limitless as we might think.
Everything that happens is a result of a trigger, which is called the cause, and the effect is the consequence.
So, the primary sequence is first a cause and then any effect.
This means, that for every effect, there is a cause, and in our universe, the geometry of space-time limits every cause and effect by the speed of 299 792 458 m/s.
So, the limit is not the speed of light, but it is the speed of causality, or we can say – the shortest time it takes for a cause to give out an effect.
Therefore, not only light, but any massless entity can travel at the speed of light but cannot exceed this speed, such as gravitational waves and gluons.
Since an effect cannot occur before the cause, it’s impossible to break the limit called the law of causality which is considered a fundamental law of the Universe.
For example, suppose you threw a ball at your friend, it cannot hurt him before you throw the ball.
Here, throwing the ball is the cause, and getting a bruise on your friend’s back is an effect.
Your friend won’t get hit until or unless you hit him; the same thing happens when light propagates.
We also have an article about the physics of light where we talk about causality, visit here.
So how does this comply with Time travel?
Just imagine yourself doing your regular chores and you get a message written on a paper that says – “Do not go downstairs now or you will fall”.
So it turns out you time-traveled to the past from the future to tell yourself to avoid going downstairs today, so that you don’t hurt yourself in the future.
If the “past you” follows the instructions, you won’t fall and not get hurt. And also then you will not Time Travel to the past to give yourself these instructions.
This is one of those many kinds of paradoxes that comes up with Time Travel theories.
Can you Disobey Thermodynamics? – The direction of Entropy
So it seems as if it might be impossible for us to travel in the past as there are many hurdles, another potent reason for this is entropy.
The entropy is the measure of the number of configurations the particles inside it can have.
The second law of thermodynamics states that the entropy of the Universe must always increase with time.
Everything in the Universe happens to progress into a relatively disordered state or gain more entropy.
Traveling back in time would result in an overall decrease in the entropy of the Universe, which would violate the fundamental laws of physics.
That’s also why increasing entropy is considered to be an indicator of the Arrow of time, the fact that you can only travel through time only in one direction, which you’re doing as always.
If you actually think about this, we have another question now –
Do you want to time travel as if the past, present, and future are destinations, or do you want to travel back and forth through time?
In either of the cases, the second law of thermodynamics is equally peculiar to think of.
Traveling THROUGH Time
If you consider traveling through time you will actually become younger as you go back in time or become older going forward into the future, you might as well encounter a memory loss or gain respectively, well are lots of parameters to imagine here.
In this case, if you go back in time, you’re just directly being a part of this decrease in the entropy of the universe. Broken cups rewind themselves, you rewind yourselves and stuff.
We basically claim to know that the second law of thermodynamics won’t allow this, is by the fact that we have never witnessed a broken cup defying gravity and assembling itself again on the table it was kept on, or any other such related events.
Without going much deeper I’ll shoot you a very clever quote –
“Entropy increases with time because we measure time in the direction in which entropy increases. You can’t have a safer bet than that.” – Stephen Hawking
Time Traveling to destinations – The Past, Present & Future
This case is even more peculiar to consider for Thermodynamics, you travel to the past, present, or the future as if they are like places to visit.
When you consider traveling to the past, you must remain unbothered by the decrease in entropy that happens to the rest of the universe.
Typically people like to think, that we could achieve this if we can go past the speed of light, and it’s for sake of the fact that the faster you move through space, the slower you move through time.
So it might be that going past the speed of light allows traveling back in time.
But as explained earlier, there seems to be no practical way to go past the speed of light as of now. Neither do we understand if going past the speed of light would work.
Ok, No-entry to the Past but how about the Future?
Now the question is, if we can’t travel to the past, why not stick with traveling to the future then?
Well, we must also acknowledge we’re always time-traveling to the future, you got me right?
For us, it’s necessary to understand that time travel will only make sense if we could travel in the past as well.
Why? Suppose you somehow made a trip around a Black Hole whose gravity is intense enough to dilate the time for you by a significant magnitude, say 100 years.
Now you come back to check out what the world looks like after a century or if it even exists, so you have this future information and want to get back to tell everyone what the future looks like.
But how will you go back?
You now realize that you didn’t travel to the future, but you just let your rate of time be slowed down so that your future would become your present earlier than everyone else.
“People like us, who believe in physics, know that the distinction between past, present, and future is only a stubbornly persistent illusion.” – Albert Einstein.
Hence, we can call such maneuvers time travel only when we could have a round trip back.
Due to the above reasons, time travel is deemed impossible but as you know, our understanding of physics is not complete so there might be some phenomena we’re missing that could get us into the past.
Popular media and we, in general, are fanboys and fangirls of time travel to the past, and we have watched so many movies and series based on some crazy paradoxes, giving adrenaline rush and goosebumps.
What if Time Travel exists? Welcome to the World of Paradoxes
Let us take the liberty that time travel is real, but as stated earlier this would lead us to some cheeky paradoxes. Let us go through them one by one
Grandfather Paradox
The Grandfather paradox is a paradoxical situation that arises when one alters the past such that their ‘present’ becomes inexistent.
The popular idea and as the name suggests is that – some weird theorist goes back in time and stabs his grandfather, and now there’s a logical paradox surrounding the existence of the theorist who went back in time.
Well getting out of the grandfather reference, you can also fit this concept in other cases, like the “falling down the stairs” case I mentioned earlier in the causality section.
We even have a movie called The Grandfather Paradox, in which a Professor’s grandson attempts to solve this paradox once and for all.
Bootstrap Paradox or Temporal Loop
Now let us assume that on one fine autumn evening while you were casually walking in the park some mysterious man hooded under a coat with a pair of dark sunglasses slid you a note.
After some inspection of the note, you get to know that it’s the secret or the formula for time traveling to the past. You use it and build a grandiose time machine.
But as it turns out the man who slid the note to you on that romantic autumn evening was actually “You” from the future.
So, where did the idea originate? This is called the Bootstrap Paradox.
Just like that In the Terminator series, they get information about advanced robots from the future and they create those robots but, the initial idea for the robots is unknown.
This is bizarre and leads to a paradoxical situation.
You can see many examples of Bootstrap Paradox in a series named Dark and in the popular movies Harry Potter – The Prisoner of Azkaban and Predestination.
The Bilker’s Paradox
Bilker’s paradox is a paradox in which you know what your future holds for you and in turn, you do all kinds of things to prevent that future from happening.
This is something that can be seen widely in the medium of fiction.
For example, if you know that you’re going to marry Jaiko (from Doraemon) but you then try and marry Shizuka instead then it leads to a paradox.
We also see this in the hyped and deemed to be an epic saga, that is Marvel’s Avengers Endgame.
Earth’s mightiest heroes travel back in time and then with their knowledge of the future try to undo something which has already been done.
Sexual paradox
“Uh..oh about sex, must be interesting..”
This will sound a bit weird and is actually in all senses weird but, in this kind of paradox, you are the father of yourself which is a biological impossibility.
In a tale written by the British Philosopher Jonathan Harrison, the main character fathers himself.
In another book called “All you Zombies”, the main character is simultaneously his mother, daughter, father, and his own son.
The same thing happens in predestination movies as well, don’t question greatness.
So, these are some of the weirdest and wildest paradoxes that arise out of the aspect of time traveling to the past.
Nevertheless, these paradoxes come off as skeptical and less sophisticated than they could actually be.
Rather than affecting just the subject, it can have an impact on the entire system.
In all the above cases, time travel can have dire consequences.
Traveling back in time is not only related to your body or any particular event.
But you also take the air molecules and dust particles in your lungs to the past which can create a chaotic effect at an atomic level which, ultimately, could affect the macroscopic world.
There are even risks to the planet’s destruction. All thanks to the butterfly effect.
Well, this certainly sounds like a plot for another science-fiction film.
The Final Verdict
Well, there are more kinds of paradoxes, above were all of the popular ones.
Even though the physics of today doesn’t quite allow time travel to the past.
We are not sure what dwelling surprises the Universe has for us.
For all we know, our universe is a peculiar place sleeping under the blanket of infinite mysteries.
There are wild theories about exotic particles, warp drives, alcubierre drives, dark fluid theory, and things we don’t have any clue about.
All of them are equally fascinating but do not too much into the fiction that you drift away from the practical realms of Science.
The Universe is a weird place with numerous exotic mysteries encompassing all of it. Isn’t that why we all love it? Until next time, dive deep into the cosmos.
What should you conclude?
- We often witness fictional works representing facts & concepts associated with time travel that have no practical scientific grounds, anyway, it is beautiful but still just fiction.
- We don’t know if we can time travel going past the speed of light, what we know currently is that we cannot go past the speed of light.
- Wormholes if exist might act as time portals, but there is no evidence of the existence of such bodies as of now.
- The law of causality states that no effect can take place faster than the speed of light, and there is a delay between a cause and its effects, and this delay cannot be faster than the speed of light and hence doesn’t allow time travel.
- Time-traveling to the past also results in a net decrease of entropy of the universe, and hence this violates the 2nd Law of Thermodynamics.
- Time dilation is time travel in some restricted sense, it’s like stepping into the future as your relative aging slows down.
- Even if time travel still becomes somehow possible, there are lots of inconsistencies that come up called time paradoxes.
So it is safe to consider time travel is not really possible for now, although this is a bit imagination-restricting fact, but yes it is true for now.
Although science fictional works have no practical scientific grounds, we must acknowledge their contributions that shape science.
Who knows today’s fiction might be tomorrow’s reality.
Explore beyond
- Astronaut Scott Kelly says – “So, whereas I used to be just 6 minutes older, now I am 6 minutes and 5 milliseconds older” – Youtube link.
- Learn about the Revolutionary Physics of Light.
- Learn more about Negative Energy.
- Learn more about time dilation – Einstein’s clocks, How people get time dilation wrong & The twin paradox.
