Time Machine: Possibilities and Modern Physics!

{mosimage}There’s a word in Bangla, ‘swarthopor’—selfish. However, someone close to me used to call it ‘swarthonij.’ And when he’d say it with a sweet smile, it really seemed like he was right. These days, we often hear people say, almost as a cliché, “Life is more precious than time.” But I’d say, “Time is more precious than life.” Of course, if I were to mention this easily on a blog like I am now, people outside might think I’m crazy. And if I say: time travel is genuinely possible, perhaps in a small town they’d want to show me the asylum, but at BUET or BUETI I might get lucky. So, today let’s learn together about the steps that could fulfill our dreams to visit the future or past and try to save my back for tomorrow!
{mosimage}In 1895, H.G. Wells wrote a storybook called The Time Machine. At the time, everyone considered it pure science fiction, and many secretly wished, “If only this were real,” while we were still busy being ruled by the British. A few years later, a clerk named Einstein shook up physics a bit and became the father of modern physics. He showed, however, that it is impossible for us to move at the speed of light. But research didn’t stop there. In time machine research, the main topic is events and their reactions. If we try to approach nature’s unified theorem, the time travel concept runs into problems there, e.g., the law of conservation of energy. Another related idea gets challenged: the ancient saying “Nature abhors a vacuum.”

Is building a time machine easy?

These polite discussions don’t take us far. Here’s an example: many of us are already familiar with the concept of time dilation. Imagine twin brothers, Ram and Sam. Sam gets a job at NASA, and soon he’s sent off in a whizzing spaceship to a nearby star. His spaceship starts traveling at an unimaginable speed due to the lack of gravity, does a loop around the star, and then returns to Earth, where Ram stayed home and fulfilled his family duties. Suppose Sam’s round trip takes him just 1 year, but 10 years pass on Earth. In that case, Ram ends up being 9 years older than Sam. Sam, after coming back to Earth after a year, is seeing the world 9 years into the future—meaning he’s already done a small bit of time travel.

Some real-life examples: Beyond our senses—a bit of JET LAG

In reality, we experience these situations constantly. For example, moving at aircraft speed, we can’t notice time dilation because it only amounts to a few nanoseconds. But with atomic clocks, if measured precisely, we’d see that time is stretched a bit by speed! So this shows it’s likely we’re constantly “time traveling” in ways beyond our perception. There are thousands of such experiments you can find online.
If we look further into this time stretching, we can consider subatomic levels, where particles are spinning at nearly light speed (here, something could be said about the Large Hadron Collider). Among these, there’s a particle called the muon, which is notable because it acts as a kind of built-in clock due to its fixed half-life (1.52 microseconds, and after disintegration muon = electron + electron antineutrino + muon neutrino). Inside the collider, when a muon is accelerated greatly, according to Einstein’s formula, its rate of decay slows down. This mismatch of time occurs in some cosmic ray particles, too. Since these particles travel close to light speed, from their perspective, they can cross a galaxy in a few minutes, while from Earth’s perspective it would seem to take more than 10,000 times longer (again, according to Einstein’s and Lorentz’s contraction formula). Without time dilation, these particles could never reach Earth.

A few words from Einstein

Let’s talk once again about Einstein’s relativity: According to this theory, Einstein the “clerk” said gravity slows down time. That is, a clock on the 10th floor runs a little faster than one on the ground floor, because it’s a bit closer to the Earth’s center and thus slightly closer to the gravity source. However, you and I will never feel this; only an atomic or seismic clock could measure it (sadly, I’ve never seen such clocks—and nowadays I barely wear a watch because of my mobile!). But one thing we can notice, for those who use GPS regularly: if this weren’t true, sailors and cruise missiles would end up miles off target.
Here’s another complex example: You’ve probably heard about neutron star density and its gravity (details for another post if there’s interest). Calculations show that time there ticks 30% slower compared to Earth. If an observer on such a star looked at Earth with a telescope, they’d see everyone here rushing around like a fast-forwarded video cassette. The black hole is similar: if someone falls into the singularity, they would get a clear idea about ideal time dilation.

Other theories: Is going to the past possible?

In 1948, Kurt Gödel solved Einstein’s gravitational equation and showed that it actually represents a rotating universe (though how he did this is beyond me, but it would have been a fantastic discovery if true). According to this theory, an astronaut could orbit back to their own past. This is possible because gravity influences the speed of light in such a way. But this solution also has many problems: for instance, it doesn’t fit with the big bang and singularity.
Another scenario appeared in 1974, when Frank Tipler of Tulane University calculated that if a gigantic infinitely long cylinder spun at the speed of light around its own axis, an astronaut could journey back to the past thanks to the same effect: light would be dragged into a closed loop. Again, in 1991, Richard Gott predicted that cosmic strings could produce the same effect (cosmologists suspect these formed during the initial moments of the Big Bang). But in the mid-eighties, the wormhole concept brought in a new dimension.

A wormhole is a kind of hypothetical object, also called a star gate, essentially a shortcut connecting two distant points in the universe. If someone jumps through a wormhole, they could find themselves at the far end of the universe. The wormhole fits well with the general theory of relativity, where gravity can twist not only time but space itself. This theory offers an alternative route and the idea of a tunnel which connects the two points directly. A wormhole could be shorter than the direct route!

Now, suppose it is possible to travel through a wormhole (the main issue here is if I go through, I might get stretched so much that my 5.8 foot body becomes 30 feet long, and what happens to my soul or breath I really don’t know, because at that time I’d lose all sensation or growth!), within this, Thorne’s existence of exotic matter is necessary. According to quantum mechanics, exotic matter has negative mass, and in gravity, it repels instead of attracts. For a wormhole’s stability, its presence is necessary, because it can create an antigravity force to prevent an explosion, which would otherwise turn the wormhole into a black hole. This exotic matter can’t be explained with our known physics (that’s why it’s hypothetical!), but quantum mechanics shows that this negative energy state does exist in some systems, though it’s still unclear exactly how much antigravity substance would be required to stabilize a wormhole!
But later, Thorne and colleagues realized that if a stable wormhole could be created, it could work as a time machine.

Stabilized Wormhole: At the threshold of a time machine

So, if we want a wormhole, one mouth would need to be kept near a neutron star, on its surface. The star’s gravity would slow time at that point, causing a time gap between the two mouths to gradually increase. If both mouths are held in place, this time gap remains!
Suppose the time difference becomes 10 years. If an astronaut jumps in through one side, they’ll arrive 10 years in the future, and if another astronaut jumps in from the other side, they’ll arrive 10 years in the past. In this way, a closed spatial loop turns into a closed temporal loop. There’s just one restriction: the astronaut can never go back to before the wormhole was created.
Alternatively, wormholes could naturally form at extremely tiny scales, like the Planck length, comparable with an atomic nucleus. Theorists believe such a minute wormhole could be stabilized with an energy pulse and then, somehow, persist in the normal world!

Paradox

Now, let’s discuss some paradoxes—which is the biggest question facing time machine theorists! Suppose, after solving all engineering problems, we manage to build a time machine, and then some villain travels to the past and kills his daughter as a child. What would happen?
There are many puzzles like this that emerge when someone tries to change the past. But let’s say someone saves a girl in the past and she grows up to become his mother in the future—that would create a causality positive loop, which can be consistent. Perhaps the actions of time travelers could be restricted by these causality consistencies, but it’s almost impossible to control the frequency of such travelers.
There can also be situations like this: someone goes back a year, reads a groundbreaking theorem in IEEE, travels to the past and thoroughly teaches it, then that same theorem gets published again. So the same piece of writing exists in two different times.
These kinds of strange stories about time travel keep it in the realm of possibility. However, Stephen Hawking proposed the “chronology protection conjecture” to exclude these causality loops. Roger Penrose has a similar conjecture. Anyway, the laws of relativity support causal loops, and with Hawking’s conjecture, certain factors would need to be considered which could affect traveling to the past. So what are these factors? One suggestion is that quantum processes might come into play. The existence of time machines might allow particles to travel in a loop back to their own past. Some calculations suggest that past paradoxes could violently reappear and create massive energy surges that would destroy the wormhole!

This chronology protection is still just theoretical, so there’s still hope for time travel. Maybe we’ll have to wait until the day quantum mechanics can unite gravitational hypotheses—perhaps in the form of string theory or its extensions, giving rise to the legendary M-theory. It’s possible that next-generation colliders will be able to create tiny subatomic wormholes, which would be mere playthings compared to H.G. Wells’ time machine! But one thing is certain—it would be a revolutionary change in physics!