What would happen if I went back in time to stop my father from smoking?

Traveling to the future is possible. But what about traveling backward in time?

A couple of months ago, I wrote a piece titled “Do You Want to Live Forever” admitting that I am passionate about the concept of time. It mentioned that physics is the only science that explicitly studies time but because equations like (Vƒ)2 = (Vo)2 + 2ax are gobbledygook to me, I didn’t become a physicist. Don’t get me wrong, being excited about what you do is important, but it’s just part of the formula. You must also be gifted — after all, most music reality show contestants passionate about music don’t become singers.

But that doesn’t stop me from spending a lot of time thinking about time. Not only do I read everything written about it, I also write whenever possible (hello, this piece!).

While my fascination with time has always been there, I probably first realized it as a kid watching a rerun of the movie The Time Machine. You can’t talk about time without mentioning that movie. Not the remake made in 2002 but the original from 1960 staring Rod Taylor as H. G. Wells, who wrote the novel in 1895.

Just to be clear, what it is about time that captivates me is less about its physical reality or beginning (something must have proceeded the big bang to set it in motion), and more about its direction, commonly referred to as the arrow of time.

Simply put, the arrow of time is the reason we always see things progressing in a particular direction, e.g. scrambled eggs always come after whole eggs, never the other way around. It may seem weird to you, but there’s nothing in the laws of physics that prevents the act of unbreaking a broken egg. But it never happens. And it’s this one-way direction of time, from past to future, that fascinates me.

So when a friend asked me what I would do if I could reverse the arrow of time, my first thought wasn’t that I’d go back and reverse some of history’s worst crimes against humanity. (New York Times Magazine asked their readers if they would kill baby Hitler if they could go back). Rather, having watched my father die from cancer, my first thought was that I’d go back to when he was a teenager and encourage him to stop smoking. It’s been thirteen years since he died and there are still times that I reach for the phone when something good happens.

We all know that time travel is often the topic of science fiction books and movies, but the truth is that it is not ruled out by scientific fact. Albert Einstein’s theory of special relativity allows for traveling forward in time. It’s as simple as hopping on a spaceship, a really fast spaceship.

Basically, it works this way: Imagine you book a five-year flight into space. Suppose your spaceship is powerful enough (don’t ask me how) to travel at 99% the speed of light. You stop for the night and pitch a tent on some distant asteroid 2.5 years out, and then in the morning hop back on your spaceship and rocket back to Earth at the same pace you left. You wouldn’t notice anything strange (as far as you are concerned time is passing as usual) but when you land, much has changed — in your five-year absence, 36 years have passed on Earth and everyone you know is now 31 years older than you thought. Which means a very fast spaceship is a time machine into the future.

Now the evidence to date suggests that it is probably impossible to build such a fast spaceship. But the theory behind it is sound — according to Einstein (he would be 17 years old today based on calculations by the American Museum of Natural History had he taken the spaceship mentioned above for a spin the year he was born and just returned), time moves at a different rate depending on how fast we’re moving. Which means it doesn’t just happen at near-light speed. Physics says your clock ticks a tiny bit slower even when you’re on an airplane.

By way of example, and you may find this hard to believe, if you’re a frequent flyer and your identical twin isn’t, you’re slightly younger than your non-flying twin. You roll your eyes but it’s true. That doesn’t mean you should spend your life on an airplane just to outlive the rest of us who only fly occasionally. Even if you did that, across your entire lifetime you wouldn’t notice the difference.

Astronauts Scott and Mark Kelly are the perfect example of this. You may have read that Scott Kelly zoomed around Earth at 17,500 mph (light speed is 669,600,000 mph) for one year on the International Space Station. When he got back, he was younger than his twin Mark Kelly who stayed behind. But only buy milliseconds because time depends on how fast you’re traveling.

If all this is confusing, take a minute to watch an excellent video from Diannna Cowen the creator of Physics Girl. You won’t be disappointed. It’s mind-boggling stuff and she does an excellent job putting it in language that’s easy to understand.

So traveling to the future is possible. But what about traveling backward in time?

Believe it or not, a new study from the University of British Columbia and the University of Maryland published in the journal Classical and Quantum Gravity says time travel to the past is possible — at least mathematically. But there’s at least one big problem. It’s called the Grandfather Paradox.

In a nutshell, this hypothetical scenario says if you go back in time to meet and kill your grandfather as a child, you wouldn’t have been born and couldn’t have killed your grandfather.

The same paradox occurs with my father. If I build a time machine and beam myself back through time to stop him from smoking, then I prevent his death and have no reason to travel back in time.

This is why some physicists believe that the past can’t be changed no matter how hard one tries. Even if you could go back, they say, incredibly unlikely things would prevent you from killing your grandfather — for example, your gun would refuse to fire right at the moment where the paradox would otherwise occur.

But other physicists say there’s a workable solution to resolve the grandfather paradox. It’s called the Many-Worlds theory, where traveling back in time creates a parallel universe.

I remember years ago when a colleague stopped me from waking into heavy traffic on a business trip. While I felt relieved by the near miss, according to Many-Worlds, in a parallel universe another version of my colleague did not act so quickly and I was killed. Yet another universe saw me recover after spending weeks in the hospital. The number of scenarios is endless.

In terms of my father, according to Many-Worlds there’s a universe in which I stop him from smoking, and in that world he goes on to live a long life. But there’s also an entirely separate universe in which my father continues to smoke, and goes on to die from cancer. And those two scenarios are just scratching the surface. There’s a world for every possible series of events, each completely separate from the other, and each existing simultaneously.

This would seem to resolve the grandfather paradox. What’s important to remember, though, in all of this is that no one has ever identified someone who has traveled back in time. There is zero evidence of it. Although the most famous physicist since Einstein tried to find proof that time travel actually exists.

In 2009, Stephen Hawking published an invitation for a party at the University of Cambridge that had already happened. His theory was that anyone who shows up for the party, which no one attended, must have read about it in the future. While everyone was invited, no one showed up.

So although the laws of physics seem to allow it, from a practical standpoint science doesn’t know how to make time travel work.

But at least one high-profile physicist would disagree. At the University of Connecticut, a professor of theoretical physics inspired by a tragedy in his life not unlike mine is already building a time machine using the center of a laser twisted into a loop.

In his documentary How To Build A Time Machine, Dr. Ronald Mallett explains why he’s fixated on the possibility of conquering time: “I would say it was to see my father again.”

Whether Mallett can actually go back and save his father from dying of a heart attack is questionable at best. But we are living in the most amazing time in that it is technically possible that we may see scientists soon send elementary particles back in time. And that’s a start.

Quantum physicist Aephraim Steinberg believes that the quantum world is the key to a working time machine. Steinberg and his colleagues at the University of Toronto performed an experiment in 2010 with a photon (that’s a particle of light) that simulated time travel. But no matter how hard it tried, the photon from the future could not kill its former self. Although the experiment does not prove that time travel is impossible, it does show that its possibility should not be rejected— and what more can you ask when trying to make sense of all this.

So to answer my headline question in three words — I don’t know.

Time travel sounds off the wall, maybe even immoral, but one thing I’ve learned over the years when it comes to science is that things that at first seem far out eventually become reality.

Who would have though, before Einstein was born, that we would have handheld devices that use satellites to pinpoint our precise location anywhere on the planet. Today we hardly think twice about asking our smartphones for directions. Without the proper application of Einstein’s theory of relativity, GPS navigation systems would crash almost immediately.

So, is time travel the stuff of science fiction or something more?

I’m skeptical, as you are. But I’ll leave you with this fun fact: H. G. Wells wrote, “Scientific people know very well that time is only a kind of space. We can move forward and backward in time just as we can move forward and backward in space.”

I suspect that’s why physicists like Ronald Mallett who study time keep trying to unlock its mystery, and discover whether or not you can actually build a time machine. “People tend to think of time travel as something fictional because we don’t actually do it,” University of British Columbia theoretical physicist and mathematician Ben Tippett was quoted by The Independent. “But, mathematically, it is possible.”

What do you think? Is time travel possible? What would you do if you could go back?