Time travel. Whether you’re reading H.G. Wells or watching Capt. James T. Kirk “slingshot around the sun” in the U.S.S. Enterprise, and it’s always a lot of fun. “What ifs” make for great stories, and time travel opens up a vast trove of possibilities.

Still, it’s just fiction. We can’t actually do it, and here’s why.

I’m no physicist, but I know the difference between an object and a unit of measurement. The first is tangible in a very real way; the second is merely a convention. It’s a human construction, entirely artificial and fully dependent on the thing it’s designed to measure.

We create such constructs all the time. They help us make sense of the world.

The words you’re reading right now represent real things. The word “box” represents a real object, but the word is not that object – and apart from the object it refers to, it would be utterly meaningless. We could have just as easily called that object a Heffalump or a Bandersnatch. Whatever we decide to call it, as long as we all agree that the word in question represents a cube-shaped object with a hollow interior, we’ll understand one another just fine … which is the purpose of communication.

The same is true for numbers. Numbers don’t exist in and of themselves; they measure things that exist. We can use Roman numerals, Arabic numerals (our own system). We can use a base-10 system, a base-5 system or whatever. Our choice. The things we’re numbering remain the same regardless of the labels we place on them, and we can’t count anything unless we have something to count.

Say we’re measuring something in space. We can use inches or centimeters or whatever, but the actual thing we’re measuring – its physical length – doesn’t change, no matter what units we devise to quantify it.

So, how does this apply to time?

Like distance, it’s something we measure, using years, centuries, hours, minutes, etc. We can base our system on a sundial or modify it for daylight savings. We can monkey around with the calendar to create a year of 12 or 13 months if we so choose. For centuries, the Western world used the Julian Calendar, devised by Julius Caesar; these days, we use a calendar promoted by Pope Gregory XIII. But whether we use one or the other has absolutely zero effect on the way Earth rotates on its axis or orbits the sun.

In the same way we talk about “distance” and “volume” to measure length or storage capacity, we use the concept of time to measure a specific aspect of our universe: change.

Without change, there would be no time, because there would be no way to tell the difference between one moment and the next. In fact, there wouldn’t be any moments, per se. The concept of time merely gives us a way to understand and document change; without change, “time” is meaningless, just as the word “box” is meaningless without the thing it describes.

You might argue that it’s still possible to travel forward in time by entering a condition of stasis. This is at least theoretically possible – although the idea of “freezing” and “unfreezing” the human body is problematic in a practical sense and has not been achieved outside of science fiction. But think about it: We’re traveling forward in time anyway, so none of this would really change the nature of the way things work: You’d merely be altering a single physical element – the body – by prolonging its viability. Other than that, change would continue in the very same manner it otherwise would have.

(One could even argue that prolonging average human life span to more than 70 years from just over 30 at the start of the 20th century constitutes a form of forward time travel.)

To “go backward in time,” by contrast, would require far more than simply placing one small element of the universe into stasis. It would mean restoring the entire universe, down to the smallest subatomic particle, to the precise state in which it existed in 1776, 1492, 10 million years BC or whenever you wanted to go. To describe such a task as Herculean would be the biggest understatement of all time (pun intended).

So while it might be great fun to talk about slingshotting your way around the sun and finding yourself back in, say, medieval England or Biblical Judea, it ain’t gonna happen, folks. That’s why they call it science fiction.

It’s also why people like authors and poets, screenwriters, musicians and visual artists are so important. They can take us on journeys beyond the limits of this universe, into the only alternate universe any of us has ever really visited: our imagination.

The trip there and back again is no less a journey of discovery than any other adventure you can …

… imagine.