Cartoonist and science fan David B. Clear explains why it’s not as simple as in the sci-fi films:

Let’s assume you’d travel back 1,000 years into the past. Where exactly in the universe was our Earth so long ago? You would have to know and you would have to know very precisely. The smallest error and you’d end up in space again or, which is not much better, you’d transport yourself into the Earth’s crust, into the middle of a mountain, or somewhere in the middle of the atmosphere…

And even if you could make sure that there’s nothing standing at your destination, you still have another problem — you need to match the Earth’s speed and direction. In other words, you have to match its velocity. Otherwise, considering how fast the Earth moves, it’s like trying to board an incoming bullet train.

David B. Clear, “Why a Time Machine Would Instantly Kill You” at Medium (March 6, 2021)

He adds that you would also need to consider where the sun and planets were in the galaxy at the time or face doom in outer space.

Hmm. That’s quite a bit different from the classic sci-fi stories. Take this 1960 adaptation of H. G. Wells’s novel, The Time Machine:

But suppose the machine didn’t kill you? A popular time travel mission, according to neuroscientist Dean Burnett, is to kill Hitler and thus prevent the Holocaust. Burnett offers some reflections on why the idea is too simplistic, including,

Stephen Fry dealt with this superbly in his book Making History. Without spoilers, the problem is that many assume Hitler was the sole cause of the second world war and all the associated horrors. Sadly, this is a gross oversimplification. Germany in the 1930s wasn’t a utopia of basket-weaving peace lovers who were suddenly and severely corrupted by Hitler’s charismatic moustache. The political tensions and strife were all there, results of a previous world war and a great depression; Hitler was just able to capitalise on this. But if he hadn’t, say because he had been eliminated by an errant time traveller, then there’s nothing to say that nobody else would.

Problems rarely exist in isolation. Just like you can’t go in and rip out a tumour because it’s connected to the wider body which will react badly to such a blunt intrusion, elimination of the main figurehead won’t necessarily prevent events that were as much a product of the wider socio-political context.

Dean Burnett, “Time travellers: please don’t kill Hitler” at The Guardian (February 24, 2014)

Hitler didn’t invent lethal anti-Semitism; he capitalized on its existing strength. Burnett adds that Hitler was actually a poor strategist and lost the war, ending the Nazi hopes for global dominance. If a time traveler had killed him, a better strategist might well have taken his place. Perhaps he might even have got the atomic bomb before the Allies did. Messing with history could turn out to be a tricky business.

Here are some scenes from Star Trek episodes that deal with time travel:

Is time travel even possible? Yes, in a very limited way. NASA explains:

We can’t use a time machine to travel hundreds of years into the past or future. That kind of time travel only happens in books and movies. But the math of time travel does affect the things we use every day.

For example, we use GPS satellites to help us figure out how to get to new places. NASA scientists also use a high-accuracy version of GPS to keep track of where satellites are in space. But did you know that GPS relies on time-travel calculations to help you get around town?

GPS satellites orbit around Earth very quickly at about 8,700 miles (14,000 kilometers) per hour. This slows down GPS satellite clocks by a small fraction of a second…

However, the satellites are also orbiting Earth about 12,550 miles (20,200 km) above the surface. This actually speeds up GPS satellite clocks by a slighter larger fraction of a second.

Here’s how: Einstein’s theory also says that gravity curves space and time, causing the passage of time to slow down. High up where the satellites orbit, Earth’s gravity is much weaker. This causes the clocks on GPS satellites to run faster than clocks on the ground.

The combined result is that the clocks on GPS satellites experience time at a rate slightly faster than 1 second per second. Luckily, scientists can use math to correct these differences in time.

“Is time travel possible?” at Space Place (April 30, 2020)

It doesn’t kill us because the time and distance are insignificant.

Some other thoughts about significant time travel:

You could theoretically go back but you couldn’t really change anything:

“Events readjust around anything that could cause a paradox, so the paradox does not happen,” Germain Tobar, the study’s author and a student at the University of Queensland, told IFLScience.

His work, published in the journal Classical and Quantum Gravity last week, suggests that according to the rules of theoretical physics, anything you tried to change in the past would be corrected by subsequent events.

Susie Neilson, “Time travel is theoretically possible, new calculations show. But that doesn’t mean you could change the past.” at Business Insider (September 30, 2020)

The illustration used is Marty McFly in Back to the Future (1985), who nearly prevents his parents from getting together, thus potentially preventing his own existence:

As Tobar and co-author Fabio Costa point out by way of example, suppose you went back to try to find Patient Zero in order to stop the COVID-19 epidemic. If you succeeded, then there would be no epidemic — but then you wouldn’t have gone back.

However, even if a change were made, time would be self-healing:

Take the coronavirus patient zero example. “You might try and stop patient zero from becoming infected, but in doing so, you would catch the virus and become patient zero, or someone else would,” Tobar told the university’s news service.

In other words, a time traveler could make changes, but the original outcome would still find a way to happen — maybe not the same way it happened in the first timeline but close enough so that the time traveler would still exist and would still be motivated to go back in time.

“No matter what you did, the salient events would just recalibrate around you,” Tobar said.

Matthew S. Schwartz, “Paradox-Free Time Travel Is Theoretically Possible, Researchers Say” at NPR (September 27, 2020)

In which case, Marty simply wouldn’t have ultimately prevented his parents from getting together. That suggests that we live in a fatalistic universe. Tobar and Costa’s paper is open access.

Science and astronomy editor Chenoa van den Boogaard suggests that time travel into the future might be possible but it is a one-way ticket:

In order for humanity to send a traveller years into the future, we would either have to take advantage of the intense gravitational acceleration caused by black holes or send the traveller rocketing into space at close to the speed of light (about 1 billion km/h). With our current technology, jumping a few microseconds into the future is all humans can manage.

But if technology one day allows us to send a human into the future by travelling close to the speed of light, would there be any way for the traveller to use time dilation to return to the past and report her findings? “Interstellar travel reaching close to the speed of light might be possible,” says Dr. Jaymie Matthews, professor of astrophysics at the University of British Columbia, “[but] this voyage is one way – into the future, not back to the past.”

Chenoa van den Boogaard, “Time travel is possible, but it’s a one-way ticket” at Science Borealis (November 16, 2020)

And Einstein-Rosen bridges into the past (wormholes,) are fraught with technical difficulties:

Einstein’s equations suggested that this bridge in space could hypothetically connect two points in time instead if it were stable enough. “At the moment, even an Einstein-Rosen bridge cannot [be used to] go back in the past because it doesn’t live long enough – it is not stable,” Matthews explains.

“Even if it was stable, it [requires] other physics, which we don’t have. Hypothetical particles and states of matter that have “exotic” physical properties that would violate known laws of physics, such as a particle having a negative mass. That is why “wormholes” are only science fiction.”

Chenoa van den Boogaard, “Time travel is possible, but it’s a one-way ticket” at Science Borealis (November 16, 2020)

Cosmologist Paul Davies tells us that time is the central mystery of the universe, precisely because, unlike other dimensions, it is not reversible (in any important way). Except, of course, in the movies.

Here are some short sci-fi flicks with a time travel theme at the Dust channel at YouTube:

Are we tethered to our time despite time travel? A very short film about a girl on a mission to save her mother raises a profound philosophical question. “Tethers” offers a different take on Ray Bradbury’s Butterfly Effect. Assuming time travel is possible, can we really use it to make a difference?

and

Dirty Machines: Short time travel flick exceeds expectations. A tense soundtrack, intriguing ending, and thoughtful stylistic choices make Dirty Machines: The End of History a thoughtful exploration of a logically tricky subject. Now, if the director can just resist the temptation to get woke… (Adam Nieri)