Now,after a long academic career studying black holes and lasers,Mallett thinks he has found a way to bend time. “It came from solving some of Einstein’s field equations. Einstein died the same year as my father:1955. They were the two giants of my life.”

So what would time travel look like if it was cooked up in a lab rather than Hollywood? Is it logically possible to return to the past – or change it? And what does all this have to do with space travel and wormholes?

What is time?

You are travelling in time right now but at the same boring old clip:second by second. Reality has a direction,things change from one moment to the next,ice melts,trees grow,and so we measure that passage by “keeping time” with clocks.

Our experience of time is subjective. Sometimes it seems to speed up. At other times,perhaps even now while reading this article,it feels punishingly slow,each moment dragging out longer than the next. But it’s not always just in our heads – under the right conditions,time can change speed.

Ten years after Wells wroteThe Time Machinein 1895,Einstein's seminal theory of relativity ruled that time and space really are part of the same fabric of the universe:spacetime. You can move in three dimensions in space (up/down,left/right and forwards/backwards) but there is a fourth dimension you need to locate yourself in too:time. And as with space,time is malleable. In the words of one of television’s most famous time travellers,Dr Who,this is where it all gets a bit “wibbly-wobbly,timey-wimey”.

Time has a beginning,Mallett says;the birth of the universe in that first cosmic explosion known as “the Big Bang” billions of years ago. But scientists (and philosophers) are split over whether there is already an end – a future stretching out in front of us as real as the present and the past. Your death may already exist somewhere in spacetime,the theory goes. It imagines all of time and space as a block with no global or fixed present,where the difference between Cleopatra’s reality back in ancient Egypt and yours in 2020 is just a matter of perspective.

From the halls of the University of Sydney’s Centre for Time,philosopher Kristie Miller explains:“It’s like one big Persian rug,there might be dinosaurs up one end and sentient robots down the other,but it’s all woven in already.”

And if the past and the future are both as real as the present,with plottable co-ordinates in spacetime,then why couldn’t you change where you are in the block? If the past really is a foreign country,as the saying goes,couldn’t we visit?

“The block theory of the universe makes time travel theoretically possible,” Miller says.

Of course,our intuition also rails against this idea. We know there is something inherently different about the past,the future and the present. We experience the present,it is what we see and feel and smell right now,we have memories of the past,ghost impressions of a former present,and we have none at all of the future. Doesn’t that suggest it hasn’t happened yet?

To account for this,some theorise the block of time is unfinished,that it grows from one moment to the next and has done so since the Big Bang. But the slippery science of trying to pin down any kind of universal present,a cliff on which the bridge of reality is then built over,renders this surprisingly inelegant. Scientists have largely given up on finding the cut-off point,Miller says.

But in looking for it,they found the first kind of time travel – forward.

Can you travel fast enough to reach the future?

To travel to the future,we typically imagine time speeding up. “Night again,day again,faster and faster still,” Wells wrote,as the years whizzed by outside his fictional time machine. But to skip ahead,you actually want time to move more slowly for you,relative to everything else.

Einstein showed that two things can slow down the usual flow of time:high speed and high gravity.

In the case of speed,the rule goes like this:the closer an object comes to travelling at the speed of light (299,792 kilometres per second),the slower time will pass for it. In 1971,scientists flew atomic clocks,the world’s most accurate timekeepers,around the Earth twice in passenger planes and compared their time with that of identical clocks on the ground. The travelling clocks had slowed,if only by a tiny fraction.

This strange dilation of time means space travellers could become time travellers,Mallett says,journeying vast distances but ageing far fewer years than those awaiting their return on Earth. “Your heart is a clock,it will keep time for you,and it will slow down too once you hit high enough speeds,” Mallett says.

He points to the classic filmPlanet of the Apes,where our heroes land thousands of years into the future on an Earth ruled by super-intelligent apes,after a lightspeed space flight of just two years. To get to some of the distant planets now being studied through telescopes by astronomers,spaceflight will have to get much,much faster.

But nothing can break the speed of light. Photons or light particles can travel this fast because they have no mass – at light speed,time itself falls away and the journey seems instantaneous. But for something with mass,such as a spaceship,to travel that fast would require an infinite (and so impossible) amount of energy. And our technology is still a long way off moving humans anywhere close to light speed.

“If we need to move faster through space,perhaps we could design a way to move space itself,instead of just us.”

“But who knows what we will build in the future,” Mallett says. “If we need to move faster through space,perhaps we could design a way to move space itself,instead of just us.”

In the subatomic world,where there is considerably less baggage slowing things down,scientists can already accelerate particles hair-raisingly close to light speed. For a tiny proton in the sprawling Large Hadron Collider beneath the Franco-Swiss border,for example,11 months of Earth time will seem like just one second.

Mass also distorts spacetime,like putting a bowling ball on a trampoline. We call this effect gravity. Out in space,where the gravitational pull of the Earth is less strong,time speeds up. It’s why the GPS satellites that guide our smartphones and cars on the ground have to be synced up with Earth clocks roughly every few minutes to stay accurate – otherwise they run fast by 38 microseconds each day.

But in a black hole,where gravity is so intense even light cannot escape,time might appear to stand still. Some scientists wonder if travelling near,but not into,a black hole could act as a “natural time machine”,slowing time for those making the journey,Mallett explains:“Yes,like the movieInterstellar.

“And if you have a rotating black hole,which we sometimes see,then it doesn’t just bend spacetime,it twists it,like stirring a coffee with a spoon. Our Earth rotates and has gravity too but a black hole has so much gravity it can twist both space and time. That could create not just a way forward,but a way back. A loop.”

Could we actually travel back to the past?

On June 28,2009,Stephen Hawking threw a party for time travellers – and no one came. The sly scientist had sent out the invitations publicly the day after the soiree,meaning only those already in the future with a means to travel back into the past could have attended.

So does Hawking’s disappointing turnout mean time travel never becomes a reality?

It’s not a definitive no,but many experts say there is an inherent problem with the logic of backwards time travel,regardless of the science.

Hawking conceded that time travel remained a very serious question.

Hawking himself said it must be impossible. To travel back in time would invite the possibility of changing the past,which doesn’t make logical sense seeing as it’s already happened. He supposed there must therefore be something about the universe we still don’t understand,probably in the strange world of quantum mechanics where subatomic particles seem to defy the usual physics,that would render backwards time travel impossible. But in his last book,published posthumously in 2018,Hawking conceded that time travel remained a very serious question.

Here’s the problem:travelling into the future doesn’t break the direction of reality – time (and causality) moves forward as one moment leads to the next. But to go back,Miller says,you’d have to turn that around “so what you do in the next minute causes you to go backward,affecting things in reverse. The tea on the counter goes from cold to hot,from being left out to just poured”. (Christopher Nolan's latest blockbuster film,Tenet,centres around this strange idea of time reversal.)

One potential solution again lies in Einstein’s mathematics. He theorised it was possible to fold spacetime,creating a tunnel between two distant locations,a shortcut known as a wormhole. Each endpoint could be a vast distance apart in both space and time,Miller says,“punching a hole through spacetime”,making the concept a convenient staple of science fiction from Star TrektoStargate.

As you read this,scientists are hunting for wormholes out in the universe with radio telescopes,but if we find one or even figure out a way to conjure up our own,Miller says,we still don’t know if we can travel through them – or where we’d end up.

To hold the mouth of a wormhole open in spacetime,say,long enough for a spaceship to travel through,scientists say we’d need a special kind of force,an energy without matter known as negative or dark energy. Since the Big Bang,the universe has been expanding but that growth is not slowing as one would expect billions of years after the initial explosion – instead,it’s accelerating. Scientists think a mysterious force known as dark energy is driving it.

Mallett,meanwhile,believes he’s found another theoretical way to fold spacetime – using light. “Einstein showed that light produces a gravitational field too,so if light has gravity,and gravity affects time,then light could affect time.”

Mallett theorises that a circulating beam of light or “ring-laser” with enough power could twist not only space but time as well. “Kind of like our rotating black hole and stirring that coffee cup,” he says.

The woman’s father wanted to know if he could send a message back in time to himself,the day of his daughter's fatal car crash …

With the right geometry,he thinks it could even fold back into a loop,allowing travel into the past.

Demonstrating this beyond maths equations will require serious funding,he concedes,but as with all such experiments with spacetime,he says it could lead to beneficial spin-off technologies too.

“I’m looking more realistically at sending information back in time,rather than people,which would require a[gigantic] amount of energy if it was proven to work,” Mallett says. “But if we could send subatomic particles back like neutrons,they have an'up'and a'down'state we could turn into binary code[and] we could warn people in the past about what was coming. We could save lives.”

He recalls a letter he once received in German. He doesn’t speak the language but from the accompanying photographs,he could already understand the story:in the first photo a young woman was pictured with her family,in the second was a grisly car wreck. When Mallett had the letter translated,he learned it was written by the woman’s father,who wanted to know if he could send a message back in time to himself,the day of his daughter's fatal car crash,to warn him of what was to come.

So what does all this mean for would-be time travellers?

Mallett agrees the twisty logic of backwards time travel is difficult to reconcile. But he notes the subatomic world – where particles can be in two places at once – may yet offer answers.

“If science has taught us anything it’s that nature is weird,” he says. “The question is open and it’s no longer taboo. It wasn’t until this century I could come out of the time travel closet,as it was,and reveal I’d been researching it,but there’s lots of people looking at it now.”

Still,if he does build his laser time machine,Mallett admits it will never fulfil the wish of his 10-year-old self.

“We would only go back as far as time has been bending,when the machine was turned on and the loop started,” he says. “I wouldn’t be able to go back to my father.”

Perhaps we will work out how to use black holes and wormholes after all,where the scale of the past they could unlock would be staggering.

Unlike using natural phenomena to time travel such as a wormhole,a machine built by humans would be expected to come with this inbuilt limit – you could only travel as far back into the past as the machine had been running. This perhaps explains,Mallett says,why we’re not already inundated with time travellers from the future. “Year zero” for time travel – marking the beginning of possible journeys – is yet to arrive.

But he’s not ruling anything out. Perhaps we will work out how to use black holes and wormholes after all,where the scale of the past they could unlock would be staggering. Perhaps an advanced alien race will reveal itself with its own[long-running] time machine or a way around time,as is imagined in the film Arrival.

Or perhaps the answer is already etched in the logic,as Hawking guessed,a painful truth of the universe waiting to be confirmed. No matter how far science may take us,we can never go back.

Also in this explainer series ....

• ‘A numbers game’:Will we ever find aliens (and what are UFOs)?
• Curing cancer,designer babies and supersoldiers:How will gene-editing change us?
• Beam me up,Scotty:Will we ever teleport or travel the universe?
• Could we resurrect mammoths,Tassie tigers and dinosaurs?
• Brain chips,cyborgs and the ‘singularity’:Will artificial intelligence rule the world?