By Helena Merriman
BBC World Service
Renowned physicist Dr Michio Kaku says that the world of science fiction may be closer to reality than fantasy.
So if you thought that invisibility cloaks, time travel and teleportation were for the silver screen only, think again.
Dr Kaku is a theoretical physicist and the co-founder of string field theory, a branch of string theory, often referred to as "the theory of everything".
In his recent published book, Physics of the Impossible, he considers the phenomena of science fiction, including time travel and invisibility.
He was shocked to find that almost all of them were consistent with the known laws of physics.
According to Dr Kaku, most of what we see on the silver screen is possible - it is just a question of time.
"A lot of the predictions made by science fiction writers have been replaced by the march of science," he told BBC World Service.
Invisibility has long been a hallmark of science fiction or fantasy.
Star Trek spacecraft use invisibility shields, and Harry Potter would have struggled in his battle against Lord Voldemort without his invisibility cloak.
But could invisibility really be future science fact rather than science fiction?
Fiction to fact
Physics optics books used to teach that invisibility was impossible. But Dr Kaku says this is wrong; it is possible.
But he concedes that there are currently technical obstacles with the Harry Potter invisibility cloak that may hold it back from wide uptake.
For example, to be able to see out of the cloak, he says, you would need to drill two holes.
But then you would see a pair of disembodied eyes floating in mid-air, which might ruin the look. Or lack of a look.
And what about teleportation?
Dr Kaku points out, this has already been done on an atomic scale.
The world record for teleportation is 600m.
Properties of photons - particles of light - have already been teleported across the River Danube and he predicts that we will be teleporting molecules within the decade, and eventually to the moon.
But he stresses that this is only on the atomic scale - no Captain Kirks have been beamed up.
"Remember Captain Kirk consists of 50 trillion cells, so a human being will take many centuries."
But Star Trek takes place in the 23rd Century. "That's enough time for us to teleport a human," he says.
So far, the silver screen has been quite accurate, except when it comes to robots, says Dr Kaku.
In fact, he says Hollywood has misled us into thinking that smart robots are just around the corner.
Fans of the emotional Sonny from I-Robot, or the depressed and bored Marvin from The Hitchiker's Guide to the Galaxy, may be disappointed to hear that they are more likely to find a cockroach composing a symphony or writing a poem than a robot.
"Stupid cockroaches are smarter than our robots," says Dr Kaku.
"Robots are nothing but tape-recorders, pre-scripted moves ahead of time.
"Digital computers have a hard time learning and that's the fundamental problem. They don't learn new skills."
And finally, what about time travel, a concept which has spawned hundreds of films, novels and plays after HG Wells first conjured the world of time machines and fourth dimensions in 1895.
"Time travel is definitely on the table, and in principle it may be possible to build a time machine," Dr Kaku says.
"So, if one day somebody knocks on your door and claims to be your great-great-great-grand-daughter, don't slam the door."
There is a logical problem though with time travel, one that offers rich pickings for films and books.
It is known as the "grandfather paradox" and asks what would happen if a time traveller were to kill their grandfather and therefore prevent themselves from being born.
If the time traveller was not born, how would they travel back in time?
Dr Kaku has a solution.
While he agrees with Einstein that time is a river, he says we now realise that the river of time can have whirlpools and fork into two rivers.
In one of these forks, in another parallel universe, a different history can exist, but the past can never be altered.
"You cannot alter your own past. You cannot go back in time and kill your own parents before you were born. Your timeline is intact."
While these inventions may be scientifically possible, Dr Kaku laments that not enough young people are being trained to become scientists of the future.
But he warns against defeatism.
"It's always dangerous to say the word impossible," he muses.
"The word impossible is a challenge to the physicist."