How time travel COULD work: Expert explains Stephen Hawking’s final prediction
- Hawking wondered if subject of time travel could be approached scientifically
- University of Nottingham researcher says he was ‘cautiously optimistic’ about it
- The late theoretical physicist touches on the subject in his posthumous book
‘If one made a research grant application to work on time travel it would be dismissed immediately,’ writes the physicist Stephen Hawking in his posthumous book Brief Answers to the Big Questions.
He was right. But he was also right that asking whether time travel is possible is a ‘very serious question’ that can still be approached scientifically.
Arguing that our current understanding cannot rule it out, Hawking, it seems, was cautiously optimistic.
So where does this leave us? We cannot build a time machine today, but could we in the future?
Scroll down for video
The late theoretical physicists Stephen Hawking was ‘cautiously optimistic’ on the subject of time travel, the researcher explains
Let’s start with our everyday experience. We take for granted the ability to call our friends and family wherever they are in the world to find out what they are up to right now. But this is something we can never actually know.
The signals carrying their voices and images travel incomprehensibly fast, but it still takes a finite time for those signals to reach us.
Our inability to access the ‘now’ of someone far away is at the heart of Albert Einstein’s theories of space and time.
Einstein told us that space and time are parts of one thing – spacetime – and that we should be as willing to think about distances in time as we are distances in space.
As odd as this might sound, we happily answer ‘about two and half hours’, when someone asks how far Birmingham is from London.
What we mean is that the journey takes that long at an average speed of 50 miles per hour.
Mathematically, our statement is equivalent to saying that Birmingham is about 125 miles from London.
As physicists Brian Cox and Jeff Forshaw write in their book Why does E=mc²?, time and distance ‘can be interchanged using something that has the currency of a speed’.
Einstein’s intellectual leap was to suppose that the exchange rate from a time to a distance in spacetime is universal – and it is the speed of light.
The speed of light is the fastest any signal can travel, putting a fundamental limit on how soon we can know what is going on elsewhere in the universe.
This gives us ‘causality’ – the law that effects must always come after their causes. It is a serious theoretical thorn in the side of time-travelling protagonists.
For me to travel back in time and set in motion events that prevent my birth is to put the effect (me) before the cause (my birth).
Now, if the speed of light is universal, we must measure it to be the same – 299,792,458 metres per second in vacuum – however fast we ourselves are moving.
Einstein realised that the consequence of the speed of light being absolute is that space and time itself cannot be.
And it turns out that moving clocks must tick slower than stationary ones.
Einstein also told us that the force of gravity is a consequence of the way mass warps space and time. The more mass we squeeze into a region of space, the more spacetime is warped and the slower nearby clocks tick. File photo
The faster you move, the slower your clock ticks relative to ones you are moving past. The word ‘relative’ is key: time will seem to pass normally to you. To everyone standing still, however, you will be in slow motion.
If you were to move at the speed of light, you would appear frozen in time – as far as you were concerned, everyone else would be in fast forward.
So what if we were to travel faster than light, would time run backwards as science fiction has taught us?
Unfortunately, it takes infinite energy to accelerate a human being to the speed of light, let alone beyond it. But even if we could, time wouldn’t simply run backwards. Instead, it would no longer make sense to talk about forward and backward at all.
The law of causality would be violated and the concept of cause and effect would lose its meaning.
Einstein also told us that the force of gravity is a consequence of the way mass warps space and time. The more mass we squeeze into a region of space, the more spacetime is warped and the slower nearby clocks tick.
If we squeeze in enough mass, spacetime becomes so warped that even light cannot escape its gravitational pull and a black hole is formed.
And if you were to approach the edge of the black hole – its event horizon – your clock would tick infinitely slowly relative to those far away from it.
So could we warp spacetime in just the right way to close it back on itself and travel back in time?
The answer is maybe, and the warping we need is a traversable wormhole. But we also need to produce regions of negative energy density to stabilise it, and the classical physics of the 19th century prevents this.
FROM THE BIG BANG TO BLACK HOLES: HOW STEPHEN HAWKING HELPED EXPLAIN THE UNIVERSE’S BIGGEST MYSTERIES
Stephen Hawking probed the very limits of human understanding both in the vastness of space and in the bizarre sub-molecular world of quantum theory.
As well as numerous best-selling books, Hawking also published several important scientific papers during an illustrious research career.
Through his groundbreaking theories, the legendary physicist examined the origins of the universe and helped explain the behaviour of black holes.
Stephen Hawking, who sought to explain some of the most complicated questions of life while working under the shadow of a likely premature death, has died at 76
1970 Space-time in black holes
One of Hawking’s first key ideas was how space and time react within the brutal confines of black holes.
Black holes are regions of space with a gravitational field so intense that no matter or radiation can escape.
The objects are so powerful they bend time and space in bizarre ways, and in 1970 Hawking showed how black holes alter ‘space-time’.
‘Space-time’ is a theory used by physicists to describe the fusion of 3D space and time into a four-dimensional continuum.
Up until the ’70s physicists had known Einstein’s theory allowed for ‘singularities’ – points where space-time appeared to be infinitely curved.
But it was unclear whether or not these singularities actually existed.
Birkbeck College physicist Sir Roger Penrose showed that singularities do exist as they can form in black holes.
Alongside Sir Penrose, Hawking applied the same idea to the universe in its entirety in 1970.
They showed that Einstein’s theory predicted a singularity in our distant past: The Big Bang.
1971-72 Black hole mechanics
Black holes are regions of space with a gravitational field so intense that no matter or radiation can escape.
Their field is so intense that they form their own set of physical laws unlike anything else in the universe.
Hawking devised the second law of black holes, which states that the total surface area of a black hole will never get smaller.
In separate work, Hawking sparked the ‘no hair’ theorem of black holes.
This states that black holes can be characterised by three numbers – their mass, charge and angular momentum.
The ‘hair’ in Hawking’s idea is other information that disappears when it falls into the black hole.
1974-75 How black holes vanish
Hawking showed that black holes emit heat and eventually vanish in an extremely slow process.
While a black hole with the mass of the sun would take longer than the age of our universe to evaporate, smaller ones disappear faster.
Near the end of their lives they release heat at a dramatic rate, with an average-sized black hole releasing the energy of a million hydrogen bombs in just a tenth of a second.
Hawking’s drew on ‘quantum theory’ for the finding – the branch of physics concerned with how the universe works at the subatomic level.
Through his groundbreaking theories, the legendary physicist helped explain the behaviour of black holes (artist’s impression) and examined the origins of the universe
1982 How galaxies arise
Many physicists believe the universe inflated rapidly shortly after the Big Bang.
Hawking was one of the first to show how galaxies may have formed during this explosion of time and space.
He found that quantum fluctuations – tiny variations in the distribution of matter – grew into the galaxies that dot the cosmos today.
This is because strong gravitational forces made matter clump together.
Hawking’s theory is supported by recent observations of the faint afterglow of the Big Bang, which spotted the sort of variations Hawking worked with.
1983 How the universe began
Hawking is best known for his attempts to combine two key theories of physics: Quantum theory and Einstein’s general relativity.
While quantum theory covers how tiny subatomic particles stitch together the fabric of our universe, general relativity deals with larger objects.
It describes how galaxies, stars, black holes, planets and more interact with one another via gravitational forces.
Much of Hawking;s work focussed on combining quantum theory and general relativity with Einstein’s theory of gravity.
He suggested that this new theory, known as quantum gravity, could fill in many of the gaps of our current understanding of physics and the universe.
In 1983 the physicist partnered with Chicago University’s Professor Jim Hartle to propose a ‘wave function of the universe’.
Known as the Hartle-Hawking state, this notion is meant to figure out how the universe began through quantum mechanics.
In theory, this could be used to understand the properties of the universe around us.
1988 A brief history of time
Hawking’s bestselling book A Brief History of Time has sold more than ten million copies since it was published in 1988.
The book, which described the structure, origin, development and eventual fate of the universe, was a surprise success for the relatively unknown physicist, staying in the Sunday Times bestseller list for 237 weeks.
Hawking wrote the book for readers with no knowledge of any scientific theories.
The physicist joked himself that many who owned the book struggled to understand its complexity and never finished it.
The book ultimately propelled Hawking to stardom, with the physicist publishing or co-publishing 15 books in total and writing or starring in multiple scientific documentaries, television shows, films and more.
What happened before the Big Bang?
At the time of the Big Bang 13.8 billion years ago, all matter in the universe erupted from a singularity to create the cosmos.
But scientists are unsure what happened before then.
In a recent TV interview, Hawking said ‘nothing was around before the Big Bang’.
Instead, time and space existed in a ‘bent state’ that was distorted along another dimension.
The physicist believes the Big Bang was the formation of what we now regard as time because the event broke down the laws of physics.
This means that anything that preceded it cannot be applied to our understanding of time and existence.
By Harry Pettit, science and technology reporter
The modern theory of quantum mechanics, however, might not. According to quantum mechanics, empty space is not empty. Instead, it is filled with pairs of particles that pop in and out of existence.
If we can make a region where fewer pairs are allowed to pop in and out than everywhere else, then this region will have negative energy density.
However, finding a consistent theory that combines quantum mechanics with Einstein’s theory of gravity remains one of the biggest challenges in theoretical physics.
One candidate, string theory (more precisely M-theory) may offer up another possibility.
M-theory requires spacetime to have 11 dimensions: the one of time and three of space that we move in and seven more, curled up invisibly small. Could we use these extra spatial dimensions to shortcut space and time? Hawking, at least, was hopeful.
So is time travel really a possibility? Our current understanding can’t rule it out, but the answer is probably no.
Einstein’s theories fail to describe the structure of spacetime at incredibly small scales.
And while the laws of nature can often be completely at odds with our everyday experience, they are always self-consistent – leaving little room for the paradoxes that abound when we mess with cause and effect in science fiction’s take on time travel.
‘If one made a research grant application to work on time travel it would be dismissed immediately,’ writes Hawking in his book. But he was also right that asking whether time travel is possible is a ‘very serious question’ that can be approached scientifically
WHAT DOES PROFESSOR STEPHEN HAWKING THINK HAPPENED BEFORE THE BIG BANG?
Professor Stephen Hawking believed that before the Big Bang 3.7 billion years ago, time and space as we know it did not exist.
In the past, the esteemed theoretical physicist predicted that as the universe goes back in time, it shrinks and closes off like a sphere.
His latest paper, however – submitted just weeks before his death – adds new constraints to the history of the universe that challenge his previous theories.
According to earlier ‘no boundaries’ theory, the universe was shrunk and condensed to an incredibly dense ball of heat and energy the size of a single atom.
Inside this speck, the laws of physics and time as we know them cease to function, and time as we understand it did not exist.
If we move back in time from the Big Bang, the ‘arrow’ of time shrinks infinitely as the universe becomes smaller, never reaching a clear starting point.
Professor Stephen Hawking believes that before the Big Bang 3.7 billion years ago, time and space as we know it did not exist
Hawking said in a recent interview that before the Big Bang, time was bent – ‘It was always reaching closer to nothing but didn’t become nothing.’
Essentially, ‘there was never a Big Bang that produced something from nothing. It just seemed that way from mankind’s point of perspective.’
In a lecture on the so-called no-boundary proposal, Hawking wrote: ‘Events before the Big Bang are simply not defined, because there’s no way one could measure what happened at them.
Since events before the Big Bang have no observational consequences, one may as well cut them out of the theory, and say that time began at the Big Bang.’
But, his new work, published posthumously, challenges this earlier ‘no boundary theory.’
Borrowing points from the string theory – the concept that the universe is a complex hologram – Hawking and colleague Thomas Hertog argue that our universe and other ‘pocket universes’ are not the infinite structures some have proposed.
Instead, Hawking says the universe is ‘reasonably smooth and globally finite,’ setting new boundaries in cosmological history that could ultimately allow the theory to be tested.
‘Now we’re saying that there is a boundary in our past,’ Hertog added.
Despite his playful optimism, Hawking recognised that the undiscovered laws of physics that will one day supersede Einstein’s may conspire to prevent large objects like you and I from hopping casually (not causally) back and forth through time.
We call this legacy his ‘chronology protection conjecture’.
Whether or not the future has time machines in store, we can comfort ourselves with the knowledge that when we climb a mountain or speed along in our cars, we change how time ticks.
So, this ‘pretend to be a time traveller day’ (December 8), remember that you already are, just not in the way you might hope.