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"If someone was applying for a research grant to work on a trip back in time, he would be immediately fired," wrote physicist Stephen Hawking in his posthumous book Brief Answers to the Big Questions. He was right. But he was also right in saying that asking if time travel is possible is a "very serious matter" that can still be addressed scientifically.
Asserting that our current understanding could not rule it out, Hawking, it seemed, was cautiously optimistic. So where does this leave us? We can not build a time machine today, but could we in the future?
<p class = "canvas-atom-canvas-text Mb (1.0em) Mb (0) – sm Mt (0.8em) – sm" type = "text" content = "Let's start with our everyday experience. Call our friends and family wherever they are in the world to find out what they are doing now. But it's something we can never know. The signals that carry their voices and their images travel at an incomprehensible speed, but it always takes a finite time for them to reach us. "Data-reactid =" 24 "> 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're up to. currently doing now. But it's something we can never know. The signals that carry their voices and images travel at an incomprehensible speed, but it takes a while longer for these signals to reach us.
Our inability to access the "now" of someone from far away is at the heart of Albert Einstein's theories about space and time.
Speed of light
Einstein told us that space and time are part of one thing – space-time – and that we should be as willing to think about distances in time as we are distances in l & # 39; space. Strange as it may seem, we enthusiastically respond to "about two and a half hours" when someone asks how far is Birmingham from London. What we mean is that the journey takes so much time at an average speed of 50 miles to the hour.
Mathematically, our statement is equivalent to saying that Birmingham is about 125 km from London. As physicists Brian Cox and Jeff Forshaw write in their book Why E = mc² ?, time and distance "can be traded using something that has the motto of a speed". Einstein's intellectual leap was to assume that the exchange rate of a time at a distance in space-time is universal – and that is the speed of light.
<p class = "canvas-atom canvas-text Mb (1.0em) Mb (0) – sm Mt (0.8em) – sm" type = "text" content = "The speed of light is the fastest speed traveling , setting a fundamental limit to how quickly we can know what is happening elsewhere in the universe, which gives us "causality" – the law that produces the effects must always come after their causes. This is a serious theoretical thorn alongside the protagonists who travel back in time. For me, traveling in time and setting in motion events that prevent my birth is to place the effect (me) before the cause (my birth). "Data-reactid =" 29 "> The speed of light is the fastest signal that can travel, setting a fundamental limit to how quickly we can know what's happening elsewhere in the universe , which gives us "causality" – the law that produces the effects must always come after their causes. This is a serious theoretical thorn alongside the protagonists who travel back in time. For me, traveling in time and setting 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 so that it is the same – 299 792 458 meters per second in the vacuum – whatever our speed of movement. Einstein realized that the speed of light being absolute, time and space can not be in itself. And it turns out that moving clocks must be slower than fixed clocks.
The faster you move, the slower your clock is compared to the one you have passed. The word "relative" is the key: the time will seem to pass normally for you. For all those who remain motionless, you will be in slow motion. If you moved at the speed of light, you would seem frozen in time – as far as you're concerned, all the others would be fast forward.
<p class = "canvas-atom canvas-text Mb (1.0em) Mb (0) – sm Mt (0.8em) – sm" type = "text" content = "And if we traveled faster as light, would time go backwards as we learned science fiction? "data-reactid =" 32 "> And if we traveled faster as light, would time go backwards as we learned science fiction?
Unfortunately, it takes infinite energy to accelerate a human being at the speed of light, not to mention its passing. But even if we could, the time would not be just back. Instead, it would not make sense to talk at all back and forth. The right of causality would be violated and the concept of cause and effect would lose its meaning.
Wormholes
Einstein also told us that the force of gravity is a consequence of how the mass distorts space and time. The larger the mass is in a region of space, the more the space-time is distorted and the closer the clocks slow ticking. If we compress enough mass, the space-time becomes so distorted that even the light can not escape its gravitational appeal and a black hole is formed. And if you approach the edge of the black hole – its horizon of events – your clock will turn infinitely slow compared to those who are far away.
Can we reduce the space-time in the right way to close it on itself and go back in time?
The answer may be, and the warping we need is a crossworm wormhole. But we must also produce regions of negative energy density to stabilize it, something that nineteenth-century classical physics prevents. The modern theory of quantum mechanics, however, might not be so.
According to quantum mechanics, the empty space is not empty. Instead, it is filled with pairs of particles that appear and disappear. If we can create an area in which fewer pairs are allowed in and out than anywhere else, then this region will have a negative energy density.
However, finding a coherent theory combining quantum mechanics with Einstein's theory of gravity remains one of the greatest challenges in theoretical physics. One candidate, string theory (more specifically the M theory) may offer another possibility.
The M theory requires that space-time has 11 dimensions: that of time and that of the space in which we move and seven others, curled up in an invisible way. Could we use these extra spatial dimensions to shorten space and time? Hawking, at least, was hopeful.
Save the story
So, is the trip in time really a possibility? Our current understanding can not exclude it, but the answer is probably no.
<p class = "canvas-atom-canvas-text Mb (1.0em) Mb (0) – sm Mt (0.8em) – sm" type = "text" content = "Einstein's theories do not describe the structure from space-time to incredibly small scales.And although the laws of nature can often be completely at odds with our daily experience, they are always coherent – leaving little room for the paradoxes that abound when one tackles the causes and effects of science fiction on the journey in time. "data-reactid =" 56 "> Einstein's theories do not describe the structure of space-time on an incredibly small scale, and even though the laws of nature can often be completely at odds with our daily experience they are always consistent – leaving little room for the paradoxes that abound when one meddles with cause and effect in science fiction on time travel.
In spite of his cheerful optimism, Hawking recognized that the undiscovered laws of physics that will one day supplant Einstein can conspire to prevent bulky objects like you and me from advancing through time. We call this inheritance its "conjecture of protection of chronology".
Whether or not the future holds us time machines, we can comfort ourselves knowing that when we climb a mountain or accelerate in our cars, we change the direction of time.
So, "pretend to be a time traveler" (December 8), remember that you already are, but not in the way you could hope.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
<p class = "canvas-atom canvas-text Mb (1.0em) Mb (0) – sm Mt (0.8em) – sm" type = "text" content = "Peter Millington receives funding from Leverhulme Trust. "data-reactid =" 72 ">Peter Millington receives funding from Leverhulme Trust.
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