"If someone were to apply for a research grant to work on time travel, it would immediately be rejected," writes the physicist Stephen Hawking in his posthumous book, Brief Answers to the Big Questions. He was right. But he was also right that questions or time travel is possible, a very serious question & # 39; is that can still be approached scientifically.

Under the assumption that our current understanding can not exclude it, Hawking seemed cautiously optimistic. Where is this going? We can not build a time machine today, but could we do that in the future?

Let's start with our daily experience. We take it for granted that we can call our friends and family wherever they are in the world to find out what they are planning *now*. But this is something we can never really know. The signals that carry their voices and images travel incomprehensibly quickly, but it still takes a finite time before these signals reach us.

Our inability to access the & # 39; nu & # 39; someone far away is central to Albert Einstein's theories about space and time.

## Speed of light

Einstein told us that space and time are parts of one thing – space-time – and that we should be equally prepared to think about distances in time when we are distances in space. Strange as this may sound, we like to answer "about two and a half hours" when someone asks how far Birmingham is coming from London. What we mean is that the journey takes so long with an average speed of 50 miles per hour.

Mathematically, our statement is equivalent to the fact that Birmingham is about 125 miles from London. As physicists Brian Cox and Jeff Forshaw write in their book. Why can E = mc² ?, time and distance "be exchanged with something that has the currency of a speed". Einstein's intellectual leap was to assume that the exchange rate of a time to a distance in space time is universal – and it is the speed of light.

The speed of light is the fastest signal that every signal can travel, setting a fundamental limit to how quickly we can know what is happening 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 midst of time-traveling protagonists. To go back in time and initiate events that prevent my birth, the effect (I) is to be placed before the cause (my birth).

Now, if the speed of light is universal, we have to measure it to be the same – 299,792,458 meters per second in vacuum – no matter how fast we move ourselves. Einstein realized that the consequence that the speed of light is absolute, space and time itself can not be. And it turns out that moving clocks have to hit slower than stationary.

The faster you move, the slower your clock taps than the one you pass. The word & # 39; relative & # 39; is central: time seems to pass you normally. For everyone who stands still, you will be in slow motion. If you were to move with the speed of light, you would seem frozen in time – as far as you know, everyone else would be fast ahead.

So what if we would travel *faster* then light, would time lapse as science fiction has taught us?

Unfortunately, it takes infinite energy to accelerate a person to the speed of light, let alone behind it. But even if that were to happen, time would not simply deteriorate. Instead, it would not be logical at all to talk about back and forth in any way. The law of causality would be violated and the concept of cause and effect would lose its meaning.

## wormholes

Einstein also told us that gravity is a consequence of the way in which mass space and time is distorted. The more we squeeze into an area of space, the more spacetime shifts and the slower clocks in the neighborhood tick. If we squeeze enough mass, the spacetime is so distorted that even the light can not escape its gravity and a black hole is created. And if you approached the edge of the black hole-the event horizon-your clock would be ticking infinitely slower than those far away.

Can we turn space time exactly the right way to close it back on its own and travel back in time?

The answer is perhaps, and the warping we need is a traversable wormhole. But we also have to produce regions with a negative energy density to stabilize it, and classical physics of the 19th century prevents this. However, the modern theory of quantum mechanics would not be possible.

According to quantum mechanics, empty space is not empty. Instead, it is filled with pairs of particles that die in and die out. If we can make a region where fewer couples can board and alight than anywhere else, this region will have a negative energy density.

However, finding a consistent theory that combines quantum mechanics with Einstein's gravity theory remains one of the greatest challenges in theoretical physics. One candidate, string theory (more specifically M-theory) can offer another possibility.

M-theory requires space-time to have 11 dimensions: that of time and three of the space in which we move and another seven, rolled up invisibly small. Can we use these extra spatial dimensions to shorten space and time? Hawking was at least hopeful.

## Save history

So is time travel really a possibility? Our current understanding can not exclude it, but the answer is probably no.

Einstein's theories do not describe the structure of spacetime on incredibly small scales. And although the laws of nature are often completely at odds with our daily experience, they are *always* self-consistent – leaves little room for the paradoxes that are abundant when we mess with cause and effect in the time travels of science fiction.

Despite his playful optimism, Hawking recognized that the undiscovered laws of nature that Einstein will ever replace are conspiring to prevent large objects such as you and me from jumping back and forth nonchalantly (through time). We call this legacy his "chronology protection presumption".

Whether the future also has time machines in the store, we can comfort ourselves with the knowledge that when we climb a mountain or make speed in our cars, we change how time ticks.

So, this "do if you are a time traveling day" (December 8), remember that you are already, just not as you would expect.