Travel to the past is probably impossible. But to the future? That’s a different story. Cathal O'Connell considers the feasibility of physics.

In 2009 the British physicist Stephen Hawking held a party for time travellers – the twist was he sent out the invites a year later.

旅行到过去可能是不可能的。但对未来？这是一个不同的故事。 Cathal O'Connell认为物理学的可行性。

在2009年，英国物理学家霍金（Stephen Hawking）为时间旅行者举办了一场聚会，一年后他转交了邀请函。

Travel into the past is probably impossible. Even if it were possible, Hawking and others have argued that you could never travel back before the moment your time machine was built.

过去旅行可能是不可能的。即使有可能，霍金和其他人也认为，在你的时间机器建成之前，你永远不会回头。

Of course, we are all time travellers as we are swept along in the current of time, from past to future, at a rate of one hour per hour.But, as with a river, the current flows at different speeds in different places. Science as we know it allows for several methods to take the fast-track into the future. Here’s a rundown.

当然，我们是所有的时间旅行者，因为我们在当前的时间内，从过去到未来，以每小时1小时的速度扫荡。但是，像一条河一样，目前在不同的地方以不同的速度流动。我们知道的科学可以通过几种方法来实现快速进入未来。这是一个漏洞。

Speed / 速度

This is the easiest and most practical way to get to the far future – go really fast.

According to Einstein’s theory of special relativity, when you travel at speeds approaching the speed of light, time slows down for you relative to the outside world.

这是实现远景的最简单和最实用的方式 - 走得真快。

根据爱因斯坦的狭义相对论，当你以接近光速的速度旅行时，相对于外界而言，时间会减缓

In the case of the aircraft, the effect is minuscule. But If you were in a spaceship travelling at 90% of the speed of light, you’d experience time passing about 2.6 times slower than it was back on Earth.

在飞机的情况下，效果是微不足道的。但是，如果你是在90％的光速下飞行的宇宙飞船，那么你会遇到比地球上慢了2.6倍的时间。

The highest speeds achieved through any human technology are probably the protons whizzing around the Large Hadron Collider at 99.9999991% of the speed of light. Using special relativity we can calculate one second for the proton is equivalent to 27,777,778 seconds, or about 11 months, for us.

通过任何人类技术实现的最高速度可能是大型强子对撞机周围的质子以99.9999991％的速度闪烁。使用狭义相对论我们可以计算出一秒的质子相当于27,777,778秒，或约11个月。

Gravity / 重力

The next method is also inspired by Einstein. According to his theory of general relativity, the stronger the gravity you feel, the slower time moves.As you get closer to the centre of the Earth, for example, the strength of gravity increases. Time runs slower for your feet than your head.

下一个方法也受到爱因斯坦的启发。根据他的广义相对论，你感受的重力越强，时间越慢。当你越来越接近地球的中心时，重力就会增加。时间比你的头慢你的脚。

Again, this effect has been measured. In 2010, physicists at the US National Institute of Standards and Technology (NIST) placed two atomic clocks on shelves, one 33 centimetres above the other, and measured the difference in their rate of ticking. The lower one ticked slower because it feels a slightly stronger gravity.

同样，这个效果已经被测量了。 2010年，美国国家标准与技术研究所（NIST）的物理学家在架子上放置了两个原子钟，比另一个高出33厘米，并测量了其滴答率的差异。较低的人因为感觉到更强的重力而慢起来。

To travel to the far future, all we need is a region of extremely strong gravity, such as a black hole. The closer you get to the event horizon, the slower time moves – but it’s risky business, cross the boundary and you can never escape.Assuming you had the technology to travel the vast distances to reach a black hole (the nearest is about 3,000 light years away), the time dilation through travelling would be far greater than any time dilation through orbiting the black hole itself.

为了旅行到未来，我们所需要的是一个非常强烈的重力的地区，如黑洞。越接近事件的距离越远，时间越慢，但它的风险越大，越过边界，你永远不会逃脱。假设你有这种技术可以漫长的距离到达一个黑洞（最近的是3000个光线几年前），通过旅行的时间扩张将远远大于通过绕黑洞本身的任何时间扩张。

Suspended animation / 停播动画

Another way to travel to the future may be to slow your perception of time by slowing down, or stopping, your bodily processes and then restarting them later.
Bacterial spores can live for millions of years in a state of suspended animation, until the right conditions of temperature, moisture, food kick start their metabolisms again. Some mammals, such as bears and squirrels, can slow down their metabolism during hibernation, dramatically reducing their cells’ requirement for food and oxygen.

旅行到未来的另一种方式可能是通过减慢或停止您的身体过程，然后稍后重新启动来减慢您的时间感。
细菌孢子可以在暂停动画的状态下生活数百万年，直到温度，水分，食物的正确条件再次启动其代谢。一些哺乳动物，如熊和松鼠，可以在冬眠期间减缓其新陈代谢，大大降低了细胞对食物和氧气的需求。

Wormholes / 虫洞

General relativity also allows for the possibility for shortcuts through spacetime, known as wormholes, which might be able to bridge distances of a billion light years or more, or different points in time.

Many physicists, including Stephen Hawking, believe wormholes are constantly popping in and out of existence at the quantum scale, far smaller than atoms. The trick would be to capture one, and inflate it to human scales - a feat that would require a huge amount of energy, but which might just be possible, in theory.

 https://youtu.be/SLUzJeto0Wo

广义相对论也允许通过时空的快捷方式（称为虫洞），这可能能够跨越十亿光年或更长时间或不同时间点的距离。

许多物理学家，包括霍金霍金斯认为，蠕虫洞不断地出现在量子尺度上，远远小于原子。诀窍将是抓住一个，并将其膨胀到人类的尺度 - 一个需要大量能量，但在理论上可能是可能的壮举。

Using light / 使用光

Another idea, put forward by the American physicist Ron Mallet, is to use a rotating cylinder of light to twist spacetime. Anything dropped inside the swirling cylinder could theoretically be dragged around in space and in time, in a similar way to how a bubble runs around on top your coffee after you swirl it with a spoon.

美国物理学家罗恩·马利特（Ron Mallet）提出的另一个想法是使用旋转圆筒来扭转时空。理论上可以在空间和时间上拖曳旋转圆筒内的任何东西，就像在用勺子旋转咖啡后，泡沫如何在咖啡顶上流动一样。

According to Mallet, the right geometry could lead to time travel into either the past and the future.Since publishing his theory in 2000, Mallet has been trying to raise the funds to pay for a proof of concept experiment, which involves dropping neutrons through a circular arrangement of spinning lasers.

据Mallet说，正确的几何可能导致时间旅行到过去和未来。自2000年发布他的理论以来，Mallet一直在试图筹集资金来支付概念验证实验，其中包括通过一个旋转激光器的圆形排列。

Next post will be regarding to why time travel will be in only one direction why not both forward and backward ?

下一篇文章将是为什么时间旅行只会在一个方向为什么不向前和向后？