Journey Through Time
"What is time? Does it, like the ancient chant,
wash away all our dreams in its flowing waves?"
“时间为何物？ 它是否像古老赞歌说的那样，
把我们所有的梦想一卷而空的东流逝波？”
——《果壳中的宇宙》
Part 1: C.P.T Aliens
第一章: 外星人
If you encounter an extraterrestrial being that looks very much like you,
and you want to extend your hand for a handshake,
please refrain from doing so!
如果你遇到一个外星人，
跟你长得很像，
你想伸出手的时候他也是，
那你千万别去握他的手！
Not because it might carry radiation,
but because it could be entirely composed of antimatter!
You both might vanish in a terrifying instant.
(Shh, this is not a horror sci-fi story, it's a popular story by Hawking.)
不是因为它可能携带着辐射，
而是他可能完全是反物质构成的！
你们可能在可怕的一瞬闪光中化为虚无。
（嘘，这不是恐怖科幻，这是霍金讲的通俗小故事）
"Physics does not distinguish between the past and the future under C.P.T."
What is C.P.T?
C stands for antiparticle.
P stands for taking a mirror image.
T stands for reversing the direction of all particle motions
- in other words, reversal.
“物理学对时间的过去和未来不加区分。”
即在C.P.T下，物理定律保持不变。
什么是C.P.T？
C指反粒子。
P指取镜像。
T指所有粒子运动反向——也就是倒退。
So, "for inhabitants on another planet,
which is our mirror image and composed of antimatter,
social activities would be the same as ours."
所以， “对于是我们的镜像且由反物质构成的
另一行星上的居民来说，
社交活动刚好是和我们相同的。”
It may sound a bit mind-boggling, but in simple terms:
There could be another version of you made of antimatter and a complete mirror image,
just like the principles of -1+1=0,
and when you both combine, it results in "nothing."
听起来可能有点烧脑，
说白了就是：
可能有一个一切物质相反又镜像的你，
就像-1+1=0的一负一正道理相同，
你们合到一起也是“无”。
Part 2: 2D People and 3D People
第二章：二维小人和三维人
Hawking said:
If you encounter a two-dimensional creature,
it definitely doesn't have a passage that goes through its body.
Because in that case, it would be split into two pieces...
霍金说：如果你遇到一只二维生物，
那它肯定没有一条贯穿身体的通道。
因为那样它就被分成两块了·······
In general relativity,
objects always move along straight lines in four-dimensional spacetime,
but to us, they appear to follow curved paths in three-dimensional space.
For a specific example:
The massive mass of the sun bends spacetime,
causing the Earth to move along a straight line in four-dimensional spacetime,
but to us, it appears to move in a circular orbit in three-dimensional space.
在广义相对论中，
物体总是沿着四维时空的直线走，
但是在我们看来它在三维空间里是沿着弯曲的路径。
举个具体栗子：
太阳的巨大质量弯曲了时空，
于是使得地球沿着四维时空中的直线运动，
但是在我们看来，它是在三维空间中做圆周运动。
"We cannot see four-dimensional time,
just like a two-dimensional creature with only length and width cannot perceive height."
“朝菌不知晦朔，蟪蛄不知春秋。”
我们看不见四维时间，
就像只有长宽的二维的生物不知道什么是高度。
Relativity revolutionized our understanding of space and time.
As an object's velocity increases, its mass also increases,
and it requires more and more energy to accelerate further.
相对论变革了我们对空间和时间的的观念。
当一个物体运动速度越来越快，
它的质量也会越来越大，
这样它需要越来越多的能量才能进一步加速。
In reality, objects cannot reach the speed of light because according to E=mc²,
as the speed approaches the speed of light,
the mass becomes infinitely large, requiring an infinite amount of energy.
It is precisely for this reason that relativity limits the speed of objects to be below the speed of light.
General relativity not only predicts that light will be bent by gravity fields
but also predicts that time will pass more slowly near massive objects.
This phenomenon is known as the "time dilation effect" in relativity.
实际上物体运动没法达到光速，
因为根据E=mc2，
质量变成无限大，就需要无限大的能量。
正是这个原因，相对论限定了物体低于光速运动。
广义相对论除了预言了光线必被引力场弯折，
也预言了大质量物体旁边，时间流逝会更慢一些。
“钟慢效应”就是指相对论中的时间膨胀效应。
Let's take another example:
someone living by the coast will age slightly slower than someone living on a mountaintop
(because they are closer to the Earth).
The difference in the strength of the gravitational field leads to a more noticeable time dilation effect.
再举个例子，
在沿海生活的我会比在山顶生活的我老得慢一点点······（因为更靠近地球）
引力场强度相差越大，钟慢越明显。
Part 3: Light Cone
第三章：光锥
"In an instant,
scattered light forms a three-dimensional cone in four-dimensional spacetime,
which is called the 'future light cone' of an event.
The universe we see now is the light from distant galaxies,
we see the past, but we don't know how they look now,
we are inside the 'past light cones' of those stars."
一瞬间散开的光，
会在四维的时空里形成一个三维的圆锥，
这个圆锥称为事件的“将来光锥”。
而我们现在看到的宇宙，都是遥远星系来的光，
我们看到过去，而不知道他们现在的样子，
我们就在那些星星的“过去光锥”里。
Part 4: Arrow of Time
第四章：时间箭头
What is the arrow of time?
Irreversibility and increasing entropy are examples of the arrow of time.
"The arrow of time distinguishes the past from the future, giving time a direction."
什么是时间箭头？
无序度和熵增就是时间箭头的一个例子。
“时间箭头将过去和将来区别开来，使时间有了方向。”
There are three types of time arrows:
Thermodynamic arrow:
for example, a broken mirror is hard to mend.
Psychological time arrow:
it's the feeling of time passing.
"In this direction, we can only remember the past, not the future."
Cosmological time arrow:
the universe expands in this direction, not contracts.
时间箭头有三种：
1热力学箭头。比如破镜难圆。
2心理学时间箭头：就是时间流逝之感。
“在这个方向上我们只能回忆过去而不是未来。”
3宇宙时间箭头：宇宙在这个方向上膨胀而不是塌缩。
Why do these three arrows coincidentally coincide?
Hawking says it's because intelligent life can only exist in an expanding universe.
For instance, when humans consume ordered energy in the form of food,
the energy transformed will always be in a disordered state.
为什么三个箭头很巧合地重合？
霍金说是因为智慧生命只能在宇宙膨胀中才能生存。
比如人消耗能量有序时的形式——食物，
转化出来的都是无序能量。
Part 5: Imaginary Time
第五章：虚时间
Why do we propose "imaginary time" when we already have "real time"?
Our perception of time is a horizontal line from left to right,
while imaginary time refers to time perpendicular to the one we feel is flowing.
Real time forces us to trace back to infinity or assume the universe began at a singularity.
But in imaginary time,
there is no creation or end, it merely exists.
Within the vortex of reality and imagination, who knows what is true?
明明有“实时间”了为什么还要提出“虚时间”？
我们认知的时间线是从左到右的水平线，
而虚时间，
是指和我们感觉到正在流逝的时间成直角的时间。
实时间让我们不得不回溯至无穷，
或者认定宇宙在奇点处开端。
但是在虚时间里，
没有创生，也没有结束，它仅仅是存在。
在虚实的漩涡里，谁又知道何为真实？
