⚛️ Everything that ever happens

Every push, every pull, every fall, every spark, every star — all of it is four forces. Not four hundred. Four. Nothing else has ever been found. 每一次推、每一次拉、每一次坠落、每一颗星星——全都来自四种力。不是四百种，就是四种。从来没有发现过第五种。

A force is not a thing. It is what happens when two particles exchange a third particle between them. The forces differ by what they can touch, how far they reach, and how hard they pull.
“力”不是一个东西。它是两个粒子之间交换第三种粒子时发生的事。四种力的区别在于：能碰到什么、能伸多远、拉得多用力。

🔬 How the four forces work

Seven machines to play with. Touch everything, and say what you see. 七台机器，随便玩。看到什么就说出来。

Click a bar. The scale is logarithmic — each step down the ladder is ten times weaker, and there are thirty-eight steps from top to bottom.
点一条。这是对数刻度——每往下一格弱十倍，从最强到最弱一共三十八格。

Its job

Say it

🤔 If gravity is the weakest, why does it run the universe? because it never cancels out

This is the best question in physics, and the answer is cancellation.

The strong force is a 10³⁸ times stronger than gravity — but it stops dead about one proton-width away, so it cannot reach out of the nucleus. Electromagnetism does reach forever, and it is 10³⁶ times stronger than gravity — but it comes in two flavours, + and −, and they cancel. You are made of an almost perfectly equal number of protons and electrons, so from a metre away your enormous electric charge adds up to nothing.

Gravity has no negative. Mass only ever attracts mass. Nothing cancels, so it just adds up — and once you pile up an Earth, or a Sun, or a galaxy, the weakest force in nature is the only one still standing at that distance. Gravity wins the long game by never losing anything.

这是物理里最好的问题，答案是抵消。强核力比引力强 10³⁸ 倍，但一个质子宽就到头了，出不了原子核。电磁力确实能伸到无限远，也比引力强 10³⁶ 倍——但它有正负两种，会互相抵消。你身上的质子和电子几乎一样多，所以隔一米看，你巨大的电荷加起来等于零。引力没有负的，质量只会吸引质量，什么都不会抵消，只会不断累加。堆出一个地球、一颗太阳、一个星系之后，在那种距离上还站着的，就只剩最弱的那个力。引力赢在从不失去任何东西。

🌍 Gravity is the one you can feel right now. It is pulling you into your chair. It is also the only force that is always attractive — there is no such thing as anti-gravity, because there is no negative mass.
引力是你现在就能感觉到的那个力，正把你压在椅子上。它也是唯一永远吸引的力——没有反引力，因为没有负质量。

Sideways speed of the planet —

Orbit is falling and missing

What you are looking at

This is a real simulation, not an animation on a fixed track. The pull is computed every frame from F = G·m₁·m₂ / r² — the actual law.

Move the slider and you get all three outcomes: too slow and the planet falls in; too fast and it escapes forever; in between it orbits — falling towards the star, and missing, forever.

这是真的模拟，不是固定轨道的动画。每一帧都用真实的公式 F = G·m₁·m₂ / r² 算出拉力。拖动滑块可以得到三种结果：太慢就掉进去，太快就逃走，中间那档就绕圈——一直朝着恒星掉，一直掉不到。

📉 The inverse square law

Distance —

Double the distance and the force does not halve — it drops to a quarter. Triple it and you get a ninth. That is what inverse square means, and electromagnetism does exactly the same thing.

The reason is beautifully simple: the force spreads out over the surface of a sphere, and a sphere's area grows as r². Same pull, more sphere to cover.

距离加倍，力不是减半，而是变成四分之一；距离三倍，变成九分之一。这就是平方反比， 电磁力也完全一样。原因很漂亮：力扩散在一个球面上，而球面的面积按 r² 增长。

🕳 The force that is not a force

Newton said gravity is a pull. Einstein said something stranger and more accurate: gravity is not a pull at all. Mass bends space and time, and things moving through bent space simply travel in the straightest line available to them.

The Earth is not being yanked by the Sun. The Earth is going perfectly straight through space that the Sun has curved. You do not feel gravity pulling you down right now — you feel your chair pushing you up, stopping your natural free fall.

This is why gravity is the odd one out. The other three are particle exchange in the quantum world. Gravity, so far, is geometry — and nobody has managed to make those two pictures agree. Its carrier, the graviton, is still only a prediction: never detected, and the only one of the four carriers we have never seen.

牛顿说引力是一种拉力。爱因斯坦说了更奇怪也更准确的话：引力根本不是拉力。质量会弯曲时空，而穿过弯曲空间的东西只是走了它能走的最直的路。地球不是被太阳拽着，地球是在太阳弄弯的空间里笔直前进。你现在感觉到的也不是引力往下拉，而是椅子往上推你，拦住了你自然的自由落体。这就是引力的特别之处：另外三种力是量子世界里的粒子交换，引力目前是几何，没有人能让这两幅图对上。它的传递粒子引力子还只是一个预言，从未被探测到。

Say it Gravity is the weakest force, but it reaches forever.

⚡ This is the force you actually live in. Every single thing you have ever touched, smelled, seen or tasted was electromagnetism. Your chair holds you up because the electrons in it repel the electrons in you. You have never truly touched anything in your life — you have only ever been pushed.
这才是你真正生活在其中的力。你摸过、闻过、看过、尝过的一切都是电磁力。椅子撑住你，是因为它里面的电子排斥你身上的电子。你这辈子从来没有真正碰到过任何东西——你只是一直被推着。

Distance —

Two flavours, one force

Electric charge comes in two kinds. Opposite charges attract; like charges repel. That single fact builds every atom, every molecule, every chemical reaction and every living thing.

The carrier is the photon — and a photon is also light. So the thing gluing electrons to nuclei, the thing coming out of your screen right now, and the thing that takes eight minutes to arrive from the Sun are all the same particle. The photon has no mass, which is exactly why this force reaches infinitely far.

电荷有两种：异性相吸，同性相斥。就这一条事实，造出了每个原子、每个分子、每个化学反应和每个生命。它的传递粒子是光子——而光子就是光。把电子粘在原子核上的东西、此刻从你屏幕射出来的东西、从太阳走八分钟到你这里的东西，是同一种粒子。光子没有质量，所以这个力能伸到无限远。

🧲 A fridge magnet beats the entire planet 10³⁶ times stronger

Stick a magnet on a fridge and let go. It hangs there.

Look at what just happened. The whole Earth — six thousand billion billion tonnes of it — is pulling that magnet down with everything it has. And it is being beaten by a two-gram piece of metal. Not narrowly. It is not even close.

That is the strength gap made visible: electromagnetism is about 10³⁶ times stronger than gravity. If gravity were a single grain of sand, electromagnetism would be a ball of sand bigger than the Sun.

So why does the Earth ever win? It wins on quantity, not strength — it has piled up uncancelled mass for four and a half billion years, and every gram of it pulls in the same direction.

把磁铁贴在冰箱上放手，它挂住了。看看刚才发生了什么：整个地球——六万万亿亿吨——正用尽全力往下拉那块磁铁，结果输给了一块两克的金属，而且不是险胜，是完全不在一个量级。这就是强度差距：电磁力比引力强约 10³⁶ 倍。如果引力是一粒沙，电磁力就是一个比太阳还大的沙球。那地球靠什么赢？靠数量，不是靠强度。

Say it Opposite charges attract. Like charges repel.

💪 The strongest thing there is — and it works backwards. Every other force gets weaker as things move apart. This one gets stronger, like a rubber band.
最强的力——而且它是反着来的。别的力越远越弱，这个力越远越强，像橡皮筋。

Pull the quarks apart —

Try to pull one quark out

Why you can never win

Pulling two quarks apart stores energy in the band between them, exactly like stretching a rubber band. Pull far enough and that stored energy becomes so large that it is cheaper for the universe to make two brand-new quarks out of it — E = mc² running backwards, energy turning into matter.

So the band snaps and you are holding two pairs, never one lone quark. This is called confinement, and it is why a single quark has never been seen.

把两个夸克拉开，会在它们之间的“橡皮筋”里存下能量。拉得够远，存的能量大到宇宙觉得 直接造两个新夸克更便宜——这就是 E = mc² 反过来用：能量变成物质。于是筋断了，你手上是两对夸克，永远不会是一个孤单的夸克。这叫夸克禁闭，也是从没有人见过单个夸克的原因。

⚖️ You weigh almost nothing — 99% of you is pure energy E = mc²

Step on the scales. Nearly everything the scales report is the strong force.

A proton is three quarks. Add up the mass of those three quarks and you get about 1% of the proton's mass. Just one percent. The other 99% is the energy of the gluon field binding them — energy that shows up as mass because E = mc².

So when you weigh yourself, you are not really weighing stuff. You are weighing bound-up energy. And let even a tiny fraction of it out and you get the Sun, and nuclear power, and the reason there is any warmth on this planet at all.

站上体重秤，秤上几乎所有的数字都是强核力。一个质子由三个夸克组成，把三个夸克的质量加起来，只有质子质量的约 1%。剩下的 99% 是把它们绑在一起的胶子场的能量——因为 E = mc²，能量表现为质量。所以你称体重时，称的不是物质，而是被束缚的能量。放出其中极小的一点点，就是太阳和核能。

🔗 The leftover bit that holds every nucleus together the residual strong force

A nucleus is a bag of protons, and every proton is positive. They should violently blow each other apart — and the electric repulsion between two protons that close together really is enormous.

What stops it is a leftover. The strong force is busy inside each proton, gripping its quarks — but a little of it spills out past the edge and grabs the neighbouring protons and neutrons. That spillover is called the residual strong force, and even the leftovers are stronger than the full electric repulsion. Just barely, and only at very close range.

That just barely is why there is a heaviest possible element. Pile in too many protons and the repulsion — which reaches forever — finally beats the glue, which does not. The nucleus breaks. That is radioactivity.

原子核是一袋质子，每个质子都带正电，本该猛烈互斥炸开。拦住它的是一点剩余：强核力主要在每个质子内部抓夸克，但有一点点溢出边界，抓住旁边的质子和中子。这叫剩余强核力—— 连剩下的都比完整的电斥力强，但只是勉强强一点，而且只在极近距离。正因为“勉强”，才存在最重的元素上限：质子太多时，能伸到无限远的电斥力最终赢过伸不远的胶水，原子核就碎了。这就是放射性。

Say it The strong force gets stronger as the quarks move apart.

☢️ The only force that can change what a particle is. The other three push and pull. This one transforms. Without it the Sun could not burn and you would not exist.
唯一能改变粒子身份的力。另外三种只是推和拉，这一种会变身。没有它，太阳烧不起来，你也不会存在。

🪶 Why is it so weak?

Not because the force itself is feeble — because its carriers are fat. The W and Z bosons weigh about 90 times a proton. The photon and the gluon weigh nothing.

A heavy carrier cannot travel far before quantum mechanics makes it pay its own energy bill back. So the weak force dies out after about 10⁻¹⁸ metres — a thousandth of the width of a proton. It is not weak so much as impossibly short-sighted: two particles have to be practically on top of each other before it can act.

And where did that mass come from? The Higgs field — which is exactly why finding the Higgs boson in 2012 mattered so much.

不是因为这个力本身软弱，而是因为它的传递粒子太胖。W 和 Z 玻色子约是质子的 90 倍重，而光子和胶子没有质量。重的传递粒子走不远，所以弱力在大约 10⁻¹⁸ 米之后就消失了，只有质子宽度的千分之一。与其说它弱，不如说它近视得离谱。那质量哪来的？希格斯场——这正是 2012 年找到希格斯玻色子如此重要的原因。

☀️ It is why the Sun shines

The Sun runs on hydrogen fusion, and the very first step is a weak one: two protons meet, and one of them must turn into a neutron before they can stick together. Only the weak force can do that.

And it is staggeringly slow. A given proton in the Sun's core waits an average of billions of years for its turn. That bottleneck is the reason the Sun burns gently for ten billion years instead of detonating in an afternoon.

The weakest-but-one force sets the pace of every star — which means it set the pace for life to appear at all.

太阳靠氢聚变发光，而第一步就是弱力的事：两个质子相遇，其中一个必须先变成中子，它们才能粘住。只有弱力做得到。而且它慢得惊人：太阳核心里的一个质子平均要等几十亿年才轮到自己。正是这个瓶颈，让太阳温和地烧一百亿年，而不是一个下午就炸掉。倒数第二弱的力，定下了每一颗恒星的节奏。

Say it The weak force changes one kind of quark into another.

🧩 Are there really four? Maybe not. Turn the energy up high enough and the forces start merging into each other. Physics thinks they may all be one force, wearing four masks because the universe today is so cold.
真的有四种吗？也许没有。把能量调得够高，这些力会开始互相融合。物理学认为它们可能本来就是一种力，只因为今天的宇宙太冷，才戴上了四张面具。

Energy / how far back towards the Big Bang —

✅ One merger is not a guess — we built it and it worked the electroweak theory

The first merger on that chart is not speculation. In the 1960s, theorists predicted that electromagnetism and the weak force are the same force above about 100 GeV, and that this required heavy carrier particles nobody had ever seen.

In 1983 CERN built a machine big enough to reach that energy and the W and Z bosons appeared, with the predicted masses. In 2012 the Higgs boson — the thing that splits the one force back into two by giving W and Z their weight — appeared too.

So four forces is already really three. The remaining steps — folding in the strong force (a Grand Unified Theory) and then gravity (quantum gravity) — are still unfinished. Nobody has done it. It is the biggest open problem in physics, and it is waiting for somebody.

图上的第一次合并不是猜测。1960 年代理论家预言：在约 100 GeV 以上，电磁力和弱力是同一种力，而这需要几个从没见过的重传递粒子。1983 年，CERN 造出了能达到那个能量的机器，W 和 Z 玻色子出现了，质量和预言一致。2012 年，把这一种力重新劈成两种的希格斯玻色子也出现了。所以四种力其实已经是三种。剩下的两步——并入强核力（大统一理论）和并入引力（量子引力）——还没有完成。这是物理学最大的未解问题，它在等人。

Say it At high energy, two of the forces become one.

🧬 A question with a real answer. DNA is a twisted ladder. The two sides never come apart on their own, but a cell can unzip them in an instant, every time it copies itself. Which of the four forces is doing that?
一个有确切答案的问题。DNA 是一把扭曲的梯子，两边不会自己散开，但细胞每次复制时都能瞬间拉开它们。这是四种力里的哪一种在做这件事？

⚡ Electromagnetism — every single bond in the picture.

It is the only one of the four that can do it. The strong and weak forces die out inside the nucleus of one atom — they cannot reach across a gap between two molecules. Gravity reaches, but at this scale it is about 10³⁴ times too weak to matter. Chemistry has only one force available, and this is it.
它是四种力里唯一做得到的。强力和弱力在一个原子的原子核内部就衰减殆尽，够不到两个分子之间的缝隙。引力伸得到，但在这个尺度上它弱了大约 10³⁴ 倍，完全不起作用。化学只有一种力可用，就是它。

Try to unzip the rung —

What a hydrogen bond actually is

It is not a real bond. It is naked electric attraction across a gap.

When a hydrogen atom is bonded to a nitrogen or an oxygen, that bigger, greedier atom drags the shared electrons towards itself. The hydrogen is left slightly positive (δ+); the N or O is left slightly negative (δ−).

A hydrogen bond is simply δ+ pulling on δ− — the same rule you flipped back and forth in the Electricity machine, at a scale of billionths of a metre. Opposite charges attract. That is the whole mechanism.

它不是真正的化学键，而是隔着一道缝的裸露的电吸引。当氢原子连在氮或氧上时，那个更大更“贪”的原子把共用电子拽向自己，氢就剩下一点正电（δ+），氮或氧带一点负电（δ−）。氢键就是 δ+ 吸引 δ−——和你在电那台机器里反复切换的规则一模一样，只是发生在十亿分之一米的尺度上。异性相吸，机制就这么简单。

💡 Why weak bonds are the whole point

A hydrogen bond is about twenty times weaker than the covalent bonds holding each base together. That sounds like bad engineering. It is the opposite.

The rungs must break — every time a cell divides, the ladder has to unzip so each half can be copied. If the rungs were strong bonds, your genetic code would be permanent, unreadable and useless.

So life uses the strong version of the electric force (covalent, sharing electrons) along the sides of the ladder where nothing must ever break, and the weak version (hydrogen bonds, just attraction) across the rungs where things must come apart on demand. Same force, two settings.

氢键比把每个碱基连起来的共价键弱大约二十倍。听起来像是设计失误，其实正相反。梯子的横档必须能断——细胞每次分裂，梯子都要拉开，两半各自复制。如果横档是强键，你的遗传密码就会是永久的、读不出来的、没用的。所以生命在绝不能断的梯子两侧用电磁力的强形式（共价键，共用电子），在必须能拉开的横档上用弱形式（氢键，只是吸引）。同一种力，两档设置。

🪜 The whole ladder, force by force

Strong — holds the nucleus of every carbon, nitrogen, oxygen and phosphorus atom together. It never reaches outside its own atom.

Electromagnetic — does everything else. It binds electrons to those nuclei, bonds the atoms into rings, builds the sugar-phosphate rails, and grips the two strands across the rungs.

Weak — plays no part in holding DNA together. But the carbon and oxygen atoms themselves were built by fusion inside a dead star, and the weak force lit that star.

Gravity — utterly irrelevant here. Between two atoms in a base pair it is roughly 10³⁴ times weaker than the electric pull — about 7 × 10⁻⁴⁴ newtons of gravity against roughly 7 × 10⁻¹⁰ newtons of electric attraction.

强核力——把每个碳、氮、氧、磷原子的原子核攥住，从不伸出自己的原子。
电磁力——其余全部：把电子绑在原子核上、把原子连成环、搭起糖-磷酸骨架、隔着横档抓住两条链。
弱核力——不参与固定 DNA。但碳原子和氧原子本身是在一颗死去的恒星内部聚变造出来的，是弱力点燃了那颗星。
引力——在这里完全无关，比电吸引弱约 10³⁴ 倍：引力约 7 × 10⁻⁴⁴ 牛顿，而电吸引约 7 × 10⁻¹⁰ 牛顿。

Say it Hydrogen bonds hold the base pairs together, and they are electromagnetic.

🔍 Then why does G–C grip harder than A–T? three bonds instead of two

Because it is simply more of the same force. Nothing new is going on.

A and T line up in two places, so a rung of A–T is held by two hydrogen bonds. G and C line up in three. Three small electric pulls instead of two — about one and a half times the grip.

You can measure the consequence. DNA that is rich in G–C needs a higher temperature to melt apart, because there is more electric attraction per rung to overcome. Organisms living in hot springs tend to have G-C-rich genomes — a fact you can predict from nothing but "opposite charges attract."

Try it: switch the pair above from A–T to G–C and unzip both. The G–C rung survives further up the slider.

因为那只是同一种力多来一份，没有任何新东西。A 和 T 有两处对得上，所以 A–T 横档由两个氢键拉住；G 和 C 有三处，也就是三份小小的电吸引，抓力约为一点五倍。这个后果可以测量：G–C 多的 DNA 需要更高的温度才能熔开。生活在热泉里的生物往往有富含 G-C 的基因组——仅凭“异性相吸”就能预言出来。 试试看：把上面的碱基对从 A–T 换成 G–C，再拉一次，G–C 能撑到滑块更靠后的位置。

📋 All four at a glance

Cover the right-hand columns and say each row out loud from memory. 遮住右边几列，凭记忆把每一行大声说出来。

Strengths are rough comparisons, not fixed constants — the real strength of every force changes with the energy you probe it at. That is the whole idea behind the last machine.
这些强度是粗略的比较，不是固定常数——每种力的真实强度会随探测能量改变。最后一台机器讲的就是这件事。

🗣 Words you need to say this in English

Tap 🔊 and repeat. Then close your eyes and use each word in your own sentence. 点 🔊 跟读。然后闭上眼睛，用每个词造一个自己的句子。