跟讀練習: The Most Misunderstood Concept in Physics - 通過影片學習英語口語

選擇練習模式

每種模式都會開啟全屏練習室。

270 句

如果句子過短或過長，請點選 Edit 進行調整。

- This is a video about one of the most important, yet least understood concepts in all of physics.

It governs everything from molecular collisions to humongous storms.

From the beginning of the universe through its entire evolution, to its inevitable end.

It may, in fact, determine the direction of time and even be the reason that life exists.

To see the confusion around this topic, you need to ask only one simple question.

What does the Earth get from the sun?

- What does the earth get from sun?

- Well, it's light rays?

- What do we get from the sun? - Heat.

- Warmth. - Warmth, light.

- Vitamin D, we get vitamin D from- - We do get vitamin D from the ultraviolet rays.

- Well, a lot of energy.

- What does the earth get from this, energy?

- Yeah, energy.

- Energy. - Nailed it.

Every day, the earth gets a certain amount of energy from the sun.

And then how much energy does the earth radiate back into space relative to that amount that it gets from the sun?

- Probably not as much, I, you know, I don't believe it's just radiating right back.

- I'd say less.

- Less. - Less. - I say less.

- I guess about 70%?

- It is a fraction.

- I'd say 20%.

- Because... - Because we use some of it.

- We use some of the energy. - Mm-hmm.

- We consume a lot, right?

- But the thing about energy is it never really goes away.

You can't really use it up.

- It would have to break even, wouldn't it?

Same amount, yeah.

- You know, cause and effect.

It'd be equal in some ways, right?

- For most of the earth's history, it should be exactly the same amount of energy in from the sun as earth radiates into space.

- Wow. - Because if we didn't do that, then the earth would get a lot hotter, that'd be a problem.

- That'd be a big problem.

- So, if that is the case...

- Yeah. - Then what are we really getting from the sun?

- That's a good question.

- Hmm. - It gives us a nice tan.

- It gives us a nice tan, I love it.

We're getting something special from the sun.

- I don't know, what do we get without the energy?

- But nobody talks about it.

To answer that, we have to go back to a discovery made two centuries ago.

In the winter of 1813, France was being invaded by the armies of Austria, Prussia, and Russia.

The son of one of Napoleon's generals was Sadi Carnot, a 17-year-old student.

On December 29th, he writes a letter to Napoleon to request to join in the fight.

Napoleon preoccupied in battle, never replies.

but Carnot gets his wish a few months later when Paris is attacked.

The students defend a chateau just east of the city, but there're no match for the advancing armies, and Paris falls after only a day of fighting.

Forced to retreat, Carnot is devastated.

Seven years later, he goes to visit his father who's fled to Prussia after Napoleon's downfall.

His father was not only a general, but also a physicist.

He wrote an essay on how energy is most efficiently transferred in mechanical systems.

When his son comes to visit, they talk at length about the big breakthrough of the time, steam engines.

Steam engines were already being used to power ships, mine ore, and excavate ports.

And it was clear that the future industrial and military might of nations depended on having the best steam engines.

But French designs were falling behind those of other countries like Britain.

So, Sadi Carnot took it upon himself to figure out why.

At the time, even the best steam engines only converted around 3% of thermal energy into useful mechanical work.

If he could improve on that, he could give France a huge advantage and restore its place in the world.

So he spends the next three years studying heat engines, and one of his key insights involves how an ideal heat engine would work, one with no friction and no losses to the environment.

It looks something like this.

Take two really big metal bars, one hot and one cold.

The engine consists of a chamber filled with air, where heat can only flow in or out through the bottom.

Inside the chamber is a piston, which is connected to a flywheel.

The air starts at a temperature just below that of the hot bar.

So first, the hot bar is brought into contact with the chamber.

The air inside expands with heat flowing into it to maintain its temperature.

This pushes the piston up, turning the flywheel.

Next, the hot bar is removed, but the air in the chamber continues to expand, except now without heat entering, the temperature decreases.

In the ideal case, until it is the temperature of the cold bar.

The cold bar is brought into contact with the chamber and the flywheel pushes the piston down.

And as the air is compressed, heat is transferred into the cold bar.

The cold bar is removed.

The flywheel compresses the gas further increasing its temperature until it is just below that of the hot bar.

Then the hot bar is connected again and the cycle repeats.

Through this process, heat from the hot bar is converted into the energy of the flywheel.

And what's interesting to note about Carnot's ideal engine is that it is completely reversible.

If you ran the engine in reverse, first the air expands lowering the temperature, then the chamber is brought into contact with the cold bar, the air expands more, drawing in heat from the cold bar.

Next, the air is compressed, increasing its temperature.

The chamber is placed on top of the hot bar and the energy of the flywheel is used to return the heat back into the hot bar.

However many cycles were run in the forward direction, you could run the same number in reverse, and at the end, everything would return to its original state with no additional input of energy required.

So by running an ideal engine, nothing really changes.

You can always undo what you did.

So what is the efficiency of this engine?

Since it's fully reversible, you might expect the efficiency to be 100%, but that is not the case.

Each cycle, the energy of the flywheel increases by the amount of heat flowing into the chamber from the hot bar, minus the heat flowing out of the chamber at the cold bar.

So to calculate the efficiency, we divide this energy by the heat input from the hot bar.

Now the heat in on the hot side is equal to the work done by the gas on the piston, and this will always be greater than the work done by the piston on the gas on the cold side, which equals the heat out.

And this is because on the hot side, the hot gas exerts a greater pressure on the piston than that same gas when cold.

To increase the efficiency of the engine, you could increase the temperature of the hot side, or decrease the temperature of the cold side, or both.

Lord Kelvin learns of Carnot's ideal heat engine and realizes it could form the basis for an absolute temperature scale.

Imagine that the gas is allowed to expand an extreme amount, so much that it cools to the point where all the gas particles effectively stop moving.

Then they would exert no pressure on the piston, and it would take no work to compress it on the cold side, so no heat would be lost.

This is the idea of absolute zero, and it would make for a 100% efficient engine.

Using this absolute temperature scale, the Kelvin scale, we can replace the amount of heat in and out with the temperature of the hot and cold side respectively, because they are directly proportional.

So we can express efficiency like this, which we can rewrite like this.

What we have learned is that the efficiency of an ideal heat engine doesn't depend on the materials or the design of the engine, but fundamentally on the temperatures of the hot and cold sides.

100

To reach 100% efficiency, you'd need infinite temperature on the hot side or absolute zero on the cold side, both of which are impossible in practice.

101

So even with no friction or losses to the environment, it's impossible to make a heat engine 100% efficient.

102

And that's because to return the piston to its original position, you need to dump heat into the cold bar.

103

So not all the energy stays in the flywheel.

104

Now, in Carnot's time, high pressure steam engines could only reach temperatures up to 160 degrees Celsius.

105

So their theoretical maximum efficiency was 32%, but their real efficiency was more like 3%.

106

That's because real engines experience friction, dissipate heat to the environment, and they don't transfer heat at constant temperatures.

107

So for just as much heat going in, less energy ends up in the flywheel.

108

The rest is spread out over the walls of the cylinder, the axle of the flywheel, and is radiated out into the environment.

109

When energy spreads out like this, it is impossible to get it back.

110

So this process is irreversible.

111

The total amount of energy didn't change, but it became less usable.

112

Energy is most usable when it is concentrated and less usable when it's spread out.

113

Decades later, German physicist, Rudolf Clausius, studies Carnot's engine, and he comes up with a way to measure how spread out the energy is.

114

He calls this quantity, entropy.

115

When all the energy is concentrated in the hot bar, that is low entropy, but as the energy spreads to the surroundings, the walls of the chamber and the axle will entropy increases.

116

This means the same amount of energy is present, but in this more dispersed form, it is less available to do work.

117

In 1865, Clausius summarizes the first two laws of thermodynamics like this.

118

First, the energy of the universe is constant.

119

And second, the entropy of the universe tends to a maximum.

120

In other words, energy spreads out over time.

121

The second law is core to so many phenomena in the world.

122

It's why hot things cool down and cool things heat up, why gas expands to fill a container, why you can't have a perpetual motion machine, because the amount of usable energy in a closed system is always decreasing.

123

The most common way to describe entropy is as disorder, which makes sense because it is associated with things becoming more mixed, random, and less ordered.

124

But I think the best way to think about entropy is as the tendency of energy to spread out.

125

So why does energy spread out over time?

126

I mean, most of the laws of physics work exactly the same way forwards or backwards in time.

127

So how does this clear time dependence arise?

128

Well, let's consider two small metal bars, one hot and one cold.

129

For this simple model, we'll consider only eight atoms per bar.

130

Each atom vibrates according to the number of energy packets it has.

131

The more packets, the more it vibrates.

132

So let's start with seven packets of energy in the left bar and three in the right.

133

The number of energy packets in each bar is what we'll call a state.

134

First, let's consider just the left bar.

135

It has seven energy packets, which are free to move around the lattice.

136

This happens nonstop.

137

The energy packets hop randomly from atom to atom giving different configurations of energy, but the total energy stays the same the whole time.

138

Now, let's bring the cold bar back in with only three packets and touch them together.

139

The energy packets can now hop around between both bars creating different configurations.

140

Each unique configuration is equally likely.

141

So what happens if we take a snapshot at one instant in time and see where all the energy packets are?

142

So stop, look at this.

143

Now there are nine energy packets in the left bar, and only one in the right bar.

144

So heat has flowed from cold to hot.

145

Shouldn't that be impossible because it decreases entropy?

146

Well, this is where Ludwig Boltzmann made an important insight.

147

Heat flowing from cold to hot is not impossible, it's just improbable.

148

There are 91,520 configurations with nine energy packets in the left bar, but 627,264 with five energy packets in each bar.

149

That is the energy is more than six times as likely to be evenly spread between the bars.

150

But if you add up all the possibilities, you find there's still a 10.5% chance that the left bar ends up with more energy packets than it started.

151

So, why don't we observe this happening around us?

152

Well, watch what happens as we increase the number of atoms to 80 per bar and the energy packets to 100, with 70 in the left bar and 30 in the right.

153

There is now only a 0.05% chance that the left solid ends up hotter than it started.

154

And this trend continues as we keep scaling up the system.

155

In everyday solids, there are around 100 trillion, trillion atoms and even more energy packets.

156

So heat flowing from cold to hot is just so unlikely that it never happens.

157

Think of it like this Rubik's cube.

158

Right now, it is completely solved, but I'm gonna close my eyes and make some turns at random.

159

If I keep doing this, it will get further and further from being solved.

160

But how can I be confident that I'm really messing this cube up?

161

Well, because there's only one way for it to be solved, a few ways for it to be almost solved, and quintillions of ways for it to be almost entirely random.

162

Without thought and effort, every turn moves the Rubik's cube from a highly unlikely state that of it being solved to a more likely state, a total mess.

163

So if the natural tendency of energy is to spread out and for things to get messier, then how is it possible to have something like air conditioning where the cold interior of a house gets cooler and the hot exterior gets hotter?

164

Energy is going from cold to hot, decreasing the entropy of the house.

165

Well, this decrease in entropy is only possible by increasing the entropy a greater amount somewhere else.

166

In this case, at a power plant, the concentrated chemical energy and coal is being released, heating up the power plant in its environment, spreading to the turbine the electric generators, heating the wires all the way to the house, and producing waste heat in the fans and compressor.

167

Whatever decrease in entropy is achieved at the house is more than paid for by an increase in entropy required to make that happen.

168

But if total entropy is constantly increasing and anything we do only accelerates that increase, then how is there any structure left on earth?

169

How are there hot parts separate from cold parts?

170

How does life exist?

171

Well, if the earth were a closed system, the energy would spread out completely, meaning, all life would cease, everything would decay and mix, and eventually, reach the same temperature.

172

But luckily, earth is not a closed system, because we have the sun.

173

What the sun really gives us is a steady stream of low entropy that is concentrated bundled up energy.

174

The energy that we get from the sun is more useful than the energy we give back.

175

It's more compact, it's more clumped together.

176

Plants capture this energy and use it to grow and create sugars.

177

Then animals eat plants and use that energy to maintain their bodies and move around.

178

Bigger animals get their energy by eating smaller animals and so on.

179

And each step of the way, the energy becomes more spread out.

180

- Okay, interesting.

181

- Yeah. - Oh wow, I did not know that.

182

- There you go.

183

Ultimately, all the energy that reaches earth from the sun is converted into thermal energy, and then it's radiated back into space.

184

But in fact, it's the same amount.

185

I know this is a- - You do know this is...

186

- I'm a PhD physicist.

187

- Oh, okay, but anyway, so... - I trust you.

188

The increase in entropy can be seen in the relative number of photons arriving at and leaving the earth.

189

For each photon received from the sun, 20 photons are emitted, and everything that happens on earth, plants growing, trees falling, herds stampeding, hurricanes and tornadoes, people eating, sleeping, and breathing.

190

All of it happens in the process of converting fewer, higher energy photons into 20 times as many lower energy photons.

191

Without a source of concentrated energy and a way to discard the spread out energy, life on earth would not be possible.

192

It has even been suggested that life itself may be a consequence of the second law of thermodynamics.

193

If the universe tends toward maximum entropy, then life offers a way to accelerate that natural tendency, because life is spectacularly good at converting low entropy into high entropy.

194

For example, the surface layer of seawater produces between 30 to 680% more entropy when cyanobacteria and other organic matter is present than when it's not.

195

Jeremy England takes this one step further.

196

He's proposed that if there is a constant stream of clumped up energy, this could favor structures that dissipate that energy.

197

And over time, this results in better and better energy dissipators, eventually resulting in life.

198

Or in his own words, "You start with a random clump of atoms, and if you shine light on it for long enough, it should not be so surprising that you get a plant." So life on earth survives on the low entropy from the sun, but then where did the sun get its low entropy?

199

The answer is the universe.

200

If we know that the total entropy of the universe is increasing with time, then it was lower entropy yesterday and even lower entropy the day before that, and so on, all the way back to the Big Bang.

201

So right after the Big Bang, that is when the entropy was lowest.

202

This is known as the past hypothesis.

203

It doesn't explain why the entropy was low, just that it must have been that way for the universe to unfold as it has.

204

But the early universe was hot, dense, and almost completely uniform.

205

I mean, everything was mixed and the temperature was basically the same everywhere, varying by at most 0.001%.

206

So how is this low entropy?

207

Well, the thing we've left out is gravity.

208

Gravity tends to clump matter together.

209

So taking gravity into account, having matter all spread out like this, would be an extremely unlikely state, and that is why it's low entropy.

210

Over time, as the universe expanded and cooled, matter started to clump together in more dense regions.

211

And in doing so, enormous amounts of potential energy were turned into kinetic energy.

212

And this energy could also be used like how water flowing downhill can power a turbine.

213

But as bits of matter started hitting each other, some of their kinetic energy was converted into heat.

214

So the amount of useful energy decreased.

215

Thereby, increasing entropy.

216

Over time, the useful energy was used.

217

In doing so, stars, planets, galaxies, and life were formed, increasing entropy all along.

218

The universe started with around 10 to the 88 Boltzmann constants worth of entropy.

219

Nowadays, all the stars in the observable universe have about 9.5 times 10 to the 80.

220

The interstellar and intergalactic medium combined have almost 10 times more, but still only a fraction of the early universe.

221

A lot more is contained in neutrinos and in photons of the cosmic microwave background.

222

In 1972, Jacob Bekenstein proposed another source of entropy, black holes.

223

He suggested that the entropy of a black hole should be proportional to its surface area.

224

So as a black hole grows, its entropy increases.

225

Famous physicists thought the idea was nonsense and for good reason.

226

According to classical thermodynamics, if black holes have entropy, then they should also have a temperature.

227

But if they have temperatures, they should emit radiation and not be black after all.

228

The person who set out to prove Bekenstein wrong was Stephen Hawking.

229

But to his surprise, his results showed that black holes do emit radiation, now known as Hawking radiation, and they do have a temperature.

230

The black hole at the center of the Milky Way has a temperature of about a hundred trillionth of a Kelvin, emitting radiation that is far too weak to detect.

231

So still pretty black.

232

But Hawking confirmed that black holes have entropy and Bekenstein was right.

233

Hawking was able to refine Bekenstein's proposal and determine just how much entropy they have.

234

The super massive black hole at the center of the Milky Way has about 10 to the 91 Boltzmann constants of entropy.

235

That is 1,000 times as much as the early observable universe, and 10 times more than all the other particles combined.

236

And that is just one black hole.

237

All black holes together account for 3 times 10 to the 104 Boltzmann constants worth of entropy.

238

So almost all the entropy of the universe is tied up in black holes.

239

That means, the early universe only had about 0.000000000000003% of the entropy it has now.

240

So the entropy was low, and everything that happens in the universe like planetary systems forming, galaxies merging, asteroids crashing, stars dying, to life itself flourishing, all of that can happen because the entropy of the universe was low and it has been increasing, and it all happens only in one direction.

241

We never see an asteroid uncrash or a planetary system unmix into the cloud of dust and gas that made it up.

242

There is a clear difference between going to the past and the future, and that difference comes from entropy.

243

The fact that we are going from unlikely to more likely states is why there is an arrow of time.

244

This is expected to continue until eventually, the energy gets spread out so completely that nothing interesting will ever happen again.

245

This is the heat death of the universe.

246

In the distant future, more than 10 to the 100 years from now, after the last black hole has evaporated, the universe will be in its most probable state.

247

Now, even on large scales, you would not be able to tell the difference between time moving forwards or backwards, and the arrow of time itself would disappear.

248

So it sounds like entropy is this awful thing that leads us inevitably towards the dullest outcome imaginable.

249

But just because maximum entropy has low complexity does not mean that low entropy has maximum complexity.

250

It's actually more like this tea and milk.

251

I mean, holding it like this is not very interesting.

252

But as I pour the milk in, the two start to mix and these beautiful patterns emerge.

253

They arise in an instant and before you know it, they're gone back to being featureless.

254

Both low and high entropy are low in complexity.

255

It's in the middle where complex structures appear and thrive.

256

And since that's where we find ourselves, let's make use of the low entropy we've got while we can.

257

With the right tools, we can understand just about anything, from a cup of tea cooling down to the evolution of the entire universe.

258

And if you're looking for a free and easy way to add powerful tools to your arsenal, then you should check out this video sponsor, brilliant.org.

259

With Brilliant, you can master key concepts in everything from math and data science to programming and physics.

260

All you need to do is set your goal, and Brilliant will design the perfect learning path for you, equipping you with all the tools you need to reach it.

261

Want to learn how to think like a programmer?

262

Then Brilliant's latest course, "Thinking in Code" is a fast and easy way to get there.

263

Using an intuitive drag and drop editor, it teaches you what you really need to know, including essential concepts like nesting and conditionals.

264

You can start by jumping right in to program a robot and then learn how to apply your new tools to your everyday life, like automating reminders on your phone or building a bot that filters your matches on a dating app.

265

What I love about Brilliant is that they connect what you learn to real world examples.

266

And because each lesson is hands-on, you'll build real intuition, so you can put what you've learned to good use.

267

To try everything Brilliant has to offer free for a full 30 days, visit brilliant.org/veritasium.

268

I will put that link down in the description.

269

And through that link, the first 200 of you to sign up will get 20% off Brilliant's annual premium subscription.

270

So I wanna thank Brilliant for sponsoring this video, and I wanna thank you for watching.

下載應用

流利口語所需的一切

AI 發音評分逐句打分

音標練習掌握每個音

詞彙學習建立你的詞庫

詞彙遊戲邊玩邊學

掃碼

Download on theApp Store Get it onGoogle Play

同頻道

FOR YOU

你能學到什麼

這段影片對話能幫你提升兩項實用口語技能：一是聽懂日常對話中的快速問答，影片里人們對“地球從太陽得到什麼”的搶答節奏緊湊，練習後能更好應對真實交流；二是用自然語調錶達疑惑與思考，比如對話中“Hmm”“That's a good question”的使用，讓你的口語更生動。通過英語影子跟讀（shadow speak）練習，這些技能能快速掌握。

注意這些發音現象

對話中存在不少連讀和弱讀，比如“what does”連讀成/ˈwɑːt dəz/，“a lot of”弱讀為/ə ˈlɑːt əv/。還有“energy”的發音，很多人會讀成/ˈenədʒi/，但影片裡更自然的讀法是/ˈenərdʒi/。這些細節是影子跟讀（shadowspeak）的關鍵，多聽多模仿能讓你的發音更地道。

像母語者一樣表達

模仿說話人的節奏和重音很重要。比如“Nailed it”要重讀“Nailed”，表達肯定；“But the thing about energy is it never really goes away”中，“never really”要輕讀，突出“goes away”。使用影子跟讀網站（shadowing site），跟著音訊逐句重複，注意停頓和語調變化。堅持練習，你會發現自己的口語越來越流暢，甚至能自然運用“Hmm”這樣的語氣詞。快來試試shadowspeaks練習，感受快速進步的樂趣吧！

影片中的語法

說話人最常用的結構，並附上影片中的原話：

結構	影片中的用法
條件句 if + 從句，will/would + 動詞 — 條件及其結果	If I keep doing this, it will get
被動語態 be + 過去分詞 — 強調發生了什麼，而不是誰做的	being invaded · is attacked · is connected
現在完成時 have/has + 過去分詞 — 過去發生但與現在仍有關聯的事	have learned · has flowed · has even been suggested
定語從句 who / which + 從句 — 補充說明人或事物	piston, which is · disorder, which makes · packets, which are

什麼是跟讀法？

跟讀法 (Shadowing) 是一種有科學依據的語言學習技巧，最初開發用於專業口譯員的培訓，並由多語言者Alexander Arguelles博士普及。這個方法簡單而強大：您在聽英語母語原聲的同時立即大聲重複——就像是一個延遲1-2秒緊跟說話者的影子。與被動聽力或語法練習不同，跟讀法強迫您的大腦和口腔肌肉同時處理並模模擬實的講話模式。研究表明它能顯著提高發音準確性，語調，節奏，連讀，聽力理解和口語流利度——使其成為雅思口語備考和真實英語交流最有效的方法之一。

影子跟讀法: 閱讀完整分步指南 →

跟讀練習: The Most Misunderstood Concept in Physics - 通過影片學習英語口語

選擇練習模式

下載應用

Something is jamming GPS over Europe. Here's what we found

Why It Was Almost Impossible to Make the Blue LED

You've (Likely) Been Playing The Game Of Life Wrong

We're trying something new...

How does light 'know' the shortest path?

The CIA's new tech doesn't make sense

Why does every mammal get 1 billion heartbeats in their life?

Why Cold Drinks Were Deadly Before 1914

Exposing The Flaw In Our Phone System

Our Greatest Delusion

The Most Controversial Problem in Philosophy

Why Are 96,000,000 Black Balls on This Reservoir?

We've Been Using The Wrong Science In Court For 50 years

Explaining Pi With Pizza

5 Fun Physics Phenomena

The Absurd Engineering Behind the Internet

Falling ladders - why does this happen?

How a slinky falls in slow motion #shorts

We're 99.9% sure this pattern is true, but no one can prove it

What is the secret of a good life? Lessons from the longest study on happiness

The Speed of Light Isn't a Limit. It's a Converter

The Evolution of Human Skin Colour | BBC Earth Science

How The Universe Could Destroy Life On Earth

The Nucleus: Crash Course Chemistry #1

Why is it Hot Underground?

The Dumbest Animal Alive

What Did Ancient Humans Do at Night?

Anti-matter & nuclear weapons: Why technology is always a double-edge sword | Don Lincoln

The Most Controversial Idea In Physics

How to Move the Sun: Stellar Engines

Joe & Matt Walsh Go Back and Forth on the Moon Landing Conspiracy

你能學到什麼

注意這些發音現象

像母語者一樣表達

影片中的語法

什麼是跟讀法？