Shadowing Practice: Why is there antimatter? - Learn English Speaking with Video

בחר מצב תרגול

כל מצב פותח חדר תרגול במסך מלא.

445 משפטים

אם המשפטים קצרים או ארוכים מדי, לחץ על ערוך כדי להתאים אותם.

In 1928, a young man shuffled onto a stage in Germany to present a lecture on his recent work.

He had a slightly unusual presentation style.

Physicist Eugene Wigner described the lecture as detached, almost like a recitation of a technical text.

He said the man spoke without giving any sign of enjoying his own lecture.

But the work this strange, unassuming man presented was about to send some of the most famous quantum physicists of the 20th century spiraling.

After the lecture, Werner Heisenberg described the man's theory as the saddest chapter in modern physics.

Heisenberg also wrote to Niels Bohr and said, I find the present situation quite absurd, and on that account, almost out of despair,

I have taken up another field.

Legend has it that Wolfgang Pauli even announced that he was abandoning quantum physics, and then he started writing a utopian novel.

What had the young man said to disrupt the world of quantum mechanics so profoundly?

Well, he had been working on a problem that physicists are still tackling today.

The unification of Einstein's relativity and quantum mechanics.

And his work had revealed something troubling.

A particle unlike any seen in the world.

A particle with negative energy.

In 1905, Albert Einstein published his special theory of relativity.

It was based on the simple idea that for anyone moving with constant speed, the laws of physics should be the same.

And this includes any measurement of the speed of light.

So it doesn't matter how you're moving relative to a beam of light, you should measure its speed to be 300 million meters per second.

This means that things that we ordinarily think of as fixed,

like time and space, have to transform so that the speed of light is always measured to have the same value.

In making this discovery, Einstein realized that space

and time are not really separate dimensions at all they're linked in a four -dimensional fabric called spacetime

when Einstein applied this idea to an object emitting light he found something peculiar

when an object loses energy by emitting photons its mass must also decrease

and the change in the objects mass is equal to the

energy of the photons emitted divided by the speed of light squared in other words he found

that energy must be equal to mc squared.

E equals mc squared.

Mass and energy are for different manifestations of the same thing.

So a particle's total energy comes from its momentum and its rest mass, giving a new relationship between energy and momentum.

Now if we take the square root of both sides,

we get an expression for the energy of a particle in terms of its mass and momentum.

And if we plot energy versus momentum in a single dimension, we get this curve, where mc squared, the rest mass energy of the object,

is the lowest possible value for energy.

But really, when we took the square root, we should have put a plus minus out front, which would give us two curves,

one for positive energies and another for negative energies.

But we don't observe things that have energy less than zero.

I mean, what would that even mean?

So in classical physics, the solution is simple.

You just ignore the negative energy solutions.

They can't physically represent anything, right, Casper?

Well, sure, that sounds right.

But around the same time that Einstein was coming up with his special theory of relativity,

physicists began observing some strange phenomena in atomic physics that were overthrowing all of these classical assumptions.

First of all, they noticed that when looking at the energy levels of subatomic particles like electrons, they weren't continuous at all.

They were discrete.

And they also didn't just behave as particles, but as waves.

So if you would take electrons

and fire them through two narrow slits

then they would create interference patterns just like light does what they were discovering was the new field of quantum mechanics

and in 1926 erwin schrodinger formalized this field by coming up with his now famous wave equation

which describes how quantum mechanical systems evolve over time this was the most radical new theory of the 20th century,

and would remain so.

This is not a bit like

Newton or a bit like Maxwell.

This was radically new.

The equation solution, called the wave function, psi, does not describe a particle with a precise position and momentum, as classical laws would.

But instead, the modulus squared can give us the probability of finding the particle in a specific location at a specific time.

Deriving the Schrodinger equation is actually surprisingly simple.

Just start by writing down the total energy, which for a free particle is just its kinetic energy, half mv squared.

And we can rewrite it in terms of momentum, mass times velocity.

So kinetic energy is just momentum squared divided by 2m.

Now we're going to make this quantum.

And to do that, we need two things.

The first is the wave function, psi, which describes how particles act as waves at quantum scales and the second is something called a quantum operator.

This is just a mathematical tool that extracts information from the wave function to reveal a specific property of the particle,

like its position, energy, or momentum.

The operators for energy and momentum look like this.

By inputting these operators, we can make each side of the equation act on the wave function.

And that gives us the Schrodinger equation for a free particle.

For particles that aren't free, like electrons and atoms, you need to factor in potential energy too, which gives you the full Schrodinger equation.

but there are some places where it doesn't produce the right

prediction for example take the element gold the schrodinger equation suggests

that it should be a silver gray color just like all the other metals

but it's not i mean it's gold and for mercury the schrodinger equation predicts

that it should be a solid at room temperature

but it's a liquid so what is wrong well i'll give you a hint

because both gold and mercury are heavy elements they have a high number of protons in their nuclei

which attract orbiting electrons more strongly so

if you're thinking in a classical sense the electrons would be bound tighter in

and they'd be whizzing around at higher speeds than in other elements

and for electrons in some orbits those speeds start to get a little too close to the speed of light

so let's go back to the schrodinger equation there we had the kinetic energy was p squared over 2m

but for particles moving at relativistic speeds close to the speed of light

that equation isn't the right one the correct equation is the

energy momentum relation from special relativity Schrodinger's equation is not consistent with the theory of relativity

so it's technically it's not right

so the solution seems simple enough just start with the relativistic energy momentum relation and use

that to derive a new wave equation well this is exactly what a physicist named oscar klein did in 1926 he

He subbed in the same energy and momentum operators that Schrodinger used and found this equation.

Klein's work was pretty well received by a number of his colleagues.

In fact, it turned out the two other physicists, Walter Gordon and Vladimir Falk, had independently arrived at the same equation in that same year.

It became known as the Klein -Gordon equation, which was a bit of a burn for Falk.

But that's often how it goes.

You do all the hard work and then no one notices.

Things have changed a little since Fox time, but the key to recognition is still visibility.

And these days, that means having a website.

Simple enough, you just need to write a couple thousand lines of CSS,

100

JavaScript, or HTML, or else pay someone a few thousand dollars to do it for you.

101

Oh, and then there's the hosting costs and the maintenance, or you could check out today's sponsor Hostinger.

102

Not only will they get your website online, their intuitive tools let you build it yourself.

103

There's no need to learn multiple coding languages.

104

The Hostinger Builder can get you most of the way there with just one prompt.

105

You can optimize the website with their built -in tools for SEO rating, logo design, and e -commerce.

106

And once you're happy, you're online immediately on a domain that's included with the plan.

107

That whole process is fast and straightforward, and a subscription is just a few dollars a month.

108

So sign up now to get an 85 % discount in the sale

109

and use the code VERITASIUM for an extra 10 % off.

110

Just scan this QR code or follow the link in the description to get started.

111

I wanna thank Hostinger for sponsoring this part of the video and now back to Klein, Gordon and Falk.

112

The Klein, Gordon, Falk equation even made its way over to one of the fathers of quantum physics,

113

Niels Bohr, who must've been impressed because at the 1927 Solvay Conference, Bohr was chatting to a promising young physicist about what he was working on.

114

The young man said, I'm trying to get a relativistic theory of the electron.

115

Bohr replied, but Klein has already solved this problem.

116

The young man disagreed.

117

So who was this bold 25 -year -old?

118

He was Paul Dirac, a man who Bohr would later give the nickname the strangest man

119

because he was truly one of a kind.

120

The first time I went to Princeton, I met a couple who invited Dirac and his wife, and Dirac for the whole meal,

121

which lasted three hours, didn't say a single word.

122

Not one word.

123

He wasn't sulking.

124

He wasn't angry.

125

He just saw no reason to speak.

126

That's crazy.

127

Dirac was a man of so few words that his colleagues invented a special unit, a dirac, equivalent to speaking one word per hour.

128

Legend also has it that he liked to relax by climbing trees in three -piece suits.

129

And that wasn't his only strange hobby.

130

As a student, Dirac had attended seminars on Einstein's theory of relativity, and he'd kind of fallen in love with it.

131

He was impressed by the elegance and beauty of its mathematics.

132

To Dirac, this got to the core of what a good theory must do.

133

He once said, It is more important to have beauty in one's equations than to have them fit experiment.

134

Do you know why Dirac was so drawn to relativity?

135

Einstein went in there and did it all, not all, but almost all by deductive reasoning.

136

That made a huge impression on Dirac.

137

He realized that it was advanced mathematics was really important to understanding the way that nature works.

138

So Dirac found a new hobby, obsessively updating classical equations with Einstein's theory of relativity,

139

so that they would now work for relativistic scenarios as well that are those approaching the speed of light.

140

He later said this was like a game for him.

141

And so it's maybe not too much of a surprise then

142

that he was one of the people trying to unite quantum physics and relativity.

143

Because a theory that would cover both of these would be truly beautiful in his eyes.

144

Dirac didn't see the beauty in Klein, Gordon, and Fox's relativistic update of the Schrodinger equation.

145

And he wasn't alone.

146

Fellow physicist Wolfgang Pauli, after reading Klein's paper, wrote him a letter wishing his physics a speedy recovery.

147

Pauli was known for being pretty savage, and actually the Klein -Gordon equation is useful in some cases.

148

But he and Dirac were both concerned with one particular feature of the equation.

149

And that's because the Klein -Gordon equation contains this term.

150

That's a second -order time derivative, so the wave function is being differentiated twice with respect to time.

151

Now, remember, the Schrodinger equation also contains a second -order derivative, but that's with space, not with time.

152

Time is just first order.

153

So why is this such a problem?

154

Well, think of a very simple second -order time derivative.

155

Something like d squared y over dt squared is equal to 3.

156

Now, to figure out what y is, the function that satisfies this equation, we just have to integrate it twice.

157

But notice, to find the solution, we now have two integration constants, c and d.

158

And the problem is that we can only find that if we know both y at a given time,

159

and if we know the first derivative of y at a given time.

160

This makes sense physically if you think about, say, throwing a ball in the air.

161

If you want to predict the ball's future motion, well, then you need to know where you threw it from,

162

that's its position, and how hard you threw it, that's its velocity.

163

Or in other words, you need to know both the position and the first derivative of the position.

164

Similarly, to predict a quantum system's future state using the Klein -Gordon equation,

165

you'd need the initial wave function and the initial first derivative of the wave function.

166

Now, the beauty of the Schrodinger equation was that

167

if you knew only the wave function of a system at a given time,

168

you could use it to determine all the future states of that system.

169

But in the Klein -Gordon equation, the wave function no longer told the whole story.

170

And this introduced a further problem.

171

The probability of finding a particle in a given area could

172

no longer be simply described by the modulus of the wave function, as it was for the Schrodinger equation.

173

For the Klein -Gordon equation, a new equation was needed to describe the probability using the wave function, which takes a while to derive,

174

so we won't get into that here, but it looks like this.

175

And the problem with that equation is that, unlike the simple modulus of the wave function, it can have negative solutions, negative probabilities.

176

How can the chance of something happening be less than zero?

177

As Dirac put it, that, of course, is physically nonsense.

178

So Dirac did what Dirac does best, and he set out to find his own solution that contained no second -order time derivatives.

179

First, he rewrote the relativistic energy -momentum relationship like this, which is a linear equation, so no squaring E, that is, no squaring the energy operator,

180

which is what gave rise to that second -order time derivative in the Klein -Gordon equation.

181

Now, he just had to solve this equation and find the coefficients alpha x, alpha y, alpha z, and beta, and he would have a solution.

182

To do this, we can start by squaring both sides of the equation.

183

Next, we can expand the momentum on the left -hand side into its three dimensions,

184

and then expand the right -hand side to end up with this long equation.

185

It might look pretty complicated, but there is a trick we can use.

186

Notice that all these terms on the right -hand side are multiplied by pmc cubed, but on the left -hand side,

187

there are no terms containing pmc cubed.

188

So that tells us that when you multiply these coefficients together, like alpha times beta and beta times alphas, all of that must be zero.

189

And similarly, notice that these terms are multiplied by two different momenta, like px and py times c squared,

190

which also don't have a counterpart on the left -hand side.

191

So these alpha times alpha terms must also be equal to zero.

192

And that just leaves this top row.

193

These terms do match on both sides, which tells us that alpha x squared must be equal to 1, and so must alpha y squared,

194

alpha z squared, and beta squared.

195

So Dirac now had a set of simultaneous equations for alpha x, alpha y, alpha z, and beta that he just needed to solve.

196

That sounds easy, right?

197

But it's not that straightforward.

198

To see why, let's do some examples.

199

So say alpha x is 1 and beta equals minus 1.

200

Well, then we get 1 squared, which is 1, equals minus 1 squared, which is also 1.

201

So that satisfies the top equation.

202

But if we fill it in here, we get 1 times minus 1, which is just minus 1, plus minus 1 times 1,

203

which is again minus 1, for a total of minus 2.

204

So that doesn't work.

205

So let's try another number.

206

Say alpha x is 1, and now it's alpha z that is minus 1.

207

Well, in that case, we get 1 times minus 1, so again we get minus 1, plus minus 1 times 1,

208

again minus 1, so we again get minus 2.

209

And in fact, if you look at this top equation, the only numbers that satisfy this relation is 1 and minus 1.

210

So all these coefficients have to be either 1 or minus 1.

211

But if you fill that in, then the only two possible answers you're going to get in these equations is minus 2 or positive 2.

212

Now, the two equations that are giving problems are these two.

213

And that's because alpha times beta is the same as beta times alpha.

214

So you're always going to get minus 2 or 2.

215

So the only way to get out of this is to somehow make the order of multiplication matter.

216

But where do you find something like that?

217

Let's think about a really simple vector, say 1 in the x -direction and 2 in the y -direction.

218

If I reflect this vector along the line y equals x, and then reflect it about the x -axis, I get this.

219

But if I start again and instead reflect it first about the x -axis, and then along y equals x,

220

I get this, which is totally different.

221

So the order matters.

222

How do we express this mathematically?

223

Mathematically, we need some way to represent transformations like reflections as a single coefficient, which means we need matrices.

224

Matrices are arrays of numbers which encapsulate these transformations, telling us how to reflect, rotate, stretch, or squish across each dimension.

225

Mathematically, the order we multiply them matters, since we multiply the rows of the first matrix by the columns of the second.

226

A matrix that reflects along y equals x looks like this, and a matrix that reflects about the x -axis looks like this.

227

And if I apply them to that original vector one by one, you can see how we end up with different results.

228

And actually, using matrices in quantum mechanics was not completely out there.

229

Dirac had already seen it done by Werner Heisenberg, his new, if unlikely, friend.

230

Dirac and Heisenberg were very different characters.

231

Heisenberg was charming and outgoing.

232

Dirac hated socializing and small talk.

233

Once, the two were on a cruise ship to a conference together, and Heisenberg spent a lot of time dancing with women on board.

234

So Dirac asked him, why do you dance?

235

Heisenberg replied, when there are nice girls, it is a pleasure.

236

Dirac thought about this for a bit before saying, how do you know beforehand that the girls are nice?

237

So they were pretty different guys, but Heisenberg had a profound impact on Dirac.

238

Do you know what drew him to quantum mechanics initially?

239

He always said that Heisenberg gave him his start.

240

He always said that.

241

In later life, he used to begin lectures when he was 70, even 80 years old with the following words.

242

Quantum mechanics was discovered by Heisenberg in 1925.

243

As a PhD student, Dirac had closely followed Heisenberg's work.

244

And a few years earlier, Heisenberg had found that for certain properties, like position and momentum, the order of multiplication matters,

245

so x times p is not the same as p times x.

246

This was actually the beginning of Heisenberg's famous uncertainty principle,

247

which says that there's a limit to how precisely we can know certain pairs of physical properties in quantum systems.

248

It turns out that if the order of multiplication matters for two properties,

249

like position and momentum, then that means there's an inherent uncertainty in their combined measurements.

250

The order we measure them in will change the outcome.

251

Heisenberg's mentor, Max Born, suggested that he could represent this mathematically by using matrices, because there the order of multiplication also matters.

252

This led Heisenberg to a new form of quantum mechanics, which was mathematically equivalent to Schrodinger's equation, but it was based on matrix algebra.

253

Dirac recognized something similar in his coefficients, where the order of multiplication clearly mattered.

254

So he thought matrices might be the solution.

255

For his coefficients, he tried two -by -two matrices, since they're the smallest, simplest matrices that could make the order of multiplication matter.

256

He found that some worked with the equations.

257

In fact, those reflection matrices we tried earlier worked well.

258

But he just couldn't find solutions for all four coefficients that all work together.

259

This is not simple.

260

You're directly one of the smartest people of the 20th century in physics, right?

261

But it was not easy.

262

You don't have just to twiddle with bits of the Schrodinger equation.

263

You have to do something completely radical.

264

His working is in there and you could see him really struggling.

265

Then Dirac had a stroke of genius.

266

He said, I suddenly realized that there was no need to stick to quantities

267

which can be represented by matrices with just two rows and columns.

268

Why not go to four rows and columns?

269

With these four 4x4 matrices, Dirac got these solutions to his simultaneous equations.

270

A matrix with 1s on the diagonal is mathematically equivalent to a 1, and a whole array of 0s is the same thing as 0.

271

So all of the equations were satisfied.

272

Dirac had found coefficients that worked.

273

Now, we can substitute those matrices back into Dirac's linear equation, then rewrite the 3 -momenta and alpha coefficients as vectors,

274

and, just as we did with the Schrodinger and Klein -Gordon equations,

275

we can use the energy and momentum operators to make both sides act on the wave function.

276

If we substitute those operators in, we get Dirac's final equation for the relativistic free electron.

277

The young man who spent his life looking for mathematical beauty had found, perhaps, his most beautiful equation of all.

278

People were expecting the solution to be horrible, and it turned out to be a thing of beauty, right?

279

It was something you look at and you think, that is absolutely amazing.

280

It's a pattern that Dirac had found deep in nature.

281

Nothing like that had been seen in physics at that time.

282

To see some of that beauty, you only have to compare it to the Schrodinger equation, which you can see right here.

283

Now, the two look very similar, but there is a lot hidden in the Dirac equation.

284

First of all, it's relativistic, so it works at really, really high speeds because it uses Einstein's energy -momentum relationship.

285

Unlike the Schrodinger equation, which breaks down there.

286

But what's even more beautiful is that if you look at this, the Ruggs equation isn't just first order in time derivatives,

287

it's also first order in spatial derivatives.

288

Whereas the Schrodinger equation was second order in spatial derivatives.

289

So you might wonder, well, why does that matter?

290

Well, because not only does going first order in all derivatives solve the second order time derivative problem of the Klein -Gordon equation,

291

it now also treats time and space symmetrically.

292

And it becomes really important if you start thinking about relativity, where time and space are not separate dimensions.

293

They're intertwined in a four -dimensional space -time.

294

And while the Schrodinger equation had just a single component wave function, to make the wave function work with these four -by -four matrices,

295

in the Ruck's case, you actually need a wave function that has four components.

296

So it will look something like this.

297

Psi 1, Psi 2, Psi 3, and Psi 4.

298

The four -component wave function means that the Dirac equation describes four possible states for any quantum system,

299

which reveals something Schrodinger's single wave function didn't.

300

An electron at a given energy level actually has two possible states due to different orientations of its intrinsic angular momentum, spin up and spin down.

301

This spin creates a tiny magnetic field, so these two states are like tiny magnets pointing in opposite directions.

302

And that has an interesting implication.

303

Take hydrogen, which has one proton and one electron.

304

Classically, the electron whizzes around the proton.

305

But if you switch to the perspective of the electron, well, now it's the proton that's moving.

306

And since it's positively charged, that moving charge creates a magnetic field in the electron's frame of reference.

307

So now you've got this electron, which has its own mini magnetic field, interacting with a larger magnetic field,

308

and that interaction will be slightly different for a spin -up and a spin -down electron.

309

And as a result, some energy levels of the electron split into two closely -spaced energy levels.

310

And you can actually see this if you zoom into the emission spectrum for a hydrogen atom.

311

Schrodinger's equation didn't predict that splitting, since it just had one solution for each energy level.

312

But with Dirac's four -component solution, the top two wave functions now described two different spin states with two slightly different energies.

313

Dirac admitted himself that he had never set out to capture spin in his equation.

314

I started out this work without any intention at all of bringing in the spin of the electron.

315

But hold on, because if an electron at a given energy level only has two possible spin states,

316

then why do we have a four -component wave function?

317

Why are there four states and not just two?

318

Well, that's what brings us back to that 1928 lecture at the start,

319

the one that seemed to drive even the most well -established quantum physicist mad, because that strange man presenting his work was actually Paul Dirac.

320

Sharing his new equation with the world it was Dirac's beautiful equation that Heisenberg called the saddest chapter in modern physics

321

To understand why all those physicists were losing their minds

322

We only have to look at the simple case when a particle is at rest

323

This term right here describes the momentum

324

So when the particle doesn't move this becomes zero and it drops out which gives us this

325

next we can sub in the energy for this quantum operator to get this

326

so now we know that beta times mc squared must be equal to the particle's energy

327

and if we write out beta

328

and multiplied by mc squared then we find two positive solutions

329

and two negative solutions for the energy

330

so negative energy solutions are baked right into the rock equation this idea

331

that a free electron could have negative energy was impossible for any of these physicists to accept

332

because think about it if electrons can have a negative energy then

333

that means that they could continually radiate positive energy

334

that is emit photons and drop into lower

335

and lower negative energy states there would be no limit to how far

336

they could fall into the negative energy of this it looked like to many very smart people

337

and dirac was smart enough to know they had a point

338

that this equation it got the mass

339

and the magnetic moment of the electron yes

340

but on the other hand it predicts this ridiculous situation where they have negative energy values right that's nonsense

341

so heisenberg said i give up this is just this is just ridiculous

342

so dirac he had in some very clear sense had to

343

rescue his equation dirac spent three years sticking to his guns

344

he tried all sorts of ways to interpret his new equation to explain where the negative energy was coming from.

345

Then, in 1931, he proposed something radical to explain the negative energy solutions.

346

A new kind of particle, unknown to experimental physics, having the same mass and opposite charge to an electron.

347

We may call such a particle an anti -electron.

348

We should not expect to find any of them in nature on account of their rapid rate of recombination with electrons.

349

So Dirac was proposing that those four states described in his four -component wave function are a spin -up electron,

350

a spin -down electron, a spin -up anti -electron, and a spin -down anti -electron.

351

When Dirac said that, people didn't stop running around saying where's this anti -electron, they just ignored it.

352

Cut to the laboratory at Caltech. In 1932,

353

a Caltech postdoc named Carl Anderson was working on a project trying to identify the charge particles produced by cosmic rays.

354

He photographed the tracks these particles left in a cloud chamber containing a uniform magnetic field.

355

Anderson noticed several instances of a similar track.

356

It looked a lot like the tracks he'd seen left by electrons, only it curved in the opposite direction in the magnetic field,

357

kind of like the tracks of positively charged protons.

358

But there was no way it could be a proton based on the length of the track.

359

It had traveled farther in air and therefore it had to be much lighter.

360

It had to be something with around the same mass as an electron, but opposite charge.

361

A positive electron, or as he named it, a positron.

362

He actually also tried to rebrand electrons as negatrons, but that one didn't quite stick.

363

Just one year after Dirac proposed the anti -electron, Carl Anderson found it, entirely by accident.

364

But this alone doesn't get rid of the negative energy problem.

365

Remember what we said earlier, if any particles, like these positrons,

366

can have negative energy, then they could continually radiate energy and drop into lower and lower negative energy states.

367

Fortunately, Dirac proposed a solution to this problem as well, although it was a little crazy.

368

He theorized something called the Dirac Sea, describing a vacuum as an infinite sea of electrons occupying all available negative energy states.

369

And since no two electrons can occupy the same state, this prevents observable positive energy electrons from falling into the negative energy states.

370

A hole or vacancy in this sea then becomes a positron.

371

When an electron and a positron meet and annihilate, well that's just an electron falling back into the sea and filling that hole.

372

The theory is mathematically sound, of course it's Dirac we're talking about, but if you feel like it's hard to come to terms with the idea

373

that we're floating on an infinite sea of electrons, well, you wouldn't be alone.

374

It's not even that crazy if, I don't know, several decades later people find that exact model in a physical system.

375

In condensed matter physics you see an exact analogy of also having electrons in the conduction

376

Band and Save Phelan's Band.

377

Yes, that's true.

378

That's true.

379

But Dirac really was, you know, out there.

380

You see what I mean?

381

Maybe it was people like, you know, really senior people saying, this is nonsense.

382

But it was a way of Dirac's rectilinear thinking, thinking, well, maybe this one.

383

And then it drove him to antimatter.

384

Thankfully, in 1941, a Swiss physicist named Ernst Stuckelberg had a clever idea.

385

The wave function contains a term that looks like this, where energy is multiplied by time.

386

So we can see that if we just change the sign of time when the energy is negative, then we get the same result,

387

because a negative multiplied by a negative gives us a positive.

388

Stuckelberg took this, and he suggested that negative energy electrons traveling backwards in time are mathematically equivalent to positive energy anti -electrons,

389

that is positrons, traveling forwards in time.

390

A few years later, around 1948, Richard Feynman took this idea and used it in one of the most powerful tools in modern physics,

391

the Feynman diagrams.

392

In his sketches of particle interactions, he showed antiparticles traveling the opposite way to particles, backwards in time.

393

It was a brilliant trick.

394

Negative energy solutions no longer had to mean there was negative energy or a Dirac -C.

395

They simply indicated the presence of an antiparticle.

396

We now know that there's a corresponding antiparticle for every subatomic particle, with the same mass but opposite charge.

397

So the proton has the antiproton, the neutrino has the antineutrino, and so on.

398

According to his friend Heisenberg, that was the biggest leap in 20th century physics,

399

to say there's a whole slew of antiparticles corresponding to particles.

400

And Dirac got that through this crazy model.

401

But all is not solved.

402

This new anti -world introduces some big questions about the very nature of our universe.

403

Because particles and their antiparticles are equal and opposite,

404

when they come together, they annihilate and produce two photons with energy equivalent to their mass and kinetic energy.

405

And this process is reversible.

406

Two photons with the right energies can produce a matter and antimatter pair.

407

This is called bright wheeler pair production.

408

During the first moments after the big bang, the universe was hot, dense, and full of these pairs popping into and out of existence.

409

If an equal number of matter and antimatter particles were created, you would expect them to all annihilate each other in this dense environment,

410

leaving behind only energy.

411

But that didn't happen.

412

We ended up with a universe full of matter.

413

If we work backwards from how much matter and antimatter is in the universe today,

414

it's actually estimated that only one particle per billion of matter needed to survive this hot, dense era and not get annihilated.

415

That tiny difference would give us the makeup of our universe today, where matter dominates.

416

so what allowed that one particle per billion to escape annihilation why did matter win out over anti -matter

417

well that's a pretty big question with some not so simple answers

418

so we're doing an entire second video where we have some pretty spectacular stuff happening

419

we're getting into the beast it's absolutely insane i feel like i should not be in here

420

So you're making anti -atoms?

421

Yes.

422

It's so cool.

423

Dirac is probably less well -known than people like Heisenberg or Schrodinger, but his contribution to quantum physics was immense, and he was recognized for it.

424

He shared the 1933

425

Nobel Prize with Schrodinger for the discovery of new productive forms of atomic theory.

426

And perhaps this gave strange, quiet Dirac some newfound confidence, because that physicist in the audience of his 1928 lecture at the start,

427

Eugene Wigner, actually became a reasonable friend of Dirac's and in 1934 introduced Dirac to his sister,

428

Majit Wigner, a woman who would change Dirac's life perhaps more than any equation or Nobel Prize.

429

They were antiparticles, right?

430

They had completely different personalities.

431

He had almost no empathy right

432

and he knew it she had buckets of it he hardly talked

433

she couldn't stop talking you could just go on like they are completely different people

434

but the marriage did work my great friend Lillia Harris Chandra who knew them

435

she came out with a great line great line

436

which is he gave her status she gave him a life

437

so I guess there is one particle antiparticle pair that that never annihilated.

438

and it's the perfect way to challenge your friends.

439

Every time we play it at Veritasium, things get a little bit heated and that is so much fun.

440

If you want to go check it out, then head over to our Kickstarter where you can pre -order your own version.

441

And right now we've enabled global shipping, so no matter where you are in the world, you can get your own version.

442

We're coming close to the final few days of the Kickstarter campaign,

443

so this is your last chance to get your hands on the exclusive launch edition.

444

So if you want to support us, head over to Kickstarter by clicking the link in the description or scanning this QR code.

445

I want to thank you for all your support and most of all, thank you for watching.

הורד אפליקציה

כל מה שאתה צריך כדי לדבר בשטף

הגיית AIציון לכל משפט

תרגול IPAשלוט בכל צליל

אוצר מיליםבנה את מאגר המילים שלך

משחק אוצר מיליםלמד תוך כדי משחק

סרוק QR

Download on theApp Store Get it onGoogle Play

אותו ערוץ

FOR YOU

המלץ על סרטון

128 משפטים

The Terrible Paradox of Self-Awareness | Fernando Pessoa

Daily ConversationUniversity StudentsWorking Adults

39 משפטים

Why Walls STOP Light Waves But Not Radio Waves?

Daily ConversationUniversity StudentsWorking Adults

279 משפטים

LET THERE BE... Voltage? | Maxwell's Equation #2 Explained for Beginners

Daily ConversationUniversity Students

370 משפטים

Are We Living Inside an Alien Control System?

Daily ConversationUniversity StudentsWorking Adults

147 משפטים

We Traveled Back in Time. Now Physicists Are Angry.

Daily ConversationUniversity StudentsWorking Adults

62 משפטים

A mind-blowing explanation of symmetry | Frank Wilczek for Big Think

Daily ConversationUniversity StudentsWorking Adults

32 משפטים

What if quantum physics could eradicate illness? | Jim Al-Khalili for Big Think

Daily ConversationUniversity StudentsWorking Adults

245 משפטים

Oppenheimer Wrote One Letter About Feynman — It Changed His Life

Daily ConversationUniversity Students

234 משפטים

The Philosophy of Rick Sanchez

Daily ConversationUniversity StudentsWorking Adults

73 משפטים

Superfluid Dark Matter

Daily ConversationUniversity StudentsWorking Adults

אוצר מילים והערות הגייה לשיעור הזה

שיעור הדיבור הזה מבוסס על הסרטון “Why is there antimatter?”. המילים שחוזרות הכי הרבה: equation, energy, dirac, electron, particle. בסרטון הזה יש 445 משפטים ו-6013 מילים לתרגול Shadowing. הדיבור נמשך 35:22. הדובר מדבר בקצב טבעי של כ-170 מילים בדקה, קרוב לשיחה יומיומית. רק 77% מהמילים נמצאות בין 3,000 המילים הנפוצות ביותר באנגלית, כך שאוצר המילים מאתגר.

אוצר מילים מרכזי בסרטון הזה

15 מילים מהסרטון שכדאי ללמוד, עם ההגייה והמשמעות שלהן:

מילה	הגייה	משמעות
equation שם עצם	/ɪˈkweɪ.ʒən/	משוואה \ מִשְׁוָאָה
particle שם עצם	/ˈpɑɹtək(ə)l/	חֶלְקִיק, גַרגִיר
electron שם עצם	/ɪˈlɛk.tɹɑn/	אֶלֶקְטְרוֹן
momentum שם עצם	/ˌmə(ʊ)ˈmɛntəm/	תֶּנַע
quantum שם עצם	/ˈkwɑntəm/	קְוַנְט
alpha שם עצם	/ˈæl.fə/	אַלְפָא
minus	/ˈmaɪ.nəs/	פחות, מינוס
physics שם עצם	/ˈfɪz.ɪks/	פִֿיזִיקָה
spin פועל	/spɪn/	הסתובב
beta שם עצם	/ˈbeɪtə/	בֵּיתָא
operator שם עצם	/ˈɑpəˌɹeɪtɚ/	מרכזיה
magnetic שם תואר	/mæɡˈnɛtɪk/	מַגְנֵטִי
mechanic שם עצם	/mɪˈkænɪk/	מְכוֹנַאי
reflect פועל	/ɹɪˈflɛkt/	החזיר
component שם עצם	/kəmˈpoʊ.nənt/	מַרְכִּיב, רְכִיב

Phrasal verbs שתשמעו

מילה	משמעות
check out פועל	בדק
drop out פועל	נָשַׁר
figure out פועל	הֵבִין, מצא תשובה
give up פועל	וִתֵּר
write down פועל	רָשַׁם

משפטים שכדאי לחזור עליהם

משפטים קצרים ושלמים מהסרטון שאפשר להשתמש בהם בשיחה יומיומית:

I mean, what would that even mean?
To see why, let's do some examples.
How do we express this mathematically?
Do you know what drew him to quantum mechanics initially?
You see what I mean?

דקדוק בסרטון הזה

המבנים שהדובר משתמש בהם הכי הרבה, עם המילים המדויקות מהסרטון:

מבנה	בסרטון
סביל (Passive voice) be + past participle — הדגש הוא על מה שקורה, לא על מי שעושה את הפעולה	was based · is always measured · would be bound
משפטי זיקה (Relative clauses) who / which + clause — מידע נוסף על אדם או דבר	equation, which was · man who Bohr · this, which is
הווה מושלם (Present perfect) have/has + past participle — פעולה שקרתה בעבר ועדיין יש לה חשיבות או השפעה עכשיו	have taken · have changed · has already solved

הגייה ששווה לשים לב אליה

הדובר משתמש ב-57 קיצורים וצורות מקוצרות, כמו didn't, you're, we're. אמרו אותם בצורה הקצרה, כפי שאתם שומעים אותם.

הצלילים של “th”: mathematics /mæθ(.ə)ˈmæt.ɪks/, mathematical /ˌmæθ(.ə)ˈmæt.ɪ.kəl/
הצלילים של “sh” ו-“zh”: equation /ɪˈkweɪ.ʒən/, dimension /daɪˈmɛn.ʃən/, relation /ɹɪˈleɪʃ(ə)n/, transformation /ˌtɹæns.fɔɹˈmeɪ.ʃən/, dimensional /daɪˈmɛn.ʃə.nəl/
מילים ארוכות — שימו לב להטעמה: momentum /ˌmə(ʊ)ˈmɛntəm/, derivative /dɪˈɹɪvətɪv/, operator /ˈɑpəˌɹeɪtɚ/, probability /ˌpɹɑ.bəˈbɪl.ə.ti/, ridiculous /ɹɪˈdɪk.jə.ləs/

צלילים שדוברי עברית מתקשים בהם:

/w/ — שפתיים מעוגלות, לא /v/: equation /ɪˈkweɪ.ʒən/, quantum /ˈkwɑntəm/, backward /ˈbækwɚd/, straightforward /ˌstɹeɪtˈfɔː(ɹ)wə(ɹ)d/, whereas /(h)wɛɹˈæz/
/æ/ — פתוח יותר מה-“אֶ” העברית, לא כמו “a” ב-“cup”: alpha /ˈæl.fə/, magnetic /mæɡˈnɛtɪk/, mechanic /mɪˈkænɪk/, classical /ˈklæs.ɪ.k(ə)l/, radical /ˈɹædɪkəl/
/r/ — הלשון לא נוגעת בשום מקום, לא ה-“ר” הגרונית: particle /ˈpɑɹtək(ə)l/, electron /ɪˈlɛk.tɹɑn/, matrix /ˈmeɪ.tɹɪks/, derivative /dɪˈɹɪvətɪv/, operator /ˈɑpəˌɹeɪtɚ/

איך לתרגל עם הסרטון הזה

הקשיבו לכל הסרטון פעם אחת בלי לדבר, ורשמו את המילים שאתם לא מכירים.
התחילו במהירות 0.75×, תרגלו Shadowing משפט אחר משפט, וחזרו למהירות רגילה כשזה מרגיש קל.
הקליטו את עצמכם והשוו למקור, תוך תשומת לב למילים כמו equation, particle, electron.

מהי טכניקת השאדואינג?

שאדואינג היא טכניקת למידת שפה מבוססת מדע שפותחה במקור להכשרת מתורגמנים מקצועיים ופופולרית על ידי הפוליגלוט ד"ר אלכסנדר ארגואלס. השיטה פשוטה אך עוצמתית: אתה מקשיב לאודיו באנגלית ילידית וחוזר עליו בקול רם מיד — כמו צל העוקב אחרי הדובר עם עיכוב של 1–2 שניות בלבד. בניגוד להאזנה פסיבית או תרגילי דקדוק, שאדואינג מכריח את המוח והשרירים בפה לעבד ולשחזר דפוסי דיבור אמיתיים בו זמנית. מחקרים מראים שזה משפר משמעותית את דיוק ההגייה, האינטונציה, הקצב, הדיבור המחובר, הבנת השמיעה ושטף הדיבור — מה שהופך אותו לאחת השיטות היעילות ביותר להכנה לIELTS Speaking ולתקשורת באנגלית בעולם האמיתי.

טכניקת Shadowing: קרא את המדריך המלא שלב אחר שלב →

Shadowing Practice: Why is there antimatter? - Learn English Speaking with Video

בחר מצב תרגול

הורד אפליקציה

White holes and wormholes are probably real. Here's why.

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

No device could prove Einstein wrong... until now

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?

Why Cold Drinks Were Deadly Before 1914

Exposing The Flaw In Our Phone System

Our Greatest Delusion

The Most Controversial Problem in Philosophy

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

The Most Misunderstood Concept in Physics

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 Terrible Paradox of Self-Awareness | Fernando Pessoa

Why Walls STOP Light Waves But Not Radio Waves?

LET THERE BE... Voltage? | Maxwell's Equation #2 Explained for Beginners

Are We Living Inside an Alien Control System?

We Traveled Back in Time. Now Physicists Are Angry.

A mind-blowing explanation of symmetry | Frank Wilczek for Big Think

What if quantum physics could eradicate illness? | Jim Al-Khalili for Big Think

Oppenheimer Wrote One Letter About Feynman — It Changed His Life

The Philosophy of Rick Sanchez

Superfluid Dark Matter

אוצר מילים והערות הגייה לשיעור הזה

אוצר מילים מרכזי בסרטון הזה

Phrasal verbs שתשמעו

משפטים שכדאי לחזור עליהם

דקדוק בסרטון הזה

הגייה ששווה לשים לב אליה

איך לתרגל עם הסרטון הזה

מהי טכניקת השאדואינג?