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Can We Describe The Whole Universe With A Single Number? - Video học tiếng Anh
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Can We Describe The Whole Universe With A Single Number?
Can We Describe The Whole Universe With A Single Number?
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0:00
Picture this: it’s 2002. You’re a particle physicist.
0:04
And you’ve grabbed your snack of choice to
0:05
relax with the latest issue of the Journal of High Energy Physics.
0:09
I’ve actually been published in that one.
0:11
But my paper’s not the one that we’re gonna talk about.
0:14
I was a child in 2002.
0:16
Sandwiched between papers about black holes and supersymmetry,
0:20
you might find an article that’s even weirder,
0:22
about something even more fundamental.
0:25
The paper had three authors…all esteemed particle physicists…
0:29
debating each other about an almost philosophical question
0:33
they’d been arguing since the early 1990s:
0:36
How many numbers do we need to describe reality?
0:40
You may think, what with reality being so big and all,
0:43
that it would be a pretty huge number.
0:45
But the debate was about whether the number was three,
0:48
or two, or…zero?
0:51
As weird as that is, fast-forward to now,
0:53
and that debate still hasn’t been settled.
0:57
In fact, a new argument has been made that the answer is…one.
1:00
So, can we really describe the whole universe with a single number?
1:05
[♪ INTRO]
1:09
If you’ve ever driven between the US and Canada,
1:12
you’ll have noticed that the speed limits change units
1:14
as you cross the border, between miles per hour and kilometers per hour.
1:19
I promise, this is relevant.
1:20
Like, a 50 mile an hour road would become an 80 kilometer an hour road.
1:25
But despite the numbers being different,
1:27
the speeds they represent are the same.
1:29
Because miles and kilometers represent
1:31
the same physical quantity: distance. ~
1:34
So even though there are countless units out there…
1:38
seconds and years for time, calories and electron volts for energy,
1:42
Celsius and Kelvin for temperature… we don’t strictly need most of them.
1:47
We have more options just for convenience, or historical reasons.
1:51
In fact, some whole types of measurement are redundant.
1:55
Like, all temperature units can be converted into energy ones,
1:59
since temperature is just a form of energy.
2:01
Or kilograms and pounds and stone.
2:04
What the heck is up with stone?
2:06
So, the question that the particle physicists
2:08
were debating in the 90s was “how low can you go?”
2:11
How many units can you plausibly get rid of?
2:13
This may seem like a strange thing for particle physicists to be debating,
2:17
but it was actually directly relevant to their work studying
2:20
the fundamental constants: the baseline numbers at the heart of reality.
2:25
If you ever took a physics class,
2:26
you’ll have seen a bunch of numbers listed on a page in your textbook.
2:30
These numbers are the constants.
2:32
They describe basic properties of reality as we know it,
2:35
like the speed of light, the strength of gravity,
2:38
the mass of the electron, and so on.
2:40
Right now, the best theory of physics predicts 30 fundamental constants
2:44
of nature…which is quite a lot more than three.
2:48
So what’s going on?
2:49
Well, these 30 constants mostly come from particle physics,
2:52
giving us things like the masses of the fundamental particles
2:56
and the strengths of the fundamental forces.
2:58
As far as we know, the 30 constants are fundamental,
3:02
in that our theories can’t predict their values.
3:05
We can only measure them.
3:06
And we think they’re all the exact same value everywhere in the universe.
3:10
Always have been, always will be.
3:12
But importantly, 27 of those constants can be written
3:15
so that they don’t have units.
3:18
They’re what’s known as dimensionless.
3:20
In physics jargon, a dimension is basically just a unit.
3:24
Think distance, time, energy, charge…
3:27
basically anything we can imagine physically measuring.
3:30
To get why this is important, consider the number 1.07.
3:35
Sounds completely random, but it’s not.
3:37
It’s the mass of the iPhone 15 Plus,
3:40
divided by the mass of the iPhone 15 Pro.
3:43
To work that number out, you could have weighed the two iPhones
3:46
in pounds, or grams, or any other unit you liked.
3:50
It wouldn’t matter, because after you divide
3:52
one mass by the other, the units disappear.
3:55
In some fictional Apple universe,
3:57
the ratio 1.07 could be a dimensionless constant dictating
4:01
the relative masses of the Plus and the Pro models for all of eternity.
4:05
In the real universe, this isn’t true at all.
4:08
For example, the ratio for the 16 Plus and Pro isn’t 1.07, it’s exactly 1.
4:15
But it’s easier for me to explain the concept using smartphones
4:18
than anything from our list of 30 fundamental constants.
4:21
But of those 30 constants, a whopping 27 are like our example 1.07:
4:28
a number with no units, where it doesn’t matter if you’re using
4:31
imperial units or metric or some other system to measure things.
4:35
And the thing is, there’s a lot we still don’t
4:37
understand about this list of 27 constants.
4:40
Especially whether 27 is the actual final amount on this list.
4:44
It’s fully possible that there are fundamental,
4:47
dimensionless numbers missing from the list,
4:49
hidden in the consistently murky areas of physics
4:52
like dark matter or cosmic inflation.
4:54
Meanwhile, physicists might find a way to
4:57
explain some constants in terms of others.
5:00
That would reduce how many numbers we truly need to put on sheets
5:03
for students to memorize or not memorize
5:06
depending upon how good of a student you are.
5:08
But no matter whether the final theory of everything has more
5:11
or fewer than 27 dimensionless constants,
5:14
we still need units to actually do physics with them.
5:17
Like, if you’re doing an experiment in the lab,
5:19
you eventually need to measure some length scale,
5:21
be it in meters or feet or fathoms.
5:24
By nature of having units, the last three constants in our list of 30 should
5:28
establish the fundamental units for physics as we know it.
5:32
But do they?
5:32
Before I can answer that, I have to show you this ad…
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5:38
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6:29
The debate published in 2002…or the “trialogue” as its authors called it…
6:35
framed everything in terms of a minimum number of constants.
6:38
But after reading the whole thing, our writer walked away
6:41
feeling it was really a debate over the minimum number of units.
6:45
So let’s try and unpack all of their arguments.
6:48
And we’ll start with the most popular position,
6:50
that you need three constants with units.
6:53
For this episode, we’ll call them Team Three.
6:55
Feel free to assign them a mascot, like a triceratops or something.
6:59
Team Three’s argument starts like this:
7:01
we know most constants with dimensions
7:03
can be derived from other constants.
7:05
For example, you may have heard of the “SI” standard of units,
7:09
which defines seven units.
7:11
Some people call them fundamental units,
7:13
even though everyone agrees they aren’t.
7:15
They’re really just a matter of convenience.
7:18
Like remember this guy from chemistry class?
7:20
The mole counts the number of particles in a specific amount of carbon.
7:24
So really, it’s just a conversion factor between big and small scales.
7:29
Meanwhile, Einstein’s famous relation E equals m c squared tells us
7:34
mass is just energy wearing a different hat,
7:36
so we don’t need separate fundamental units for both energy and mass.
7:40
In fact, in particle physics, scientists write particle masses
7:44
using a unit of energy: the electron volt.
7:47
Team Three argues this trick can work for most constants with units,
7:51
but you’ll always need at least one unit for space,
7:54
one unit for time, and one unit for the stuff going on
7:58
within spacetime: an energy unit.
8:00
If you have fewer units than that, there are some
8:03
measurements in physics you won’t be able to do.
8:05
Like, if you don’t have a unit of time, you can’t measure any speeds.
8:08
What’s extra appealing here is, there’s a very natural choice for which
8:12
three physical constants are the fundamental ones you can derive all the
8:16
others from, and which ones get put on that list of 30 necessary constants:
8:21
The speed of light, c, the universal gravity constant, G,
8:25
and the Planck constant, h, which pops up
8:28
in a lot of quantum mechanics stuff.
8:29
Each of these constants is made by combining dimensions of length,
8:33
time, and energy…or in G’s case, energy wearing a mass hat.
8:38
That means you can reverse the process,
8:40
and combine the constants together to get three special units.
8:43
Physicists named them the Planck length, the Planck time,
8:47
and the Planck energy, after the guy who first proposed them:
8:50
German theoretical physicist Max Planck.
8:53
They’re special because they each represent a fundamental physical limit
8:56
in reality: the shortest span of length or time the universe
9:00
allows you to measure, and the most energy you can
9:03
possibly cram into one single particle.
9:06
So our constants c, G, and h have a solid case for being a fundamental
9:11
component of reality as we know it.
9:13
But the trialogue paper makes clear,
9:16
not everyone agrees this is as simple as it gets.
9:19
Team Two Constants chose their answer
9:21
after redefining the laws of physics.
9:23
More specifically, they assumed the weird and entirely
9:26
hypothetical world of string theory was true.
9:29
String theory is just one avenue scientists have explored
9:32
to try and explain all of physics in one go.
9:35
To unite the tiny world of quantum particles
9:38
with the massive world of gravity.
9:40
Because our current laws of physics cannot do that.
9:43
According to string theory,
9:44
reality is filled with fundamental, one-dimensional, vibrating strings.
9:49
The way these strings vibrate corresponds to
9:51
different kinds of particles and forces.
9:54
So, for instance, a particle of light is just a string vibrating one way,
9:58
and an electron is a string vibrating a different way.
10:01
Behind this simple idea is a vast library
10:04
of complicated mathematical theories.
10:06
But I’m not a string theorist, and there’s a good chance you aren’t either,
10:10
so this is all we need right now.
10:12
Over at Camp Triceratops, Team Three needs a separate unit of energy
10:16
to describe all the stuff going on in spacetime.
10:18
But if everything is just vibrating strings,
10:20
then the energy of each string is fixed by its length.
10:23
Energy no longer needs a separate unit;
10:26
it’s just length wearing a different hat.
10:29
That means we can get rid of energy and express everything
10:31
in terms of just two constants: the speed of light, and a special length scale.
10:36
Unfortunately, while lots of people work in and believe in string theory,
10:39
there’s no direct evidence backing it up, yet.
10:42
And as far as science can tell,
10:44
there’s little hope of that changing any time soon.
10:46
So we’ll have to wait a while longer to find out if Team Two
10:49
was just stringing us along.
10:51
Next, let’s look at the newest argument, from Team One Constant,
10:55
which instead of turning to a new version of reality,
10:58
tried to redefine the original question that was being debated.
11:01
And remember, Team One Constant is the new team
11:04
that came about decades after the original trialogue.
11:07
Instead of answering how many constants we need to describe reality,
11:10
they said the question is really: what is the minimum number
11:14
of instruments you need to make any possible measurement?
11:17
Because if you can do any possible measurement using just one device,
11:21
then the units of that measurement would be all you need.
11:24
Everything else would just boil down to that.
11:26
According to this paper’s authors, all you need are clocks.
11:30
Every measurement a physicist could ever want to
11:32
make can be done using some form of timing information.
11:36
Take mass, for instance. In 2010,
11:39
a team of astrophysicists used timing information from a kind of dead star
11:43
called a pulsar to calculate the masses of planets in our solar system.
11:47
In other words, a measurement of mass
11:50
was derived from a measurement of time.
11:52
If you can measure everything just using timing, that would mean c, G,
11:56
and h would all be expressed in terms of some timing unit,
12:00
bringing us from three down to one fundamental constant.
12:04
But what about that guy from the original trialogue
12:06
who went even simpler than that?
12:08
How the heck can he justify zero fundamental units?
12:11
Well, he basically went super existential with it all:
12:15
what if I redefine what even matters?
12:17
To Team Zero, any constant that depends on arbitrary things like units
12:22
can’t possibly be something real and fundamental.
12:25
It must be a human convention.
12:28
Remember how some of the SI units can be explained away?
12:31
Well here, the same logic applies to all units.
12:35
Any constant with units is just a conversion factor
12:38
between two different, related concepts.
12:41
Like, going back to E equals m c squared,
12:44
imagine defining c to be equal to 1.
12:46
And in fact, in some areas of physics, they really do set C equal to 1!
12:50
So the equation is now just E equals m.
12:54
It tells us mass is energy.
12:56
They’re equivalent.
12:57
And that revelation is the real physics behind the equation.
13:01
Everything else is just arbitrary labelling.
13:03
We lose nothing physical by ignoring c.
13:06
Team Zero says every constant with units is like this.
13:10
None of them are meaningful constants
13:12
that represent something fundamental about the world.
13:14
Instead, we’re just left with the dimensionless constants.
13:18
Which I will remind you currently stands at 27.
13:21
And 27 is not, you may have noticed, equal to zero.
13:25
In fact, that one author admits that we may need
13:28
a few dimensionless constants by the time we figure out
13:31
which are truly fundamental.
13:33
This is why our writer came out convinced that
13:35
this was ultimately an argument about units, not constants. ~
13:38
But whether you’re thinking in terms of constants, or in terms of units,
13:42
one thing is clear.
13:44
And that is that nothing is clear.
13:46
The arguments over this topic range from practical questions about
13:49
how we measure things, to speculative questions about future theories
13:53
of physics, to philosophical questions about the nature of reality.
13:57
But that’s okay, because these questions all get to the heart of
14:00
why we even do science.
14:02
And why we keep doing it.
14:04
In fact, you could even say our search for
14:06
a better understanding of the universe is, itself, a constant.
14:11
[♪ OUTRO]
