Hello to everyone,
Hello everyone.
I am very happy to be here with you
for reasons specific to today,
in fact quite particular,
I am very very happy to be with you.
(Applause)
I would like to explain to you how thought,
finally, a certain way of thinking,
to question the real,
to ask questions,
allows to go beyond the limits of the empirical world,
of the observable world,
of the world as it is already given to us.
And the example I would like to support is Albert Einstein.
Before mentioning the father of relativity,
I would like to talk a bit about what we call big data,
which you hear a lot about,
and you know they are already changing our lives, our ways of working.
Some people think that they will also change the way we do science.
Many think that,
instead of theorizing,
we will yield to the facilities of induction.
That is to say, we will observe patterns of behavior,
for example car traffic, for example consumers,
climate, weather,
economic indices, etc … –
and from these regularities,
we will infer laws, which we will consider as general,
even universal,
even though they will only condense what has already been given.
And so, instead of theorizing,
we could no longer predict, because, as you observed in the 20th century,
thanks to physical theories, for example,
we have been able to predict
the existence of new kinds of reality elements.
For example, thanks to the equations of particle physics,
we have been able to predict the existence of the Higgs Boson,
this particle detected for the first time in 2012.
Thanks to Einstein’s equations of gravitation,
we have been able to predict the existence of gravitational waves,
that were detected this year,
that is to say a century after their prediction in 1916 by Einstein.
And so big data is obviously a fantastic opportunity,
but they may lead us to look only at correlations,
correlations between the data that is available.
Now, a correlation,
it’s not the same thing as a cause and effect relationship.
It’s not because there are frogs after the rain
that we have the right to say that it has rained frogs.
So now,
I come to Einstein himself.
1915, Einstein publishes a paper, even several,
in which he develops a new theory of universal gravitation,
completely revolutionary,
with equations, therefore, associated with this theory.
And this theory is,
a century later,
the one we continue to use to make scientific cosmology.
That is to say, the countless data collected for a century,
by the satellites,
by telescopes, etc.,
have never had to
to question Einstein’s equations.
What did we know about the universe in 1915?
In fact, nothing.
We did not know that there were other galaxies than ours,
we did not know where the stars are coming from,
– we had not yet spotted the nuclear forces –
we obviously did not know that the universe is expanding.
And, as heavenly objects,
we only knew stars, planets and comets.
That’s all.
And so there was very little data,
which did not stop Einstein from elaborating his theory,
which continues to be valid,
despite all the data we have accumulated.
Now we can have fun making a counterfactual story,
imagine that things would have happened in reverse.
Imagine that we have all the data collected for a century,
by satellites and telescopes;
the question is :
“From this data,
could we find Einstein’s equations? “
And the answer is: no, at least it’s a negative answer
that Einstein would have brought to this question.
I quote it, I open the quotation marks.
He writes to Maurice Solovine,
one of his friends, with whom he had founded Olympia Academy:
“No inductive method can lead to the fundamental concepts of physics,
the inability to understand it is the most serious philosophical mistake
many nineteenth-century thinkers. “
End quote.
So how did Einstein proceed to develop a theory without data?
Well, he did it by so-called thought experiments.
Thought experiments,
it’s experiences you can imagine,
which invite to deviate from empirical reality,
to be transposed into a sort of elsewhere,
to see how a theory, which is assumed true,
would imply certain phenomena in a situation that one can imagine.
And Einstein started doing that as a teenager,
at the age of 15.
He was in Munich,
in a high school with military discipline, like those in Germany at the time.
He resigned at age 15.
He joins his parents in Italy.
And there, he is out of school for several months.
He writes a letter to an uncle,
where he explains the questions that will interest him throughout his life.
In this letter,
he says that the issues that interest him are
relating to light and its propagation in space.
And he asks weird questions.
“What would happen if the light emitted light? “
The light goes at the speed of light,
and so if we have the right to add speeds,
as we usually do,
the speed of the light emitted would be twice the speed of light.
But if the light emitted in turn emits light, that’s 3 times.
And so we build a kinematic staircase to infinity.
Is not that a problem?
One year later,
he’s still in school again, in Switzerland,
in a high school that prepares him for the Zurich Polytechnic Competition,
in a much more liberal school.
There he makes a thought experiment
– he will often do it later –
in which he stages his own body.
He asks himself the question:
“What would I perceive if I were riding on a light beam? “
At the time, we know that light,
finally, we think that light is made of electromagnetic waves,
and so, if I’m riding a light beam,
I move along with the electromagnetic wave,
I surf on her.
So this wave,
compared to me,
is motionless.
So I see a standing wave.
But at 16, Einstein already knew
that there is no stationary solution of Maxwell’s equations,
which describe the light.
So, of two things one,
or this thought experiment, which I made, has no meaning,
in the sense that it does not correspond to any possible reality,
or there is a problem
with Maxwell’s equations.
And he’ll solve the question ten years later he’s in Bern,
in 1905, the miraculous year,
5 great papers, including one,
on the special relativity,
the article of June 1905,
in which he says that the speed of light is an invariant.
It is the same in all the repositories.
It is therefore independent of the speed of the source that emits it,
and the speed of the observer who measures it.
The speed of light is also a speed limit.
It has to do with our subject:
a particle can not exceed it.
Moreover, the anagram of the speed of light already says it:
the speed of light has for anagram:
limits dreams beyond.
In 1907, Einstein is still in Berne.
He still does not have a university job,
despite the 5 extraordinary papers he published two years earlier.
Returning from lunch in November,
in a semi-awake dream,
he’s doing a thought experiment again,
who will be, he says later,
the happiest idea of his life.
He understands what?
Just read it.
“I understood that a person,
free fall,
does not feel his own weight. “
Bizarre.
If you fall,
it is because of your weight that is the cause of your fall.
At the same time, this weight causes your fall,
and, while you fall,
all the things that fall with you, for example, your phone,
in relation to you, do not fall,
since all bodies fall at the same speed in a vacuum.
This is what Galileo showed.
So your phone falling with you,
you seem to be weightless,
even though it is gravity that makes you fall.
Bizarre.
He states what he will call the principle of equivalence.
Equivalence between acceleration and gravity,
and he simply says that,
if I’m in a cabin,
completely closed,
there is no porthole to see the outside,
this cabin, without that inside I know it,
is placed on the surface of the Earth,
motionless, in the earth’s gravity field.
First experience.
2nd experience:
the same cabin is far from any massive object,
it is outside any gravitational field,
simply, it is accelerated by a superhero,
with an acceleration equal to the acceleration of gravity.
So it is accelerated in the void,
outside any field of gravitation:
it’s the 2nd experience.
And what Einstein says,
is that, if I’m in the cabin,
there is no experience I can do,
which allows me to know if I am in the first situation,
or in the second situation:
they are equivalent.
One year later,
2 years later, rather,
he goes to Prague:
his first university post,
he continues to think about all this,
and he gets into the 2nd situation,
the cabin is accelerated in a certain direction,
with constant acceleration.
A hole is drilled in the wall,
– I look at the time –
light enters.
She does not have infinite speed,
it takes a while to cross the cabin.
During that time,
the cabin moves in space.
And so the point of impact of the light on the opposite wall,
is not exactly opposite the point of entry.
It is a little lower.
So, who am I in the cabin,
I see that the light is curved in the elevator.
It’s just an acceleration effect.
That, everyone can understand it.
Where the genius of Einstein comes in,
there is still no equation.
Principle of equivalence.
What I see in the accelerated elevator car,
it’s the same thing I would see if I were in the stationary cabin on the ground.
So if the cabin was motionless on the ground,
the light would be deflected,
not by acceleration,
but by gravitation.
Which means that light,
although having no mass,
undergoes gravitation.
Einstein including that,
without having to change the geometry of the space-time,
to use a curved space-time,
to account for this effect.
Three years later,
he will have written the equations of general relativity,
published in 1915,
that will allow him to predict gravitational waves,
as early as 1916.
So, the moral of the story,
it is that a well-done theory makes it possible to bring out new data,
but the reciprocal is not true.
Data does not always make it possible to emerge
a theory that makes it possible to understand them.
I’ll let you think about that.
I put it in quotation marks,
because it has already been said.
Thank you.
(Applause)
”
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