What was Einstein's biggest blunder?
Humans had always been asking themselves how the universe began, but for the great majority of
history, we didn't have the right mathematical and physical instruments to consistently describe it.
Things changed only at the beginning of the 20th
century when Einstein formulated his theory of general relativity.
According to this theory, the fabric of space-time was curved by gravity;
time was not universal, but relative – as well as space. In addition, the speed of light in a vacuum
was the same in all reference frames. General relativity was also later shown to
predict the bending of light due to a massive object and the existence of black holes.
Over the years, this became the best theory we have to describe the world we live in.
As soon as he introduced his theory, Einstein began
testing the universe but, in doing so, he made what he called
his “biggest blunder” ever. But what was this blunder? And was it an actual mistake?
Follow me in this video to get to know the answers to these questions, and more!
==========================
In 1917, Albert Einstein inserted a term called the cosmological constant into his
theory of general relativity to force the equations to predict a stationary universe
in keeping with physicists' thinking at the time.
When it became clear that the universe wasn't actually static, but was expanding instead,
Einstein abandoned the constant, calling it the '"biggest blunder" of his life.
But lately, scientists have revived Einstein's cosmological constant
(denoted by the Greek capital letter lambda) to explain a mysterious force called dark energy that
seems to be counteracting gravity -- causing the universe to expand at an accelerating pace.
So, in short terms, this lambda term, the cosmological constant,
firstly introduced by Einstein, does not seem to count as an actual mistake,
because it rather helped astrophysicists explain an important feature of the universe.
And this video could end here, but... why did Einstein and other physicists like the idea
of a stationary universe? And what do we know about Dark Energy?
To answer the first question, we have to make a step back and think of how the world was
before Einstein. Prior to Einstein, the best theory we had to describe the universe was
Newton's theory, and the best cosmological model was the one proposed by Newton as well.
In Newton's theory of gravitation, mass is the source of gravity and the resulting force is
universally attractive. The empirical successes of Newton's theory are numerous and familiar.
All observations of gravitational phenomena prior to this century
were consistent with Newtonian theory, with some small expections.
According to Newton's cosmological model, the universe couldn't be finite.
In fact, observations at that time were showing that, if you looked up in the sky beyond the moon,
the sun, and the planets, you would not be able to detect any motion of objects.
This, according to Newton, meant that the universe was static.
In a nutshell, it didn't appear to move on a large scale. Starting from this assumption, apparently
supported by his observations, Newton deduced that the universe must be infinite in size.
If everything in the universe is attracted to everything else ( according to Newton's law of
gravitation), then the matter at the edge of a finite Universe would be attracted by all of the
other mass in the Universe, and would eventually be pulled toward the center of the Universe. If
that happens, one would see the stuff on the edge of the universe moving toward the middle.
But this wasn't seen during observations. The natural consequence is that the universe
wasn't falling inwards. The only way for the most external mass not to fall in was to have
more mass further out, which prevented the other material from being pulled inwards.
Then you might ask: what about the mass further out? Won't it be pulled in?
Well...not if there is even more stuff. If you keep following this line of reasoning,
you will soon find yourself in Newton's cosmological model of an infinite universe!
So Newton naturally explained the observed static universe
within the framework of an infinite universe. The problem is that he didn't know about all
of the motions we see today, since his technology wasn't able to detect it:
up to date, we know that the universe IS NOT static.
However, this cosmological model had some problems.
For instance, if the universe was infinite, then that would mean there must be an infinite amount
of mass as well, and if there was an infinite amount of mass, you would be able to go outside
at night and see the sky filled with objects. But what when we go outside we don't see something
in every single direction of the sky: there are actually a lot of places where nothing is seen.
This is known as the “Olbers paradox”, and another way of seeing it is to ask ourselves the question:
“If the universe is infinite, why is the night sky dark?”. If there was an infinite universe then we
would see a much brighter sky, characterized by a constant brightness, day or night.
So this was Newton's cosmological model. But as soon as Einstein introduced his
theory of general relativity, he tried to come up with a new cosmological model,
to see if it would solve some questions left open within the framework of the Newtonian theory.
Hey, if you are still here, it means you are really enjoying this video!
Why don't you subscribe now and press the bell notification?
According to Einstein, the universe contained a finite amount of matter,
and it was a closed universe, similar to a sphere,
even if it's hard to picture it because it should be a 4-th dimensional kind of object.
For the purposes of this video, imagine Einstein's universe to be spherical.
Starting from every point on this sphere, you could make your way through the universe
and, eventually, end up at the point where you started from. Basically, one could have crossed
this universe in every direction, and see no edge, even though the space was finite in size.
Einstein was giving us a new way of seeing the universe. Metter gives the universe a curvature,
which makes it spherical, and the amount of mass (and therefore, luminous objects) in it
doesn't need to be infinite anymore. This was a nice way to solve the Olbers paradox!
But what about the stationary universe? At the beginning of the 20th century,
astronomers were still thinking the universe was eternal and static.
How could Einstein's theory explain this?
Well, in order to justify this, a consistent solution necessitated
the introduction of a new term to the field equations of relativity.
Some regarded the new term, known as the cosmological constant, as something of a fudge
factor and claimed that it marred the symmetry and simplicity of the original field equations.
However, general relativity certainly allowed the term; indeed it is a little-known fact
that Einstein had noted the possibility of such an extension to the field equations
in his original exposition of 1916. However, it didn't take long for Einstein to be proved wrong.
In 1922, the young Russian physicist Alexander Friedman suggested that non-static solutions of
the Einstein field equations should be considered in models of the universe. Starting from the
modified field equations, Friedman derived two differential equations linking the time
evolution of the universe with the density of matter and the cosmological constant.
Einstein did not welcome Friedman’s
contribution. His first reaction was that the Russian had made a mathematical error.
Then, in 1927, a Belgian physicist - Georges Lemaître - also derived time-varying equations
for the radius of the universe from the modified field equations.
At that time, observations of the recession velocity of the spiral nebulae
were suggesting that the universe was not static, but it was rather expanding.
Such nebulae were later understood to be completely different galaxies than ours,
laying far beyond the confines of the Milky Way.
Lemaître linked the new astronomical observations with general relativity
by suggesting that the data were evidence of a universe that was not static but expanding.
Einstein was not happy with this suggestion either:
Lemaître recalled that Einstein described his model of an expanding universe as “abominable.”
But things changed forever in 1929.
During the course of that year, astronomer Edwin Hubble published the first definitive evidence
of a linear relation between the recession of the nebulae and their radial distance.
Many physicists accepted Hubble’s results as evidence of a universe that was expanding on
the largest scales, and a variety of relativistic time-varying models of the cosmos were proposed.
Einstein finally gave up and in the early 1930s, he published two distinct models of
the expanding universe, one of closed spatial curvature, and a flat one.
In both cases, he decided to abandon the cosmological constant,
stating that this term was giving redundant and unstable solutions.
This was Einstein's biggest blunder. ==================
He wrote that the introduction of a constant cosmological term in the equation of gravity
has to be rejected and that if Hubble's law had been known at that time,
the cosmological constant would have never been introduced.
But what scientists later found out is that...the biggest blunder,
wasn't a real mistake. In 1998, scientists Perlmutter, Schmidt,
and Riess discovered that the expansion of the universe is accelerating.
Something is driving the acceleration, and that something is usually referred to
as "dark energy". This dark energy is now thought to constitute 73% of the stuff of the Universe,
dwarfing the 23% that is dark matter and the 4% that is ordinary matter.
What astrophysicists found was that the accelerated expansion of the universe
can be explained by the introduction -again!- of the cosmological constant.
As in Einstein's static universe, this constant would have the meaning of a negative pressure
acting on the universe, which would make it expand in an accelerated fashion.
Perlmutter, Schmidt, and Riess were awarded the Nobel Prize in 2011 for their discovery.
Jokingly, we could say that Einstein was right...even when he thought he was wrong!
What we can actually learn from this story is that no one is perfect,
and the road to science is filled with obstacles. Our job in this case is to persevere,
and never give up, because we can always learn from our mistakes.
This video ends here! What do you think about Einstein's greatest blunder?
Let me know in the comments below and be sure
to watch our other videos through youtube endscreens and playlists!