Proof of the existence of atoms

[Thecla]

It is said that some physicists doubted the existence of atoms in 1900 until Einstein proved their existence a few years later. Did Mendeleev's creation of the periodic table in the 1870s already prove the reality of atoms by giving the known elements atomic masses?

 

[Delta2]

Sorry for spoiling this but the Ancient Greek scientist and philosopher Democritus was the first that hypothesized the existence of atoms. The existence of atoms was proved experimentally somewhere in the beginning of the 20th century somewhere around 1920s I think.

I have to admit though that Democritus was wrong as for him, atoms were the smallest unit of matter that cannot be decomposed further(something like the smallest elementary units). But we know today that atoms consist of even smaller units: electron, protons, neutrons, and protons consist of quarks which are elementary particles, at least according to current mainstream theory.

 

[Ibix]

According to my statistical mechanics lectures, Brownian motion was a real problem if one proposed atoms. Why do smoke particles bounce around? If we propose the high school "because atoms are colliding with it" answer we immediately run into a problem: atoms cannot cause a smoke particle to bounce around for the same reason that a fly colliding with a truck does not make the truck bounce around. So Brownian motion could only happen in an atomic theory if there was some coordinated motion of billions upon billions of atoms. And there was no such mechanism, so atomic theory was very doubtful on this basis, whatever results were coming from the study of chemical reactions.

So not believing atomic theory, or at least having strong reservations about it, was reasonably defensible. Then Einstein worked out the maths of the fluctuations in atom fluxes and showed that by chance billions more atoms could collide with one side of a smoke particle than the other for short periods, and it became a lot less defensible.

 

[WernerQH]

It is said that some physicists doubted the existence of atoms in 1900 until Einstein proved their existence a few years later. Did Mendeleev's creation of the periodic table in the 1870s already prove the reality of atoms by giving the known elements atomic masses?

Of course chemists had little doubt in the existence of atoms. But they could not determine their masses. All "atomic masses" were relative to that of hydrogen. (And they still are, at least approximately, given that 1/12 of carbon-12 is almost the same.)

The physicists opposed to the atomic hypothesis were thermodynamicists, and were concerned that the force of thermodynamic considerations could be weakened through the introduction of dubious model assumptions. Thermodynamics is the theory of bulk matter, and is (from the thermodynamic viewpoint) largely independent of the precise nature of the microscopic constituents. For those thermodynamicists the 90 or so chemical elenents could have been 90 different sorts of jelly down to the smallest conceivable dimensions. But of course not all physicists were so narrow-minded. Even within thermodynamics itself, combining for example the surface tension of a liquid with its heat of evaporation gives you (on dimensional grounds) a handle on the size of molecules. Maxwell and Loschmidt were able to estimate Avogadro's number by combining information from the liquid and gase phases (kinetic theory). After 1900 the determinations of the size of atoms quickly converged and became consistent.

 

[jbriggs444]

According to my statistical mechanics lectures, Brownian motion was a real problem if one proposed atoms. [...]
Then Einstein worked out the maths of the fluctuations in atom fluxes and showed that by chance billions more atoms could collide with one side of a smoke particle than the other for short periods, and it became a lot less defensible.

To condense that a bit, the idea is roughly that the relative variance goes as $\sqrt{\frac{1}{n}}$ rather than as $\frac{1}{n}$ where n is the number of atoms per smoke particle?

But that seems too obvious to have been ignored.

 

[vanhees71]

The breakthrough with Einstein's Brownian-motion article was to take fluctuations into account, which indicate the presence of atoms/molecules. I think the most important and convincing argument is the validity of the dissipation-fluctuation relation, relating two transport coefficients via the kinetic theory of matter. Another famous paper by Einstein is on critical opalescence and Rayleigh scattering.

 

[Vanadium 50]

It is said

By whom>

that some physicists

Who?

I am very, very skeptical of this. We had the discover of the electron in 1897. As you say, we had the periodic table before then. We had the law of definite proportions. Heck, we had Avogadro's number for three decades, This sounds very, very unlikely.

I don't think Brownian motion was considered a problem for atoms. It was considered a problem for brownian motion. Many-body theory was in its infancy - J.W. Gibbs was still alive in 1900.

 

[PeroK]

By whom>

Who?

I am very, very skeptical of this. We had the discover of the electron in 1897. As you say, we had the periodic table before then. We had the law of definite proportions. Heck, we had Avogadro's number for three decades, This sounds very, very unlikely.

I don't think Brownian motion was considered a problem for atoms. It was considered a problem for brownian motion. Many-body theory was in its infancy - J.W. Gibbs was still alive in 1900.

I believe that Max Planck was skeptical of atomic theory until the 1890's. There's a piece here, for example:

https://physicsworld.com/a/max-planck-the-reluctant-revolutionary/

In the 1890s the debate about the second law centred on the statistical (or probabilistic) interpretation that Ludwig Boltzmann had originally proposed back in 1872 and expanded in 1877. According to Boltzmann’s molecular-mechanical interpretation, the entropy of a system is the collective result of molecular motions. The second law is valid only in a statistical sense. Boltzmann’s theory, which presupposed the existence of atoms and molecules, was challenged by Wilhelm Ostwald and other “energeticists”, who wanted to free physics from the notion of atoms and base it on energy and related quantities.

What was Planck’s position in this debate? One might expect that he sided with the winners, or those who soon turned out to be the winners – namely Boltzmann and the “atomists”. But this was not the case. Planck’s belief in the absolute validity of the second law made him not only reject Boltzmann’s statistical version of thermodynamics but also doubt the atomic hypothesis on which it rested. As early as 1882, Planck concluded that the atomic conception of matter was irreconcilably opposed to the law of entropy increase. “There will be a fight between these two hypotheses that will cause the life of one of them,” he predicted. As to the outcome of the fight, he wrote that “in spite of the great successes of the atomistic theory in the past, we will finally have to give it up and to decide in favour of the assumption of continuous matter”.

However, Planck’s opposition to atomism waned during the 1890s as he realized the power of the hypothesis and the unification it brought to a variety of physical and chemical phenomena.

 

[Vanadium 50]

But even here it says his opposition waned in the 1890s. And he was in his 30's then. He didn't make a name for himself until the later 1890's.

 

[gmax137]

It is said that some physicists doubted the existence of atoms in 1900 until Einstein proved their existence a few years later. Did Mendeleev's creation of the periodic table in the 1870s already prove the reality of atoms by giving the known elements atomic masses?

I am very, very skeptical of this.

I remember being told the same thing in a physics class, late 1970s. I was surprised to hear this, especially that Planck was among the sceptics. How could the father of quantum theory not believe atoms exist??

I believe that Max Planck was skeptical of atomic theory until the 1890's. There's a piece here, for example:

This is a very interesting addition to the "story." Especially when combined with

But even here it says his opposition waned in the 1890s.

Thanks too to @anorlunda for "promoting" this thread.

 

[anorlunda]

There was pressure from different directions. Clausius' thermodynamics, then Boltzman's early statistical mechanics. Even solids obey the laws of thermodynamics. So if the foundation is statistics, statistics of what? That hinted at something profoundly wrong with non-atomic theories of matter.

 

[bob012345]

It is said that some physicists doubted the existence of atoms in 1900 until Einstein proved their existence a few years later.

As discussed in David Lindley's book Boltzmann's Atom, The Great Debate That Launched A Revolution In Physics, and above, Einstein explained Brownian Motion using Boltzmann's atomic theory in 1905. The book has the history of the development of these ideas.

 

[vanhees71]

As I said above, the breakthrough of Einstein's Brownian-Motion-paper of 1905 was to consider (thermal) fluctuations, and Brownian motion directly makes these fluctuations observable. If phenomenological thermodynamics and classical continuum theories of matter were right, you'd not observe these fluctuations.

On top Einstein's paper was the first showing that the fluctuations are related with dissipation and that there are fundamental, quantitative "fluctuation-dissipation relations" between transport coefficients (in this case viscosity and diffusion coefficients).

 

[Mister T]

It is said that some physicists doubted the existence of atoms in 1900 until Einstein proved their existence a few years later.

Many physicists of the time were not convinced of the existence of atoms. This persisted well into the 1900's. Einstein presented an argument for the existence of atoms, but these things are never proofs. They are simply pieces of evidence that support the existence of atoms. It's not like one day there's suddenly proof and everyone instantaneously becomes convinced.

Did Mendeleev's creation of the periodic table in the 1870s already prove the reality of atoms by giving the known elements atomic masses?

Again, not a proof. Just an argument that seemed to support the existence of atoms. Some were convinced, others weren't.

In the early 1900's the evidence for the existence of atoms continued to grow, and eventually it became accepted by almost all physicists. There were still some who refused to accept it, but they eventually died!

 

[PeroK]

There were still some who refused to accept it, but they eventually died!

I suspect even those who did accept the existence of the atom eventually died as well!

 

[bob012345]

An interesting modern parallel to the debate of the existence of atoms was the 1990's-2000's era debate on how far one can push the technological use of atoms i.e. is molecular nanotechnology possible. IBM proved you can push them around on a surface.

Feynman initiated the idea of hacking atoms back in 1959. Later, Drexler and others put numbers to it and claimed it would be possible to construct useful machines at the molecular level. Nobel scientist Richard Smalley said that would be impossible. A public debate followed which was printed in the form of articles and responses in Scientific American and finished in Chemical and Engineering news.

https://en.wikipedia.org/wiki/Drexler–Smalley_debate_on_molecular_nanotechnology

Feynman's talk;
https://web.pa.msu.edu/people/yang/RFeynman_plentySpace.pdf

Drexler's thesis;
https://dspace.mit.edu/handle/1721.1/27999

 

[vanhees71]

Many physicists of the time were not convinced of the existence of atoms. This persisted well into the 1900's. Einstein presented an argument for the existence of atoms, but these things are never proofs. They are simply pieces of evidence that support the existence of atoms. It's not like one day there's suddenly proof and everyone instantaneously becomes convinced.
Again, not a proof. Just an argument that seemed to support the existence of atoms. Some were convinced, others weren't.

In the early 1900's the evidence for the existence of atoms continued to grow, and eventually it became accepted by almost all physicists. There were still some who refused to accept it, but they eventually died!

Historically the "atomic hypothesis" in the 19th century was much more accepted by chemists, because the then well-established stoichometrics found a very natural explanation from it, i.e., why chemical reactions go in integer proportions involving "elements", which could be (at least roughly) ordered in systems like Mendeleev's "periodic table". Of course, the details are not always that clear. Even today there is some discussion about the correct ordering in some parts (lanthanides and actinides etc.). Nevertheless the idea of "discrete building blocks" was much more natural for chemists than physicists. The latter had more continuum/field theories of matter, including phenomenological thermodynamics, in mind than atomistic ones. That's why the early "atomists", using statistical methods (kinetic theory of gases a la the Bernoullis, Gibbs, Maxwell, Boltzmann et al), had a hard time to convince their more conservative colleagues.

This only slowly changed, when the electron has been discovered in 1897, and this also makes a lot of trouble in the context of electrodynamics, as was clear from the very beginning with Lorentz's "electron theory".

 

[Mister T]

I suspect even those who did accept the existence of the atom eventually died as well!

Yes, but the point is that those who remained shared their acceptance of the atom.

 

[bob012345]

Yes, but the point is that those who remained shared their acceptance of the atom.

That was understood but @PeroK 's joke was clever and very funny.

 

[andresB]

I remember being told the same thing in a physics class, late 1970s. I was surprised to hear this, especially that Planck was among the sceptics. How could the father of quantum theory not believe atoms exist??

I heard the same, and I'm way younger. It might be a myth, but if so, it's a well-extended myth.

 

[anorlunda]

I heard the same, and I'm way younger. It might be a myth, but if so, it's a well-extended myth.

This entertaining video about Max Plank's history may interest you and @gmax137

 

[Hornbein]

Sorry for spoiling this but the Ancient Greek scientist and philosopher Democritus was the first that hypothesized the existence of atoms. The existence of atoms was proved experimentally somewhere in the beginning of the 20th century somewhere around 1920s I think.

I have to admit though that Democritus was wrong as for him, atoms were the smallest unit of matter that cannot be decomposed further(something like the smallest elementary units). But we know today that atoms consist of even smaller units: electron, protons, neutrons, and protons consist of quarks which are elementary particles, at least according to current mainstream theory.

No no, Democritus was right. It's not his fault that our atoms are misnamed. Using his terminology, neutrinos and quarks and electrons and so forth are atoms.

 

[nasu]

No no, Democritus was right. It's not his fault that our atoms are misnamed. Using his terminology, neutrinos and quarks and electrons and so forth are atoms.

For now.

 

[sophiecentaur]

Did Mendeleev's creation of the periodic table in the 1870s already prove the reality of atoms by giving the known elements atomic masses?

He certainly managed to tidy things up a lot but IMO Lavoisier more or less introduced the idea of atoms before he had his head removed by those nasty French Revolutionaries in 1794. The experimental evidence didn't all arrive at once and it's a matter of opinion what constituted an actual proof. The existence of isotopes didn't exactly help things.

 

[MathematicalPhysicist]

The real question is can we not divide matter indefinitely?

LQG says Space Time at the quantum level is discrete, it's hard for me to believe it because of my younger self who argues:"what can stop you from keep on smashing the stone/kicking the stones uphill?".

 

[TeethWhitener]

He certainly managed to tidy things up a lot but IMO Lavoisier more or less introduced the idea of atoms before he had his head removed by those nasty French Revolutionaries in 1794. The experimental evidence didn't all arrive at once and it's a matter of opinion what constituted an actual proof. The existence of isotopes didn't exactly help things.

Dalton generally gets the credit for atomic theory in chemistry, followed closely by Avogadro. Lavoisier is more known for his formulation of the law of mass conservation.

 

[Demystifier]

No no, Democritus was right. It's not his fault that our atoms are misnamed. Using his terminology, neutrinos and quarks and electrons and so forth are atoms.

Did Democritus said that one kind of atom can turn into another? In other words, that atoms can be created and destroyed?

 

[Mister T]

No no, Democritus was right. It's not his fault that our atoms are misnamed. Using his terminology, neutrinos and quarks and electrons and so forth are atoms.

To Democritus an atom was the smallest amount of something possible. Neutrinos, quarks, and electrons don't fit this definition. For example the smallest amount of gold is one atom of gold. That part he got right. He thought of these atoms as indivisible. He got that part wrong.

 

[sophiecentaur]

He got that part wrong.

What a loser. ?

 

[epenguin]

I've long forgotten my readings about this, but I'm sure it did exist. Among scientists it was something of a Sunday philosophy. you might hear it in the annual address to a national chemical Society or something like that, I have run into it in a place like that. I think you will find that it did get itself into textbooks, where it lasted longer than it did among the creators of science and practitioners of scientific research.

It was a 'philosophical' rather than 'scientific' view point of view. It's emphasises that our scientific beliefs are not some absolute truth but are deduced from experiment etc. this could lead onto various forms of e.g. extreme empiricist philosophy, scientific theories were nothing but a sort of shorthand summary of the facts as known. It is not complete stupidity, apart maybe from extremist formulations there is a sound insight and it has to be considered one, well severa, of the varieties of empiricism. Will be considered in every programme or book on philosophy of science. A classical exponent was Ernst Mach. Well-known name, the philosopher most frequently referred to by Einstein, but who has read him today? But in some form his ideas certainly come down to and through Popper.

Extreme formulations were welcome to anti-scientists, those upset by Scientific discovery, e.g. Evolution. If you listen today to their successors, creationists or climate change deniers, you will likewise find them starting off by earnestly explaining to you the scientific method, and then why crediting Evolution by natural selection, or anthropogenic climate change, do not satisfy its requirements.

 

[bhobba]

Then Einstein worked out the maths of the fluctuations in atom fluxes and showed that by chance billions more atoms could collide with one side of a smoke particle than the other for short periods, and it became a lot less defensible.

Interestingly this led to the theory of random walks that has many applications in many areas of applied mathematics, not just physics. Mostly these days, mathematicians lead the math used in applied math and physics in general, but this is a case where physicists lead the way. Even today, it sometimes happens.

Thanks
Bill

 

[Son Goku]

No no, Democritus was right. It's not his fault that our atoms are misnamed. Using his terminology, neutrinos and quarks and electrons and so forth are atoms.

I think modern particles are so far from anything he imagined, e.g. in typical states their properties and even their number are undefined, that one can't really say he was right.

 

[PeroK]

I think modern particles are so far from anything he imagined, e.g. in typical states their properties and even their number are undefined, that one can't really say he was right.

I suspect that even if Democritus had come up with the basics of QFT, you'd give him no credit!

 

[Son Goku]

I suspect that even if Democritus had come up with the basics of QFT, you'd give him no credit!

Hardly, he's my favourite 5th Century Thracian.

 

[bhobba]

I think modern particles are so far from anything he imagined, e.g. in typical states their properties and even their number are undefined, that one can't really say he was right.

Yes, QM has changed our view of the world a lot. In hindsight, we only have experience with 'clumps' of quantum stuff and can only probe the quantum world by interacting with those clumps. That this led to a theory predicting the probabilities of those interactions (aka observations) is only expected. We are lucky, though, in that mathematical analysis shows it is just an example of a broader class of probabilistic theories than ordinary probability theory and led to the creation of a new area of mathematical investigation called Generalised Probability Theories:
https://arxiv.org/pdf/2103.07469.pdf

Some arguments suggest it is just the most straightforward generalised probability theory after ordinary probability theory:
https://arxiv.org/abs/0911.0695

Note this in no way explains what is 'really' going on at the quantum level. Quantum foundations are still an active area of research. It just elucidates why we ended up with the theory we have. The bottom line is none of us has direct experience with the quantum world, and QM reflects this.

Thanks
Bill