The Present State of Physics

Two opposite tendencies may be distinguished in the history of the development of physics.

On the one hand, new relations are continually being discovered between objects which seemed destined to remain for ever unconnected; scattered facts cease to be strangers to each other and tend to be marshalled into an imposing synthesis. The march of science is towards unity and simplicity.

On the other hand, new phenomena are continually being revealed; it will be long before they can be assigned their place—sometimes it may happen that to find them a place a corner of the edifice must be demolished.

In the same way, we are continually perceiving details ever more varied in the phenomena we know, where our crude senses used to be unable to detect any lack of unity.

What we thought to be simple becomes complex, and the march of science seems to be towards diversity and complication.

Here, then, are two opposing tendencies, each of which seems to triumph in turn. Which will win?

If the first wins, science is possible; but nothing proves this à priori, and it may be that after unsuccessful efforts to bend Nature to our ideal of unity in spite of herself, we shall be submerged by the ever-rising flood of our new riches and compelled to renounce all idea of classification—to abandon our ideal, and to reduce science to the mere recording of innumerable recipes.

In fact, we can give this question no answer.

All that we can do is to observe the science of to-day, and compare it with that of yesterday. No doubt after this examination we shall be in a position to offer a few conjectures.

Half a century ago, hopes ran high. The unity of force had just been revealed to us by the discovery of the conservation of energy and of its transformation.

This discovery also showed that the phenomena of heat could be explained by molecular movements. Although the nature of these movements was not exactly known, no one doubted but that they would be ascertained before long.

As for light, the work seemed entirely completed.

So far as electricity was concerned, there was not so great an advance. Electricity had just annexed magnetism.

This was a considerable and a definitive step towards unity.

But how was electricity in its turn to be brought into the general unity, and how was it to be included in the general universal mechanism? No one had the slightest idea.

As to the possibility of the inclusion, all were agreed; they had faith. Finally, as far as the molecular properties of material bodies are concerned, the inclusion seemed easier, but the details were very hazy.

Hopes were vast and strong, but vague.

Today, immense advancement is in progress.

The relations between light and electricity are now known.

The 3 domains of light, electricity, and magnetism are now one.

Nevertheless, the conquest has caused us some sacrifices.

Optical phenomena become particular cases in electric phenomena; as long as the former remained isolated, it was easy to explain them by movements which were thought to be known in all their details. That was easy enough.

But any explanation must now cover the whole domain of electricity. This is difficult.

The most satisfactory theory is that of Lorentz. It best explains the known facts. But it still has a serious fault as I have shown above.

It contradicts Newton’s law that action and reaction are equal and opposite. According to Lorentz this principle cannot be applicable to matter alone.

If it be true, it must take into account:

• the action of the ether on matter
• the reaction of the matter on the ether

In the new order, it is very likely that things do not happen in this way.

Nevertheless, Lorentz was able to connect:

• the results of Fizeau on the optics of moving bodies
• the laws of dispersion and absorption

• the other properties of the ether

These are connected by bonds which will not be readily severed.

It explained the new Zeeman phenomenon and even aided the classification of Faraday’s magnetic rotation, which had defied all Maxwell’s efforts.

This proves that Lorentz’s theory is not a mere artificial combination which must eventually find its solvent.

It will probably have to be modified, but not destroyed.

The only goal of Lorentz was to include in a single whole all the optics and electrodynamics of moving bodies.

He did not claim to give a mechanical explanation.

Larmor goes further. He kept the essential part of Lorentz’s theory and grafts on it MacCullagh’s ideas on the direction of the movement of the ether.

MacCullagh held that the ether’s speed is the same in magnitude and direction as the magnetic force.

But this aggravates the fault in Lorentz’s theory.

According to Lorentz, we do not know what the movements of the ether are.

We may suppose them to be movements compensating those of matter, and re-affirming that action and reaction are equal and opposite.

According to Larmor we know the movements of the ether, and we can prove that the compensation does not take place.

If Larmor has failed, as in my opinion he has, does it necessarily follow that a mechanical explanation is impossible? Far from it.

As long as a phenomenon obeys the 2 principles of energy and least action, so long it allows of an unlimited number of mechanical explanations. And so with the phenomena of optics and electricity.

But this is not enough. For a mechanical explanation to be good it must be simple; to choose it from among all the explanations that are possible there must be other reasons than the necessity of making a choice.

We have no theory as yet which will satisfy this condition and the theories of modern physics.

consequently be of any use. Are we then to complain?

That would be to forget the end we seek, which is not the mechanism; the true and only aim is unity.

We should therefore set some limits to our ambition.

Let us not seek to formulate a mechanical explanation.

Let us be content to show that we can always find one if we wish.

In this we have succeeded. The principle of the conservation of energy has always been confirmed, and now it has a fellow in the principle of least action, stated in the form appropriate to physics.

This has also been verified, at least as far as concerns the reversible phenomena which obey Lagrange’s equations—in other words, which obey the most general laws of physics.

The irreversible phenomena are much more difficult to bring into line; but they, too, are being co-ordinated and tend to come into the unity. The light which illuminates them comes from Carnot’s principle. For a long time thermodynamics was confined to the study of the dilatations of bodies and of their change of state.

For some time past it has been growing bolder, and has considerably extended its domain. We owe to it the theories of the voltaic cell and of their thermo-electric phenomena; there is not a corner in physics which it has not explored, and it has even attacked chemistry itself.

The same laws hold good. Everywhere, disguised in some form or other, we find Carnot’s principle; everywhere also appears that eminently abstract concept of entropy which is as universal as the concept of energy, and like it, seems to conceal a reality.

It seemed that radiant heat must escape, but recently that, too, has been brought under the same laws.

In this way fresh analogies are revealed which may be often pursued in detail; electric resistance resembles the viscosity of fluids; hysteresis would rather be like the friction of solids. In all cases friction appears to be the type most imitated by the most diverse irreversible phenomena, and this relationship is real and profound.

A strictly mechanical explanation of these phenomena has also been sought, but, owing to their nature, it is hardly likely that it will be found.

To find it, it has been necessary to suppose that the irreversibility is but apparent, that the elementary phenomena are reversible and obey the known laws of dynamics.

But the elements are extremely numerous, and become blended more and more, so that to our crude sight all appears to tend towards uniformity—i.e., all seems to progress in the same direction, and that without hope of return.

The apparent irreversibility is therefore but an effect of the law of great numbers. Only a being of infinitely subtle senses, such as Maxwell’s demon, could unravel this tangled skein and turn back the course of the universe.

This conception, which is connected with the kinetic theory of gases, has cost great effort and has not, on the whole, been fruitful; it may become so.

This is not the place to examine if it leads to contradictions, and if it is in conformity with the true nature of things.