In the two previous lectures I have discussed the relations towards one another of physical and biological interpretation, but in this discussion no account whatever has been taken of conscious behaviour, including perception and voluntary action. We must now take conscious behaviour into account.
In the first course of lectures it was pointed out that conscious behaviour differs from what we interpret as mere biological behaviour owing to the fact that conscious behaviour is no mere immediate response to momentary happenings, but involves both retrospect and foresight. A conscious organism is responding, not to chaotic impulses of a physically interpreted world, nor simply to the spatially co-ordinated stimuli of a biologically interpreted world, but to perceptions of a world which is co-ordinated not merely as regards space-relations, but also as regards time-relations. Past and future happenings are definitely co-ordinated as progressive interest with the spatially co-ordinated present, so that past and future are bound up with the present. Thus the perceived world is a world of co-ordinated duration or progress.
When co-ordination expresses itself not merely in momentary behaviour, but in relations to past and future behaviour, it expresses what we call interest and constituent values. A conscious organism or centre of life is thus a centre of values or interest. Its behaviour expresses far more than that of what we regard as a mere living organism, since it is “responsible” behaviour, co-ordinated in such a way as to bring past and future happenings into the sphere of co-ordination. The responsibility is no less in respect of perception than of voluntary action: for perception is no less an active response than voluntary movement. If, through want of care, we have failed to perceive the proper occasion for action, we are just as responsible as if, seeing the occasion, we had not taken proper action. To what we interpret as a mere living organism, responsibility is meaningless.
In discussing the relation of the psychologically interpreted world of interest and values to the physically or biologically interpreted worlds we must dismiss from our minds the idea that the psychologically interpreted world has no real existence of its own, so that we could regard it as merely a physically or biologically interpreted world accompanied by a mysterious something called consciousness. This idea is just as groundless and inconsistent with experience as the idea that we can reduce biological to physical interpretation. We can neglect or strip away the psychological aspects of experience, just as we can neglect the biological aspects; but all that is left is a travesty of actual experience. We cannot reconstruct actual experience out of this travesty. The facts embodied in psychological interpretation are not only clear and undeniable, but they do not admit of statement in terms of either physical or biological interpretation.
This is because of their very nature. A perception, or conscious response to it, cannot be described as a mere physical impression or result of it, nor as a merely spatially co-ordinated response to a spatially co-ordinated stimulus. Such an impression or stimulus would carry with it no reference to past or future impressions or stimuli—not even a bare qualitative difference. It was Kant who, in his criticism of Hume and the eighteenth-century English school of psychology and philosophy, pointed this out clearly, and his criticism was fundamental. Perception is no mere occurrence in time, for time-relations are included in perception. Kant expressed this conclusion by saying that time-relations constitute a form imposed on objects in their perception. Thus for Kant, though as a former professor of physics he was greatly interested in astronomy and had put forward the nebular theory of the origin of planetary systems, time was not something within which mind exists, but only an expression of mind itself.
Kant did not infer that mind makes the universe, but only that mind gives to it the form which it takes in our perception of it. Behind the visible and tangible world of our perceptual experience there was for Kant a world of “things-in-themselves,” while the visible and tangible Newtonian world of bodies existing in time and space and acting on one another was the form given in perception to the world of things-in-themselves.
This was Kant's mode of reconciling the facts relating to perception with the Newtonian interpretation of the visible world. But we have already seen that there is a visible world of life coinciding with the visible Newtonian world and not reducible to it. Had there been a biological Newton, Kant would certainly have hesitated about his postulate of a real world of things-in-themselves, since for biological interpretation things-in-themselves do not exist. We can even speculate as to how Kant's view might have been altered if Goethe, with his deep poetic insight into the phenomena of life, had written before or alongside of him. Actually, however, biological conceptions were for Kant only “heuristic principles,” incapable of systematic scientific application to the visible world of perception.
Not only is there a visible world of life, but also a visible psychological or spiritual world of interest and values. What is interpreted as belonging to this spiritual world does not manifest itself as separate events succeeding one another in time, but as events inseparable from one another in both space and time, just as the parts and environment which participate in life are inseparable in space. The world regarded as a perceived world is thus a world of interest and values. It is only through interest that our perceived world is unified in perception and conscious action. Physical and biological interpretation are incapable by themselves of unifying it as we find in our actual experience that it is unified. But it is unified in our interest, which extends, as perceived, throughout both space- and time-relations.
The interest and values which give unity to our experience are not in reality separable from one another, though by artificial distinction we may regard them separately. We become hungry and thirsty no less if we are poets or men of science than if we are manual workers, and satisfaction of hunger or thirst is an element unified in the interest of the poet or man of science no less than in that of the manual worker. The smell of food or the sight of drink appeals to them all, and so, probably, do poetry, science, and craftsmanship, though the degrees of interest they take in these may be different.
Interest and values are not mere interest and values of individuals. This is evident at once when we consider actual conscious behaviour, whether in men or animals. Perceptions and conscious actions include perceptions and actions of far more than individual interest, and what has this wider interest has a value which cannot be regarded as a sum of individual values. Unselfish actions are not such as we count on an ultimate personal return for. The really unselfish person likes doing them, but only because he is inspired by an interest which is far more than his mere individual interest, and which is also an interest only showing itself in his relations with other persons. This subject will be discussed more fully in the succeeding lecture, but meanwhile I only wish to point out that interest and values and the corresponding perceptions and actions are not merely centred round individual persons as such.
We seem to find ourselves in presence of a physically interpreted, a biologically interpreted, and a psychologically or spiritually interpreted world, the latter being a world of interest, values, and responsibilities. Each of these interpretations bases itself on our actual experience, and thus lays claim to objective significance. But as regards the experience appealed to we must remember, with Kant, that this is perceived experience. There is nothing else that we can appeal to. We cannot jump out of our own skins. Since, however, it is only perceived experience that we can appeal to, the character of perception must in reality enter into even our physically interpreted world, and perception implies that whatever is perceived is so in virtue of co-ordinated relation, both spatial and temporal, to the rest of experience.
In view of this conclusion, a physical universe of self-existent bodies, such as we ordinarily, following Newton, imagine to exist, can have no real self-existence in its parts. The assumed self-existence can represent no more than a working hypothesis which is convenient in our interest in so far as it works, but is not ultimately correct.
Recent developments of experimental physical investigation, apart altogether from biological, psychological, or philosophical considerations, have been tending more and more to undermine completely the foundations of the Newtonian conception of physical reality, although in other directions the application of this conception is being extended very fruitfully. I am neither sufficient of a physicist nor sufficient of a mathematician to follow in detail all the developments which are inconsistent with the Newtonian conception; but I must at least attempt to discuss these developments in a general manner, and it will be convenient to do so in this lecture.
On the Newtonian conception, as fully developed during the two centuries following the publication of the Principia, the visible world consists of indestructible “bodies” or collections of “substance,” acting on one another in different ways, according to their properties, but all possessing “inertia,” which is a measure of their substance or mass. They can act, either on what we picture to ourselves as actual contact, or at a distance in various ways; but all bodies attract one another in proportion to their masses and inversely as the squares of their distances apart in accordance with the law of gravitation, so that inertial mass and gravitational mass are identical. In acting on one another the bodies communicate “energy” or the power of action to one another, and this energy is just as indestructible as the substance of the bodies.
When we examine the bodies we find that they consist of atoms with certain quite definite properties and amounts of substance. Moreover, the different atoms have relative masses which led Prout, more than a century ago, to suggest that they are in some way additive compounds of an elementary unit of mass. This idea has been definitely verified recently through Aston's discovery that elementary substances which seem to contradict the hypothesis are in reality mixtures of “isotope” elements, each of which accords with the theory. Thus we might say that not merely does mass occur in the form of atoms, but that atoms are multiples of a single more fundamental “quantum” of mass.
So far there is nothing inconsistent with the Newtonian conception; and the general theory which was reached about the middle of last century as to the nature of the gaseous, liquid, and solid states of matter, and as to the nature of heat and temperature, was also consistent with the Newtonian conception, and constituted extremely important extensions of its application. But the Newtonian conception tells us nothing as to why atoms have masses which are multiples of a definite unit, and further investigation of both the atom itself and the manner in which it communicates energy to its surroundings has progressively revealed additional difficulties for the Newtonian conception.
To take one example, the discovery of radio-active elements by the Curés, and of the disintegration of atoms in the process of radio-activity, has shown that an atom itself contains enormous stores of internal energy, part of which is liberated when an atom disintegrates. But why, when atoms come into contact with one another, does this energy not share itself with the environment under ordinary conditions? In other words, why does an atom ordinarily maintain its internal energy indefinitely? In a gas, solid, or liquid, the energy of translation and rotation is regarded as sharing itself mutually and between the molecules, as we should expect on Newtonian principles; but if the internal atomic energy of which radio-activity gives us a glimpse were suddenly to share itself similarly, our world would be dispersed into the depths of surrounding space.
Faraday's investigations of the process of electrolysis showed that each atom or other component part of a dissolved substance deposited at either pole deposits also a definite electrical charge. From independent evidence we know that dissolved molecules in the solution have been split up so as to form “ions,” which we can regard as each having gained or lost one or more “electrons” by exchange with a corresponding ion or ions. Atoms can thus lose or gain electrons; and the conclusion has been gradually built up through the investigations of J. J. Thomson, Rutherford, Niels Bohr, and others that the atom is a system consisting of a relatively very minute but extremely massive central core or “proton” charged positively, with minute negatively charged electrons of very little mass revolving round it in definite orbits, the distances of these bodies from one another, as compared with their sizes, being comparable to the distances from one another, as compared with their sizes, of the sun and planets of our planetary system. Electrons, or even protons, can easily be made to shoot through very large numbers of atoms without encountering anything.
It is not the mere existence of this marvellous intra-atomic system, but the fact that its energy does not spread chaotically to the atomic environment that is unintelligible from the Newtonian standpoint. Energy is, however, exchanged between the inside of the atom and its environment by what we know as radiation, and the study of radiation has brought still further insight into the nature of intra-atomic activity. The investigations of Young and of Fresnel more than a century ago had shown that rays of light and heat could be interpreted as due to wave-motion at right angles to the ray in an all-pervasive and perfectly elastic medium, the luminiferous ether. The phenomena of interference made it possible to estimate the wave-lengths of light corresponding to different parts of the spectrum. With the introduction of good spectroscopes it was found, first, that there were a large number of black lines (Fraunhofer lines) in the otherwise continuous spectrum of sunlight, and then that these black lines corresponded to definite bright lines coming from definite chemical elements when they are strongly heated or exposed to the kathode discharge in a vacuum tube. Thus atoms, when they do take up or give off internal energy, only give it off and take it up as radiant energy of certain definite wave-lengths. These critical wave-lengths form, moreover, a characteristic interconnected series.
In order to explain the general relations between the temperature of radiating bodies and the prevalent wave-lengths of the emitted or absorbed radiant energy, Planck was led to the conclusion that radiant energy is emitted, not with continuously graduated wave-lengths, but with emissions each of which is a multiple of a fundamental unit or quantum, so that if radiation from an atom is occurring at all, it is only by increasing the frequency of emission, or diminishing the wave-length, that the rate of emission of energy can be increased. This must be the reason why, as the temperature of a radiating body rises, the prevailing wave-length diminishes. At low temperatures all the radiation is heat of low, infra-red frequency, while as the temperature rises the radiation passes more and more towards the high, ultra-violet frequency.
The discontinuity in the atomic spectrum is satisfactorily explained by Bohr's conclusion that the orbits of electrons in an atom are not just any orbits corresponding to the amount of heat-energy which may chance to be communicated to the inside of the atom, but only certain specific orbits, so that if the atom gives off or takes up internal energy at all, this will occur in definite amounts, to each of which, in accordance with Planck's conclusion, a certain wave-length will correspond. The application of this hypothesis in detail, particularly in connexion with the relatively simple hydrogen atom, has proved very fertile.
It is not merely as regards the internal energy and mass of atoms, but also as regards their external energy, that selective distribution shows itself. It was discovered early last century by Dulong and Petit that the specific heat of a number of solid elements, when divided by the atomic weight, gives a constant figure. The atomic heat of all these solid elements is thus the same. This was not mechanically intelligible at the time, but became intelligible in the light of the dynamical theory of gases. On that theory the kinetic energy per particle in an extremely numerous collection must, on an average, be evenly distributed and vary with the temperature. This gives us Dulong and Petit's principle at once, though the actual specific heat of a solid must depend also on the heat-energy absorbed in the work of either separating the atoms or compressing them as the temperature rises. It was found by Kopp that the specific heats of many solid compounds are also related similarly to the atomic weights of the atoms present.
For various of the lighter elements, such as carbon or boron, and their solid compounds, the law did not hold. Thus the atomic specific heat of carbon is only a fraction of what the law requires, but, as the temperature is raised, diverges less and less from the law. In the investigation of the specific heats of the solid compounds of other lighter atoms, such as those of oxygen, nitrogen, and hydrogen, Kopp found similar divergences in the atomic specific heats. These lighter atoms seemed to afford anomalous and unintelligible exceptions to the law; but when the atomic specific heats of various elements were determined by Dewar at the very low temperatures obtainable by the evaporation of liquid air, various atomic specific heats were found to diminish; and further, more exact experiments by Nernst and others showed that at very low temperatures the specific heat of solid carbon became inappreciable, while with all other substances the atomic specific heat became very low.
To explain these facts it had to be assumed that a certain amount of “push” is required to produce any effect at all on the movements of the atoms, so that at sufficiently low temperatures, when the pushes become feeble, fewer and fewer of the pushes are effective, and finally a substance like carbon pays no attention to temperature as indicated by the behaviour of a perfect gas. Thus selective distribution of energy and quantum relations again show themselves here in a manner which is inconsistent with the Newtonian conception.
Since the energy of rotation (with its three theoretical degrees of freedom) of an atom is internal energy, a monatomic gas does not take up energy of rotation in proportion to rise of temperature, and thus has only about half the molecular specific heat of a triatomic gas, which can spin freely in all three degrees of freedom, besides being able to move freely in all three degrees of freedom of translational movement. This is only intelligible in view of the selective distribution of energy within atoms.
In studying quantum phenomena of atomic radiation we are in the presence of an extended and more comprehensive study of exchange of energy between the inside of an atom or group of atoms and the environment. It is only, however, statistical knowledge of this exchange that we obtain by such study. If we could study an individual atom, the phenomena might seem less anomalous, since they might disclose analogies with the exchange of energy between a living organism and its environment. The form and internal activity of a living organism tend to maintain a normal level in spite of what we interpret physically as actual continuous exchange of energy with the environment. Variation in the characters of living organism tend also, as shown by the investigations initiated by Mendel, to occur in what we might call quanta. The body of a living organism has also what might be described as a quantum structure, perhaps analogous to that of a complex atom. The quantum units are cells and much smaller units, of the existence of which the phenomena of heredity have in recent times disclosed particularly clear evidence.
In the study of biology we are studying life as such from within, while in physics we can only study statistically enormous numbers of molecules from without, and therefore from a physical and not biological standpoint. Nevertheless the disclosure of quantum relations in both structure and communication of energy seems to have partially brought physical investigation within sight of distinctively biological investigation, though not within sight of psychological investigation. The point of connexion is that among what we regard as purely inorganic phenomena mechanically unintelligible co-ordination has been found in the distribution of mass and energy, so that mass and energy seem to be manifestations of a deeper reality, as in the case of life.
In a further and still more fundamental respect the Newtonian conception has been undermined by recent developments in experimental physics. In order to explain the propagation of radiant energy it was necessary on Newtonian principles to assume that in addition to the ordinary matter which is always moving about there exists a “luminiferous ether” which is not moving, so that, as the observations of astronomers demanded, light is propagated in it at the same rate in all directions, this rate being dependent on the properties of the ether. But if this ether is stationary and an observer is moving with the earth, it must, on Newtonian principles, be possible to measure the absolute velocity of the earth's motion.
The surface of the earth is moving with great and variable velocity relatively to the sun; and the whole planetary system may be moving at an unknown further velocity. But neglecting this further velocity, the velocity of the earth's surface relatively to the sun is an appreciable fraction of that of light. Since we could easily detect a difference of the order of this fraction in the apparent velocity of light, and since light in travelling from a given point to a mirror and back ought to take longer when the ray is sent out in a direction against that of motion of the earth's surface than when sent out with it, we also ought, by measuring the difference in apparent velocity with the ray sent out in different directions, to be able to deduce the absolute velocity of motion of the earth's surface. When, however, the experiment was made by Michelson in 1887, he found no difference in the apparent velocities, and accurate repetition of the experiment gives always the same result. The measurements thus seemed to point to the impossible conclusion that the earth's surface is at absolute rest—a conclusion which would flatly contradict our other experience as interpreted on Newtonian principles.
This result gave rise to much speculation, which culminated in the inference drawn by Einstein in 1905 that there is no such thing as absolute velocity, since velocity is only relative. This, however, carried with it the conclusion that there is also no such thing as absolute time or simultaneity. To persons travelling at widely varying relative velocities the lapse of absolute time would be different and there can be no such thing as real or absolute simultaneity at different places. Thus time does not exist apart from space and our own movement; nor space apart from time and our own movement. We must therefore think of physical events as occurring, not in space and time separately, but in a four-dimensional space-time.
Einstein pointed out a few years later that since velocity of rotation must also be merely relative, this affects our conception of attractive forces, including gravitation. On the Newtonian interpretation gravitation is an attractive force universally present between any two bodies, and varying directly as the product of their masses and inversely as the square of their distance apart. But the observed attractive force is interpreted as dependent also on centrifugal force due to absolute rotational movement. We allow for this in estimating the true value of purely gravitational attraction. If, for instance, the earth were rotating sufficiently much faster, bodies at the equator would have no apparent weight at all, or a negative apparent weight, in spite of gravitation, though we should still regard it as existing. When we reject the conception of absolute velocity of rotation we must therefore also alter our conception of gravitation.
We cannot by any direct means distinguish between an effect due to gravitational attraction and one due to acceleration. If, for instance, one is in a colliery cage starting rapidly from the pit-bottom, the effects are the same as if the force of gravity had been much increased; and when the cage is near the surface and slowing down rapidly, the effects are of an opposite character, and are, in fact, so like those of a positive acceleration downwards that persons unaccustomed to pits often imagine that the winding-rope has snapped and they are falling rapidly. If, now, gravitation is in reality due to nothing else than acceleration, this acceleration can be no mere acceleration in space, as bodies are subject to gravitation when they are at rest relatively to one another in space. Einstein concluded that it is acceleration in space-time, and due to curvatures or distortions corresponding in space-time to the masses of bodies. Thus bodies moving, as they are always moving, in space-time are deflected towards one another, just as if they were undergoing an acceleration by a force acting on them in the Newtonian sense. It is, however, the distortion of space-time, and not a so-called “force” acting between them, that produces the acceleration.
At first sight this conception may seem to be a rather far-fetched attempt at an explanation of the disconcerting result of Michelson's experiment. By working out mathematically the implications of the conception Einstein showed, however, that it furnishes, in addition, an explanation of what had hitherto proved inexplicable on the Newtonian theory—namely, the variation in the perihelion of Mercury. What was still more striking, however, was that the theory implied a certain definite deflection in rays of light passing close to the sun, and that this deflection, which had never before been suspected, was actually verified in eclipse observations carefully carried out for the purpose of testing the prediction.
On the new physical conception initiated by Einstein space and time do not exist independently of one another. Only a space-time medium is real. Nor do bodies act on one another at a distance. For the conception of action at a distance we have to substitute the conception of distortion in the space-time medium; and owing to the distortions in this medium the shortest path may not be a straight path. A light-impulse sent out by an observer in one direction, and travelling by the shortest path, might even ultimately return to him from the opposite direction. The old conception of a stationary luminiferous ether is also no longer needed.
Physical conceptions of the universe have, as already pointed out, been framed without taking into account the fact that the world of our experience is a perceived world of interest and values. Not only space-relations, but time-relations also, enter into perception and interest. At the outset of this lecture I pointed out that when we take conscious behaviour into account the only ultimate conclusion we can come to as regards the physical universe as pictured on the Newtonian conception is that, though very useful for practical purposes, it does not represent reality. The universe as we perceive it is at any rate perceived not merely as co-ordinated and so unified according to space-relations, but also according to time-relations. We cannot, and do not, separate space-relations from time-relations in perception. In so far as Einstein's conception insists on the unreality of the separation, it seems to represent a great step forward in bringing the physically conceived universe nearer to reality as it appears when the fact that our experience is conscious experience is taken into account. It still remains the case, however, that even this amended conception of a physical universe does not take into consideration the fact that reality is for us perceived reality, and therefore embodies co-ordination of the phenomena which constitute it. On the amended conception, perception of the world would still be impossible; and the universe would still be a meaningless physical, and not the actual spiritual, universe.
A different criticism of a more practical character may perhaps be made on Einstein's conception. The old Newtonian conception is so very simple and useful practically that we can hardly get on without it. It is at least a great mental effort to translate Newtonian into Einsteinian conceptions, and for the limited practical purposes of physical science it is nearly always unnecessary to do so. Further experience alone will show whether the deeper and less untrue conceptions of Einstein will come into extensive practical use. When we regard neither the Newtonian nor Einsteinian interpretation of reality as more than a useful working hypothesis which must in any case be discarded when we take life and conscious experience into consideration, it may prove more generally useful to retain the Newtonian conception for the practical purposes which it meets so well on the whole, though it fails to meet some of them.
Many physicists and other writers are still under the impression that it is the duty of physical science to reveal a complete representation of visible reality or Nature. Newton thought that he had done so; but, as I have pointed out already, he failed because he left out life and conscious behaviour, which his representation of reality was incapable of interpreting. Einstein's representation is an improvement, but still fails, for just the same reason as Newton's representation failed. Nature just mocks at us, as she mocked at Michelson, when we attempt to divorce her from conscious perception; and she still mocks at Einstein. There is no such thing as a physical world existing apart from consciousness; no such thing as absolute space or time or space-time, or absolute mass, motion, or energy. When we realize that perception is no mere event in a physical universe, and no mere individual perception, we begin to realize that physical science does not really set out to interpret reality, but only to discover and make use of such a provisional conception as can be used for certain limited practical purposes.