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Physics is the science of the natural world, which deals with the fundamental particles the universe is made of, the interactions between them, and the interactions of objects composed of them (nuclei, atoms, molecules, etc).
- The number of continuous transformations that exist between one set of real numbers and another is a whole order of infinity lower than the total number of possible transformations which are not continuous. The requirement of continuity provides a vast and surprising reduction in scope. Since these continuous transformations include the catalogue of possible relationships from which we draw that class of transformations (or 'equations') called the laws of physics, we see that a discontinuous world will be infinitely more complex in its potentiality. It is less constrained in what it is allowed to do. At present, physicists are enamoured of symmetry and search only for continuous pictures of fundamental physics. Maybe, one day, they will be motivated to look at possible structures of a fundamentally discreet world.
- John D. Barrow, Theories of Everything: The Quest for Ultimate Explanation (1991) p. 37.
- Theoretical physicists live in a classical world, looking out into a quantum-mechanical world. The latter we describe only subjectively, in terms of procedures and results in our classical domain.
- John Stewart Bell "Introduction to the hidden-variable question" (1971), included in Speakable and Unspeakable in Quantum Mechanics (1987), p. 29
- Physics is to be regarded not so much as the study of something a priori given, but rather as the development of methods of ordering and surveying human experience. In this respect our task must be to account for such experience in a manner independent of individual subjective judgement and therefor objective in the sense that it can be unambiguously communicated in ordinary human language.
- Niels Bohr, "The Unity of Human Knowledge" (October 1960)
- Physicists use the wave theory on Mondays, Wednesdays and Fridays and the particle theory on Tuesdays, Thursdays and Saturdays
- William Henry Bragg; quoted in Dictionary of Scientific Quotations by Alan L. Mackay, Institute of Physics Publishing, Bristol, 1994, p. 37 
- Variant: On Mondays, Wednesdays and Fridays we teach the wave theory and on Tuesday, Thursdays and Saturdays the corpuscular theory.
- Quoted in Physically Speaking: A Dictionary of Quotations on Physics and Astronomy by C.C. Gaither, 1997, ISBN 0750304707. 
- unsourced variant: God runs electromagnetics by wave theory on Monday, Wednesday, and Friday, and the Devil runs them by quantum theory on Tuesday, Thursday, and Saturday.
- I was an atheist, finding no reason to postulate the existence of any truths outside of mathematics, physics and chemistry. But then I went to medical school, and encountered life and death issues at the bedsides of my patients. Challenged by one of those patients, who asked "What do you believe, doctor?", I began searching for answers.
- In all these theories of mathematical physics, the same type of procedure is invariably followed. Experimenters establish certain definite facts and detect precise numerical relationships between magnitudes... The mathematical physicist then... translates the numerical relationships discovered by the experimenter into mathematical form. He thus obtains a mathematical relationship or equation α which is assumed to constitute the mathematical image of the concrete physical phenomenon A. His task will now be to extract from his mathematical equation or equations α all their necessary mathematical consequences. In this way... he may be led to new equations β. These new equations β, when translated back from the mathematical to the physical, will express new physical relationships B.
The mathematician assumes that just as his equations β were the necessary mathematical consequences of his original equations α, so also must the physical translation of β constitute a physical phenomenon B, which follows as a necessary consequence of the existence of the physical phenomanon A. If A occurs B must ensue. ...
Its utility is to allow us to forsee and to fortell physical phenomena. ...it suggests definite experiments which might never have been thought of, and permits us to anticipate new relationships and new laws and to discover new facts. ...by establishing a rational connection between seemingly unconnected phenomena, it enables us to detect the harmony and unity of nature which lie concealed under an outward appearance of chaos.
- A. D'Abro, The Evolution of Scientific Thought from Newton to Einstein (1927) Forward
- Maxwell's equations had proved themselves incapable of accounting for dispersion. It appeared necessary to conceive of some structure for dielectrics which would act selectively, imposing different degrees of retardation on light waves of different frequencies. Lorentz achieved this result by assuming that electricity was atomic and that matter was constituted by more or less complicated groupings of these electric atoms or electrons.
Phenomena were accounted for by taking into consideration the frictional resistances that would interfere with rapid vibrations of the electrons. When these frictional resistances were weak, oscillatory disturbances, such as rays of light, could be propogated through the dielectric, which was then termed transparent (glass). When these frictional forces were considerable, the light ray was unable to set the electrons into vibration; its energy was consumed in the attempt, and as a result it could not proceed; the dielectric was then opaque (ebonite, sulphur).
In the case of conductors such as metals, the electrons were assumed to be very loosely held to their atoms so that the slightest difference of potential would tear them away and cause them to rush in the same direction, thereby producing an electric current. It was precisely because electrons in conductors were not tied down to fixed positions by elastic forces that they were incapable of vibrating; and so conductors were necessarily opaque to electromagnetic vibrations or to light. Conversely, it was because the electrons were all tied down to fixed positions in the dielectrics, that they could not rush along in one direction. As a result dielectrics were opaque to currents, and hence were non-conductors. According to these views of Lorentz, an electric current passing through matter was nothing but a rush of electrons.
- A. D'Abro, The Evolution of Scientific Thought from Newton to Einstein (1927) p. 130.
- The human imagination, including the creative scientific imagination, can ultimately function only by evoking potential or imagined sense impressions... I confess I never met an experimental physicist who does not think of the hydrogen atom by evoking a visual image of what he would see if the particular atomic model with which he is working existed literally on a scale accessible to the sense impressions - even while realizing that in fact the so-called internal structure of the hydrogen atom is in principle inaccessible to direct sensory perception. This situation has far-reaching consequences for the method of experimental investigation.
- Martin Deutsch, "Evidence and Inference in Nuclear Research," Daedalus (Fall 1958) 87, p. 167.
- Mathematical physics represents the purest image that the view of nature may generate in the human mind; this image presents all the character of the product of art; it begets some unity, it is true and has the quality of sublimity; this image is to physical nature what music is to the thousand noises of which the air is full...
- And so in its actual procedure physics studies not these inscrutable qualities, but pointer-readings which we can observe, The readings, it is true, reflect the fluctuations of the world-qualities; but our exact knowledge is of the readings, not of the qualities. The former have as much resemblance to the latter as a telephone number has to a subscriber.
- Arthur Stanley Eddington, The Domain of Physical Science (1925)
- The supreme task of the physicist is the discovery of the most general elementary laws from which the world-picture can be deduced logically. But there is no logical way to the discovery of these elemental laws. There is only the way of intuition, which is helped by a feeling for the order lying behind the appearance, and this Einfühlung [literally, empathy or 'feeling one's way in'] is developed by experience.
- Albert Einstein, Preface to Max Planck's Where is Science Going? (1933)
- How strange is the lot of us mortals! Each of us is here for a brief sojourn; for what purpose he knows not, though he sometimes thinks he senses it. But without deeper reflection one knows from daily life that one exists for other people — first of all for those upon whose smiles and well-being our own happiness is wholly dependent, and then for the many, unknown to us, to whose destinies we are bound by the ties of sympathy. A hundred times every day I remind myself that my inner and outer life are based on the labors of other men, living and dead, and that I must exert myself in order to give in the same measure as I have received and am still receiving...
- Albert Einstein, Mein Weltbild (1931)
- The "paradox" is only a conflict between reality and your feeling of what reality "ought to be."
- The concepts which now prove to be fundamental to our understanding of nature—a space which is finite; a space which is empty, so that one point [of our 'material' world] differs from another solely in the properties of space itself; four-dimensional, seven- and more dimensional spaces; a space which for ever expands; a sequence of events which follows the laws of probability instead of the law of causation—or alternatively, a sequence of events which can only be fully and consistently described by going outside of space and time—all these concepts seem to my mind to be structures of pure thought, incapable of realisation in any sense which would properly be described as material.
- Today there is a wide measure of agreement, which on the physical side of science approaches almost to unanimity, that the stream of knowledge is heading towards a non-mechanical reality; the universe begins to look more like a great thought than like a great machine. Mind no longer appears as an accidental intruder into the realm of matter; we are beginning to suspect that we ought rather to hail it as a creator and governor of the realm of matter...
- James Jeans, The Mysterious Universe (1930, 1937)
- And the substance out of which this bubble is blown, the soap-film, is empty space welded onto empty time.
- James Jeans, The Mysterious Universe (1930, 1937)
- Physics and philosophy are at most a few thousand years old, but probably have lives of thousands of millions of years stretching away in front of them. They are only just beginning to get under way.
- James Jeans (1942) Physics and Philosophy, p. 217.
- It is impossible, and it has always been impossible, to grasp the meaning of what we nowadays call physics independently of its mathematical form.
- Jacob Klein, Greek Mathematical Thought and the Origin of Algebra (1968)
- "If I were forced to sum up in one sentence what the Copenhagen interpretation says to me, it would be 'Shut up and calculate!'"
- For every symmetry there comes a constraint. ...If physics is to look the same when the origin of time is shifted... [o]nly those processes that conserve energy are allowed. ...If physical law is to be immune to the arbitrary displacement of our spatial axes, then nature requires the conservation of linear momentum. ...If the laws are to be unaffected by the arbitrary rotation of a coordinate system, then angular momentum must be conserved. ...If the laws are to be the same for all inertial observers, then the space-time interval must be invariant. ...[A]nother constraint ...so beautiful as to make one jaw drop in wonder ...symmetry creates force. ...[T]he symmetry of identical particles forces matter ...to be enrolled as either fermion or boson ...Bosons, typified by the photon, carry the fundamental forces that cause fermions to attract and repel. Fermions, led by electrons and quarks, become constituents of ordinary matter. ...Gravity. Electromagnetism. The strong force. The weak force. Each fundamental interaction is called into being by the requirements of a particular local symmetry.
- Michael Munowitz, Knowing: The Nature of Physical Law (2005)
- There have indeed been civilizations upon your planet that understood as well as you, and without your kind of technology, the workings of the planets, the positioning of the stars -- people who even foresaw "later" global changes. They used a mental physics. There were men before you who brought back data quite as "scientific" and pertinent. There were those who understood the "origin" of your solar system far better than you. Some of these civilizations did not need spaceships. Instead, highly trained men combining the abilities of dream-art scientists and mental physicists cooperated at journeys not only through time but through space.
- Jane Roberts, in The “Unknown” Reality: Volume One, p. 196, Session 702
- The physicist ... engages in complex and difficult calculations, involving the manipulating of ideal, mathematical quantities that, at first glance, are wholly lacking in the music of the living world and the beauty of the resplendent cosmos. It would seem as if there exists no relationship between these quantities and reality. Yet these ideal numbers that cannot be grasped by one's senses, these numbers that only are meaningful from within the system itself, only meaningful as part of abstract mathematical functions, symbolize the image of existence. ... As a result of scientific man's creativity there arises an ordered, illumined, determined world, imprinted with the stamp of creative intellect, of pure reason and clear cognition. From the midst of the order and lawfulness we hear a new song, the song of the creature to the Creator, the song of the cosmos to its Maker.
- Joseph B. Soloveitchik, Halakhic Man (1983), pp. 83-84
- Science deals with but a partial aspect of reality, and... there is no faintest reason for supposing that everything science ignores is less real than what it accepts. ...Why is it that science forms a closed system? Why is is that the elements of reality it ignores never come in to disturb it? The reason is that all the terms of physics are defined in terms of one another. The abstractions with which physics begins are all it ever has to do with...
- J. W. N. Sullivan, The Limitations of Science (1933)
- As soon as we venture on the paths of the physicist, we learn to weigh and measure, to deal with time and space and mass and their related concepts, and to find more and more our knowledge expressed and our needs satisfied through the concept of number, as in the dreams of Plato and Pythagoras.
- Our judge is not God or governments, but Nature.
- Do parallel universes exist? We don't know, uhm parallel universes are losing favor to the multiverse we have some cogent theoretical expectations that our universe might be just one of many spawned from this, sort of, this hyper-dimensional medium which we'll call the multiverse there's no data to support it but we have good theoretical premise to think that it's there and we have philosophical precedent we used to think Earth was special and unique. It wasn't, we got 8 .. 9 .. 8 planet we thought the Sun was special it's one of a hundred billion suns, the galaxy's special, no there's a hundred billion galaxies we have one universeor do we? The track record said why should there only be one? be open to the possibility that you don't live in the majoritylooking? universe that's out there Would a separate universe .. when you say "different universe" slightly different laws of physics which (that's what I'm asking) oh this is the fun part because if you find, if you manage to get a portal to another universe don't be the first one to volunteer to go through because your atoms are working in this universe if a slightly different law of physics.. you could implode, explode come out with three heads who knows?
- All science is either physics or stamp collecting.
- As quoted in Rutherford at Manchester (1962) by J. B. Birks
- Unsourced variants:
- That which is not measurable is not science. That which is not physics is stamp collecting.
- Physics is the only real science. The rest are just stamp collecting.
- That which is not measurable is not science. — (which is also attributed to Lord Kelvin)
- I had spent six years slugging my way through many dozens of physics textbooks that were carefully written with the best of pedagogical plans, but there was something missing. Physics is the most interesting subject in the world because it is about how the world works, and yet the textbooks had been thoroughly wrung of any connection with the real world. The fun was missing.
- Jearl Walker, David Halliday, and Robert Resnick, Fundamentals of Physics (10th ed., 2014), Preface
- The physical doctrine of the atom has got into a state which is strongly suggestive of the epicycles of astronomy before Copernicus.
- Alfred North Whitehead, Science and the Modern World (1925)