Philosophy of Physics Quotes

“that philosophy makes no progress, but to a large extent the creation of autonomous disciplines is how philosophy progresses.”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“progress,”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“In the case of Newtonian physics, velocity boosts—transformations where all bodies are increased in speed by the same amount, in the same direction—are, provably, symmetries. And so Galileo is right: velocity boosts are undetectable in Newtonian mechanics. The postulate that velocity boosts are symmetries is called the principle of relativity. In popular culture it is of course associated with Albert Einstein, but the basic idea is hundreds of years older. And it creates a potentially severe problem for Newton’s physics: it tells us that, contra Newton’s suggestion, it is in principle impossible, according to physics itself, to detect whether or not something is moving with respect to the rest frame.”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“Consider this famous passage from Galileo: Shut yourself up with some friend in the main cabin below decks on some large ship, and have with you there some flies, butterflies, and other small flying animals. Have a large bowl of water with some fish in it; hang up a bottle that empties drop by drop into a wide vessel beneath it. With the ship standing still, observe carefully how the little animals fly with equal speed to all sides of the cabin. The fish swim indifferently in all directions; the drops fall into the vessel beneath; and, in throwing something to your friend, you need throw it no more strongly in one direction than another, the distances being equal; jumping with your feet together, you pass equal spaces in every direction. When you have observed all these things carefully (though doubtless when the ship is standing still everything must happen in this way), have the ship proceed with any speed you like, so long as the motion is uniform and not fluctuating this way and that. You will discover not the least change in all the effects named, nor could you tell from any of them whether the ship was moving or standing still Galileo’s point is that the absolute velocity of a system of bodies is not detectable by any means available to a scientist who is part of that very system, because the relative motions of the bodies are unaffected by their overall velocity. Only by relating the bodies to some external system can the motion be detected”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“Newton himself was well aware of the problem: in his Principia Mathematica he writes It is indeed a matter of great difficulty to discover, and effectually to distinguish, the true motions of particular bodies from the apparent [i.e., the relative motions]; because the parts of that immovable space, in which those motions are performed, do by no means come under the observation of our senses.”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“But there is something a little strange about using motion relative to an absolute rest frame as the basis of mechanics, rather than motion relative to other material bodies. After all, we can see material bodies, and so we can see whether something is moving relative to them. We can’t see the rest frame. (In Newton’s terms: the points of absolute space are invisible.)”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“Substantivalists believe space is a substance, a thing itself over and above the material contents of the world; relationists believe ‘space’ is just a pretty way of talking about the relations that hold between bodies. This might seem to be an arcane, even a semantic debate, but Newton’s arguments show its significance for physics: if all that exists is matter, we don’t seem to have any way to define the rest frame that we need to do physics.”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“Newton himself believed—and forcefully argued—that the only way to define ‘motion’ adequately was to admit something else to our picture of the world, something additional to all of the moving matter, something which would persist even if the matter was to vanish: absolute space.”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“Underdetermination—where two different theories give the same predictions—is rarely an all-or-nothing affair, because the distinction between theoretical and observational claims is blurry. Apparent cases of underdetermination often get resolved over time as one theory turns out to be more powerful as a framework. The only realistic cases of exact underdetermination seem to be where two theories are mathematically equivalent; in these cases, physicists—and some, but not all, philosophers of physics—regard them as the same theory.”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“Falsification is a big improvement on induction as a description of the scientific method, but it is still only a crude approximation—a given observation usually only falsifies a theory given a host of background assumptions. So there is no simple, one-off test for when something is science:”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“physicists used to think heat was a fluid; now they think it’s random motion of molecules; they used to think light was a vibration in an all-encompassing ‘aether’; now they think it can exist in the absence of any such thing. This pessimistic argument has historically been the main objection raised to realism.”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“The other reason comes from the history of science—especially physics. Repeatedly in that history, an established theory has been overthrown, even though it was highly successful at making predictions. Newtonian gravity, for instance, is in an important sense wrong, replaced by the general theory of relativity;”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“Scientific realism might seem just obvious: isn’t doubting our well-established theories a sort of anti-scientific scepticism? But there are reasons to treat it cautiously. The first we have already met: the threat of underdetermination. If we have two theories that make the same observational predictions but which contradict one another, then (at least) one of them must be false—in which case the no-miracles argument can’t be right.”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“there is no remotely plausible way to understand why those theories are so successful, other than by assuming that they are at least roughly correct. This is sometimes called the no miracles argument, following philosopher Hilary Putnam’s observation that it would be a miracle for scientific theories to work so well if they weren’t true.”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“the success of our current scientific theories gives us good reason to think that they are correct (and not merely useful gadgets to make predictions). Electrons, or quarks, or black holes, cannot be directly observed—that is, you can’t see, hear, or touch them—but (say scientific realists) we still have good reason to think that there are such things.”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“observations are theory-laden: even to describe an observation, we need the language of theory. ‘This detector will display the number 5.228’ … which detector? Only a detector built the right way—and ‘the right way’ inevitably involves theoretical ideas.”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“The appeal of positivism to philosophers in the early 20th century—and to some physicists even today—is that it dismisses as meaningless any claim about a theory that does not have experimental consequences.”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“To the positivists, non-observational claims cannot be understood independently of their observational consequences: to say, for example, ‘atoms are made up of electrons and protons’ is just to say ‘if I make this measurement I’ll get this result, if I make that measurement I’ll get that result, …’. In effect, the content of a scientific theory just is the collection of observational claims it makes: the rest of the theory is just a calculational tool to get from one set of observations to another.”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“If the choice between two theories is scientific, evidence must bear on it; but what about where two distinct theories make exactly the same predictions? Philosophers call this case ‘underdetermination of theory by evidence’:”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“Popper’s own interest in falsificationism, and in the scientific method, was only partly for its own sake. He also sought a criterion for when an approach to knowledge-collection counted as science, and found it in the requirement of falsifiability (so, supposedly, neither Freudian psychology nor Marxist economics—two of his bugbears—counted as scientific). Modern physicists often seem to do likewise: to dismiss a question, or sometimes an entire sub-discipline (like string theory) or field of study (like philosophy!), as ‘unfalsifiable’ and thus unscientific.”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“Traditionally, understanding the deep nature of the world was the task of metaphysics, but in modern times that understanding relies critically on our best physics theories—yet those theories do not wear their meaning on their sleeve. In this sense, philosophy of physics provides a bridge between the metaphysician and the physicist—or, put another way, philosophy of physics tells us how to do a metaphysics that is scientifically informed.”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“few professionally trained physicists learn much philosophy; few philosophers know more than the rudiments of physics. How, then, can there remain a philosophy of physics? The clearest and simplest reason is that while the conceptual foundations of physics are clearer by far than they were before Newton’s time, there remains much that we do not understand. Physics is not simply mindless calculation: good physicists are alive to the conceptual questions and paradoxes that arise in their work.”
― David Wallace, Philosophy of Physics: A Very Short Introduction

“It is often said that philosophy makes no progress, but to a large extent the creation of autonomous disciplines is how philosophy progresses. Mathematics in antiquity; physics in the Renaissance; biology after Darwin; logic in the early 20th century; computer science in mid-20th century; cognitive science still more recently; in each case, so much progress was made, so many controversies resolved, so many confusions clarified, that a self-contained subject was created and equipped to progress further. The philosopher Daniel Dennett defines philosophy as what we do when we don’t know what questions to ask; when we understand enough to work out what the questions are and can start answering them, a new science buds off from philosophy.”
― David Wallace, Philosophy of Physics: A Very Short Introduction