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I consider myself a (metaphysical) materialist or, to use the synonymous term that is more fashionable nowadays, physicalist, and I'm familiar with the academic literature on contemporary materialism/physicalism. But in no paper or book did I find really satisfying, fully adequate definitions of the central concepts of a material/physical object and of a material/physical property. (A material/physical property certainly isn't material/physical in the same sense as a material/physical object.) Does this mean that there actually aren't any such definitions, and that materialism/physicalism is therefore a virtually vacuous doctrine? Material/physical objects (substances) could be defined in terms of material/physical properties: x is a material/physical object =def x has some (intrinsic) material/physical properties. But then the big problem is how to properly define the concept of a material/physical property. I've been trying to devise and formulate a fully adequate definition of it for several years...

This is indeed a difficult question. If we say that a physical object is an object with intrinsic physical properties, then you are right: we have left ourselves with the question of what a physical property is. If we say that a physical object is an object with spatiotemporal properties (such as position and velocity), then someone who believed in irreducible minds or souls that have spatial locations could presumably still count as a physicalist, which seems inappropriate. If we say that a material object is an object that is made of matter, then we need an account of what matter is. Are electric fields made of matter? They have mass, after all. Would Newtonian space be made of matter? It doesn't seem like it would be ... but its existence does not compromise materialism, does it? More generally, materialism and physicalism seem to be motivated by the idea that the entities described by physics are all of the entities that there are -- or, more precisely, are all of the fundamental entities there...

We know that when we see Alpha Centauri with the naked eye we are seeing light that left that star over 4 years ago when Bush was still President. Other stars are obviously much farther away and we’re looking at light that originated, say, when Galileo was still around or when the pyramids were being built. When we’re told that telescopes help us see into ‘deep space’ I’m wondering what that means: do they simply magnify the detail of images or do they help us see the detailed images earlier than we would with the naked eye? The difference that I have in mind is this: a friend comes to my house who I know has been travelling an hour to see me. I first see him when I open the front door. But suppose I’m looking forward to the reunion and I set out to meet him half way so as to abbreviate his journey. Suppose further I have the capacity/technology to meet him at his place of origin so I can see him immediately. Now, does a telescope, say Hubble, allow astronomers and cosmologists to see ‘earlier’ into...

A telescope collects more light than an ordinary human eye. It is a larger "light bucket". Consequently, a telescope helps us to see things that are fainter (as seen from earth) than we can with the naked eye. Consequently, a telescope helps us to see things that are more distant (and hence helps us to see things as they were longer ago). A terrestrial telescope does not "meet" light somewhere along the way, unfortunately. The light must still manage to arrive at Earth.

Stephen Hawking has claimed in his new book that "...philosophy is dead...(it) has not kept up with the developments in science, particularly physics". What do philosophers think of this claim?

Well, I cannot speak for all philosophers. But it seems to me that Hawking has not kept up with the developments in philosophy. Of course, he need not do so ... unless he plans to say something about them, as he apparently did. There is a tremendous amount of very scientifically informed philosophy of science. People in philosophy departments and people in physics departments both work on the conceptual, logical, and metaphysical foundations of physics (and analogous points could be made about evolutionary biology or economics, for instance). Even a cursory glance at the literature would bear this out. I apologize if this sounds somewhat defensive. I guess it is. But physicists do tend to deprecate philosophy of science without having taken the trouble to familiarize themselves with it. See, for instance, Steven Weinberg's book "Dreams of a Final Theory" (1992), to which Wesley Salmon replies in "Dreams of a Famous Physicist", an article reprinted in his book "Causality and Explanation".

Are symmetry principles laws of nature, or meta-laws of nature? The intuition is that laws of nature are contingent. That is, it could be different in different logically possible worlds. Does this hold true for symmetry principles? Could there be some symmetric principles that had to hold in all possible worlds?

My view (which I defended in my recent book, "Laws and Lawmakers" from Oxford University Press) is that symmetry principles in physics are widely regarded as meta-laws. For instance, the principle that all first-order laws must be invariant under arbitrary displacement in time or space explains why all first-order laws have this feature (and, in a Hamiltonian framework, ultimately explains why various physical quantities are conserved). The symmetry principles function as constraints upon what first-order laws there could have been. Had there been an additional force, for instance, then the laws governing its operation would have obeyed these symmetry principles, since these symmetry principles are meta-laws. Eugene Wigner and others have suggested that the relation of symmetry principles to the first-order laws they govern is like the relation of those first-order laws to the particular events they govern. I see no reason why symmetry principles would differ from first-order laws by holding in...

Many of my science professors have remarked that the law of conservation of mass and energy is unprovable (or at least unproven); is this really the case, however? Isn't the problem of the conservation law precisely the problem of induction? (I.e., we observe that the mass and energy of every system we have ever examined has remained constant, but how do we know that this will hold true (1) in the future and (2) of all systems?) But presumably when my professors have said that the conservation law is unproven, they didn't mean that this is so because of the problem of induction (after all, if they took this route then all of science would be "unproven"!). I feel as though they are treating the conservation law as exceptional when in fact it is not. -ace

I agree with you that observational evidence for the hypothesis that all processes conserve energy (or mass, or mass-energy) inevitably fails to prove that hypothesis (though succeeds in confirming the hypothesis strongly), just as our observational evidence for the hypothesis that (say) "All bolts of lightning are followed by claps of thunder" inevitably fails to prove that hypothesis. Even if every lightning bolt we have observed so far has been followed by a thunder clap, no contradiction would result from the next lightning bolt occurring without a thunder clap. So the "problem of induction" applies to both examples equally. Perhaps your science professors had something else in mind in emphasizing the "unprovability" of energy conservation. Suppose we observe a process that apparently fails to conserve energy. The system's energy before the process occurs seems to exceed the system's energy after the process occurs. Rather than rejecting the conservation of energy, couldn't we always respond by...

This may be a silly question displaying only my ignorance on the subject. My question has to do with point-particles and spatiality. Physicists say that point particles have causal powers, i.e. photons striking someone’s eyes at certain wavelengths cause them to see. Perhaps photons are only contributory causes to one’s seeing. Physicists also say point particles are objects that are both concrete and physical. That is, they can be located in space which entails they are spatial objects too. However, by definition a point-particle lacks width, length, and depth, the three spatial dimensions. My question is how can this be? Is this a conceptual incoherence, or am I missing something? Does spatiality entail physicality or conversely, does physicality entail spatiality? Alternatively, is it that these two concepts have no intimate connection? Please explain. Thanks.

Your question seems to concern the connections between being spatial, being concrete, and being physical. Part of your question seems also to concern the idea of point particles. Now it might be that ordinary material objects really do consist of point particles. If that's true, then point particles are concrete, physical, spatial entites. (That a point body has no dimensions does not keep it from being spatial: it has a location in space.) On the other hand, it might be that the fundamental, elementary objects that physics seeks are not point sized. (Perhaps they are strings or whatever.) In that case, it might still be that certain physical theories that invoke point bodies do a pretty good job for certain purposes. Point bodies would then be idealizations -- useful ones. They would be abstract rather than concrete objects, in one sense of "abstract." (It could work the other way too: hydrodynamical theories that treat water as a continuous fluid may work very well even if a body of water...

Is the universe infinite? And if it isn't, what is outside it? Are there lots of universes, or is it all just fractals? And what about other dimensions? Is it possible that time and laws of physics work differently in other universes? Helen from Worcester, age 12

Hello Helen from Worcester! Thank you for your excellent questions! Let's start with whether the universe is infinite. The answer is: We don't know! But suppse it is NOT infinite. Then what is outside of it? Perhaps the answer is that there is no such thing as "outside" of it. The universe consists of all of space and time. An "outside" would be a location -- in space -- that is outside of the universe -- and so outside of all space. That is a contradiction, right? (It would be in space and outside of space at the same, um, time.) Suppose that the universe is not infinite. Suppose you head off in your spaceship in a particular direction, and just keep going straight. Since the universe is not infinite, will you eventually get to a wall that you cannot penetrate: the edge of the universe? No! Instead, you might just find yourself approaching the place from which you started, but from the opposite direction. Space might be curved so that there is only a finite amount of it, but there is no edge...

I read all the questions and responses related to determinism, quantum mechanics and chaos theory that you have posted, but I am still unclear exactly how they relate. Supposedly, quantum mechanics and chaos theory refute any hard case for determinism, but I am still unclear as to how. Could anyone add to this or suggest some reading on the subject?

Determinism is the view that the state of the world at any moment, plus the laws of nature, determine (i.e., logically entail) the state of the world at any other moment. Quantum mechanics and chaos theory relate to determinism in rather different ways. Chaos theory concerns systems whose development is exquisitely sensitive to their current state -- in that a very small change to their current state would produce enormous changes to their later state. A chaotic system is not incompatible with determinism as I have defined it above. But the existence of chaotic systems entails that any small uncertainty in our knowledge of that system's initial conditions (and some such uncertainty is always present, for grubby practical reasons) will quickly ramify into great uncertainty in our predictions regarding that system, even if we know all of the relevant laws of nature. None of this threatens determinism as a view about prediction "in principle." But quantum mechanics does that. The complete state...

Could a newly discovered law of physics ever change/affect a law of logic?

Very good question! Let's begin by drawing an important distinction. By "changing a law of logic", you might mean (i) our changing our minds about what the laws of logic are, or (ii) the actual laws of logic changing -- one set of laws was in force at one time and another set is in force at another time. I will assume you had option (i) in mind, since the idea that the laws of logic change is at least as weird as the idea that the laws of physics change (which is to say: pretty weird), and in any case, the change would surely not be a result of something as cosmically inconsequential as our making a certain scientific discovery! So, your question now is: Could we be justified in changing our minds about what the laws of logic are as a result of a discovery in physics? This is a controversial question. Some philosophers have said that we know the laws of logic a priori -- that is, independent of sensory input. In general, such philosophers do not think that we could justly change our minds...

Did Einstein ever engage the "scientific method" of empirical investigation in the course of his work on special and general relativity; and if not, wasn't he more a philosopher of science (albeit an exceptionally productive and influential one) than a scientist? If Einstein simply engaged in a priori reasoning and conceptual analysis (using his famous thought-experiments) then I don't see why the physics community has any more claim to him than the philosophical community. After all, it seems that his methodolgy bore a much stronger resemblance to that of contemporary philosophical efforts than it does to anything going on in or commonly associated with physics departments. -Will Leonard

An excellent question! Many of Einstein's most famous papers make shockingly few references to the details of previous empirical work by other scientists. To put the same point in another way, many of Einstein's most famous arguments arise largely from "philosophical" considerations. For instance, Einstein's 1905 special theory of relativity paper begins by noting a symmetry in electromagnetism: that the current induced by a magnet moving relative to a loop of conducting wire is the same, according to electromagnetic theory, whether the magnet is moving and the conductor is at rest, or vice versa, as long as their relative motion is the same in both cases. However, Maxwell's electromagnetic theory (as it was then understood) assigns the induced current different causes in the two cases. Einstein suggests that the current should be understood as having the same cause in the two cases, which leads him to suppose that there is no fact about whether a force is really electric or magnetic. Clearly, this...

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