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...

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.

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".

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...

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...

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...

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...

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...

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...

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...