First off, it is important to define terms. In order for my readers to understand me (and to avoid unnecessary objections), it would be wise for me to begin by making sure we're all on the same page. Any time I use one of these words or phrases, then, they should be understood in light of the definitions which I will give them, unless explicitly stated that I am referring to another person's definition.
If I appeal to an authority, as I will make a habit of doing (and of using footnotes, either of superscript form or of bracketed form), then I will note it below. If you feel that I have overlooked something, please comment to let me know.
The Universe: The volume which contains all the stars, galaxies, and the space between them, within which everything that we can observe is contained. The Universe might very well extend beyond the visible universe, which is significantly different only because physical (i.e. pertaining to the laws of physics) restrictions preclude us from seeing beyond a certain distance away from the earth. The physical boundary of the Universe is simply that--the point in three-dimensional space beyond which there exists no matter or energy at all. Granted that the Universe is expanding, this point will change in its distance from earth and the center of the Universe; however, the Universe is expanding into what is presumably composed of absolute nothingness, that is, a void, and it is this void that the Universe expands into, whereas the actual matter and energy (and the space between them) is WHAT it is that is doing the expanding. To summarize, the Universe is the totality of matter and energy in existence that is bounded by the outer edge of itself, beyond which is not-the-Universe, but something different. A void, perhaps.
Volume: The term I italicized above, meaning simply a three-dimensional area (area not being used in a mathematical sense here). On that note, I used volume (above) exactly in the mathematical sense.
Space: The area (again, simply meaning "zone," in this sense) between the visible, observable (and possibly interactive) matter and energy in our universe. It is considered to be totally empty by some, whereas others consider it to be negative, in some sense (not merely zero matter). At any rate, I am going to make a distinction betweenspace and a void, with the difference being that space is the areas between matter/energy particles inside our universe, whereas a void is a three-dimensional area that has absolutely nothing in it, not even negative stuff.
(Three-Dimensional) Space: A distinction is made between this 'space' and the previous 'space.' This variety corresponds to a coordinate system such as in mathematics, whereas the previous corresponds to the literally existing stuff (whether matter, energy or 'emptiness') between solid particles in our Universe.
Matter/Energy: One and the same, though different permutations of the same substance. Matter and Energy are bound by the laws of physics (the ones we know of and the ones we haven't discovered yet). This is an important distinction, because this is the only substance which we know of that obeys such laws. As far as we know, 1) They do not disobey these laws, and 2) nothing else (that is, any other type of substance) has been observed or measured empirically which either obeys or disobeys these laws. Matter (I may also refer to it as mass) and energy are related by the equation E = mc^2 [I don't yet know how to do superscripts, if I can]. Of course, new information may arise to change the specifics of this equation, but it is widely known that certain materials (I'm talking about radioactive ones) may be accelerated into one another at high speeds in order to convert a small portion of the matter directly into energy. Examples of this are atomic bombs (See 'tsar bomba' on youtube for a video of the biggest yield recorded so far)
Void: Absolute nothingness. Space is a vacuum, for example, but there is evidence that even empty space may be filled with something called "zero point energy" (more on that later). A void has nothing whatsoever in it. In fact, it might be entirely impossible for us to understand the concept of a void, considering that space probably is not one (despite that we consider space, in casual thought, to be a void).
Note: AstroPhysicists often talk of space as being able to be "curved," (often in reference to gravity) and they speak of "open" and "closed" universes. Also for this reason, I make a distinction between space, which apparently can be "affected" in some way, and a void, which is defined by its nonexistence, and as such can exert no effect, nor be affected.
Substance: A thing which exists. I should take care not to refer to it as "physical," considering that this is a specific reference to the behavior of mass (a particular type of substance) in our universe. Mass interacts with other mass in a physical manner, due, incidentally, to the fact that it is bound by (it obeys) the laws of physics. Consider the fact that a substance which is not mass would not have to behave as mass does. Quite to the point, if there existed (in our Universe or outside it) another type of substance, then there is no reason to expect that it would have to obey the laws of physics at all. In fact, considering that we have never empirically observed something in operation that is not composed of mass-energy, we have no comparison to make--we cannot presume to know anything about how a non-mass-energy substance would behave.
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Something additional to note about non-mass/energy substances: Everything in our universe that we are empirically aware of is composed of mass and energy. It is important to recognize that we ourselves are made of matter (and energy, but I'm starting to feel a little redundant saying that--just remember that they're generally the same thing). We are also bound by time. It is conspicuous that this Universe of matter/energy appears to be constrained by the phenomenon of time. For example, it marches inexorably forward (all fantasies to the contrary, time has never moved backwards as a video may, so whereas there is no conceivable reason why it shouldn't, there appears to be no observational evidence to support this--for example, if time travel is possible, where are the time travelers? An age-old question, indeed), and shows no indication of being able to be controlled. However, we have recently (in the last century, if you can call that 'recently') discovered that time is affected by at least two things: the phenomenon of gravity and the behavior of photons.
It is known that time passes faster in areas of lower gravity. For now, I apologize for the lack of citations, but I'm sure you'll have little problem accepting my statements for now--I have little doubt that you can find this information on line with little difficulty. Huh, I sure used "little" a lot there, didn't I? Well, moving on...I watched a show (I think it was National Geographic, but I could be wrong) where the host went to visit GPS headquarters in the American Southwest, and discovered that in locations all around the world, people sit in front of computers and punch the "reset" button several times a day, to set their clocks to be even with Greenwich Mean Time. Why is this important? Well, it's because various nations and companies have satellites in space, and since there is less of a gravitational effect in orbit, as opposed to at sea level, the clocks on-board the satellites/space stations, etc tend to run ahead, since 'time flows faster' for the satellites. The practical issue is that if the clocks on the satellites are not synchronized with the clocks on the surface (and each other, of course), there is high risk of collision, among other problems (notably such as being able to accurately send payloads up into space and not hit anything and still get where you need to go), because a few seconds off can be several miles when you're traveling at 1.3 miles per second (for a geosynchronous orbit at 36,000 km distant from the surface)[1]. Satellites closer to the earth move even faster (some polar orbits have an altitude of 900 km [1]).
So we've established that apparent time moves faster in areas of low gravity, and anyone familiar with Einstein's laws of relativity know that time slows to a stop at light speed. A significant result of this is that photons do not experience the passage of time, or so it seems. For photons, the trip from a 2.5-mly distant galaxy to our eyes is instant.
Note: we are now at a position where we can combine our knowledge of gravity and light. Consider that time "flows" slower the higher the gravity, and slower the higher the speed. A practical implication of this relationship is that it is impossible for anything of mass to travel at light speed, because its gravitational attractiveness (it's weight) would be infinite. [Stating the obvious for clarity: "Time stops" at infinite speed (light speed), and it stops at infinite mass] Considering that an infinite mass would take up an infinite amount of space, and also that space is not infinite (considering that it is expanding, which something can only do if it has something to expand into, thereby implying that it is not infinite) and that there is not an infinite amount of matter in the Universe (since that would take up an infinite amount of space), it is thereby physically impossible for anything that possesses mass to travel at light speed. Now, the mathematics may be wrong, though this is doubtful, since math is the most pure form of knowledge that exists (it is only logic--there is no error in pure math, though error creeps in when you involve empirical data). Realizing that there's currently no reason to doubt the mathematical results, we are faced with the realization that photons must possess either infinite mass (which is ludicrous, because then we would be destroyed if we came into contact with just one photon), or no mass (since anything having no mass will not increase its mass the faster it travels--see the math for details; I suspect it's got something to do with the fact that you can't multiply 0 by anything and get anything besides 0).
With this in mind, I'll have to refine my previous statement about mass/energy. Obviously photons, relative to everything else at least, have no mass; however, I will still lump them into any reference to matter(or mass)/energy, since they do, after all, interact physically with other mass particles in the universe. Photons are still things which exist, and so they're included in matter-and-energy considerations. Also recall that photons are related to mass and energy by Einstein's famous equation as well, so they're not off in their own world or anything. I will still personally refer to them as matter, for the time being, whether they have any practical mass or not.
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Okay, those last two paragraphs may have been just a little off topic. Hopefully I'm still keeping my audience awake.
I would like to make a proposal: Time doesn't exist. It is clear that our perception of time is affected by gravity and light (or the speed thereof). Obviously, time is not a fundamental concept, but a derived one--that is to say, it is a function of gravity and light. It does not exist in and of itself, but it is merely a PRODUCT of other physical laws in operation in our Universe. This observation is crucial to understanding some things which I'll talk about later.
A practical result of the fact that time does not exist is the realization that you cannot travel backward, forward, or in any other way through it. Whereas it is possible to make some funky things happen to your own perception, this requires that you either put yourself in a gravitational well[2], or gravitational void (in this case I mean a place with little gravity), or approach light speed. This is hardly practical in the typical time-traveling sense that we see popularized in movies and shows. Since we can't do any one of those things while remaining on earth, without others on earth experiencing it as well, and thereby negating any differences in observed effects (e.g. if you travel in time, everything else does, too, so it's just as if you didn't travel in time at all), then time travel is totally impossible. There is nothing to flow through. Time simply doesn't exist, yet we are still bound by the effects of what we refer to as time.
Clarification of my last sentence: We are causal-effective beings. Matter and energy obey the laws of physics, remember? One of them is the law of cause and effect. Everything that happens (in time--remember, this is crucial) has a cause. Also, the cause, if bound by time, must exist/happen in time before the effect. (Sounds like common-sense, right? This logic is also crucial to refuting some rather wacky arguments).
Existence: The state of being, or of possessing the quality of existence. Existing, whether by your own means or by some other means. It's pretty self-explanatory and hard to explain it in any other way. Anything that IS, can be said to be in existence, whether it is made of matter, energy, or something else.
Substance: What anything in existence is composed of. I cannot conceive of anything being able to exist withoutbeing something--that is, having some property of existence, which I define as having/being composed ofsubstance. Not all substance is matter and energy (there is no such logical requirement); however, it is probable that our entire Universe only contains the substance of matter and energy. Lacking any conceivable way to test for other substances at the moment, we cannot know. For now, matter and energy appears to be all there is, at least in the eyes of a significant portion of mankind.
I may be making a logical leap here--if so, correct me--but I believe that if something exists, then it would have the ability to interact with something; if nothing else, at least with itself--so if it has substance, then the substance can interact with more of said substance. I believe this is a logical conclusion (though I could be wrong), considering that matter of the same properties can interact with each other (ex: two gold atoms colliding). For this reason, I am going to make the claim that anything that exists must have substance. If anyone disagrees, please comment and I may incorporate your suggestion into editing my post. :-)
***I will use the words "stuff" and "things" to aid my presentation--these will mean what they usually mean, but should not be tied to the concepts of matter and energy by the reader. Otherwise you may be confused by my usage of them.
Footnotes:
[1]: Henbest, Nigel, and Couper, Heather. DK Space Encyclopedia.(New York, NY. DK Publishing, Inc. 1999) p. 53.
[2]: A gravitational well is a place where there is more matter than elsewhere (example: the surface of a planet, center of a galaxy, or anywhere within a star cluster). For example, a lone solar system in space, not near a galaxy or star cluster, would be in a gravitational void and time would travel much faster for that system (from our perspective, if I've got it right), in comparison to the passage of time for an observer on Earth. Earth is in a gravitational well, being nearby many billions of stars which exert a gravitational effect on the space nearby, causing our perspective of time to be different than it would be if we were not in such a well.
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