Most of our modern conceptions of math defined in terms of a universe in which there are only three dimensions. In some advanced math classes, I have learned to generalize my math skills to any number of variables- which means more dimensions. Still, let's assume that some alternate theory of the universe, such as string theory is true. Does any of our math still hold true? How would our math need to be altered to match the true physics of the universe?

Let's start with a quick comment about string theory. My knowledge is only journalistic, but it's clear that string theory is a mathematical theory and states its hypotheses about extra dimensions using mathematics. And as your comment about additional variables already suggests, there's nothing mathematically esoteric about higher dimensions. When variables have the right sort of independence, they represent distinct mathematical dimensions in a mathematical space, though not necessarily a physical space. (Quantum theory uses abstract spaces called Hilbert spaces that can have infinitely many dimensions. But these mathematical spaces don't represent space as we usually think of it.) Of course, it might be that getting the right physics will call for the development of new branches of math. Remember, for example, that Newtonian physics called for the invention of Calculus, and though earlier thinkers had insights that helped pave the way, Calculus was something new. Just what sort of new...

Henry Stapp (a physicist at Berkeley) in his book The Mindful Universe states: "Let there be no doubt about this point. The original form of quantum theory is subjective, in the sense that it is forthrightly about relationships among conscious human experiences, and it expressly recommends to scientists that they resist the temptation to try to understand the reality responsible for the correlations between our experiences that the theory correctly describes. The following brief collection of quotations by the founders gives a conspectus of the Copenhagen philosophy: Heisenberg (1958a, p. 100): The conception of objective reality of the elementary particles has thus evaporated not into the cloud of some obscure new reality concept but into the transparent clarity of a mathematics that represents no longer the behavior of particles but rather our knowledge of this behavior" As philosophers, what is your take on these statements? It appears to me that these quite distinguished physicists are saying...

It's certainly true that Bohr and Heisenberg, among others, interpreted quantum theory in a way that put the knowing subject center stage, but this is just one part of a controversy that continues to this day. Einstein and Schrödinger, for rather different reasons, resisted these more epistemic interpretations, and while some would say that Einstein lost in the wake of the investigations of Bell's inequality, Bell himself was very attracted to realist interpretations of quantum theory. "Collapse" interpretations, such as the so-called GRW theory, are not epistemological interpretations, nor is Bohmian mechanics (a development of de Broglie's pilot wave idea), nor, for that matter, is the Everett interpretation (roughly, the "many-worlds" interpretation.) So the simplest thing to say is that there partisans on both sides and the controversy is ongoing. If you'd like to read more, you could do worse than to get a copy of Alistair Rae's Quantum Physics: Illusion or Reality? or John Polkinghorne's ...

My question is about quantum theory and the afterlife. In the many worlds interpretation of quantum mechanics, even if I die in *this* branch of the multiverse then "I" will still exist in some parallel universes. If we subscribe to the theistic position that every individual has a soul, then what happens to my soul upon death? Will it go the afterlife? What about the parallel "me's"; do they each have their own soul? I'm confused.

The obvious response is that there isn't a single response, and for a simple reason: quantum theory doesn't have anything to say (or not obviously, anyway) about souls -- at least not if a soul is some non-material thing that doesn't fit into the equations we use to do physics. There is a view that's rather like many worlds and that allows for something soul-like. It's called the Many Minds interpretation, and you can read a short account of it (and get further references) by following this link: http://plato.stanford.edu/entries/qm-everett/#6 However, this won't address your worries about the afterlife. And since this is a topic that physics has even less to say about than it says about souls, it's even clearer that there's no good answer. That said, a handful of extra thoughts. The first is that IF there is a non-physical soul (a very big "if), then we can start by asking what happens to it after death on our usual non-quantum picture. And then we could say that whatever the story...

If quantum mechanics or other fundamental theories of physics have it that small physical entities which make up everything else do not behave deterministically, does that indeterminism inherited by all other larger entities, whether those are molecules, gases, instantiated computer programs, and people? In general, does indetermism on one "lower" physical level imply indetermism on a "higher" one?

The answer to the general question is that indeterminism at the "lower" level doesn't have to mean indeterminism at "higher" levels. Here's an abstract way to think about it. Suppose some theory has a set of possible states -- call it S -- and a strict deterministic law governing how the states change over time. Let's suppose that this theory is both true and know to be true. But suppose, unbeknownst to us, each of the states in S can be realized in many different ways, at some sub-microscopic level that we don't have access to. And suppose that even though the law that tells us how we get from one state in S to another is deterministic, there's no deterministic law governing exactlywhich way states in S will be realized as the system moves from one state to another.We might never have any reason to believe any such thing, but it could be true all the same. That's one story about how indeterminism at the micro level might not infect the macro level. Another way is a "for all practical...

If the universe is expanding, what is it expanding into ?

My closet. More seriously, it isn't expanding into anything. The universe contains all of space-time. If it's expanding, this is a fact about the stuff that makes up the universe itself, and not about its relation to some other place/thing/void. So what does it mean to say that the universe is expanding? At least this: that distances between things (on average) are getting larger. One common analogy: think about the surface of a balloon that's being blown up. Imagine the ballon as covered with little dots. Then as the balloon expands, each dot will get further away from each other dot. The obvious objection is that the balloon is expanding into the space around it -- there's more than just the surface of the balloon. That's true. But that's why it's an analogy rather than a perfect fit. Although it puzzles the imagination, the math of space-time doesn't call for positing a container space for the expanding universe. The geometry is internal to the universe itself.

Hi. This is a question about Logic. I've read in a book by Michio Kaku, _The Physics of the Impossible_, that it may be possible to receive a signal before it was sent. This to my way of thinking would violate the logic behind causality. And yet on a social level we are effected by what happens in the future. An example would be Christmas shopping. My question is can an effect precede a cause, and if so what does that mean in relation to actuality and reality? Cheers, Pasquale

We normally assume that causes can't precede their effects, but this isn't a logical truth, and in fact it's possible to tell coherent stories where the principle fails. By "tell coherent stories," I don't just mean tell science fiction. As your author may point out (I haven't read the book), it's possible to say how causal loops and backward causation might fit into physical theory, even though there's no strong case for saying that such things actually happen. So there's no issue about the "logic" of causality being violated. As for your Christmas shopping, this isn't really an example of the future affecting the present. Your present intentions do the causing. You want to make sure that come Christmas day, Granny gets that gorgeous pair of Manolo Blahniks, and it's that present desire and intention that gets you to head off to the shoe store. Granny's beaming grin isn't reaching back from the future to get you to the mall.

How does the temperature ever change? If we assume that temperature is a continuous measurement, then we know that it has an infinite number of potential values. In order for temperature to transition between two values, it must then pass over the infinite set of values that lies between whichever two values the temperature is transitioning between. It now seems that temperature should not be able to change at all because before it may change to a given value, it must first reach a value between the desired and the current. Since we can make this claim infinitely, it would seem that temperature becomes "trapped", in a sense, at its current value, unable to change at all. Of course this problem can be applied to other concepts as well, and we might easily draw comparisons to Zeno's ancient thought experiment of Achilles and the tortoise. But the logic here is slightly different; the desired temperature is not continuously fleeing from the present as the tortoise is from Achilles. I simply raise the...

I'll have to confess that I'm one of those people who was early on seduced by a particular sort of solution to this sort of problem, and since then I've never been able to feel the force of the puzzle. Here's a somewhat fanciful example that conveys the idea. Suppose that we have a body whose temperature at 12:00 midnite is o° Celsius. And imagine that the body's temperature is increasing at a steady rate as follows. Let T t be the body's temperature at time t. And let the temperature at any given time over the hour after midnite be given by T t = t where t is the number of minutes after midnite. In other words, the temperature rises steadily at the rate of one degree Celsius per minute. It goes through all the intermediate values in finite time. If the arithmetic of real numbers makes sense, so does this. And so I can't find the puzzle. Pick any time you like over that one hour period. There's an answer to the question "What is the body's temperature at that instant?" And in...

I am a very ordinary art teacher, one who breaks into a rash at the merest glimpse of an equation, and one who is trying to get to grips with the quantum world. Can you answer me this question about the double slit experiment? In the Double Slit experiment, why is it assumed that the particle splits and then reconverges at a point in between the two expected points, rather than a single particle merely bending and curving its trajectory to arrive at the in between point? Yours, Keith

Hi Keith. A perfectly good question. The short answer is that any such assumption is, to put it mildly, controversial. There are respectable ways of thinking about quantum theory that look at it in more or less the way you suggest. Those ways, however, come at a price: they require us to assume that there are influences on the particile that propogate faster than light. They also go beyond what the theory itself has to say, and add some extra physical/interpretive machinery. That's not a criticism of such views; it's just a remark. Quantum theory, alas, is a topic that leaves us in a peculiar lurch. As physics, it's fantastically well-confirmed and scientists and engineers make use of it in a vast number of ways that give us things like cell phones, laptop computers, MRI machines and a good deal else. But there are weird things about the math of the theory (I'll spare you any equations) that make it quite different from "classical" theories in ways that are still enormously controversial. Just to...

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