Speaking as a physics grad student who studies Quantum Field Theory (what happens when you unify Quantum Mechanics and Special Relativity into a single consistent framework) and General Relativity (what happens when you unify gravity and Relativity into a consistent, non-quantum theory), I'm going to correct some of the claims being made on here. I may not be clear enough, so please ask questions. If people want to argue points, I enjoy that, too, just do me a favor:

If your perspective (I'm not saying anyone here has this or is giving me this impression, but I've discussed this with a lot of people) is that scientists are wrong or stupid and just don't understand, and you have no real interest in changing your mind on this point or you are unwilling to cordially discuss this (even if you won't agree), then I would kindly ask you to politely say "Well, Rishi, I see what you're saying, but I don't agree and I likely never will." and leave it at that.

VikingBoyBilly wrote:It's laid out in one simple sentance right here:

http://en.wikipedia.org/wiki/Dimension# ... dimensions
[indent]"In physics, three dimensions of space and one of time is the accepted norm."[/indent]

This means that time is just being used as a dimension in FORMULAS to calculate things like distance, speed, acceleration, change, etc. of an object in the three dimensions of space.

It does not mean "oh, hey, you can time travel if you insert a date in the fourth dimension here and zap there. Pretty cool, huh? Also you can create funny paradoxes by playing around with it."

This has to be the biggest misconception of a scientific concept since... uh... I don't know, something else extremely cool and stupid at the same time.

Now

this is the fourth dimension of space.

This is a misunderstanding of why time is a different kind of dimension than space, not that time isn't a dimension. Mathematically, it is definitely a "dimension", and that's not really a disputable maths point. Crucially, the Pythagorean theorem (Euclidean inner product or metric) does not apply to it, but a generalization of the Pythagorean theorem holds in Minkowski spacetime, which is the correct description of spacetime at low energies and for weak gravitational fields. But unless you understand advanced linear algebra, this probably isn't necessarily going to make sense, so I'll rephrase it this way:

When you write down a vector describing where in spacetime you sit, then you

*are* specifying three numbers to indicate the spatial point and one number to specify the time. In principle if one could build a machine (though that almost certainly can't happen) that would allow you to input four numbers that was the current point in space and the current time that you're at, then yes, if you plugged in t_now - 5 seconds, you would jump back in time to five seconds ago. However, time travel isn't really a reasonable possibility for a different set of reasons, which has to do with quantum mechanics and the

Hawking chronology protection conjecture, which has a lot of non-trivial theoretical support.

However, when people say that it's spacetime and not "space + time", if you're moving at a constant offset speed from someone else, your space and time information gets mixed up from their frame of reference. The best example of this is

the relativity of simultaneity. This effect can be demonstrated in the following example: if I take two rockets a thousand meters apart and blast them off at the same time (according to my frame), then according to a car's frame moving velocity v (parallel to the two rockets), one will go off before the other one. This is because the spatial distance between the two rockets in your frame gets mixed up into the temporal periods of the moving car's frame. The effect is not perceptible, however, from the velocities capable of produced by cars. For instance, satellites move 8,000 meters per second around the earth (Which is, needless to say, way faster than any car you've driven), and the difference in the time it between them going off is order .001 nanoseconds, or .00000000001 seconds.

For those who are math majors or have a math degree,

Minkowski spacetime does form a vector space and a differentiable manifold with well-formed notions of linear and topological dimension.

wiivn wrote:In order to prove that, we need to prove that E = mc2 is wrong. And there's where the string theory comes.

The universe in a nutshell.

Well, "E = mc^2" is actually already false by the time we get into General Relativity (the generalized theory for physics involving strong gravitational fields) so we know that this equation is itself an approximation without any mentioning of String Theory. And we know that General Relativity must also be an approximation to Quantum Gravity. This is where String Theory

*might* come in. With that said, there's a very mild generalization of E = mc^2 for particles which is true (i.e. m \sqrt{ -g_{\mu\nu}u^{\mu}u^{\nu} } = mc^2) that accounts for the corrections that GR introduces. But that formula is less catchy, so people don't discuss it as much. =P

BTW, I might suggest taking Michio Kaku's public presentation skills with a grain of salt. He really likes the sound of his own voice, and doesn't mind saying really ambiguous things because he knows the audience will think he sounds really smart. Sean Carroll is probably the best that I can think of off the top of my head, though Lawrence Krauss is pretty good for any non-quantum gravity issues (i.e. take what he says about String Theory with a grain of salt).

Lava89 wrote:Thanks for posting this! As a student in math, it always bugged me that people treat Time like it's Depth or Width. I always preferred to separate time and spatial dimensions.

But it is depth or width, or at least via a boost, it will turn into depth and width. If you know linear algebra (I don't know how far you're into your math program), you can read the article I gave above from Wikipedia on Minkowski space. A great point here is that if you choose units where the speed of light is "1" (which are the most natural units in Special Relativity), then intervals of time are measured in length scales, such as meters.

candyjack wrote:Nowadays there even are cosmologists (most notably Lawrence Krauss) who claim that "the universe came forth out of nothing." Not only do they claim that you can go from nothing to something; they claim that this necessarily happens. The basis for this is that, when observing a volume of space that is an absolute vacuum (i.e. there are no particles in this space), it turns out that even then particles arise and disappear in this space, seemingly out of nowhere. This may seem counterintuitive, but it really is a property of physical space, a phenomenon called

quantum fluctuation. From what I understand, it's hypothesized that this principle is what originally caused the emergence of the universe. But what is ignored is that 'nothing' in physics stopped reflecting how people originally and intuitively used that word the moment quantum fluctuation was discovered. 'Something out of nothing' is possible, they argue, because of the laws of nature, but those laws are something unto themselves and real 'nothingness' would exclude the existence of even those laws.

1.) Lawrence Krauss is referring to what is known as the

Hawking-Hartle no-boundary proposal. It uses something in quantum mechanics known as "instanton tunneling" to quantum mechanically tunnel from no spacetime at all (which is a trivial solution to Einstein's equations) to an expanding spacetime (a non-trivial solution to Einstein's equations). The derivation uses something called the mini-superspace approximation, which most cosmologists and quantum gravity experts don't believe is justified and thus the model cannot be relied upon because it's ignoring extremely important physics associated to quantum gravity.

2.) So it's not necessary true. It's not really accepted by almost any cosmologists because of the unjustified approximation that it uses.

candyjack wrote:
So it is with the idea of time as a dimension. Physicists discover that certain calculations are easier when time is defined in such and such a way, and thus they try to impose that change in definition even on the domains of philosophy and linguistics, even though those domains are more fundamental.

If you really believe that physicists' are simply defining things to make their calculations easier by defining time this way and that's the end of it, then I strongly suggest that, if you haven't done so already, you should really pick up a book on Special Relativity or read Wikipedia articles on

Special Relativity (specifically on

time dilation,

length contraction, and the simultaneity of relativity). You may disagree with the mainstream scientific community on this, but I feel that if you're going to tell physicists (and indirectly the majority of philosophers) that they're wrong about an issue, you should be aware of the issues present.

Also, it'd be hard to argue that linguistics is more fundamental than physics in terms of reality. Linguistics is a statement about how the human mind organizes information and thinks, which makes it more fundamental in terms how humans are and think, but not more fundamental in terms of objective reality. Philosophy is more fundamental than physics, again, in terms how humans approach questions, but philosophy is not necessarily more fundamental in terms of truth statements. That's an open question in metaphysics and the philosophy of science, but most philosophers are not ignorant of Special Relativity and the

vast majority of philosophers accept scientific realism (Question 25).