My impression of the fourth dimension is that it’s basically traces of the third dimension moving. So if I take a cube, and move it, then the fourth dimension (which we don’t see) is basically the trail behind it showing how it has moved.

So basically, if I walk down a hallway, if you can envision the fourth dimension, you can see traces of me, behind me, and basically see a path of me walking.

[Correct the above part as needed. This next part assumes that the above is correct.]

Now if you think about it, doesn’t this “trail” of me moving [the fourth dimension (possibly)] basically show the history of my movement. In other words, doesn’t it represent the “past”? So if we could somehow manipulate that, wouldn’t that basically be like controlling time? Then we would be manipulating the “past”, and as a result, could we travel [back] in time?

I know this sounds really imaginary/hypothetical and all, but I’ve been thinking about the fourth dimension lately and watched a few videos on this subject, and I find it intriguing.

The problem with this is that it requires an additional dimension of time, wherein we perform our manipulations. Since we, ourselves, as far as we can know, are stuck in our timestream – flies in amber – we don’t have the higher-dimensional functionality to do this.

Fritz Leiber, in his time-travel fantasy stories, referred to “The Big Time” (title of book) and used this dimension as a “plotline” dimension.

e.g., at Big Time 1:00, I go back in time to 1938 and kill Hitler.
At Big Time 2:00, you go back in time to 1937, and kill my assassin before he can kill Hitler!

Your event happens “before” mine in ordinary time, but “after” mine in Big Time, and this meta-causality is what time travellers manipulate.

The problem is…there doesn’t actually seem to be any such thing.

The notion of “Big Time”, or something like it, is often invoked by SF authors, but it’s a cheap dodge. If you’re willing to accept time travel, and you’ve got “Big Time”, then why not Big Time travel?

You’re basically just saying that time is the fourth dimension - which isn’t a new idea at all - and then saying that we can just manipulate that dimension to time travel. Do you not see the circularity there? “If we can just hop around from time to time, we could travel through time.” Well, yes, obviously. But that observation doesn’t help us get there.

Using “Big Big Time?” Like turtles, it’s “Big Times” all the way up?

(But, quips aside, is it logical? We’ve only got three spatial dimensions, and – apparently – no more. So why not two “time” dimensions and no more? It could be. Yeah, it’s just cheesey sci-fi, but…)

When you say I lack understanding, are you referring to the part where I explain what I think the fourth dimension is, the part where I bring in time travel, or both?

Perhaps a better quesrion would be: is the fourth dimension of any relevance to time travel?

Another side question I have: does the tesseract support the theory that the fourth dimension is time, or a different one? What is the other theory on the fourth dimension?

The video is pretty odd. Reminds me of some pop physics from a few decades ago and the sort of conversations people doing arts degrees would have about it. The video is sort of OK up to about 3 minutes in, then starts to play fast and loose with terminology, and then at about 7 minutes goes completely off the rails.

There is a specific assertion made that suggests the writer has missed something very important. Ha asserts that if the fourth dimension is somehow different then the entire edifice is on shaky ground - and implies that this proves his point about the extensibility of dimensions, and the manner in which time can be treated identically to the others. The logic alone is faulty, but the line of reasoning also suggests that the writer really doesn’t understand, and is just plucking misunderstood factoids out of the ether and joining them together in some random manner.

Time is often considered a fourth dimension, but usually a distinction is made between a spatial and a temporal dimension. Either way – yes, if we COULD treat a temporal dimension as a spatial dimension, we could time travel*. But as mentioned above, this is equivalent to saying “if we could travel through time, we could time travel.” It’s kind of a tautological thought.

There are ways that this theoretically happens, that are far too complex for me to understand (wormholes warping space time and such), but no, there’s no way to just concentrate REALLY hard and suddenly see a fourth dimension you can walk down like a school hallway.

Assuming motion in that dimensions doesn’t require absurd amounts of energy or something for some reason.

There’s no particular unique numbering. There are three dimensions of space and one of time (or at least, that many readily-apparent ones), this is known. What labels you put on those four is arbitrary, though. In fact, in practice, the usual convention among relativists is not to call time the fourth dimension: Instead, we usually call it the zeroth.

I wonder about the so-called “Accepted Definition” of anti-matter mentioned in the video (1:05). I have not heard of Michio Kaku, but the book title cited in the video (“Physics of the Impossible”) leaves a little to be desired in terms of scientific rigor.

I don’t claim to have the background to discuss theoretical physics, but I would take that video as entertainment rather than meaningful information.

All of the above. There are misconceptions and false assumptions galore, and it’s difficult to begin to untangle it.

Let’s take a different analogy, which I half-alluded to in my previous post. Instead of using ‘time’ as the dimension you manipulate, let’s say you wish to manipulate one dimension of space. So, your three “set” dimensions are two of space and 1 of time. Using the same reasoning you originally employ, you can simply “manipulate” the remaining spatial dimension to arbitrarily travel through space along that dimension. That’s basically a form of teleportation, which is the spatial analogue to time-travel.

That doesn’t really begin to address the misconceptions but really is just a cheap-o thought experiment on why the idea is implausible. Start with this kind of teleportation (we have better intuition with spatial dimensions, anyway) and work your way to time-travel.

But also, the idea of manipulation of dimensions themselves is wrong in several different ways. One way is that a “dimension” is just that - a dimension of measure. As mentioned above, it’s rather arbitrary how we number them. Or even how we measure them. You can set ‘x,y,z’ as your spatial dimensions. Or ‘r, theta, phi’. Or some other system. They’re not “set”. They’re simply a set of numbers for determining a space-time position in our universe.

IIRC shortly after Special Relativity was first published the mathematical physicist Hermann Minkowski discovered it could be described mathematically by a system of four-dimensional geometry wherein time was treated as the fourth.

Minkowski had been a professor at the school Einstein attended as an (unimpressive) undergraduate. Amusingly and ironically Minkowski was reported to have said his pupil Einstein was a “lazy dog who never paid attention to mathematics.” Einstein retorted “I no longer recognize my own theory now that the mathematicians have gotten ahold of it.”

SR makes no provision for time travel, as far as I know. As for Quantum Mechanics its so-called “sum over histories” approach (developed by Feynman) treats all particle movement as if all possible paths were taken an the same time, including paths moving backward in time. I do not think these time-reversed paths are considered to be physically real, although real reverse time travel is not forbidden.

One thing about reverse time travel is that it would imply such occurences as the shards of dropped teacups flying from the floor back onto the table into their pre-drop teacup form. I have read such increases in order would run afoul of thermodynamics if they took place on a large enough scale. Maybe that leaves enough room, theoretically, for a sizeable number of bodies to merrily sashay back and forth in time. I hope not because I am no fan of backward time travel, unless as a disembodied observer having no chance to take part in events.

I understand what the OP is saying and he’s essentially correct. But this is the catch point:

That “somehow” contains the central problem. The way you’ve defined the terms “manipulating the fourth dimension” is just another way of saying “traveling in time”. So obviously, if you could do one, you could do the other. But you haven’t discovered a means of doing it.

Michio Kaku is the Henry Semat Professor of Theoretical Physics in the City College of New York and the author of a large number of excellent popular science books.

OK, I have to say it. That you haven’t heard of Michio Kaku leaves a little to be desired in your opinions in physics threads.

Right, I know it’s arbitrary, but laymen and sci fi writers often call it the fourth because it’s distinguished from the “classical” three. (Though I actually really like calling it the zeroth, since it’s more common to switch between working in 1D/2D/3D in many contexts, but time is just kind of a “given”)

It’s not quite that simple, after all, we’ve proven things moving at wildly different speeds relative to each other experience time much differently (or rather, experience it at different rates). See Time Dilation.

Well, yes and no. Within any reference frame, time always moves at one second per second. It’s only the apparent motion of others that speeds up or slows down. The invariance of time inside a reference frame is as basic to relativity as the invariance of the speed of light, from my understanding.