OK, the universe is expanding. The red-shift shows that everything is moving away from everything else. So the implication is that space itself is expanding. Or is it the universe that’s expanding but space-time is static? What I really want to know is if space is expanding, mustn’t time also be expanding? That is, if all objects in the universe are moving away from one another in 3-space, might not all events be moving away from one another in time?
Got to thinking about this while reading the September Scientific American, which is a special issue all about time.
IAAP, but not a relativist (so that while I’m fairly certain I’m right, if someone like JSPrinceton or Chronos disagrees with me, assume they’re right and I’m wrong)… And no, time isn’t expanding also.
This is hard to demonstrate without looking a bit into relativity and doing some fairly gory math, but the upshot of all of it is that there’s a 1 in front of the intervals in time and a time-dependent “scale factor” in front of spacial intervals, in the traditional “Robertson-Walker” version of things. I believe the same holds true in typical inflationary models, but won’t swear to it.
g8rguy’s got it. Since time is one-dimensional as we currently see it, this makes it impossible for it to expand. Indeed, for every reference frame you can think of, time marches on at one second per second by definition. If time did expand two-fold it would march on at a rate of two seconds per two seconds. This isn’t expansion at all.
Throw in another dimension of time and we may be able to work out some expansions for you. Unfortunately, right now extra dimensions of time are either non-existing or continue to be stubbornly unobserved.
One little game you can play is the following, what if there were three dimensions of time and one dimension of space? Would there be any way to distinguish between the that kind of universe and our own?
I tried to do some Googling to point to some references that studies of far away (therefore ancient) supernovae indicate that the speed of light was a little slower in the very earliest days of the universe. Now since speed is distance over time, a change can be caused by changing the distance or the time. Ergo, perhaps “time” once ran faster.
Now, back to my Googling: I kept getting creationist crap. These idjits really eat up this stuff. In particular some bozo with terrible math skills did a formula showing that the speed of light changed significantly in the last 300 years. If I can find multiple problems within 30 seconds of starting to read it, it’s pretty awful. So there is just a hoard of links to this sort of (debunked) stuff that I get when Googling.
Perhaps someone with a better idea of what keywords to use will come along and find links to real science.
This doesn’t quite cut it, you see, because distance and time are related causally right now by the speed of light. If you’re going to get rid of that relationship (by throwing away the constant) you have to come up with a new way of relating the two of them. There are two proposals I know of right now that have any merit to them which say the speed of light isn’t constant. One ignores metric considerations entirely, instead focusing on the quantum end. The other doesn’t simply proposes a proportionate shift which doesn’t answer the question at all and, if convention serves us well, it’s best just to push all factors onto the other parts of the Robertson-Walker metric, as g8rguy pointed out.
So changing the speed of light really won’t get you to expand (or contract) time. It will still march on at one second per second.
But if the universe is expanding, what does that mean? I presume it to mean one of two things:
The amount of space that the universe takes up is increasing, that is, the universe is buying up more real estate. That implies that the objects in the universe spread out to take up the available space.
Space itself is expanding, causing the objects in the universe to be farther away from one another. This would imply that all space is expanding, including space between atoms, not just space between stars.
If #2 were to be true, then the definition of length would still be “1 meter per meter”. So why couldn’t time expand yet it still be true that it passes at “1 second per second”? However, #2 may not be the correct interpretation. I open myself to your tutelage.
Relativity says that in a moving inertial frame, time expands (i.e., passes more slowly than that in a “stationary” intertial frame) and the linear dimension in the direction of motion contracts. But to the observer within that frame, time still passes at 1 second per second and length is still 1 meter per meter. We cannot tell that time is slowed unless we have a way to compare it against time in another frame.
So I think my core question can be answered by a succint explanation of what exactly is expanding when we say the universe expands.
I had general physics and quantum mechanics in school but that was about 25 years ago so I’m a bit rusty
The correct interpretation is indeed #2 in that space itself is expanding, but you have to realize that the Hubble flow is typically weaker than the attraction between objects, which tends to counteract it, so that you and I, for example, are NOT expanding. The upshot is that it in some sense looks like #1. Sorry to be unhelpful, but that, AFAIK, is the way it is.
g8rguy’s got it. The expansion of the Universe is on cosmological scales only, for the time being. Check the thread citation I made in the “The Expanding Universe, as it relates to Me thread” (link in the OP).
I don’t think it is meaningful to say time has increased. In order to say that something, say distance, has expanded, we use some measuring device, like rigid rods, to take measurements at different times. If we notice that as time increases, the measured distance increases, we say that the distance is increasing. In order for time to expand, we need to use a measuring device, like a clock, to measure some time interval at different times. The problem is what do we compare our time interval to? The only way we have to measure a time interval is with a clock. You don’t have anything to compare the clock to.
Also events cannot grow apart in time or space. An event is defined as a point in space at a moment in time. Once we establish a reference frame, the distance and the time between events is fixed.
Argh, not that again! There is some very weak evidence that something called the fine structure constant has been changing over cosmological time scales. By “very weak evidence”, I mean that even the authors of the paper don’t put too much stock in it, but are just tossing the idea out there. Now, the fine structure constant is constructed out of a few other constants of nature: Namely, the Speed of Light, Planck’s constant (significant in quantum mechanics), the charge of the electron, and Coulomb’s constant (which regulates how charges interact). If (and that’s a big if) the fine structure constant is changing, then that means that one of those other constants is changing. For some reason, though, the popular media seems to have jumped on the “speed of light” explanation.
Furthermore, even before you define a reference frame, there’s a combination of distance and time known as the proper interval which remains unchanged in any reference frame. But that’s only dealing with events. An event is a point in four dimensions, specified by a place and a time. Of more interest when considering objects or particles is the world line, and it is meaningful to talk about two world lines getting farther apart.
Hate to say it (because I’m actually something of a fan of his), but you can blame Paul Davies for this one. His paper does make some arguments as to why it is the speed of light and not the other constants that is changing, but it’s still very much speculation. He’s something of a media darling, and so whether or not his explanation will stand the test of time (for this phenomenon we aren’t even sure we’ve observed), he will be immortalized as the “guy that showed the speed of light is changing”.
Ah well… doesn’t change the universe at all, just our perception of it.
Sorry to be coming to this thread late, but I just have to ask this. Perhaps JS can elaborate on his comment.
While we could certainly just label what we consider our three spatial dimensions time1, time2, and time3 and what we consider time as space1, I get the impression that JS was implying something less trivial than this.
Since what we call time seems to have an “arrow” making that dimension unidirectional, wouldn’t a universe with three dimensions of time and one of space have significantly fewer degrees of freedom? Wouldn’t any configuration space in such a universe look a lot different than anything in our universe?
rsa, the whole point of the way the “dimesions” are portrayed in current theory is that they are not indistinguishable when they are spatial, only time-like.
Consider, if you will, a second time-like dimension (Rich Gott, in his book Time Travel in Einstein’s Universe refers to this dimensions as “dreamtime”). If there were two dimensions of time, would could simply circle around in the plane of time and dreamtime and visit whatever point in the past or future they cared to. This is not the way our unverse seems to be set up.
If you further consider three dimensions of time and three dimensions of space, why then, the richness of space would be available to us in time as well. This would make things very peculiar, and it isn’t all that clear how to explain this “six-dimensional” universe (if it indeed exists).
So now, if you contract the spatial dimensions down to “one” you can still move through the time-dimensions as before, but now you are confined linearly (due to the fact that there are two “type” of dimensions) to the last spatial dimension. In effect, you are now in the same universe as we are in. There is no way to distinguish between the two different “universes” because the only we we know of right now of distinguishing time and space is to say that the sign of the invariant metric measurement determines whether a separation of events is time-like or space-like, and it happens that the time-like separation is associated with the same sign as the dimension to which we are confined to one while the space like separation is the same sign as the dimension to which we are defined to three. As far as we can see, there is no other difference. So, in our 3 time 1 space dimensional universe the only difference is that the signs on the invariant no indicate the opposite dimensional confinement as in our universe, but since the signs are convention only (ways of reminding ourselves that the two types of dimensions are different) we can simply switch the two of them. Then we have exactly the same formalism and no difference. So, there is a peculiar thing about the way these dimensions of time and space interact, they’re (as far as we know) theoretically symmetrically related.
[QOUTE]they are not indistinguishable when they are spatial, only time-like.
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Talk about a confusing way to put it! What I’m trying to say here is there is no difference between height, length, or depth but there is a difference between space and time.
I was just adding the assumption that movement in those time dimensions would be constrained to the forward direction only, but as you make clear that’s not part of the formalism.
I would just like to add that I appreciate the quality of your responses and the fact that you don’t “dumb them down” unnecessarily.
If we assume that time is not constant, and allow the speed of light to vary, do the equations of special relativity still work?
Suppose in addition, that time is actually slowing down-would this explain the red shift as well?
