Expanding photons.

Actually that was pretty helpful, particularly with posit's question.Is there an easy explanation of how they came up with 1 sec = 300,000,000? You mentioned something about Foucault's method, but isn't it more or less just making time equivalent to distance via the velocity of light? What would be the effect of changing the ratio arbitrarily to something else (perhaps speed of sound in air to deliver a "sound cone")?

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holmes
"What a fool believes he sees, no wise man has the power to reason away." (D.Bros)

I know this is slightly tangential to the topic but I hope it's close enough, as it's something I've wondered about for ages.Like Larni, I understand that the recession of galaxies which we observe is not the result of them flying away "through" space, but the expansion of space between them. What I've never been able to figure out is why then the space within galaxies, indeed the very space within matter itself, is not expanding also. Matter occupies space, and even solid matter itself is mostly empty space so why does this not happen?Or does the space within matter also expand? At the very least, given that we observe universal expansion, I don't see how it could be happening at the same rate. If all of space - both within and without matter - were expanding at the same rate then we wouldn't notice any expansion at all, would we? That is, empty space would be expanding, but so too would matter, including us and any yardstick we might use, effectively cancelling out the expansion from our reference frame.So maybe expansion within matter does occur but at a slower rate? Perhaps the warping of space caused by the presence of mass also retards its expansion?

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Ferret brain activity increased just 20 percent when looking at Keanu Reeves compared to looking at darkness, the study found.

I think the explanation is that space within matter does expand, but the expansion does not pull the matter itself with it, because locally, the forces holding matter together are more than enough to counteract any pull the expansion of space would exert on the matter.

The curvature of space-time depends upon the matter distribution. Far away from matter (in the immense voids of the large-scale structure of the universe, far away from the clusters and super-clusters which are confined to the filaments), the global distribution of matter in the universe dominates and gives rise to expanding space (which is just how we see a particular form of space-time curvature from our 3d POV).In proximity to matter (inside the clusters, galaxies, solar systems, planets, atoms), it those distributions which dominate and dictate the local space-time curvature. The Earth itself dominates our curvature here, and local space-time takes on the Schwarzschild geometry.Consider the global curavture as a hill, and local Schwarzschild curvature as a hole. From inside the hole, the hole looks like a hole, with no clue to the fact that it is situated on the side of a hill. From afar, you simply see the slopes of the hill, and no hole.

Sorry for the late response,.....Anyway, yes, it essentially just measuring the speed of light. However if you accept relativity, then relativity changes the interpretation of Foucault's experiment into an experiment to find how many meters are in a second rather than a test for the speed of light.If this isn't clear enough just say so, as there are a few subtleties here I'm glossing over.For your other question:
The effect of changing the ratio would basically give the wrong results and wouldn't match experiment, which is one of the independant means to validate the claim that one second is 300,000,000 meters in the temporal direction.

So that book that said gravity is the expansion of matter toward other matter, for examlpe, the Earth toward a "falling" ball, was just joshing me then? Or do I have something confused?Anyone else ready to say they are "star struck" from all this space talk? J0N

What book was that?
It wasn't by somebody called Mark McCutcheon was it?

Actually, it was called Time Travel in Einstein's Universe by Richard Gott. I do not recall if the author supported the idea; I only remember him refering to it as something Einstein proposed as a possible explanation to gravity, and which was loosely supported by the evidence that the Universe is expanding.You see, I always thought the Universe was expanding because everything inside of it was expanding, right down to the atom. And that when each of gazillions of atoms expand a little, it makes a big expansion. Guess I was wrong?J0N

'Fraid so... Go read some Brian Greene and you'll soon be on the right track

The big bang model implies that the universe is expanding. At least, that's what I think it implies, though if the universe is meant to mean the entire 4D spacetime structure, i'm not sure.My question is, then: is time itself expanding the same way in which space is expanding? Are two points in the time direction moving further apart and would this have any observational effects?It would seem this would have to be the case since according to Special and General relativity space and time are equivalent. I'm especially thinking about the Lorentz transformations, where one man's space is another man's time and vice versa.Ah the trouble one gets into when they ponder these subjects without a formal education in modern physics.I assume this has something to do with projecting a 4D geometry into a 3D+1 picture, but I still can't figure it out. Depending on how you project the 4D geometry, do you get different "pictures" of the expansion?

Exactly! There are no dynamics in GR. Each space-time solution of GR is a fixed, static, immuatable object. So there can be no expansion of the 4d universe. Just as there is no expansion of the Earth. But you could think of the lines of latitude as expanding if you were to journey South from the North Pole... It is our 3d perspective and our passage along the time axis that gives rise to the impression of expansion. Yes. Different projections will represent a different observers with differnt velocities wrt the local rest (comoving) frame. Some projections will result in "unphysical" observers.Edited by cavediver, : added a bit

would this be a fair way to think of it.From each persons perspective, they are travelling along the time dimension, and the other three dimensions can change in various ways, depending on the geometry of the 4D structure.Now, if the observer changes his frame of reference, he gets a new perspective on this 4D structure. Some of what he considered to be space is now his time, and vice versa. They are just seeing the same shape from different perspectives, and they can theoretically get very different answers for how "space" is evolving through time.For instance, imagine a sphere that is stretched out, an oblong. You can travel straight up the object's long axis, and you find that the rate of expansion of the latitude lines (space) is a certain amount.But, if you had a different perspective and were travelling along the short axis, you'd find that the "latitude" lines were expanding at a much faster rate.Now, i'm not sure such a radical shift in perspective is possible with the 4D structure, but is this an apt analogy?

Yeah, it's a very good analogy. To expand on this. Getting a different perspective in a 3D space could involve a rotation which will change your perspective on the surrounding space. (Such as turning your head around.)
In a 4D space it's pretty much the same, there are more possible roations, but nothing radically different.However, if one of the dimensions in the 4D space is a time dimension, rotations act in a completely different manner and have subtle new properties.
A change in velocity is actually a rotation in the 4D space with 1 time dimension that we live in. (Spacetime)
It's the fact that one of the dimensions is a temporal one that accounts for such a radical shift in perspective, because it wouldn't be all that different from 3D space if all the dimensions were spatial ones.Hope that helps a bit.

quote:

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However, if one of the dimensions in the 4D space is a time dimension, rotations act in a completely different manner and have subtle new properties.

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A change in velocity is actually a rotation in the 4D space with 1 time dimension that we live in. (Spacetime)
It's the fact that one of the dimensions is a temporal one that accounts for such a radical shift in perspective, because it wouldn't be all that different from 3D space if all the dimensions were spatial ones.

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Hope that helps a bit.

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This is what I struggle with. What is the relationship between time and space?I get confused because distance is in Minkowski space is:ds^2=dx^2+dy^2+dz^2-cdt^2So, as you said, this is 4D space with 3 spatial and 1 temporal dimension. The temporal term is distinct from the spatial ones. This seems to tell me there is a fundamental difference between space and time.But then I see the Lorentz transformation, which show that space and time are equivalent (I think). Since one persons time is another persons space, and vice vera, the difference doesn't seem to be fundamental, but merely dependent on the observer.Basically, one tells me the difference is fundamental, the other tells me its not. Where am I going wrong?

No where really, there's just a detail I haven't been explicit on. I only did things in one frame.When we Lorentz transform we go from one guys frame, let's call it F, to another guys frame, let's call it F'.The coordinates in F are (t,x,y,z) and in F' they are (t',x',y',z').Let's say the Lorentz transformation is just a boost in speed along the x-direction.Then t' = at + bx and x' = ct + dx. Where a,b,c and d are just some numbers, it doesn't matter about the details.
So you can see how the t'(time in the new frame) is a mix of t(time in the old frame) and x(space in the old frame).So space and time don't really matter, because they're frame dependant. Now what about the distance rule, ds^2=dx^2+dy^2+dz^2-dt^2? (I prefer to leave out c^2)Well as you might imagine the rule in the F' frame is:
ds'^2 = dx'^2+dy'^2+dz'^2-dt'^2
Which we can compare with the distance rule in F:
ds^2 = dx^2+dy^2+dz^2-dt^2And, believe it or not, ds'^2 = ds^2.
So the answer to your question is that although time is important in the distance rule, it doesn't matter whose frame we are in, everybody gets the same answer from the distance rule. Basically no particular person's time matters, it's just that their time is important in their distance rule and everybody’s distance rule gives the same answer.The technical term for the fact that the distance rule gives the same answer in every frame is that the distance rule is Lorentz invariant.Hopefully that might clear something up, any questions ask away.Edited by Son Goku, : Slight modification.