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Originally Posted By: gan
I don think we can travel so far... it's only a prediction, isn't it???

It remains only a prediction because nobody's actually done it.

Likewise: 'If you fall into the sun you'll burn up'. That, too, is only a prediction, because nobody's done that either.

Both, though, are constantly proven correct in principle.


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Two points that arise out of the foregoing discussion may be worth considering.
1. Einstein did not say that no material object could travel faster than light; what SR forbids is the acceleration of any object through space from sub-light speed to super-light speed. Current thinking about the expanding Universe has distant galaxy groups moving away from us faster than light, because they are not moving through space; space is expanding and carrying them with it.
2. There seems to be a tacit acceptance that time passes, but if time moves, to what is that movement relative? Surely it is more reasonable to think of time as a static entity through which we all move. This leads to the possibility that everything moves through spacetime at the same speed; the speed of light. Light does all its travelling through space, which means it is static in time, whereas we do most of our travelling through time, and a very limited amount through space, because we are so slow (relatively speaking, of course).
Bill S.


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"Junior member"....at 70! I like it.


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Originally Posted By: Bill S.
1...galaxy groups...space is expanding and carrying them with it.
Right, and not understood by people who see the Big Bang as an explosion of material through space.

(2) seems a bit confused (confuses me anyway smile ):

'everything moves through spacetime at...the speed of light'

- Photons do. We don't.

'Light does all its travelling through space, which means it is static in time'

- Time passes as photons go from A to B.

'we do most of our travelling through time, and a very limited amount through space'

- When we travel faster through space we also travel faster through time?


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""Junior member"....at 70! I like it."

Glad that you like it;-)

Our new moderating system will allow us to get rid of spam before it shows up but we had to rejig the membership system... so now you have, perhaps, a bit of a misnomer... but hopefully it makes you feel young again!:-)

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Originally Posted By: Bill S.
"Junior member"....at 70! I like it.

That's nothing, Bill. When you reach Megastar they give you a limo and chauffeur.


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Time passes as photons go from A to B.

Time passes in our frame of reference, but what is "true" in one F of R is not necessarily the case in another.

When we travel faster through space we also travel faster through time?

If we could travel at superluminal speed we would be travelling backwards in time, or so the popular science books tell us. If the speed of light is the changeover point, then logically we must be stationary relative to time when travelling at "c", in the same way that a vehicle that is travelling forward must stop before it can reverse. If we are stationary in time, but moving in space, we must be in more than one place at a time. It follows that a photon would be at A and B at the same time, in its own F of R.


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"It follows that a photon would be at A and B at the same time, in its own F of R."

Yes, Bill, fascinating isn't it. Theory does appear to lead to that conclusion - that each photon (in its own frame of reference, as you say) must exist at every point in the universe; and one might say that they inhabit eternity. A discussion here that you might find interesting:

http://www.thenakedscientists.com/forum/index.php?topic=18981


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Redewenur. Thanks for the link. I have not struggled through all 5 pages yet, but I found a quote that says exactly what I was trying to put into words. I hope Lee E will not mind if I reproduce part of it.

"Although we can move through three spatial dimensions, at any point in time the movement vectors for all three spatial directions can be summed to a single vector. Thus movement is essentially in a single direction and can be expressed by a single value, just as when we drive heading North-West we don't say we are driving West at x mph and North at y mph; we just use the summed vector.

With movement induced time dilation, the same thing is happening, except this time the two vectors being summed are the summed spatial movement vector and the temporal movement vector. The reason we get time dilation is because it is the sum of these two vectors, spatial and temporal, which cannot exceed 'c', so as the spatial movement vector increases, the temporal vector must decrease. With zero spatial movement then, we move temporally at 'c', which in turn implies we have zero length in that direction."


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"With zero spatial movement then, we move temporally at 'c'"

I take 'zero spatial movement' to mean what it says, i.e. not moving at all (except in time) - and nothing ever does move in it's own F of R, but only relative to other things.


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"nothing ever does move in it's own F of R"

I see what you mean, but isn't this taking a somewhat restricted view of a F of R?

I am sitting down, if I stand up I am moving, but I do not move out of my frame of reference. Admittedly, my movement is measurable only in relation to other objects, but I still perceive myself as moving, while still being in my own frame of reference.


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When you change posture from sitting to standing, various parts of your body are in motion relative to each other. That is to say, each of those parts exists within its own F of R.


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Originally Posted By: redewenur
When you change posture from sitting to standing, various parts of your body are in motion relative to each other. That is to say, each of those parts exists within its own F of R.


Yea but more relevently you're accelerating. If you stand up without accelerating (assuming no relative motion of body parts), then you wouldn't perceive yourself as moving, rather you'd feel the room was moving around you.

I think that very clear explanation of time and space vectors implied the thing's being viewed from some arbitrary reference frame, not necessarily its own.

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kallog: "rather you'd feel the room was moving around you"

Yes, that's commonly known as 'the morning after syndrome' smile . - or you could achieve a similar effect by reading through this:

http://www-history.mcs.st-and.ac.uk/HistTopics/Newton_bucket.html

But seriously, yes, acceleration applies to any object moving at non-constant velocity - which includes all things in a gravitational field, so that's just about everything I suppose. Which is why I find the 'Newton's Bucket' so interesting. Mach's ideas on rotational acceleration (taken up by Einstein as 'Mach's principle') are also worth a bit of thought.

"In 1985 further progress by H Pfister and K Braun showed that sufficient centrifugal forces would be induced at the centre of the hollow massive sphere to cause water to form a concave surface in a bucket which is not rotating with respect to the distant stars. Here at last was a form of the symmetry that Mach was seeking."

Weird, innit. And dead fascinating.


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Dragging the discussion back in the direction of the F of R, one has to wonder who, or what, occupies my F of R? The simple answer might seem to be "I do", but as pointed out above, parts of me move relative to other parts, and there can be no movement within a F of R. Even my brain cannot occupy my F of R, because there is activity in there (I'm sticking to that, in spite of any contrary evidence). Every synapse has its own activity. Following this line of thought, it begins to look as though no physical entity can claim to occupy a F of R. Increasingly, the F of R looks like the "present", the closer you try to look at it, the more elusive it becomes.


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Originally Posted By: redewenur

"In 1985 further progress by H Pfister and K Braun showed that sufficient centrifugal forces would be induced at the centre of the hollow massive sphere to cause water to form a concave surface in a bucket which is not rotating with respect to the distant stars. Here at last was a form of the symmetry that Mach was seeking."


I find it all quite uninteresting. The frame dragging isn't really relevant. If you want a bucket that's not rotating relative to the stars, and yet the water has a concave surface, it's easy, just set up some sort of gravitational field which does that. It could be by frame dragging, or be classical gravity.

Although I suppose it shows that just looking at the shape of the water can't tell you if it's accelerating or in a gravitational field. That just comes back to the equivalence principle which is kind of fascinating but also kind of obvious.


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Originally Posted By: Bill S.
claim to occupy a F of R. Increasingly, the F of R looks like the "present", the closer you try to look at it, the more elusive it becomes.


True. But for many purposes you can ignore internal motion. Even tho there's thermal motion of the atoms in a brick, their overall momentum adds up to the momentum of the whole thing.

Inertial frames of reference are just convenient for simplifying things like SR does. If we were all really smart we could do away with them and treat everything and it's parts as accelerating in whatever way they are.

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Originally Posted By: kallog
I find it all quite uninteresting. The frame dragging isn't really relevant
Uninteresting? - sure, whatever lights your fire; but [frame dragging] 'not relevant'? - any distortion of spacetime is relevant to frames of reference.

Originally Posted By: kallog
Inertial frames of reference are just convenient for simplifying things like SR does.
Convenient, and actual - and I would hazard a guess that Bill's intention is to try to gain insight into what happens specifically in terms of frames of reference.


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Originally Posted By: redewenur
Uninteresting? - sure, whatever lights your fire; but [frame dragging] 'not relevant'? - any distortion of spacetime is relevant to frames of reference.


It sounded like the article used that frame dragging to show that rotation is relative to the massive things around it. But surely it isn't?

Or is it really? Do we feel centrifugal force on a roundabout because it's spinning relative to loads of massive material in the universe? And it would be equally valid to say that other stuff is spinning while we're standing still? That's amazing. But if it's not that then the whole frame dragging thing is going off on a tangent for the article, which is what I found a bit frustrating.

Originally Posted By: redewenur

azard a guess that Bill's intention is to try to gain insight into what happens specifically in terms of frames of reference.


Yea and I think that's what he's done, reminded us that pretty much everything is accelerating pretty much all the time, at least its constituent parts are.

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"Do we feel centrifugal force on a roundabout because it's spinning relative to loads of massive material in the universe? And it would be equally valid to say that other stuff is spinning while we're standing still?"
Could it not be argued that on the roundabout we know we are moving because we have experienced acceleration, whereas the world around us has not. I have never been very happy with that reasoning when applied to the twin paradox, so I would be interested to see someone shoot it down.


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