[helene_t]

Not sure what you mean, Jack. The second statement assumes that something changes size relative to something else. If y=1000x yesterday and y=1001x today we can arbitrarily say that y is constant and x has been shrinking, or we can say that x is constant and y has been expanding. I suppose with more than two variables (say the x=diameter of the universe, y=the diameter of a hydrogen atom, z=the wavelength of the radiation emited by the decay of U238) it becomes non-arbitrary since Ocam's razor requires us to keep as many variables constant as possible.

[Wackojack]

The speed of light is one of our "measuring sticks". If it is constant and matter expands we would notice it.

[helene_t]

Wackojack, on Dec 27 2007, 01:00 PM, said:

The speed of light is one of our "measuring sticks". If it is constant and matter expands we would notice it.

But how do we know that the speed of light is constant? If the diameter of the universe was 1000000000 lightyears yesterday and 1000000001 today, we could arbitrarily say that light has become slower or that the universe has expanded, or that the instruments we use to measure the speed of light (in particular clocks) have been altered.

[Wackojack]

Whether or not the speed of light is constant is another question. (Perhaps worth going into... I happen to believe Einstein). In your first post Helene your 2 statements contradict because the 2nd statement assumes the speed of light to be constant.

[Fluffy]

helene_t, on Dec 27 2007, 10:19 AM, said:

No, if everything was expanding all our measurement sticks would expand as well so we wouldn't notice anything. In fact nothing would be expanding, since expansion is relative.

I think the same, but if universe expands faster than galaxies and they expand faster than systems ..... and atoms.

From our point of view particles might be decreasing

[kenberg]

About the speed of light:

First: I haven't thought much about this for roughly forty years, so take what I say with a grain of salt.

Now: "The speed of light is constant" is shorthand, and perhaps misleading. Since space and time are hopelessly intertwined it takes some work to sort out even what it should mean. As I understand the original issue it went something like this: If I guy is standing on a platform on a train that is moving away from you at 10 mph and throws a ball towards you at, from his viewpoint, 25 mph then it approaches you at 15 mph. The thought was that a light source moving away from you should behave similarly and the experimental fact was that it didn't. If a light was flashed towards you from an object that was a certain distance away then the time required for the light to reach you was independent of the motion of the object. Thi fact was summarized as "The velocity of light is constant". The experimental fact preceded the development of the theory of relativity.

Now to relativity: Sometimes it is said that Einstein saw time as the fourth dimension and this is illustrated by the fact that if you agree to meet someone you not only have to specify the spatial coordinate but also the time coordinate. Of course it did not take Einstein to note that if I say that I will meet you at Starbucks it would be useful if I also said when we should meet. Rather the Einstein idea was this: In discussing velocity we might rotate the coordinate system so that, say, we speak of so many degrees off of East instead of 90 more degrees off of North. Or we might measure time in hours instead of minutes. But no one had thought it reasonable to set things up so that a change in coordinates might involve an interplay between time and space. (Actually I think Poincare had thought a bit along these lines but leave that be. He didn't get to where Einstein did.) In 1905, AE suggested this theory and in 1915 he carried the idea much further. The result was a theory that was consistent with the previously observed phenomena and, importantly, predicted new behavior (the slowing down of the decay of elementary particles as their velocity approaches c for example, although that was not available at the time) that have been experimentally verified. Already you can see the difficulties in getting the statements right: Really it should be that the decay of particles, measure as we measure time, slows down if the particles are moving at high speeds relative to us. Someone moving with the particles notices no change in the time of decay. So the statement "How long did it take for the particles to decay" is meaningless unless the frame of reference for the time measurement is stipulated.

This is, I think, an approximately correct summary of "The speed of light is constant". The experimental fact preceded Einstein, the theory he developed to explain it postulated an interplay between space and time that predicted other phenomena that have been experimentally tested and found to be correct.

[barmar]

"The speed of light is constant" actually turns out to be an AXIOM, not a discovery.

We need to measure space and time in terms of something. We need fundamental units with which to describe these measurements. What has been done is to select the vibration rate of a particular atom as the standard for time, and then define space in terms of the distance that light travels in this period of time.

A good book that explains this well for laymen is The Comprehensible Cosmos: Where Do the Laws of Physics Come From? by Victor J. Stenger. Basically, he explains that all the "Laws" of physics (e.g. Newton's Laws of Motion, Special and General Relativity, Conservation of Energy/Momentum/Angular Momentum) ultimately derive from general principles of "point-of-view invariance". Each law comes from a different invariant or symmetry.

[kenberg]

barmar, on Dec 27 2007, 10:46 AM, said:

"The speed of light is constant" actually turns out to be an AXIOM, not a discovery.

We need to measure space and time in terms of something. We need fundamental units with which to describe these measurements. What has been done is to select the vibration rate of a particular atom as the standard for time, and then define space in terms of the distance that light travels in this period of time.

A good book that explains this well for laymen is The Comprehensible Cosmos: Where Do the Laws of Physics Come From? by Victor J. Stenger. Basically, he explains that all the "Laws" of physics (e.g. Newton's Laws of Motion, Special and General Relativity, Conservation of Energy/Momentum/Angular Momentum) ultimately derive from general principles of "point-of-view invariance". Each law comes from a different invariant or symmetry.

Axiom? In the theory, perhaps yes. But a physical theory is not constructed with arbitrary axioms. Maybe it's a bit of the chicken and the egg sort of argument but I believe that it is historically correct that there were observations preceding relativity theory that could and were described as the constancy of the speed of light. Some considerable effort was put into explaining these observations. Once relativity explained it, and predicted other phenomena correctly, then the constancy of the speed of light became an axiom in at least some tellings if the theory although I may prefer invariance under Lorentz transformations as a fundamental statement.



"Point of view invariance" is, I think, only a starting point. The Galilean transformations dealt with point of view invariance but they kept time and space separate. Lorentz transformations destroy the separateness of these two entities and lead to local clock changes for moving objects.


Anyway, I should probably hold off a bit here. I think that everything I have said so far is correct but it's best I don't push my luck.

[mike777]

Is time slowing down?
21 December 2007
Zeeya Merali
Magazine issue 2635
IT CAN drag or it can race, but what if time stopped altogether? It now seems that time could disappear from our universe - and we may already have found evidence of its forthcoming demise.

When astronomers observed a decade ago that supernovae are apparently spreading apart faster as the universe ages, they assumed that something must be causing the expansion of the universe to speed up. But so far, nobody has been able to explain where the "dark energy" causing this acceleration comes from.

Now José Senovilla at the University of the Basque Country in Bilbao, Spain, and his colleagues have a radical answer - we are fooled into thinking that the expansion of the universe is accelerating, because time itself is slowing down.

Senovilla's work, which will appear in Physical Review D, is based on speculative string theory models in which our universe is confined to the surface ...

http://www.newscientist.com/channel/fundam...owing-down.html

[Gerben42]

Quote

The 2 statements contradict. 2nd statement assumes that space expands and we would notice if everything in it expanded also.

Nothing in it expands as it is bound by gravity on the large scale and the other 3 forces on the small scale. A galaxy does not grow in size as it cannot escape its own gravity well. Ditto for galaxy clusters. Only the different clusters (gravitationally unbound!) increase their distance to each other.

It is nice to think of the universe as a balloon but the stars and galaxies and all are not drawn onto the balloon like in many illustrations, they are like little flies sitting on the surface, not expanding with the balloon as it is inflated.

[kenrexford]

Gerben42, on Dec 28 2007, 06:33 PM, said:

Quote

The 2 statements contradict. 2nd statement assumes that space expands and we would notice if everything in it expanded also.

Nothing in it expands as it is bound by gravity on the large scale and the other 3 forces on the small scale. A galaxy does not grow in size as it cannot escape its own gravity well. Ditto for galaxy clusters. Only the different clusters (gravitationally unbound!) increase their distance to each other.

It is nice to think of the universe as a balloon but the stars and galaxies and all are not drawn onto the balloon like in many illustrations, they are like little flies sitting on the surface, not expanding with the balloon as it is inflated.

But, wouldn't the little flies have to shuffle their feet? Otherwise, their feet would spread apart, causing them to fall off off the balloon.

[barmar]

kenrexford, on Dec 28 2007, 07:49 PM, said:

Gerben42, on Dec 28 2007, 06:33 PM, said:

Quote

The 2 statements contradict. 2nd statement assumes that space expands and we would notice if everything in it expanded also.

Nothing in it expands as it is bound by gravity on the large scale and the other 3 forces on the small scale. A galaxy does not grow in size as it cannot escape its own gravity well. Ditto for galaxy clusters. Only the different clusters (gravitationally unbound!) increase their distance to each other.

It is nice to think of the universe as a balloon but the stars and galaxies and all are not drawn onto the balloon like in many illustrations, they are like little flies sitting on the surface, not expanding with the balloon as it is inflated.

But, wouldn't the little flies have to shuffle their feet? Otherwise, their feet would spread apart, causing them to fall off off the balloon.

Yes, they do. The connections between their body parts is analogous to gravity, which holds structurs like solar systems, galaxies, and galaxy clusters together while the universe around them expands. Thus, even though the universe is expanding, the Milky Way and Andromeda galaxies are actually approaching each other.

[Al_U_Card]

Its hard to relate quantum phenomena to macrocosmic observation. Remember that your intention condenses out those nasty eventualities so that no feet shuffling is required.

Concentrate on parking between the lines......fewer headaches and lawsuits.

[Winstonm]

If there is a big bang to start the universe and no one is there is hear it, does it still make a sound?

[Al_U_Card]

If you listen carefully, you can hear the echo....

[matmat]

Winstonm, on Dec 30 2007, 08:28 AM, said:

If there is a big bang to start the universe and no one is there is hear it, does it still make a sound?

YES!
and you can see the echo!

[Winstonm]

ECHO, Echo, echo, e-c-h-o. I don't see any ECHO, Echo, echo, e-c-h-o....

[matmat]

Winstonm, on Dec 30 2007, 04:02 PM, said:

ECHO, Echo, echo, e-c-h-o. I don't see any ECHO, Echo, echo, e-c-h-o....

wtf is gnome?

[Fluffy]

Sound only travels through mass doesn't it?, it would not happen except into the exploding point of mass

[Al_U_Card]

And since the singularity was infinitely dense, you could hear it everywhere at once!