Philosophy Now Forum

[quote="Obvious Leo]
Do you know what the term "physical" means? What physical properties does empty space have which allow it to perform these miraculous feats? [quote="nix"]

I don't claim that empty space is performing miraculous feats. To say space is defined physically is only to say that physical measurements with meter rules and clocks define that space, and measure the geometry of that space empirically. The point I was making was that the logically consistent geometry of space could have been Euclidean, or curved with positive curvature or curved with negative curvature and that the case which actually pertains can only be determined by physical measurement.

nix wrote:[quote="Obvious Leo]
Do you know what the term "physical" means? What physical properties does empty space have which allow it to perform these miraculous feats?

nix wrote:
I don't claim that empty space is performing miraculous feats. To say space is defined physically is only to say that physical measurements with meter rules and clocks define that space, and measure the geometry of that space empirically. The point I was making was that the logically consistent geometry of space could have been Euclidean, or curved with positive curvature or curved with negative curvature and that the case which actually pertains can only be determined by physical measurement.

I understood your meaning well enough but this is a philosophy forum and in matters of philosophy precision of language is very important. A physicist may choose to say that the curvature of empty space causes matter and energy to behave in a particular way but a philosopher is not entitled to such a semantic latitude. A philosopher must say that the behaviour of matter and energy can be modelled mathematically in terms of a curved space. The distinction is not a trivial one.

Leo,

in the discussion about the age of the universe, the point I was trying to make is that the time from the big bang until here and now is well defined (13.8 byr) and has meaning as the proper time of an observer carried along on the expanding spacetime from BB until here and now. This time is not infinite as would be expected if scotts absolute time and absolute space existed.

I make no claims about what might have happened before the big bang and whether this is a meaningful or meaningless question.

nix wrote:in the discussion about the age of the universe, the point I was trying to make is that the time from the big bang until here and now is well defined (13.8 byr) and has meaning as the proper time of an observer carried along on the expanding spacetime from BB until here and now. This time is not infinite as would be expected if scotts absolute time and absolute space existed.

Agreed.

nix wrote: I make no claims about what might have happened before the big bang and whether this is a meaningful or meaningless question.

My reference to the eternal universe refers to this question. Although the cyclical universe paradigm couldn't yet be regarded as mainstream it's certainly gaining in acceptance and has the advantage of being metaphysically coherent, which the universe with a beginning is transparently not.

Leo,

Ok. but a couple of points I don't follow;

1) you say "However SR is NOT compatible with GR " but SR is a limiting case of GR when gravity is weak and generally SR is locally equivalent to GR (though not globally) so in what does this incompatibility consist?

2) you say "the speed of light cannot possibly be a constant. The reason why it cannot possibly be a constant is because it is OBSERVED TO BE a constant in all referential frames". Is the rest mass of the electron not a constant then?; it is observed to be the same in all reference frames. What would qualify then as being a constant?

A better term is "the speed of light is an invariant" would you accept that?

nix wrote:1) you say "However SR is NOT compatible with GR " but SR is a limiting case of GR when gravity is weak and generally SR is locally equivalent to GR (though not globally) so in what does this incompatibility consist?

This statement is false. SR is not a limiting case of GR when gravity is weak. SR is a limiting case of GR when gravity is ABSENT. SR is treated as a special case of GR in the so-called "flat" space but there is no such region as a flat space in a hypersphere. SR is purely a mathematical abstraction which has no real analogue in the universe because there is nowhere in the universe where gravity is absent. This is of fundamental importance in sub-atomic physics because it means that time passes more quickly on the electron than it does on the nucleus it orbits. In fact this is quantum gravity.

nix wrote:What would qualify then as being a constant?

Nothing. There is no such thing as a constant in a relativistic universe. Constants are a useful mathematical convenience but no more than pragmatic approximations. The problem with SR is that it isn't relativistic enough.

nix wrote:A better term is "the speed of light is an invariant" would you accept that?

I can't see how the term could have any meaning without a referential frame. Invariant compared to what?

Obvious Leo wrote:

nix wrote: Constants are a useful mathematical convenience but no more than pragmatic approximations.

Approximations to what if not constant quantities in nature?


I can't see how the term could have any meaning without a referential frame. Invariant compared to what?

invariant to transformations of spacetime coordinates in accordance with the Lorenz group or the poincaire group?

Scott Mayers wrote: This raises a lot of problems when considering the 'age' of the universe. Taking Einstein's view, we should have then recognized that the determined 'age' of our universe is only an illusion of the observer.

The "age of the universe", or rather the time since the big bang, is dependent on the observer, but in such a way that many observers will deduce the same age: The age is the proper time of an observer stationary with respect to the frame of expanding space (eg stationary on the surface of an expanding balloon of space i.e. stationary in the co-moving coordinate system) from the big bang to the observers here and now. This is the maximum experienced time for the physical stuff of the universe. Any additional motion of the observer against this expanding background frame leads to a reduction of the observers experienced time and hence age estimate. We sit in a uniform bath of cosmic microwave radiation from the big bang so it looks like we are not moving significantly fast compared to the co-moving coordinate system so our 13.8Billion year estimate will agree with that made by other observers co-moving with the expansion.

nix wrote:Approximations to what if not constant quantities in nature?

How can there be constant quantities in nature if nature is relativistic? Surely the notion of immutable physical laws is absurd enough without having to assume immutable constants as well? The constants you refer to are mathematical objects and solely derived from observation. This grants them great utility in the prediction of future observations but this method is tautologous which means that the origin of the constants themselves remains beyond explanation by definition. I know of very few modern physicists who still cling to the idea of constants in nature and can name plenty who are convinced that no unification model can ever be achievable under this unworkable assumption.

nix wrote: invariant to transformations of spacetime coordinates in accordance with the Lorenz group or the poincaire group?

Poincare would turn in his grave to discover that his name has been linked with the bogus Minkowski model. He never bought the 4D manifold for an instant and it was he who was the true father of modern relativity theory, not Einstein. The Lorenz transformations are useful predictive tools but cannot possibly be reflective of a physically real system. The Lorenz-Fitzgerald contraction hypothesis is even more non-physical and the reasons are bloody obvious. SR uses two different definitions of time within the same model, a logical fallacy which any philosophy undergraduate could drive a truck through sideways.

nix wrote:
The "age of the universe", or rather the time since the big bang, is dependent on the observer, but in such a way that many observers will deduce the same age:

Every observer in the universe will deduce the same age because we are all equidistant from the big bang. The observer is always right in the middle of the universe because regardless of which "direction" he looks in the big bang is precisely the same "distance" away. There is no "location" in the universe where this is not a true statement.

Having got the serious stuff out of the way it's time for a piss-take.

nix wrote:The age is the proper time of an observer stationary with respect to the frame of expanding space

How does space expand? Does it just thin itself out a bit or do new bits of space arrive from somewhere to fill in the gaps?

If you stand at the roadside and watch a car driving away from you is the space between you and the car expanding or is the car just driving away from you?

Obvious Leo wrote:

nix wrote:Approximations to what if not constant quantities in nature?

How can there be constant quantities in nature if nature is relativistic? Surely the notion of immutable physical laws is absurd enough without having to assume immutable constants as well? The constants you refer to are mathematical objects and solely derived from observation.

To say there are constants of nature (lists of which can be found in any physics or chemistry text book or data book with the best measured values of them, as you well know) is not to maintain any metaphysical position as to their origins. Relativity is forced on us because there are such invariant quantities which are found to be the same by all observers, that is what led to the need for revisions of the concepts of space and time to account for this observation.

Do you mean by there are no constants of nature that they may be time varying over very long periods? Well that is testable by measureing say the fine structure constant from spectra of hydrogen in stars of different ages. These measurements put limits on the possibility of variation of the natural constants.

I know of no physicists at all who would maintain your position ( and I know a lot of professional physicists , at Stanford, Imperial Colledge and Durham universities!)

Obvious Leo wrote:

nix wrote:
The "age of the universe", or rather the time since the big bang, is dependent on the observer, but in such a way that many observers will deduce the same age:

Every observer in the universe will deduce the same age because we are all equidistant from the big bang. The observer is always right in the middle of the universe because regardless of which "direction" he looks in the big bang is precisely the same "distance" away. There is no "location" in the universe where this is not a true statement.

Not quite true for the following reason, if the observer, A, is moving relative to the expansion his clocks run slow compared to an observer,B, moving with the expansion so, A's proper time since BB is less than B's. If A is moving significantly fast compared to the expansion he will detect this because the cosmic background radiation will no longer be distributed across his sky in a homogeneous way. (see the discussion of this on the physics forum!).

Obvious Leo wrote: SR uses two different definitions of time within the same model, a logical fallacy which any philosophy undergraduate could drive a truck through sideways.

what are these two contradictory definitions of time?

nix wrote:

Obvious Leo wrote: SR uses two different definitions of time within the same model, a logical fallacy which any philosophy undergraduate could drive a truck through sideways.

what are these two contradictory definitions of time?

Proper time and co-ordinate time. They can't possibly both be real so SR assumes that neither is real.

Obvious Leo wrote:

nix wrote:

Obvious Leo wrote: SR uses two different definitions of time within the same model, a logical fallacy which any philosophy undergraduate could drive a truck through sideways.

what are these two contradictory definitions of time?

Proper time and co-ordinate time. They can't possibly both be real so SR assumes that neither is real.

Both are well defined concepts and involve no logical contradiction, only a recognition that there is a relativity of simultanaity of space separated events (which in turn is forced on us by the experimental observation that light has the same speed in all frames of reference)!

Proper time is the time between two 'events' on a clock moving along with an object A (at rest with the object) and coordinate time is the time on a clock of an observer B watching A's movement between those same two events. Both are physically measureable times and so I would say they are both real and can be reconciled by recognition that space and time are not independent things but the combination 'spacetime' is the appropriate reality (the spacetime interval between the two events is the same for observer A and B). The fact that proper and coordinate times disagree tells us something about the nature of space and time as measured quantities.

What then do you mean by real?

nix wrote: The fact that proper and coordinate times disagree tells us something about the nature of space and time as measured quantities.

It certainly does. It tells us that they are mutually exclusive constructs and thus cannot both be physically real.

Are you seriously asking me to define what is physically real and what isn't or are you just taking the piss?