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- Thread starter boffinwannabe
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Simon Bridge

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Thatif an event happens at the exact moment of another for a single observer, what does instantaneous mean?

Of course - within the limits of the measurement. Is there any other kind?So do we know that events can happen with zero time between them for a single observer?

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ghwellsjr

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Nugatory

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It's measurable, just not measurable with unlimited precision.

There's no deep quantum-mechanical or metaphysical magic going on here, it's just that every measurement of everything contains some uncertainty. If we were being really strictly careful with the language, we would never say "simultaneous" without adding the qualifier "to the limits of our best available clocks".

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My question is whether two events can ever be said to be simultaneous

Relativity of simultaneity

http://en.wikipedia.org/wiki/Relativity_of_simultaneity

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Nugatory

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Relativity of simultaneity

http://en.wikipedia.org/wiki/Relativity_of_simultaneity

So when we say that two events are "simultaneous", we're either: 1) being sloppy by not specifying the particular frame because we think it's clear which frame is intended; or 2) being devious and not specifying the frame because we're trying to sneak a "paradox" through; or 3) demonstrating misunderstanding by thinking that it doesn't matter.

(Are there any other reasons not to specify the reference frame?)

And then, even after we've correctly said "simultaneous as observed by this observer" we STILL have to add the qualification about the limits of precision. But as long as we've been through all that, if two events have time coordinates in one reference frame that are equal, to the limits of precision of our time time measurement... That's what we mean by simultaneous.

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LURCH

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Simon Bridge

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We can deal in absolutes only in models - eg. absolute simultaneity is fine in

Context provides the meaning.

if I go through a set of doors

If we synchronize watches from the same manufacturer then their ticks will probably be less than a small fraction of a second apart ... so we can say they read the same time. The context is that we are close together when we make that reading (so the time light takes from the watches to my eyes and the time it takes to get to your eyes are about the same). This is a common scenario in SR thought experiments.

We can extend this further by using a third observer - so we don't have to be close together.

The concept of "at the tame time as" is well defined in SR.

Whenever we start talking about measurement we start introducing uncertainties and statistics. While we are talking about the mathematical models - then we are fine.

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So much like string theory its all works mathematically but is untestable and fails newtons grounds for being science. Working mathematically does not make it a reality.

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ghwellsjr

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So much like string theory its all works mathematically but is untestable and fails newtons grounds for being science. Working mathematically does not make it a reality.

Where did you get the idea that "zero elapsed time" is what simultaneous means or that it requires two separately synchronized clocks with less than perfect precision?

You don't need any clocks, synchronized or not, to establish that two remote events happen at the same time, as

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So much like string theory its all works mathematically but is untestable and fails newtons grounds for being science. Working mathematically does not make it a reality.

You seem to be asking for simultaneity to have some demonstrable absolute meaning. We are not stuck simply because we defined simultaneity a certain way, because we know it's only a procedural definition. To think we are somehow stuck implies that properly defined simultaneity has real meaning. We could laugh at the Chinese for pointing up and saying down, but it would make no more sense than saying simultaneity should be defined such that it is the same for everybody.

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Simon Bridge

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You seem to think that a physical model is only demonstrated if the exact prediction from the model exactly matches the outcome of the experiment.yes but once you define simultaneous as zero elapsed time and not an approximation then you are stuck. You can never demonstrate this experimentally because you have no way to measure it.

That is not what we do. All an experiment has to do is distinguish between different models. It does not need to be 100% accurate to do that.

If model A predicts an exact value X=5units and model B predicts an exact value X=6units ... wee need our experimental design to result in a measurement of X which is accurate enough to distinguish X=6 from X=5.

From the design of the experiment we can work out an uncertainty "e".

To find out which model, if either, is "on the right track" (note: we van never tell if a model is "True" <---<<< deliberate caps.) we have to design the experiment accordingly. eg.

Lets say the experiment got a measurement for X of 5.2 units:

If e~1 then we could say that the models appear to be on the right track even though the experiment fails to distinguish them: we cannot tell which is better. If the measurement was 100 ... this tells us that both the models are way wayy off and we need to rethink them.

If e=0.1, then the experiment lends more support to model A than to model B (model A is accepted to 95% confidence, while model B is rejected).

The uncertainty is in

i.e. We only need the stopwatch button-press to be "simultaneous" with the event being timed to the extent that this distinguishes between different theories about what we are timing. The model tells us how close-a match that needs to be. The confusion you are experiencing is doe to not being precise enough about your terms.

It is this sort of attention to detail - separating the models from the reality - allows us to refine our experiments so they can tell us more detail about the phenomena under study. It lets us, also, refine our models to more closely imitate nature. It is what allows us to spot Heisenberg's uncertainty as something that has to be accounted for in the design of really fine detail experiments.

If you like to think in terms of absolutes, think of it this way: we have a model for the physics we are studying, which may be absolute if we want; and we have

However - you should read more about "empiricism": we can never actually prove a physical theory ... your observation about the impossibility of an absolute measurement boils down to this in the end. But we can disprove them.

The trouble with string theory is not the impossibility of measuring anything: it predicts every result of every experiment ever performed after all. The trouble is that it is difficult to come up with some result from string theory that canSo much like string theory its all works mathematically but is untestable and fails newtons grounds for being science. Working mathematically does not make it a reality.

We need (re above) some value X which string theory says is 5 when everything else says it is 6 and we just happen to have not measured X accurately enough yet. IFAIK: there is no such prediction... as yet. We are able to do this in the case of special and general relativity though.

If you want to discuss string theory and why it is not just dismissed as a pseudo-science; that's a big topic: you want a new thread in another forum.

Special and General Relativity, this forum, is only a small modification of classical physics. Within the framework for S/GR, the model allows absolute measurements and provides a scheme for comparing them between different observers.

We have another model with statistics built-in to it - Quantum Mechanics.

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