David H. Burns wrote:
>> view water waves going in the same direction as he is appear to be 
>> slower than those going in the opposite direction.

> Does he? 

Yes.  As I've said, you can boat faster than your wake, and you can fly 
supersonic. In both cases, all the waves look to be going backwards.

-- 
   Darren New, San Diego CA, USA (PST)
   Understanding the structure of the universe
    via religion is like understanding the
     structure of computers via Tron.

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Darren New wrote:
> You can put an atomic clock on Southwest Airlines and measure the error 
> you caused at the other end by being out of the gravity well and 
> traveling at velocity.
> 
> You can just leave one in the basement and one at the top of a 
> skyscraper for a couple months and measure the difference.

Nahhh, that just shows the clocks are inaccurate, with moving them, 
uh...jostling the atoms so they vibrate differently.

...yeah!

> Assuming X and Y are both the same distance from Z when you start 
> counting, why wouldn't they? Even with regular waves, that'll happen.

Well, sure, if you make *that* particular assumption...but the example I 
was reading didn't specify, so I was confused.

Still am.  Observation being one thing and actuality being another, 
things do happen simultaneously.  Saying that they don't because you 
can't have an 'outside' frame of reference is hubris, imo.  Like saying 
nothing in the universe exists outside the radius of light being able to 
travel since the big bang, with the observer (us) at the centre.  It's 
the whole earth-centric universe all over again.

--
Tim Cook
http://empyrean.freesitespace.net

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Tim Cook wrote:
> Well, sure, if you make *that* particular assumption...but the example I 
> was reading didn't specify, so I was confused.

Well, if they're not the same distance from you when they flash, why would 
you expect to see the flashes at the same time?

Or maybe I'm confused. You might be right. There's a "regardless of 
distance" clause at the end that makes the thing nonsensical except for 
c==infinity.

I read it more along the lines of "the bullet goes the same speed regardless 
of how fast the gun is moving when you fire it" sort of thing.

> Still am.  Observation being one thing and actuality being another, 
> things do happen simultaneously. 

Only if they're close to each other. There really is no "at the same time" 
for objects widely separated.

> Saying that they don't because you 
> can't have an 'outside' frame of reference is hubris, imo. 

No. If you can have two events happen in one order for me and another order 
for you, and there's *nobody* who can say what "at the same time" means, 
then how do you have simultaneity?

Basically, you can't have "equal times" unless space has "equal places".

OK, so take a vector. What you're saying is "I can't believe the Z component 
of a vector is sometimes zero and sometimes not zero, regardless of where 
you put the origin and how you rotate the axes."  The only way the Z 
component stays zero regardless of where you put your axes is if the other 
components are zero also.

 > Like saying
> nothing in the universe exists outside the radius of light being able to 
> travel since the big bang, with the observer (us) at the centre.  It's 
> the whole earth-centric universe all over again.

It's not earth centric because it's true of everyone. And "exists" means 
"capable of having any sort of affect on the observer", in which case the 
sentence is true. :-)  It's as "non-existent" as the future is.

-- 
   Darren New, San Diego CA, USA (PST)
   Understanding the structure of the universe
    via religion is like understanding the
     structure of computers via Tron.

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Darren New schrieb:
>> As for /arriving/ at the same time: Yes. As for being /sent/ at the 
>> same time: No.
> 
> If X and Y are very close together in space when the beam is sent, sure. 

Not even then. It would reduce the "simultaneity window" accordingly, 
but never reduce it to zero. And note that the other effects you want to 
observe diminish with the scale of your experiment, too.

> If X is on the tracks and Y is on the train, and they each set off the 
> flashbulb as the arm on the side of the train strikes the pole stuck by 
> the side of the tracks, wouldn't that be a simultaneous event? I mean, 
> the contacts touch once, and there's only one contact, so how could it 
> not be the same time for both flashbulbs?

Note that the flashbulbs won't go off simultaneously with the 
establishment of the contact: Electric signals, too, only propagate at 
the speed of light (if not slower).

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Tim Cook schrieb:
> Still am.  Observation being one thing and actuality being another, 
> things do happen simultaneously.  Saying that they don't because you 
> can't have an 'outside' frame of reference is hubris, imo.  Like saying 
> nothing in the universe exists outside the radius of light being able to 
> travel since the big bang, with the observer (us) at the centre.  It's 
> the whole earth-centric universe all over again.

Yes, but this time we have a solid scientific reason: Because /we/ are 
living on the earth, and it's where /our/ frame of reference happens to 
be "centered" at.

So what's wrong with /us earthlings/ to be earth-centric? (After all, 
that's the place where the vast majority of us is bound to be living in 
for the next couple of millennia - so maybe a bit more earth-centricity 
would do us some good, or we may not make it that long.)

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David H. Burns schrieb:
>> Well, then compare it with the perspective of a person actively 
>> swimming: Despite obviously being inside the medium, from his point of 
>> view water waves going in the same direction as he is appear to be 
>> slower than those going in the opposite direction.
> 
> Does he? Or does he just observe a frequency change, i.e. see the crests 
> and
>  troughs closer together or further apart, as with the doppler effect in 
> sound and light?

Try it out, and you'll see that the crests and troughs will actually 
apear to be moving at different speeds depending on direction - while 
the distance between them will remain constant (which is /why/ they'll 
appear to change their frequency).


> A curious result of this is that any mass measurements contain two 
> components,
> the inertial mass (the mass that would be measured if the object were at 
> rest with
> respect to the measurer) and the relativistic mass due to the relative 
> velocity of the
> object with respect to the observer.

Yup. E = mc^2: From an outside observer's point of view, pumping energy 
into some thing to speed it up to light speed also pumps up its mass.

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clipka wrote:
> Darren New schrieb:
>>> As for /arriving/ at the same time: Yes. As for being /sent/ at the 
>>> same time: No.
>>
>> If X and Y are very close together in space when the beam is sent, sure. 
> 
> Not even then. It would reduce the "simultaneity window" accordingly,

Well, sure. Epsilon and all that.


> And note that the other effects you want to 
> observe diminish with the scale of your experiment, too.

X and Y, not X and Z. :-)

> Note that the flashbulbs won't go off simultaneously with the 
> establishment of the contact: Electric signals, too, only propagate at 
> the speed of light (if not slower).

Certainly, modulo that stuff, yes. But if Z is light-days away, it's going 
to be close to simultaneous.  Unlike if, for example, X is light-days from 
Y, where it makes no sense to talk about "one event."

It's a thought experiment, see.

-- 
   Darren New, San Diego CA, USA (PST)
   Understanding the structure of the universe
    via religion is like understanding the
     structure of computers via Tron.

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Darren New schrieb:
> Certainly, modulo that stuff, yes. But if Z is light-days away, it's 
> going to be close to simultaneous.  Unlike if, for example, X is 
> light-days from Y, where it makes no sense to talk about "one event."

Still, those minor differences make... well, the difference.

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