[ExI] Gravity waves?

[John Clark]

On Fri, Mar 21, 2014 at 4:59 PM, Ben <bbenzai at yahoo.com> wrote:

>>>> Shouldn't this be "gravity waves have been inferred from the
>>>> detectionof variations of the polarisation...", or "variations in the
>>>> polarisationof microwave radiation is consistent with the existence of
>>>> gravity waves...",
>>>>
>>>
>>
> >>> How is that fundamentally different from "electromagnetic light waves
>>> have been inferred from the chemical changes made in the retina of our eye"?
>>>
>>
>> >> The difference is the degree of separation between the thing and the
>> observer.  The more steps there are, the more uncertainty there is about
>> what is causing the result.  There's a difference between a brick falling
>> on your foot, and you reading a message where someone claims that a friend
>> of a friend suffered a brick to the foot. There are fewer possible
>> interpretations for one than for the other, and a different degree of
>> certainty about the existence of the brick.
>>
>
> > Observations of polarisation of microwaves is detection of gravity waves
> in the same sense that observation of blonde hairs on a lapel is detection
> of a cheating husband.
>

In one case we observe the polarization of microwaves and deduce that the
polarization was caused by gravity waves, in the other case we observe
movement of the mirrors of a LIGO detector and deduce that the movement was
caused by gravity waves. It seems like a comparable degree of separation
between the thing and the observer to me, except that we can observe the
mirrors with our eyes (although we'd need instruments to see them in
sufficient detail to see them move) but we can't see microwaves.

  John K Clark
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