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From: tomyee3@gmail.com (ProkaryoticCaspaseHomolog)
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Subject: Re: E =?UTF-8?B?PSAzLzQgbWM/IG9yIEUgPSBtYz8/IFRoZSBmb3Jnb3R0ZW4gSGFzc2Vu?=
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Date: Sat, 7 Dec 2024 00:34:35 +0000
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On Thu, 5 Dec 2024 18:42:24 +0000, J. J. Lodder wrote:
> Paul B. Andersen <relativity@paulba.no> wrote:

>> So if the speed of light, measured with instruments with better
>> precision than they had in 1983 is found to be 299792458?.000001 m/s,
>> then that only means that the real speed of light (measured with
>> SI metre and SI second) is different from the defined one.
>
> So this is completely, absolutely, and totally wrong.
> Such a result does not mean that the speed of light
> is off its defined value,
> it means that your meter standard is off,
> and that you must use your measurement result to recalibrate it.
> (so that the speed of light comes out to its defined value)

Not necessarily.

It is still possible that despite there having been 1 1/2 centuries
of experiment supporting the constancy of the speed of light, at some
level of precision of measurement, some variation may be discovered.
To quantify such variation, it would obviously be necessary to perform
such analysis using an EARLIER definition of the meter.

> In other words, it means that you can nowadays
> calibrate a frequency standard, aka secundary meter standard
> to better accuracy than was possible 1n 1983.
> This is no doubt true,
> but it cannot possibly change the (defined!) speed of light.
>
> In still other words, there is no such thing as an independent SI meter.
> The SI meter is that meter, and only that meter,
> that makes the speed of light equal to 299792458? m/s (exactly)

Yes.

Going back to the OP thread topic, the fact that since 2018, E=mc^2
is true BY DEFINITION does not render irrelevant experiments designed
to test its validity.

Here is a thought experiment:
1) Take a gram of electrons and a gram of positrons, converting them
   completely to electromagnetic energy.
2) Take a gram of protons and a gram of antiprotons, converting them
   completely to electromagnetic energy.

Do we know absolutely for sure that all masses, whatever their form,
are equivalent when converted to electromagnetic energy? I believe it
to be true, but I don't KNOW it to be true.

Certainly in terms of behavior in a gravitational field, most
alternative theories of gravitation generically predict violations of
the weak equivalence principle in the 10^-16 to 10^-18 range. The
Galileo Galilei mission will be the first experiment to explore this
range:
http://eotvos.dm.unipi.it/nobili/

Could violations of the "mass-energy equivalence principle" exist?