J.C.MacSwell

Hi Janus I took the first par as c/2 being the invariant and everything that had to adjust would be based on that...essentially the same universe on a different scale (wrt what? I have no idea...but my responses were based on that) More particularly can you give me context to the bold? That line of thinking is foreign to me. I realize it is not just about light, and the rest of electromagnetism is tied to it as well.

That's quite an assertion.

So...another Universe superimposed on ours that works essentially the same as ours except their light is c/2?...but c/2 relative to what? It can't be relative to all our frames and be consistent with relativity for us. c/2 in some of our frames would be FTL in others, so this is just a more complicated version of saying "what if FTL travel/communication".... Relativity or causality would have to break if there was any interaction between our Universe and this hypothetical universe. If there is no interaction then it is just as welcome to exist as the undetected pink unicorns that no doubt dance in front of our noses everyday. I am trying hard to find something in this that isn't wrong, but with regard to the bolded: A clock that relied on round trips of sound waves in a stationary medium would run fastest if it was at rest with respect to the medium, the water...yet that would be a moving clock with respect to other frames.. Light doesn't work that way...if it did we would still have the aether theory, but it doesn't so we have Relativity.

It would have to have it's own set of frames. Ours can't measure something something as c/2 in all frames.

So the new light would measure as c/2 in all frames? That would be mathematically inconsistent with our frames. They would have to live in a different universe.

Theres no circular reasoning. Relativity is assumed to be correct. There is no attempt to prove it. You asked how causality could be violated by FTL information travel.

So...you now have your answer?

Here are examples https://en.wikipedia.org/wiki/Tachyonic_antitelephone http://dumbscientist.com/archives/any-ftl-signal-can-be-sent-back-in-time

Measured wrt your base, lab, space station etc Some reference frame at a fraction 1% of c. (wrt anything else is calculated) It is very reasonable to assume the tests would have the same results if done from a base at greater speed (wrt Earth or wrt the CMBR) given that we don't have much choice...but if we did have a choice, if we could just as easily make the tests from some spaceship at greater speeds, would we be 100% confident that those assumptions are reasonable and not bother?

Measured from a base with start and finish points moving how fast wrt to theCMBR?

right...and if you attach your removed arm to the boat and rotate it to different positions and make a few tests and assumptions you can glean that information as well...and then start to wonder why you chopped off your arm...

It is all to do with simultaneity and past present and future in different frames. If you took information and sent it faster than the speed of light to a distant point in your reference frame, this would be backwards in time wrt some other frames. It could then be relayed to you at a point in your past by doing this in other frames.

In principle you could run tests from a base that is at a much greater speed wrt the CMBR than have ever taken place in the >100 years and see if the relativistic symmetry still holds. So far we have only done this from a base of a fraction of a percent of c wrt to the CMBR. This is just an example. I am not arguing against relativity per se. It is obviously the best we have, and it's based on our best assumptions. But you, I or anyone else having no reason to disbelieve it does not make something 100% certain. The level of certainty can still increase from whatever it might be today, or we might find something unexpected.

In practice what I am suggesting may not work as experimental errors may be higher than simply estimating from what you have already done...but in theory you could get the info for forearm plus hand...and then get it for just hand and subtract the difference...this all assuming your interests are from joint to joint....so get results for body...rotate at elbow and so get difference for forearm plus hand...then rotate at wrist to get results for hand,

Pretty hard without chopping your limb off, however: Using the principles of a "Lamboley test"(it has been almost 30 years since I was involved with one) which was devised to find the c.g. and moment of inertia of small sailing dinghys. (to control weight distribution and therefore costs and durability of the boats, as lightness in the ends was preferred in wave conditions) So if you had a pendulum apparatus you sat "fixed in" and testing at two different height settings...Knowing the mass distribution of the apparatus you should have enough information to know your mass, the height of the centre of gravity of your mass in that position, and the radius of gyration/moment of inertia with respect to the axis of rotation. Then, rotating only your one limb to a different position on the plain of rotation do the two tests again, and find the difference. That should give you almost enough to tell you the c.g. and moment of inertia of your limb with a few assumptions and calculations. You might need extra data points as you don't know the mass of the limb as I would when moving a fitting or lead corrector weight to make the boat "class legal". Less accurately (maybe more accurately if you can't sit rigidly during the swing test?) you could do a static test on a board just to check the change in c.g. of your "system", but with two unknowns, mass and position, you need more assumptions.

With all due respect that seems backwards. How would you devise, or even give consideration for, an experiment to confirm something if you don't look at it as a possibility first.

I didn't take it that way at all. Mistermack can correct me but I think he meant it as to hold it, or look at it, as a possibility.

If you "go there", and set up an experiment that fails to show any difference are you not doing physics?

Edit : Rethinking this, the gravitational force is still a finite value on the surface, so that statement of Janus's should be correct.

I don't think this is correct. Obviously this is an idealized case, but as the thickness approaches zero, the gravitational gradient across the thickness would tend to infinite. Assuming that is correct, the inside and outside of the surface cannot be taken as equivalent unless the shell is massless.

Of course. You also don't accept current theory as dogma and stop doing physics either.

Well so far I would say there have been, Michelson-Morely for example...they have just been negative so far or pointing that way. Physics has not been proven to the nth degree.

Surely if you do not know for certain something does or does not exist, claiming that it doesn't is not on the same standing as claiming you don't know, or claiming that the possibility has not been eliminated.

Thanks Is it zero also infinitely far away? I would expect it to be above zero at the midway point in that case, in somewhat of a gravity well by comparison. (not doing any math as you might guess)