J.C.MacSwell

It was originally stated by Mordred and I believe it is correct in context of expansion.

That would be correct. Your entire body but for your left foot doesn't include your left foot, but it doesn't mean your left foot is not part of your body.

I didn't actually say what you just quoted me as saying, but I am assuming it is based on multiplication, not addition. It was originally stated by Mordred and I believe it is correct in context of expansion.

The known universe referred to (obviously not known at that time LOL, it's the known now as it was then) was, and therefore still is finite. The remainder of the Universe may have been finite, or may not have been. We don't know, but the model says it would have expanded like the known part appears to have, so it would remain finite or infinite, as the case may be.

My understanding (again using that term) Is that there is an established way to do this, such that expansion effects can be set aside. (So Now is 14ish billion years ago here but also at the edge of the observable universe, regardless of how long the em radiation might take to here) I'm not sure what query you are referring to. We seem to disagree in principle whether a COM of a system of finite size but less than the Universe is definable.

To the bold yes. For reasons I have stated in all my posts in this thread. Everything else is also my (correct I believe) interpretation of current understanding and I don't believe any of it is controversial. If you can define a observable universe as a system it would have a COM. I cannot observe any part of the Universe in my present but I don't take it as blind faith that the Sun still exists. I guess we will know for sure in 8 minutes whether it does/did. We will never have the same certainty about the observable universe but the principles are the same.

The present day observable universe, however you choose define it, would have a COM. Your (admittedly our) difficulty defining it, or keeping track of it, does not change that. Unlike the Universe, it conceptually would have a COM. Why do you think it has ceased to exist? Conservation of mass and energy...the equations are different but the same principles apply in relativity as they do in Newtonian physics.

The observable Universe would in fact have a centre of mass, somewhere in the vicinity of where we are. But the observable universe is not the whole thing and the Universe, even conceptually, is not understood to have one. If you take the balloon analogy of the expanding universe, there is no centre of mass on the 2D surface (a balloon would have a centre of mass at it's centre inside the balloon, but there is no inside or outside to the Universe that is analogous to that of the model)

First you define your system, rocket plus exhaust,..or you and your dog...or the solar system...or all of the solar system except you and your dog etc etc. It matters not if the systems parts are already in motion with respect to one another, or parts are displaced from one another. https://en.wikipedia.org/wiki/Center_of_mass Then choose a coordinate system where the centre of mass is at rest, Now define your force. It applies to any external forces. F=ma, Any external force on any part of the system accelerates the centre of mass of the system. It can obviously become impossible to keep track of the whole thing but conservation of momentum still applies.

None of this changes the concept of the centre of mass of a system. It just complicates it. For the bold it is very simple classically (Newtonian). F=ma applies to the centre of mass of the rocket/exhaust system. It does not matter how much or how little of the system the force is applied to. Now, the whole Universe is another matter. I don't believe it has a centre as we understand it.

Need more info... Stick yourself underwater on a scale, adding weight if necessary to hold you in place and get a reading. Then stick the part of your arm out of the water that you wish to weigh. The difference will be the weight of that part of the arm, plus the weight of the displaced water. So then deduct the weight of the displaced water and you have the weight of that part of the arm.

It might suggest Burma, Liberia or USA... http://mentalfloss.com/article/55895/countries-havent-adopted-metric-system On Topic. UV degradation is a permanent affect. It doesn't go away at night.

Isn't that the point of a limited liability company though? Even the owners are exempt a s long as they are operating in good faith...or at least able to claim to be. If it is in the public interest overall then it should work itself out, even if the vehicles initially require a higher minimum of insurance...which could in time reverse where driver cars require more.

To SJ: The idea being the ability to look past the vehicle in front of you (or in your example pulling out in front of you) to anticipate what might be reasonably required? To anyone: How does current technology handle someone lane changing right in front of you, faster than you could respond if you had to brake? (maybe given the faster response this is not a problem given equal braking power??)

That is a very good point. Perhaps the automation system could produce an alarm if it senses some "confusion" or doubt on it's part, or when approaching such things as pedestrian crosswalks.

Thanks Janus. Understood.

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