geordief

I think it was to reassure myself that something like this could be done and so that I had acquired something of an understanding of the subject (even flawed understandings can be helpful if corrected)😉

When I wrote the "physical system evolves or travels" I see that that can be interpreted as mixing frames but I had it in mind that there could be a series of measurements in different frames that could be compared even if this was not strictly accurate. I was imagining the picture of the gravitational field from one frame morphing into the next,subsequent one albeit with errors creeping in at each step but still the "adjacent " pictures taken from 2 very close frames could still be run,visually like a movie ,even if with mistakes. I see there is probably no practical benefit to this; but that is what I had in mind.

EDIT: either "travels" because the physical system evolves or "travels" because we decide to place the frame elsewhere (hope that makes sense)

1)Yes I felt the title was a bit mangled but found it had to articulate my thought If the frame "travels" and the coordinate system is unchanged the resulting diagram of the field will look different ,won't it?Will the curves be different in different areas? 4) so,say for a BH we can have a frame at any distance and watch the (local?) gravitational field change in real time as the frame moves wrt it?

In the ongoing Gravity and Space thread,Swansont wrote "Photons have no mass, and even if they did, the effect would not be due to velocity, since all photons travel at c. They have energy and momentum, owing to their frequency (or wavelength). The effects on our observations is real, but spacetime is a coordinate system. Like latitude and longitude on a globe, the coordinate system is not flat, it is curved, because that's the proper coordinate system to use to describe the very real effects. When you look at them on a flat map (with a projection that lacks distortion), the lines are not straight. But latitude and longitude are not physical objects." In any particular scenario ,does this curved coordinate system have a natural point of "origin" or can one ,in theory set the origin at any point one wants? Can one have different (all curved) coordinate systems that allow one to describe any particular gravitational scenario? Say there is a body of mass and one wants to show the gravitational field in its vicinity,must we set the origin of the coordinate system at the centre of gravity of the massive object?

I would have to defer to others who have greater knowledge than I do on this subject.Hopefully Janus will reply in due course.

Would it be helpful to say "the spacetime coordinate system" instead of just "spacetime"? Would people be less tempted to interpret it physically ?

Interesting "trivia" Was also a talented footballer. Perhaps lucky to escape the Munich Air Disaster in another life. https://www.mirror.co.uk/sport/football/news/sean-connery-man-utd-bond-22939245

Let's not denigrate our equine companions. Otherwise ,fair comment.

@md65536 yes ,I suppose the looseness in my scenario was not such a big deal. I was trying to show a concrete example of this spacetime interval and it appears that I was along the right lines. You seem to ave a better feel for the ramifications of the issue than I do. @Markus Hanke thanks for that video.It dealt with a few things I had had on my mind for some time. I like the "5 minute from the shop " explanation I notice that ,as formulated the spacetime intervals has units of spatial distance.Is there a related formulation that would have units of pure time (even if convoluted)? I mean ,can the s^2= [ct]^2 -r^2 formula be rearranged or rejigged so that both [ct]^2 and r^2 are expressed in terms of time ( with c understood) ? Maybe T^2 =t^2 -[r/c]^2 where T is the temporal distance? Can I also ask whether you think that adopting the invariance of the spacetime interval as a postulate leads to the requirement of a universal and invariant speed limit ? I know you said you didn't think that it would work as a way to derive Relativity itself but might it be enough to require the existence of a "c" of some value?

@joigus @md65536I see now that my scenario was not water tight. I won't attempt to patch it up as I would probably make a bad job worse. Still I hope I may have learned a little overall (about 4-vectors, perhaps - or at least when a 3-vector does not do the job a of a 4-vector)

If I differentiate the spacetime interval formula ** with respect to time I think I get the expression dr/dt =c^2.t/r Am I correct in the math and what might be the physical significance or interpretation of that expression? To me it looks as if, when t and r are measured in infinitesimally small amounts they seem to "switch places" (dr being in the numerator and r being in the denominator in that expression) Is it simplistic to suggest that time and space might "switch places" when the measurements are completely local rather than extended? If that is ,as I expect wrong headed what physical situation might that dr/dt=c^2.t/r be describing mathematically? ** s^2 = [ct]^2 - r^2 0 =2c ^2.t -2r.dr/dt dr/dt=c^2.t/r Edit: hope I was right that d[s^2]/dt =0

jump2021?

Thanks for explaining the convention. So ,in my example the spacetime interval is as per s^2=[ct]^2 -r^2 and I make that to be [200ct]^2 -1000^2 ,which is measured in metres. [200ct]^2 and 1000^2 seem to me to represent the time and the spatial components of the spacetime interval.... Can I think of those 2 components as 2 spatial distances ,the former vastly greater than the latter and the ratio between the lengths of those 2 lines could represent the time /space ratio of the spacetime interval?

On a pike.

I was thinking along the lines of a posthumous message that explained how we had managed to survive as a group or what led up to our self destruction. @Area54 maybe advanced civilizations might have language/signalling abilities more akin to primitive life forms. I doubt we have understood all or much of these mechanisms here on the Earth . If only we could come to a modus vivendi with our new friend Covid-19....

@MarkusHanke @zapatos I think there for that we should train ourselves by attempting to learn the languages of the creatures on the Earth first. Then ,to be more sure that the message is understood as a message we might send 2 versions of the message;the first just a plain object without a message and the second the same object but with the message encoded

Suppose we wanted to leave a message to another civilization in the event that our's came to a bad end and so we could inform others of our failure (or our successes) ,how could we be sure that our message would be understood if it was ever come upon and read? Would we need to invent some kind of a universal language that we could be confident that any (or at least many/ most ) advanced civilizations would be able to decipher? What basic principles could be employed to ensure that the "reader" would firstly understand that our message was worth the effort to understand and then that it could be understood? Has this subject been already explored perhaps?(probably ,no doubt)

https://m.youtube.com/watch?v=wTP2RUD_cL0 Money for Nothing (and get your chicks for free) Apparently Trump's election team wanted to use that to open his rallies but Sting was insisting on royalties. Then Trump said he expected the royalties from them but Sting said he wasn't even American, just an Englishman in New York 🤔

Hopefully an extremely simple scenario. Let's have an observer A with another relatively stationary observer B at 100 light seconds distance A emits a pulse of light in the direction of B and that pulse is reflected and captured again by A at a distance of 1000 metres to his or her right (just in his or her vicinity, really) So we have 2 events ; the emission and the recapture of the signal. Am I right to say that the spatial measurement component of the spacetime interval between the 2 events is the 1000 metres and the time component ( ie the ct component) is the 200 light.seconds taken by the signal to make the round trip and return to the vicinity of its emission? If I am right so far ,can I ask : How does observer B calculate this spacetime interval ? (It must be the same as for A ,no?) Also can this same scenario be used whereby B is moving relative to A? Does it show that B still measures the spacetime interval the same as when he or she was stationary with respect to A?

I find dreaming gives me great hallucinations (never had hallucinations "in the dream though ;wonder if that is recorded as happening) edit:I suppose sleep is "drug induced"; just not "artificial" drugs

Are there any connections between the Black Hole model and this model (is it a model?) of Penrose? Would the latter model imply that there was no singularity at the centre of a BH? Some mechanism whereby all the surrounding forces ,Leonidas- or Dutch- boy- with- finger- in- the -hole of -a -dyke like were held at bay in the smallest of confines.

Any particular example? (sorry to drag you into the debate if I am doing that)

If your mass is greater than that of the ball then I think you will experience proportionally less acceleration than the ball. And vice versa. I don't think though that you experience acceleration relative to the ball as acceleration is absolute even though the magnitude of your initial acceleration was relative to the magnitude of the initial acceleration of the ball.

But what about the Moon being in freefall around the Earth ?It does not accelerate Should we say that because it is following a geodesic in spacetime that it is moving in a straight line? Is it just in Newtonian mechanic that it appears to be changing its velocity wrt the Earth and so apparently accelerating?