geordief

Is it not possible to run a simulation of GR and NM (=Newtonian Mechanics?) side by side bottom the "bottom up" ? Each simulation would start with n identical particles and n would increase indefinitely. Would it serve any purpose to compare "results" at various stages or would the curvature be "baked into" GR from the start and NM would just produce a ball of increasing density towards to centre? Since we already know that NM is a less complete theory ,then would this be a waste of (quantum?) computer power?

In Feynman's thought experiment how is he calculating distances (ideally, if the Earth or equivalent body were amenable to precise radius measurement)? Would it be by a laser (neutrino?) beam ? Would these be 3D or 4D measurements ?

Ha ha . Do I get points for naivete ? I realize that Newtonian mechanics falls/has fallen at one or more hurdles but is it out of the question to run a computer simulation where we model the Earth -or any test body as a group of plastic massive bodies that settle into a ball shape under the action of Newtonian gravity? Is it possible to compare the results obtained by such a simulation with that obtained by another that ran a GR algorithm? It that as daunting a project as physical measurement? What about quantum computers ? Would they be up to the task?

I heard that on the BBC.It was said with a knowing world weary smile. I am sure we can "get over it" . After all "trust me" and "I know this folks" ,"this is going to be good"

Interesting the degree of disparity between the expected radius (of the Earth) and what is measured. based on the surface area. I millimetre. That is seriously tiny but does it begin to give us a sense of the scale of this phenomenon ? I would also be interested in how these measurements were arrived at. It must be a long process and I can't see how you can easily measure the radius of the Earth and its surface area . How do they account for the different densities as you go through the Earth's interior? How do they know where the centre is? It reminds me a bit of that thread where they were measuring the curvature of the Lake Balaton http://www.scienceforums.net/topic/98386-laser-curvature-test-on-lake-balaton/?hl=%2Blake+%2Bcurvature That seems simple in comparison.

I was being sarcastic. It comes easy these days (dripping slow) The "All politicians are liars" line is a recruiting sergeant for passive acceptors of Trump

This news just in from a Feynman lecture http://www.feynmanlectures.caltech.edu/II_42.html "Suppose we take the earth as an example and forget that the density varies from point to point—so we won’t have to do any integrals. Suppose we were to measure the surface of the earth very carefully, and then dig a hole to the center and measure the radius. From the surface area we could calculate the predicted radius we would get from setting the area equal to 4πr24πr2. When we compared the predicted radius with the actual radius, we would find that the actual radius exceeded the predicted radius by the amount given in Eq. (42.3). The constant G/3c2G/3c2 is about 2.5×10−292.5×10−29 cm per gram, so for each gram of material the measured radius is off by 2.5×10−292.5×10−29 cm. Putting in the mass of the earth, which is about 6×10276×1027 grams, it turns out that the earth has 1.51.5 millimeters more radius than it should have for its surface area.4 Doing the same calculation for the sun, you find that the sun’s radius is one-half a kilometer too long." Can things really be as simple as this? Is this the famous curvature that launched a thousand "is spacetime "real" " threads? Sitting there below our feet all along... All our "spheres" are squashed** by the tiniest fraction ,but measurably so as a result of the mass of the Earth? Our space is intrinsically curved? ** ie not Euclidean

Trump doesn't care about his detractors . He is focused on building his reputation. http://www.bbc.com/news/world-us-canada-38021820 Still ,all politicians lie ,don't they?

You wouldn't have any more to say about that with you I opened a question on that subject on Stack Exchange but am not sure I got satisfaction http://physics.stackexchange.com/questions/206505/the-speed-of-information. I think I understand it is the geometry of spacetime that causes this speed limit... Can any more be said?

Thanks to you both.Clear enough.

I thought Janus was calculating speeds as a fraction of c in a simple mathematical way. There was no implication that that anything might actually travel at c in anything other than a vacuum Hope I understood your point correctly.

Perhaps a trivial and slightly specialist question . We are told that it is "real" as opposed to being an artifact of observational techniques. (I am not disputing this) Does this definition of "real" coincide with the meaning of "real" that comes up in numerous threads when the question of whether we "can know reality" comes up. ? There seems to be a dichotomy in the case of "Time Dilation " where we are warned not to consider it as an artifact of our sensory or observational apparatus but embedded in the reality of the (geometry of ) the universe. Can anyone clear up this confusion in my mind (as to the way the word "really" is being used in this particular but rather important case?

Is anti mass a purely speculative phenomenon unlike anti matter which has been experimentally verified? Anti -mass ,if it existed would lead to anti -gravity , would it?

If you read Janus' reply (post#2)I think you will see that this is not the case "Now assume we have another frame moving to the right at .09c.It will measure the light moving to the left as moving at 0.188 c relative to itself and the light moving to the right as moving at 0.1009 c relative to itself. " I think that I understood that response correctly.... It is only really the speed limit c which is the same for all (inertially moving ) observers although the relativistic addition of velocities does apply to all values of "v" . -and also in all mediums apparently.

So negative spacetime curvature is not a concept in GR?

Are there examples of Negative Spacetime Curvature? Is it a term that only applies to the universe as a whole or are there local regions when spacetime curvature is negative (I think there are local regions where it is positive -such as planetary systems and galaxies?) Might it apply to the interior space left behind by an explosion such as a supernova ?

Suppose we have a medium which limits the speed of light to some constant speed -a fraction of c ,10% as an example.(ie c/10) Will any observer in this medium ,in any inertial frame of reference see light in this medium moving at this same speed? Is it possible to extend the question to an observer outside the medium -an observer in a vacuum for example ,again moving wrt the light source at a constant rate?. As usual ,I hope my question "makes sense" and is interesting.

If we calculate the time dilation caused by a trip to Alpha Centauri and back , is the total amount of time dilation a function of the speed or is it simply a function of the distance traveled? Would a twin snail ages by an equal amount less than its twin snail than would an astronaut aboard a rocket completing the same journey at a relativistic speed?

Well I know it can't be what I think it is because I don't have a consistent set of opinions about it. Perhaps ( in all likelihood) I never will and so I have to withhold judgement and simply hold to the observations relating to it that seem uncontroversial Special Relativity seems to provide a lesson that cannot be ignored and the fact that Trump cannot be "unelected" now seems to be another but how all that "hangs together" and defines "time" I don't know..

For primitive animals is it not the latter? Whether or not higher animals have a mentation that we would recognize as similar to our own notion of time is moot. For most humans,can it not be said that they have an erroneous understanding of time since they probably consider it to have an absolute status ? For myself I find myself in no man's land because I think my understanding is incomplete but I feel that I have left behind that particular misconception. Something (we call time) is there , but it may not be what we think it is.

Could we say that "time" is a man made concept that attempts to describe a natural process ? That concept does indeed depend on an intelligent observer whereas the (undefined) natural processes predate (ironically?) or exist independently of this concept.

Yes ,difficult and convoluted but does a hands off approach to social media exacerbate the problem? Should/could unmoderated sites be outlawed?

It may not be operated under those formal requirements but it is a human activity and honesty /truthfullness is a principle which is widely acknowledged across societies This should (and likely does) permeate all industries as a matter of course. It is not the only principle that operates in human affairs and your Goebell's quote is sadly apposite. Another quote is that “Eternal vigilance is the price of liberty” as well as that one about "good men doing nothing.." I feel I am looking into a can of worms that I am not equipped to flesh out, but even so the internet may be one of the most powerful tools developed to date and in the wrong (or nobody's ) hands it surely has the potential to do untold harm without it being at first apparent.

Sadly,yes. I understand (I am not a user apart from this kind of a forum) that social media (esp Facebook) feeds users with "information" that it (the algorithm) judges will appeal to them . On the other hand it is a weakness in the human psyche (in my opinion) to wish to be validated even when this is not a rigorous process (we cannot handle the psychological truth and the temptation is to live in cotton wool) "Living is easy ,with eyes closed,misunderstanding all you see" (see ,I have fallen into the trap ) Thanks. I hope your experience carries weight in the overall scheme. Edit: I just realized I was largely excluding this kind of a (human moderated) forum from this phenomenon. rIghtly or wrongly I consider it to be more benign than those characterized by what I have understood to be the mo of Facebook and similar social media sites

Maybe I was being Narcissistic.... Seriously, I think it is a big problem. Not new ,but is it being exacerbated by the internet algorithms telling us and reinforcing our ideas of who we are? Is tribal mentality getting institutionalised in the digital age?