Cassandre

My first reaction is that space "warping" can be done with mass and this results in more time required to reach a destination. The riddle is how to achieve the contrary effect. Looking at your references, I see "the negative vacuum energy ring". Hmmm... Next, the paper looks serious, but it has the interesting remarks that "exotic matter is needed" that delivers negative energy densities and which is suggested to be "forbidden classically" but allowed in QM "in special circumstances".. To me that all looks like pure speculation, although interesting.

Good one! Indeed I should have written "deal with", "experience" or "see".

Don't you wonder why people pay scant regard to the Lampyris noctiluca? Everyone deals with light, few people encounter neutrinos (but sometimes glow worms).

To add a little to the comments of others, what you say is too general - even impossible. At constant gravitational potential and relative to an inertial reference system, the return speed of light that is bounced off a mirror is always measured as c, independent of the motion of the source. And if we use the "Einstein synchronisation method" then this is also true for one-way light rays. For details see §1 of http://www.fourmilab.ch/etexts/einstein/specrel/www/

Sorry but no, photons can't be "at rest": see replies #3 and #7. And this has nothing to do with "space warp" or wormholes.

I certainly understood the text that I quoted, and Wikipedia isn't a scientific reference. However see an older version of the same page: http://en.wikipedia.org/w/index.php?title=Relative_velocity&oldid=360389838 As well as the current page on "Velocity": http://en.wikipedia.org/wiki/Velocity#Relative_velocity By chance (and contrary to that article's current claims!), the first reference in that "Relative velocity" article is such a text that you don't know. I can send you a copy of the definition in it if you like, and you can also find it in many libraries. I will not comment more on different definitions in this thread except if the OP asks for it. Hi leveni, That can't work as was briefly mentioned [edit, also somewhat discussed]. Was the explanation sufficiently clear or do you need more clarification?

The word "space" has slightly different meanings; it quickly gets very philosophical. It can't be a material substance like earth, water or air - that is an "ether" concept that nobody could match with all observations (I think that this has been argued by Lorentz and Michelson). However "empty space" can hardly be truly empty either, if we believe in the validity of field theories. One discussion by Einstein (a bit dated though, of almost a century ago) can be found here: https://en.wikisource.org/wiki/Ether_and_the_Theory_of_Relativity Oops I now see that you posted your question in classical physics; however many clues come form relativity (as you can see in the link) and also quantum mechanics. Anyway, the linked paper also mentions a clue from classical mechanics, as forwarded by Newton.

Regretfully common textbooks say little about "time", and thus I have no other references than common history and logic for answering your question. I think (pretty sure) that "time" first off all related to the daily and yearly cycles. Thus sun dials and mo(o)nths are among the oldest time sensors. Common time sensors measure the progress of change of something observable, such as the change of position of the sun or moon or the recurring motion of a quartz crystal; and the rate of such a change is a frequency. Constant frequency devices are therefore popular for clocks. However for example a C14 "clock" is based on a (in that sample) non-recurring change of radioactivity, and the change rate is not constant. So, "dying" could make some kind of clock, but that certainly isn't a fundamental property of "time". "Change" as in "time" refers to observable changes in repeatable physical processes that can serve as a basis for comparison ("time measurement"). Just my 2 cts.

Elfmotat, I learned that "relative velocity" means the difference in velocities (compare Einstein's 1905 paper*, third section: the ray moves relatively [..] with the velocity c-v). I asked if you know an old physics text that uses "separation velocity". From your answer I get that just like me you don't know one; and with the above definition there is little need for such. Thus, once more, it appears that the definition of "relative velocity" that you use is the more recent one. And that definition lacks distinction between "relative veloctiy" and "velocity" as also your comment illustrates. Leveni, do you like me to elaborate or does this go too far off-topic? *http://www.fourmilab.ch/etexts/einstein/specrel/www/

Hi isn't it the other way round? Do you know an old physics text that uses separation velocity? I had learned it many years ago as timo understood it in post #3, and "separation velocity" is a term that I had not heard of until well after my studies. In contrast, Einstein used one century ago "relative motion" in the sense of leveni and timo here.