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There are two kinds of measurements. Absolute, as in temperature, where there is a well defined zero point, and all temps are in relation to it, and, gauge, where the measurement depends on where and how the measurement is made; think of a bird on a 15000 v hydro line, that feels zero potential. Time, and distance, are not absolute ( as Studiot has mentioned ), and it makes no sense to say this point is 12 km, or this instant is 17 sec. We specify differences in length with separation, and differences in time with duration. And just like the bird on the wire, we can set the origin of the separation at r=0 and the origin of the duration with t=0 to ease our calculations. Similarly, as INow has mentioned, there is no universal now, or universal present, because there cannot be simultaneity. My 'now' or 'present' differs from the person standing two feet away from me, never mind a galaxy a billion LY away. My now is your future and someone else's past; how do you argue that only the present, or now, exists but past ans future do not ?? And to all those who think that time is simply an effect of motion, that time is emergent from the three spatial dimensions, I challenge you, as Markus did, ( since GR is our map/model of the real terrain ) to find the Panama Canal on a map of Central America from the 1800s. Question for Markus, Mordred and anyone else who may know... We have a separation interval between events as a distance, so we convert the time coordinates by multiplying by c to get a 'distance'. Could we also specify the interval length as a time ( by dividing distances by c ) and would this make any difference whatsoever to GR ? Would some then argue that time is fundamental, and lengths are emergent ?

As i have said i no longer keep marine fish, I move around too much to provide a stable environment for them.

! Moderator Note Similar threads merged

As Salvador Dali once said: "whoever wants to engage people’s interest must provoke them." So the title of this forum thread was just to get your attention, now that I already have it, let me introduce what I've done. In 2020 during quarantine I discovered a method that successfully connects the different dimensions at a visual level. That way it is possible to convert 2D into 3D and vice versa. This method not only completes to perfection linear perspective, but it could also lead to an endless number of different applications. In addition, it could be useful in different sciences. Recently, I uploaded a video to Youtube showing how it works: This is the cube that I created in the video, it looks like a render made with a 3D design program but it's just a drawing. I'm not sure what I found, but I tried to explain it the best I could. And this video is not a tutorial, that's why I don't explain what I'm really doing. But if there is enough interest, I can do tutorials. From now on, we can discover together the potential of all of this. We only need the right person to see it, that's why the best way to contribute to this discovery is by sharing the video. Thank you very much for your time! and feel free to let me know your opinion.

http://gamelab.mit.edu/games/a-slower-speed-of-light/ Have fun folks

It is a scalar field that's needed in inflationary models of cosmology to provide a mechanism for the vacuum to go through different phases of expansion. A scalar field is a field represented by a number at every point of space and every instant of time, so that it doesn't change at all under rotations. In a way, it's very much like the Higgs field. It affords you to "imprint" needed properties on the other fields and space-time itself to account for observed properties that don't quite fit the model without it. It's considered by some to be somewhat ad hoc --convenient, but not very well understood or logically compelling.

Good question. Been a while. Not this site, used one of the countless others like bustedtees, but same outcome: https://www.redbubble.com/shop/63+earths+can+fit+inside+uranus+64+if+you+relax

Because all the energy that built the past universe was consumed in constructing the present, and the future is no more than a set of alternate building plans awaiting selection (wave function collapse if you prefer) and availability of materials for construction. Just an interpretation. Consider observing a supernova inside some huge stellar nursery from not too close but close enough to study in reasonable detail. After the early fireworks have subsided we may expect to see a growing bright edged circle as the surrounding dust is progressively illuminated. Is it unreasonable to perceive the arrival of intense radiation at that bright edge as being experienced simultaneously around the entire perimeter even when it's several light years in diameter? And what exactly is going on at that edge other than a colossal number of particle interactions being randomly chosen from an even more colossal number of possible interactions. It may even be viewed as a phase change, but in addition to a change in physical properties it's a condensation from causes to effects. Or a condensation of an unobservable, abstract 'many worlds' future into a single throughly deterministic past with the present being a thin slice of quantum fuzz between the two. Again, just a personal visualisation to help get my head around it. Perhaps, the ambiguity of 'now' for different observers is simply an SR induced form of observer bias. Not one of them is able to observe a causal reversal, and to me, that seems to be the critical issue. Is this a reasonable view?

Perhaps you can get a hold of this older degree level book. The Students Physics VolI Acoustics Alexander Wood Blackie. Wood goes into great detail about the subject including lots of experimental material. He treats many musical instruments including brass in general and trumpets in particular and offers measurements on them. Here are some pages from the more basic section involving the velocity of sound, both measured and linked to theory includng corrected gas laws.

I do not believe you, I was keeping coral more than 45 years ago and all your links do not support your claims. Coral in a tiny container without a huge support system is simply not possible in the way you are suggesting.