Physicists use the word “spacetime” as if it were a well-defined entity with unique properties. There are more branded spacetimes than there are shampoo brands.
There is a shampoo for each type of hair. You have oily hair, there is a shampoo for it. You are pregnant, and there is a shampoo for pregnant women, helping you to have the best looking hair during pregnancy.
Same with spacetimes. Visit the Einsteinium Spacetimes Supermarket and in the shelves you can find a spacetime for the requirements of any academic paper. Do you need a stationary spacetime, there is one just for you. This simple spacetime will solve your stationary spacetime needs very efficiently.
Maybe you need an asymptotically flat spacetime because you just bought a shiny new black hole from our Singularities Department and you need a matching asymptotically flat spacetime, then you are in the right place. Einsteinium has it for you.
Maybe you need spherically symmetric spacetimes. You are a beginner and you are writing your first spacetimes paper and you are too nervous and you wanted a simple spacetime. We have you covered with our spherically symmetric spacetime.
If you cannot decide which Lambda to use for your Cosmological Constant, our Cosmological Constants shelves are the best stocked in the industry; we have the usual zero, positive and negative Lambdas but also many many more; for instance, you can try our Lamda gauged to a variable Newtonian constant G which is well suited for Multiscale Diffusion spacetimes. Be the first to try this exciting new product in your institution.
Maybe you are a mid-career spacetimeologist and you need a spacetime conforming to the quality specifications of the Petrov classifications system, then we recommend a Petrov type II spacetime. But don’t use Petrov type I spacetimes because it has been shown with a definitive null experiment that Petrov type I spacetimes clearly violate General Relativity’s Type LXCDM-II Einsteinian Railroad Embankment Simultaneity Paradox Anomaly. So stay away from it. (You see, we have a great customer service and each and every one of our sales force has a PhD on Spacetimeology. Don’t hesitate to ask any questions that you may have on any products we sell.)
We have many spacetimes on sale. You might want to look at them and buy them in bulk to be used later in any boilerplate spacetimes paper you may need to churn out in the future.
Good news! Our Minkowsky spacetimes are on sale. Minkowsky spacetime is one of the oldest spacetimes, the spacetime that started it all, and it is on sale now. In fact, if you buy Thick Branes Type Warped spacetimes we’ll throw in a Minkowsky spacetime for you.
On our upscale spacetimes shelves, you can find organic, hand made, Bianchi type spacetimes. Buy one, you won’t regret it.
We also have very nice exotic spacetimes. One is called the classic Newtonian spacetime where space and time are absolute; we have a spacetime without the space coordinates, real exotic stuff; and we have spacetimes with space, time, vacuum, mass, multiverse and quintessence coordinates all combined in a Cartesian Vortexial Coordinate System. This is the Swiss Army Knife of spacetimes, all values can be adjusted with steampunk-type knobs. Just to understand the Cartesian Vortexial Coordinates System you will need half a century of study in a monastery in the French countryside.
But we have plain vanilla spacetimes too. One is coordinateless spacetime, this is the anti-reified coordinate system, there are no reified coordinates, no space and no time. This spacetime is suitable only for blackholes that were approved by Hawking before he died. We have a spacetime used by NASA where only photographable black holes live. This is very expensive because it is a proprietary spacetime. Ask your institution for availability.
We have all kinds of reference frames. Inertial and otherwise. We stock the best inertial frames in the industry with the best official General Relativistic observers wearing the official lab coats with Einstein’s portrait on the back done in the style of the famous Che Guevera poster. After all Einstein was no less revolutonary than Che. These obervers observe using only the best quality Zeiss-made observational tools. These officially sanctioned observers only use simultaneity gauges made simultaneously in China and Germany. Nothing but the best. All your simultaneity needs is covered in Einsteinium, the market of choice of all discerning spacetimeologists.
Do you like Gowdy spacetimes? We have it. We have this type of spacetime in two flavors: Polarised and non-polarized. You can mix and match Gowdy spacetimes with Multiwarped spacetimes (either polarized or not). The possibilities are endless.
If you are in a classical mood, you can have our Riemannian spacetimes and complete it with a Gödel-type spacetime and publish a nice looking paper without even trying.
And finally, if you are a real connoiseur of spacetimes and a fan of homogeneity and isotropy you may try the spacetime with the best provenance ever: the Friedmann–Lemaître–Robertson–Walker spacetime.
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And then a physicist can write a sentence like this:
«The ultimate origin of the speed of light limit lies in the structure of spacetime.»
Which spacetime?
Which fabric of which spacetime?
All shampoos at least have something in common: water. The fabric of all shampoos have water in it. But the spacetimes in the spacetimes zoo have nothing in common. They were invented to be different so that their inventors could collect some academic points. They were just invented out of the blue.
“Out of the blue” meaning, “Einstein’s Field Equations” because you can massage Einstein’s Equations to get any solution you want. Einstein’s equations is the goose that lays golden spacetimes for golden careers. The large number of spacetimes mentioned above proves this fact. So, the mantra “according to Einstein’s Equations” means nothing more than “I made up some spacetimes to fit my needs”.
“Spacetime” is the best example of scholastic hair splitting. You take an innocent word and corrupt it by loading it with infinite number of meanings. This is casuistry, the favorite tool of scholastic doctors of philosophy and theology since the beginning of time. Load a word with multiple meanings and pick and choose a meaning that confirms to what you want to prove in your paper. Casuistry can’t fail.
And hair splitting in physics is not limited to spacetimes. How many species of Big Bangs are there, do you know? Hot, cold, tepid Big Bangs... just to name a few. Then there is vacuum with its countless species. Let’s not forget black holes. There are so many species of black holes that even physicists cannot name them all. The list of “physical” concepts hair-split to charlatanism goes on and on.
Disgusting! I don’t know what other word describes modern academic physics.
References:
2. Asymptotically flat spacetime
3. Spherically symmetric spacetimes
6. Thick Branes Type Warped spacetime
11. Friedmann–Lemaître–Robertson–Walker spacetime
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