Revolutions spawn from, and contributes to, the revolutionary mood. It is no coincidence that many revolutionary ideas in science: Chemistry (Lavoisier), Biological Evolution (Lamarck), Lagrangians, Black Holes,, Fourier Analysis, Thermodynamics (Carnot), Wave Optics, (Young, Poisson), Ampere’s Electrodynamics spawned roughly at the same time and place, around the French Revolution.
In the Encyclopedie, under the term dimension Jean le Rond d’Alembert speculated that time might be considered a fourth dimension… if the idea was not too novel. Joseph Louis Lagrange in his ), wrote that: “One may view mechanics as a geometry of four dimensions…” (Theory of Analytic Functions, 1797.) The idea of spacetime is to view reality as a four dimensional manifold, something measured by the “Real Line” going in four directions.
There is, it turns out a huge problem with this: R, the real line, has what is called a separated topology: points have distinct neighborhoods. However, the QUANTUM world is not like that, not at all. Countless experiments, and the most basic logic, show this:
Manifolds were defined by Bernhard Riemann in 1866 (shortly before he died, still young, of tuberculosis). A manifold is made of chunks (technically: neighborhoods), each of them diffeomorphic to a neighborhood in R^n (thus a deformed piece of R^n, see tech annex).
Einstein admitted that there was a huge problem with the “now” in physics (even if one confines oneself to his own set-ups in Relativity theories). Worse: the Quantum changes completely the problem of the “now”… Let alone the “here”.
In 1905, Henri Poincaré showed that by taking time to be an imaginary fourth spacetime coordinate (√−1 c t), a Lorentz transformation can be regarded as a rotation of coordinates in a four-dimensional Euclidean space with three real coordinates representing space, and one imaginary coordinate, representing time, as the fourth dimension.
— Hermann Minkowski, 1907, Einstein’s professor in Zurich concluded: “The views of space and time which I wish to lay before you have sprung from the soil of experimental physics, and therein lies their strength. They are radical. Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.”
This remark rests on Lorentz’s work, how to go from coordinates (x, t) to (x’, t’). In the simplest case:
C is the speed of light. Lorentz found one needed such transformations to respect electrodynamics. If v/c is zero (as it is if one suppose the speed v to be negligible relative to c, the speed of light infinite), one gets:
t = t’
x’ = x – vt
The first equation exhibits universal time: time does not depend upon the frame of reference. But notice that the second equation mixes space and time already. Thus, philosophically speaking, proclaiming “spacetime” could have been done before. Now, in so-called “General Relativity”, there are problems with “time-like” geodesics (but they would surface long after Minkowski’s death).
Another problem with conceptually equating time and space is that time is not space: space dimensions have a plus sign, time a minus sign (something Quantum Field Theory often ignores by putting pluses everywhere in computations)
In any case, I hope this makes clear that, philosophically, just looking at the equations, “spacetime” does not have to be an important concept.
And Quantum Physics seems to say that it is not: the QUANTUM INTERACTION (QI; my neologism) is (apparently, so far) INSTANTANEOUS (like old fashion time).
As we saw precedingly (“Can Space Be Faster Than Light“), the top cosmologists are arguing whether the speed of space can be viewed as faster than light. Call that the Cosmic Inflation Interaction (CII; it has its own hypothesized exchange particle, the “Inflaton”). We see that c, the speed of light is less than CII, and may, or may not be related to QI (standard Quantum Physics implicitly assumes that the speed of the Quantum Interaction QI is infinite).
One thing is sure: we are very far from TOE, the “Theory Of Everything”, which physicists anxious to appear as the world’s smartest organisms, with all the power and wealth to go with it, taunted for decades.
Tech Annex: R is the real line, RxR = R^2, the plane, RxRxR = R^3 the usual three dimensional space, etc. Spacetime was initially viewed as just RxRxRxR = R^4.]What does diffeomorphic mean? It means a copy which can be shrunk or dilated somewhat in all imaginable ways, perhaps (but without breaks, and so that all points can be tracked; a diffeomorphism does this, and so do all its derivatives).