Without Einstein, would General Relativity be discovered by now?

From the horse's mouth:

"There is no doubt, that the special theory of relativity, if we regard its development in retrospect, was ripe for discovery in 1905. Lorentz had already recognized that the transformations named after him are essential for the analysis of Maxwell's equations, and Poincaré deepened this insight still further. Concerning myself, I knew only Lorentz's important work of 1895 [...] but not Lorentz's later work, nor the consecutive investigations by Poincaré. In this sense my work of 1905 was independent. [..] The new feature of it was the realization of the fact that the bearing of the Lorentz transformation transcended its connection with Maxwell's equations and was concerned with the nature of space and time in general. A further new result was that the "Lorentz invariance" is a general condition for any physical theory."

-- Albert Einstein

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In "La théorie de Lorentz et le principe de réaction", Poincaré published a paper in which he said that radiation could be considered as a fictitious fluid with an equivalent mass of m = E/ c^2. He derived this interpretation from Lorentz's 'theory of electrons' which incorporated Maxwell's radiation pressure.

Plenty of material here: https://en.wikipedia.org/wiki/Relativity_priority_dispute

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TL;DR: it definitely did not happen in a vacuum. IMHO his contribution is making science happen 1-2 years earlier (which is still a tremendous feat by itself).

Completely practically: clocks in orbit keep consistently different time from clocks on the surface. The discrepancy requires accounting for relative frames of reference.

" Because an observer on the ground sees the satellites in motion relative to them, Special Relativity predicts that we should see their clocks ticking more slowly (see the Special Relativity lecture). Special Relativity predicts that the on-board atomic clocks on the satellites should fall behind clocks on the ground by about 7 microseconds per day because of the slower ticking rate due to the time dilation effect of their relative motion [2].

Further, the satellites are in orbits high above the Earth, where the curvature of spacetime due to the Earth's mass is less than it is at the Earth's surface. A prediction of General Relativity is that clocks closer to a massive object will seem to tick more slowly than those located further away (see the Black Holes lecture). As such, when viewed from the surface of the Earth, the clocks on the satellites appear to be ticking faster than identical clocks on the ground. A calculation using General Relativity predicts that the clocks in each GPS satellite should get ahead of ground-based clocks by 45 microseconds per day.

The combination of these two relativitic effects means that the clocks on-board each satellite should tick faster than identical clocks on the ground by about 38 microseconds per day (45-7=38)! This sounds small, but the high-precision required of the GPS system requires nanosecond accuracy, and 38 microseconds is 38,000 nanoseconds. If these effects were not properly taken into account, a navigational fix based on the GPS constellation would be false after only 2 minutes, and errors in global positions would continue to accumulate at a rate of about 10 kilometers each day! The whole system would be utterly worthless for navigation in a very short time. "

-- from http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps....

David Hilbert discovered general relativity at practically the same moment as Einstein.

Some interesting reads on the subject:

- https://en.wikipedia.org/wiki/Relativity_priority_dispute#Th...

- https://www.mathpages.com/home/kmath650/kmath650.htm

I forgot who said that Henri Poincaré was modest to a fault and often attributed some of his work and discoveries to others.

The Wikipedia entry for "Relativity priority dispute" may have more information for you[1]

[1]: https://en.wikipedia.org/wiki/Relativity_priority_dispute

Whether or not someone would have formulated an explanation for physically observable discrepancies as quickly is maybe what you mean?

Because all those pesky missing factors of 1/2 were already being seen in astronomical phenomena, correct? Orbit of Mercury, aberration of light, etc. A postulate / explanation was what was needed (of course, maybe some more examples too).

But yes, there is also the question of whether the publicizing of the theory (and correct predictions) led to an explosion of interest that caused lots of technological advances due to the excitement around the new field.

Without Einstein, I think we would have parts of General Relativity figured out, but not grounded in the indistinguishability of acceleration from gravity.

Richard Feynman said he could not conceive of how Einstein came up with Relativity Theory with the level of scientific advancement of the time. So there is that.

Yes. It's even disputed whether Hilbert discovered it first. But regardless of einstein or hilbert, technological advancements would have necessitated the discovery of general relativity. Just like the advances of telescopes/observation lead to kepler's planetary laws or the advances of computing technology led to the theory of computation.

Science can lead to new technologies, but new technologies lead to new science as well.

https://undsci.berkeley.edu/article/whathassciencedone_03

The general direction of science and technology in the first half of the 20th century indicates that general relativity, theory of computation, etc would have been discovered.