Is it possible that a group of "elite" humans have highly advanced technology, but have kept it hidden?
For example, is it possible that some people already had powerful computers and automobiles (like modern one) several decades ago (say, in the 1950s), but hid it from "common people" ?
And maybe later they gradually introduced these technologies for the "common people". Or maybe they didn't introduce those technology and they still have more powerful equipment.
Possible, yes. If you don't mind altering history a bit. However, beware introducing things that are used publicly. Computers, sure! You can have one in your basement/vault/secret hideout. Automobiles, no. You're going to drive it around and you need space for that. It's much harder to protect.
Back in 1950, communication on a global scale was still, well, bad. I can't source this claim, to a quantifiable degree, but there was no internet. There might be global communications for politics, industry and academics, but certainly not for the common people.
To get superior technology, without fantasy elements, you need:
Or more simply put, you need materials to experiment on, tools to experiment with, and scientists to do the actual research and develop the goods.
I imagine that any "elite" group of people is very, very, very rich.
So a lab and shell companies that buy a truckload of supplies more or less each month are easily set up.
But the brainpower, that's a tricky one. Academics is based on peer research. You (for the academics here, I hope I got it right) spend your life working in a field, trying to find out why something works why it does. A lot of time goes into this. You work based on the research of others - others that have researched the things you research before.
Academics is a culture based on sharing.
This contradicts the culture where an elite group secretly has superior tech.
Communication enables sharing. Thus, for this to be viable, you'll have to take away communication. This could work in 1950, but in modern times, I don't think it's viable anymore. Nearly everybody in developed countries carries a camera and a global communication device.
Only for a short time.
The word here is "scale". Unless you have reached the point of singularity, it is highly unprobable that the few dozens or hundred of scientifics/technicians working secretly for you can keep any advantage against hundreds of thousands or millions of scientifics/technicians that work "publicly". Even if you have in your pockets "the best of the best", that would not be enough.
And, you will want not to have more than dozens or hundreds and not more because otherwise you could not keep such project secret. Also, your project is generating (almost) not revenue compared with those projects that end up being used to build cars, washing machines and instant pudding.
Add into the mix that technological advances are unpredictable. Maybe in the 40s you had the best experts in the design of electronical valves, and gloated about how you could build the best televisor sets in the world. But you could not foresee that some degenerate would invent the transistor, making your advantage moot.
There is one well known example in the real world - computers.
A lot of computing technologies that is considered to have been invented in the US was actually already implemented and used at Bletchley Park (or its successor GCHQ) in the UK. But due to the Official Secrets Act was not allowed to be disclosed to the public.
One specific example is public key exchange. Today, we call the algorithm for public key exchange the Diffie-Helman key exchange due to the first published practical algorithm by Whitfield Diffie and Martin Helman. But the first implemented algorithm (which turns out to be essentially the same) came almost a decade earlier out of work done by James Ellis at GCHQ.
There is a slight difference between what happened here and the scenario you mentioned. Instead of gradually introducing computing technologies to the public the UK government never introduced them. Instead they were only declassified long after "the public" have re-invented all those technologies themselves.
The net effect is that while the UK government spent a lot of money developing computers and algorithms none of them could be commercialized, thus none were commercially successful. Instead, re-inventions of those technologies in the US became commercially successful and were re-imported into the UK for public use.
For a fiction story, one could, of course, postulate a lone genius or a small group of geniuses, living in some isolated or segregated community, and keeping all their inventions secret.
Some technologies are not useful on a small scale. There'd be little point in building a railroad network, for example, if your secret cabal all live in one building. Where will the trains go? An assembly line to produce one car per week isn't really an assembly line. Etc.
Some technologies are hard to hide. If you invented the airplane 50 years before anyone else and you are flying it around, wouldn't someone see you sooner or later? Maybe you hide behind stories of UFOs or some such, but some things are hard to really use in hiding.
But the big catch in practice is that advanced technology often requires a large and complex infrastructure. Suppose someone in the mid-1800s -- let's call him "Charles" -- got a stroke of genius and figured out how to design a computer. How could he really build one? It's awfully tough to build a factory to make integrated circuits in your garage using a crowbar and a sledgehammer. He could try to build a mechanical computer using the tools and materials available, but even this would be very difficult. The supporting technology just isn't there, and it would be very hard to build it all from scratch.
A lot of modern technology requires bringing together materials and expertise from many people. The economist Milton Friedman once said that no one in the world can even make something as simple as a pencil. It takes many people. The wood may come from the Pacific Northwest. The lead may come from West Virginia. The metal may come from Minnesota. The rubber for the eraser probably comes from India. And then other people have to cut and plane the wood, smelt the melt, etc. People have to ship all these materials around. There are probably ultimately hundreds of people involved in making a simple pencil.
You could do it if you had some huge organization that can draw resources from a large number of places and people, and then operate in secret without having to account to all these people for what they're doing. Like a government.
