The next big growth engine could and should, if the Congress were to choose to emprace it, be a large infrastructure and public works budget (especially given the current low interest rate environment, and the large number of people unemployed and underemployed). Our bridges and highways and air traffic control system could really use it. Of course, keeping the waste, fraud, and abuse in that budget down to a minimum would be difficult, given current Congresspeople and their priorities and agendas.

I nominate cleantech, energy efficiency, and renewable energy.

Over the next 30ish years, the entire globe will have to transition from fossil fuels to sustainable energy. The amount of growth that needs to happen is probably bigger than what we experienced in the internet boom. Already in California, green jobs outnumber Hollywood jobs, and solar is now profitable without subsidy.

These are the areas that are going to have the largest growth and innovation over the next few decades. The next Google is going to be an energy company.

I don't know how you can qualify this as growth. The transition to renewable energy should be applauded, but at the end of the day it is a displacement of the fossil fuel industry, which is massive. It seems less like a wealth creation engine and more like a "we will destroy the earth otherwise" necessity.

I actually think the the Block-chain could be such growth engine.

It basically holds the promise to potentially take out any middlemen from server to bank, from company to nation which in itself would cause a major shift in the economy.

Although Google has done a much better PR around its research efforts, IBM and Microsoft are both investing much more in R&D. Microsoft is the tech company with the most research investment.

I'd caution using R&D investment as a metric. Historically, it seems the huge game changers (Google, Apple, Facebook, etc.) came from motivated individuals and not corporate research labs. Microsoft's early and enormous investments did not help them catch/contribute to the search/social/tablet/mobile wave which generated much of the value in the tech economy the past decade.

What about, say, the solid-state transistor?

The massive catalog of Bell Labs innovations came from a heavily-regulated monopoly, not a half-forgotten research arm of a software or advertising company whose mission can best be summarized as "Hire smart people so the competition doesn't get them."

Why the Bell Labs approach worked so well while most such heavily-subsidized and centralized research efforts fail is something I don't entirely understand. It could be because their charter was centered on the improvement of telecommunications in general. In the twentieth century, that covered endless acres of fertile intellectual ground. To extend the cliche', there was a lot of low-hanging fruit on the physics trees, and once it was picked, that was it for Bell Labs.

This is an important point. There are times when confidence in your future enables a longer term investment horizon. And from that good things may often follow. But neither that confidence nor the good things may be enough to keep any social/economic position in perpetuity.

I'm not trying to be snide but imo hoping the economy will improve by someone finding a growth engine is wishful thinking. I would like to see tax policy and government spending formulated for sustainability based on domestic consumption and have any potential innovation income as windfalls.

Hinging the economy on the creation of new grown engines is a risky strategy and I don't think the government should be in the business of making bets, at least at the macro economic scale (hell yeah on research spending).

I think the most likely next big growth engine will be drones. However the opportunity will probably go to China, who will make up for lack of innovation with deregulation, meanwhile western countries will be too anxious about terrorism to allow drones to fly about their cities/countryside making deliveries, spraying crops, etc.

That's easy: cheap energy, particularly thorium reactors. Driving down the cost of energy creates a defacto boom, not to mention the industry to build out such infrastructure.