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We’re about to see another change near the top of the rankings of the most valuable publicly traded U.S. companies. Apple Inc. (NASDAQ: AAPL) looks secure for a while at number 1 with a market cap of about $463 billion on Wednesday afternoon. But number 2 Exxon Mobil Corp. (NYSE: XOM) is about to be caught — and likely passed — by Google Inc. (NASDAQ: GOOG). Exxon’s market cap is about $416 billion and Google’s is about $412 billion.

Exxon was founded in 1882 as part of the Standard Oil Trust and operated as Standard Oil of New Jersey until 1972 when it changed its name to Exxon. It acquired Mobil Corp., which was founded in 1879 as Vacuum Oil Company, in 1999.

Google was incorporated as a privately held company in 1998 just a couple of years after its founders started working on a new search engine for the worldwide web. The company held its IPO in 2004.

The old and the new, fossil fuel versus the ether. Where oil and the multitude of products that come from it are tangible, a search engine is software and software for all its millions of lines of code is basically something we believe in even though we can’t really see it. A search engine has a virtually infinite well of possibilities.

And while a company like Exxon may seem to have virtually unlimited resources, those resources are in fact limited both by their abundance and by Exxon’s ability to extract the resources at a price that people are willing to pay. Looked at one way, consumers pay for the relative scarcity of oil even though global production last year was more than 89 million barrels a day. Exxon’s share of that was about 2.2 million barrels a day.

As oil prices rise — and they surely will rise to levels we don’t even want to think about — efforts to find alternatives will double and then redouble. At some point that is probably still several decades off, fossil fuels will either be exhausted (unlikely) or remain in the ground forever (likely). There will be alternatives aplenty and oil’s 200-year run as an energy source will be over.

But people will still have to find things and for all the changes that technology will bring to how we go about finding things, it’s a fair bet that Google will remain at the pinnacle of the search world. It may not be the best or the fastest or whatever, but it will have so permeated everything that people will be using it even when they aren’t aware of doing so.

Google handles more than 100 billion search queries a month and about 15% of daily queries have never been seen by the search engine before. Advertisers pay for space that is essentially limitless, and users pay for all this abundance by giving up information about themselves. There’s a limit to how much information users will be willing to give up, of course, but we’re not there yet.

Abundance versus scarcity. There’s really no limit to what Google could be worth. Exxon’s value is tied to a physical world that has limits. That’s the difference.