Offer your thoughts or opinion on the input financial query or topic using your financial background.

How (or is it necessary) to rebalance a 401k with only one index fund?
There's nothing to rebalance, the index fund rebalances itself to continue matching the index. However, you need to understand that such an investment is not diversified and you only invest in a very specific market, and very specific stocks on that market. S&P 500 is large (500 different companies, most of the time), but still not as broadly diversified as your retirement investment portfolio should be. You should talk to a financial adviser (CFP for example), many companies provide access to these for 401k plan participants. But in any case, I'd suggest considering "target date" funds - funds that are investing based on your expected retirement year, and become more conservative as you get closer to that year.