The S&P 500 is beginning to look like a massive bet on artificial intelligence. That makes me nervous.

Billions are pouring into data centres, chips, power infrastructure and ever more powerful AI models. The sums are too big for my small brain. So I asked ChatGPT whether AI was in a bubble and today's frenzy could end in the S&P 500 crashing, say, 50%.

Is it daft to ask AI if AI is in a bubble? Possibly. I'll let you decide that. Here's its response: "Yes, the S&P 500 could crash 50%. But that doesn't mean it will."

The chatbot went onto say there are reasons to worry. "US tech giants Google, Amazon, Microsoft and Meta have collectively invested more than $1.1trn in capital expenditure since the AI boom began, with around $745bn planned for 2026 alone."

All investors should be worried

Today, US tech stocks make up almost 40% of the S&P 500. Once related industries are included, some put that as high as 50%. So the wider market is at risk.

But is it really a bubble? AI said: "A classic bubble is built on excitement, speculation and companies that don't make any money. AI doesn't quite fit that description."

On Wednesday, US chip-maker Nvidia dazzled again. Q2 revenues more than doubled to $96.2bn and it's forecasting another 70% increase next year. The hyperscalers are making money too. Microsoft, Alphabet and Amazon are seeing their cloud businesses benefit from rising AI demand.

ChatGPT said: "This isn't the dotcom bubble, where investors were piling into companies with little more than a website and a dream."

Valuations can still become excessive though. The more money that pours into AI, the more spectacular returns need to be to justify it.

Other factors could burst its bubble, such as rising inflation and interest rates. Even if AI conquers all, there will be winners and losers among the hyperscalers. Some could suffer massive losses.

Many investors, including me, are already heavily exposed to US tech. That's why I'm looking more closely at FTSE 100 shares today.

Check out NatWest shares

I particularly like NatWest Group (LSE: NWG). Its shares are up 216% over five years, and 30% in the last 12 months. It's also paid generous dividends on top. The trailing yield today is 4.8%.

Obviously, banks would also get caught up in a wider stock market crash. After such a strong run, NatWest shares may slow of their own accord. Yet the company is still making good money. Q2 results (31 July) showed operating profit up 29% to £2.3bn, beating the £2bn expected.

NatWest looks financially strong, with a CET1 ratio of 13.2%. The price-to-earnings ratio is a modest 9.98. That looks much better value than many of the US technology giants. And if interest rates do climb, that will allow it to widen its net interest margins, and make even more profit. However, that could attract higher UK windfall taxes.