Rampant enthusiasm is buoying tech shares to levels that defy gravity. Invest with caution, our columnist says.

© Janet Mac

Strong investor interest in SpaceX could be good news for the A.I. rivals, which have signaled that they, too, intend to go public this year.

If you have index funds — such as in a 401(k) — they may soon include shares of the mega company. Here’s a rough calculation.

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