|I heard some time ago about a car that uses microwaves to heat the air in front of it so that it creates a vacuum. The relatively higher pressure behind then pushes it forward. Is this possible? — RM, Toronto, Ontario
Even if microwaves were effective at heating air, which they are not, this heating would not propel the car forward. The air in front of the car would become hot, but its pressure would remain almost unchanged. Instead, the air would expand to occupy a larger volume and would then be lifted upward by the cooler air around it ("hot air rises"). Cooler air would flow in to replace the escaping hot air and the car would simply sit there with a steady stream of hot air rising in front of it.