Think of a heat engine as a runner sprinting through hot and cold air — once they pick up speed, the temperature feels different. Quantum particles acting as the working fluid also begin to 'see' heat in their own way when moving at near-light speeds. This sensory trick is a consequence of Einstein’s special relativity.
In a standard heat engine, efficiency depends on the temperature difference and can’t exceed the Carnot limit. But if quantum probes race close to the speed of light, a hot field appears even hotter to them, and a cold one turns icier. Scientists built a model with simple DeWitt detectors and showed that the right acceleration boosts useful work and surpasses the classical limit. Speed itself becomes fuel.
In interstellar space, such an engine could recharge from the cosmic microwave background, the cooled light of the Big Bang, simply by moving at high speed.
The idea of harvesting energy from the vacuum isn’t new: it pops up in Stanisław Lem’s 'Star Diaries' and other sci-fi stories.