An uncertainty relation exists between energy and time; this property is measured in Joules.

• A. Energy
• B. Momentum
• C. Mass
• D. Temperature

Answer: (A)

The key idea is that energy and time form a fundamental pair in quantum mechanics: the uncertainty in energy and the uncertainty in the time interval over which you measure are linked by a relation that looks like ΔE Δt ≈ ħ/2. Since energy is what carries the SI unit joules, the property whose uncertainty is constrained here is energy. The other options don’t fit this specific relation: momentum is linked to position in its own uncertainty principle, mass isn’t the quantity tied to a time-related uncertainty, and temperature is a thermodynamic quantity, not the quantum observable described by this energy–time trade-off.