Einstein's Fridge: How the Difference Between Hot and Cold Explains the Universe

by Paul Sen

To follow Hawking’s exact reasoning is tricky. For a rough intuition for what he did, we must consider one of the most outlandish consequences of the famous “uncertainty principle” of quantum physics—something known as “vacuum energy.” As the name implies, far from being inert, the vacuum is seething with activity. At any instant, bursts of energy appear from nowhere by borrowing equivalent bursts of energy from a tiny instant in the future. Mostly, we are unaware of these fluctuations because the positive burst of energy that appears at one instant is cancelled out by the negative burst that immediately follows it. Negative energy is a strange concept, but it does exist! These energy bursts can appear in many forms. They can appear as particles, such as electrons and positrons, and as photons of electromagnetic energy. Hawking surmised that on and just outside the event horizon of black hole this “cancelling out” is disrupted. The extreme curvature of space and time here means some of the negative energy created is torn away from the positive energy that it normally would have annihilated. This positive energy, having survived, is free to radiate away from the black hole. The negative energy falls into it. And because it’s negative energy, it has the effect of making the black hole less massive.