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Since the Sun rises late in the fall season, my early morning jog today started in complete darkness with stars sprinkled overhead. The stars appeared like remote city lights. When looking up, I wished gravity did not keep my feet grounded to Earth, enabling me to continue my jog all the way to the stars.  This would have been possible if gravity was repulsive.

According to Albert Einstein’s equations of General Relativity, gravity is sourced not only by mass or its equivalent energy but also by pressure. In a uniform medium, the strength of gravity is dictated by the energy density plus three times the pressure. For radiation, the pressure is a third of the energy density, so gravity is doubled compared to cold matter of the same energy density.

But the vacuum is qualitatively different from matter or radiation. It has a negative pressure with a magnitude that is equal to its energy density. As a result, its energy density plus three times its pressure induces repulsive gravity. If Isaac Newton sat under an apple tree in a gravitational background dominated by the vacuum, the apple would rise up instead of falling down. Moreover, the apple’s speed would keep increasing as it moves up all the way to the speed of light.

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