However, every single exchange must obey the full suite of quantum rules, and these strong force interaction are time-reversal symmetric: you cannot tell whether the animated movie here is shown moving forward or backward in time. Gluons can not only be exchanged between the individual gluons within a proton or neutron, but in combinations between protons and neutrons, leading to nuclear binding. Individual protons and neutrons may be colorless entities, but the quarks within them are colored. Even the strong nuclear force, which binds atomic nuclei together, is identical forward and backward in time.Electromagnetic interactions behave identically forward and backward in time.Gravitational interactions work the same forward and backward.A purely inelastic collision, where two objects smash into each other and stick together, is exactly the same as a purely inelastic explosion in reverse, where the energy absorbed or released by the materials is identical.A purely elastic collision, like two billiard balls colliding, would behave exactly the same if your ran the clock forward and backward, right down to the speed and angle that the balls will go off at.There are many examples where one phenomenon, if you run the clock forward, corresponds to an equally valid phenomenon if you run the clock backward. Put simply, it says that the laws of physics obey the same rules whether you run the clock forward or backward. One of the most important symmetries in all of physics is known as time-reversal symmetry. ( Credit: MichaelMaggs Edit by Richard Bartz/Wikimedia Commons) While Newton’s laws of motion are the same whether you run the clock forward or backward in time, not all of the rules of physics behave identically if you run the clock forward or backward, indicating a violation of time-reversal (T) symmetry where it occurs. What happens, then, if we put these speculations together?Ī ball in mid-bounce has its past and future trajectories determined by the laws of physics, but time will only flow into the future for us. Still others have speculated that dark energy might evolve over time, rather than being a constant, leaving the door open to the possibility that it might someday counteract and reverse our Universe’s expansion. Many have speculated - although there is no proof - that the arrow of thermodynamics and the arrow of time may be related. And everywhere we look, the entropy of the Universe always goes up.Īs the story of our cosmic evolution continues, we think all of these things will continue: the laws of physics will still apply just as they do today, dark energy’s presence ensures that the Universe will keep on expanding, and entropy will keep increasing, as dictated by the laws of thermodynamics. They change their motion due to the effects of gravity and the other forces. Objects move through the Universe proportional to their velocity. Still, if we look for it, we can find that a number of things also happen to always move in the same direction, from moment-to-moment, exactly the way time does. And yet, it isn’t particularly clear exactly why this is the case. Each successive instant gives way to the next, with time continuously appearing to flow in the same direction - forward - without fail. With each passing moment in the Universe, we’re constantly stepping forward in time.
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