How time travel to the future is possible (Easy Explanation)

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Back in the day, Galileo came up with the “Galilean Equivalence Principle” (GEP) which states that all the laws of physics work exactly the same, regardless of how fast you’re moving, or indeed whether or not you’re moving. (Acceleration is a different story. Acceleration screws everything up.) What Einstein did was to tenaciously hold onto the GEP, regardless of what common sense and everyone around told him. It turns out that the speed of light can be derived from a study of physical laws. But if physics is the same for everybody, then the speed of light (hereafter “C”) must be the same for everybody. The new principle, that the laws of physics are independent of velocity and that C is the same for everybody, is called the Einstein Equivalence Principle (EEP). Moving faster makes time slow down: I’ve found that the best way to understand this is to actually do the calculation, then sit back and think about it. Now, if a relativistic argument doesn’t hinge on the invariance of C, then it isn’t relativistic. So ask yourself “What do the speed of light and time have to do with each other?”. A good way to explore the connection is a “light clock”. A light clock is a pair of mirrors, a fixed distance d apart, that bounce a photon back and forth and *clicks* at every bounce. What follows is essentially the exact thought experiment that Einstein proposed to derive how time is affected by movement.

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Let’s say Alice is holding a light clock, and Bob is watching her run by, while holding it, with speed V. Alice is standing still (according to Alice), and the time, \tau, between ticks is easy to figure out: it’s just \tau = \frac{d}{C}. From Bob’s perspective the photon in the clock doesn’t just travel up and down, it must also travel sideways, to keep up with Alice. The additional sideways motion means that the photon has to cover a greater distance, and since it travels at a fixed speed (EEP y’all!) it must take more time. The exact amount of time can be figured out by thinking about the distances involved. Mix in a pinch of Pythagoras and Boom!: the time between ticks for Bob. So Bob sees Alice’s clock ticking slower than Alice does. You can easily reverse this experiment (just give Bob a clock), and you’ll see that Alice sees Bob’s clock running slow in exactly the same way.

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It may seem at first glance that the different measurements are an illusion of some kind, like things in the distance looking smaller and slower, but unfortunately that’s not the case. For Alice the light definitely travels a shorter distance, and the clock ticks faster. For Bob the light really does travel a greater distance, and the clock ticks slower. If you’re wondering why there’s no paradox, or want more details, then find yourself a book on relativity. There are plenty. Or look up Lorentz boosts. (The very short answer is that position is also important.)

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