**Where Time Breaks Down**

A thought experiment of Einstein. **The Photon Clock**.

Imagine a clock made form two mirrors and a photon bouncing between them.

Every back and forth bounce of the photon results in a tick of the clock.

The tick rate depends on the speed of the light , and on the distance the photon has to travel between the mirrors. The rate of tick is consistent. Time flows smoothly until the clock starts moving relative to me.

Now imagine moving the mirror sideways.

From my perspective, light now takes a longer diagonal path. But here’s the weird thing.

As we know that speed of light is what we call invariant, so from my point of view, the photon takes a longer time to make the up-down journey.

It appears to travel more distance, but at the same old speed.

**Do you know how one can look, say, 10 years into the past?**

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