Can you travel into the future? To answer this question, we need to understand the concept of time relativity. According to Einstein, time is relative. Every object has its own time. The rate at which time passes for me might be different from the rate at which time passes for you.

### What is Relativity?

Imagine you are inside a flying airplane. For someone standing on earth, he will see that you are moving along with the airplane at a particular speed. While from your perspective, you are actually at rest. This might be counterintuitive because you know you are definitely moving along with the airplane. However, we are talking about the airplane's point of view, not the earth's point of view. Try to close your eyes when you are inside an airplane. You will feel that you are not moving at all.

Another example: suppose there are two cars, A and B, moving in opposite directions at the same speed. Someone on the ground can observe that A and B are moving at the same speed, let's say 40 km/h each. But, since they are moving in the opposite direction, someone in car A will feel that car B is moving at 80 km/h. The speed of car A has been added to the speed of car B. The same thing applies for observers in car B when they see car A.

From both cases, we know that speed or motion is relative to the frame of reference. Then, how does time relativity work? Can time become slower or faster in different frames of reference?

### The Photon Clock

The relativity of time can be demonstrated using a photon clock. A photon clock is a clock that consists of two parallel mirrors with a photon particle (a light particle) bouncing back and forth between them.

Are they experiencing any difference in time? Well, both of them obviously feel that time hasn't changed at all. For Alice, one second feels like one second. She can see her clock is moving normally at a constant speed. Inside the spacecraft, Bob can also feel that time is passing normally, just like when he was inside an airplane or when he was standing on earth. He can also see that nothing out of the ordinary has happened to his clock. At this point, time seems to be the same for everyone.

But now, Alice starts to observe Bob's photon clock. She can actually see something different. She notices that the photon in Bob's clock has no longer maintained its path. It is not moving straight up and down; instead, it takes a zig-zag path due to the moving spacecraft.

The implication of this motion is that the distance over which the photon needs to travel between the mirrors has become longer.

Recall that when Bob observed his own clock inside the spacecraft, he thought his photon moved at a constant speed, just like when he was on earth. Interestingly, Alice could also see Bob's photon moving at the same constant speed. That is because the photon is moving at the speed of light. According to Maxwell's equation, the speed of light is a universal constant, 299,792,458 m/s. Whichever direction the light moves and whoever observes it, the speed still remains, and nothing can move faster.

This behavior is different from what we understood earlier from observing moving airplanes and cars: that speed is relative to the frame of reference. If photons were not moving at the speed of light, Alice would see Bob's photon move faster than hers because of the relativity of motion. The speed would be fractionally increased by the speed of the spacecraft. But that can't be the case for the speed of light; it can't get any faster. It is the speed limit of any physical object in the universe, regardless of who observes it.

In summary, Bob's photon now needs to travel a longer distance between mirrors while the speed remains constant. What is the consequence of it? The photon requires more time to travel between mirrors. Thus, his clock has become slower. Time has actually slowed down for Bob.

### Time Dilation

This phenomenon is known as time dilation. Any moving object experiences time dilation. The faster an object moves through space, the slower it moves through time. We can imagine that a faster speed will produce a longer distance for the photon in the photon clock to travel, hence the longer time needed to travel between mirrors.

This is a remarkable consequence of the universal speed of light. Time needs to bend in order to accommodate the constant speed of light in all directions and all frames of reference. Please note that time is not bent by the light but by the moving object. It doesn't matter whether you don't bring a photon clock with you or don't have any light inside your spacecraft; time will still bend.

Several experiments have been conducted by scientists to prove Einstein's time relativity theory. Indeed, they found that time passed slower on moving objects. But on earth, the difference is a matter of nanoseconds. We know that we are nothing compared to the rest of the universe, and everything on earth usually doesn't travel fast enough compared to the speed of light.

If Bob's spacecraft can travel fast enough, he will experience a good amount of time dilation. But, inside the spacecraft, Bob won't notice his time has slowed down. This is because any time-based processes are affected by time dilation. Any clocks, electronic devices, and even biological activities in Bob's body (including his rate of aging) and his perception of time itself have slowed down. But by the time he comes back to earth, he will find Alice has aged faster than him. So, in a sense, he has traveled into the future.

Time dilation can also happen as an effect of gravity, since it accelerates us toward the earth. In fact, the acceleration is getting faster the closer we are to Earth. Because of this difference in acceleration, time passes slower for those closer to earth. As a result, biological processes in our feet are slower than those in our heads. Of course, the difference is so small. But it makes your head slightly older than your feet.

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