What is Relativity?

Before 1900, many phenomena had no explanation, especially those that occurred at speeds close to those of light. This is where the Theory of General Relativity comes in. This Theory also allows us to grasp understanding of the Fourth Dimension, called spacetime.

The Speed of Light

“What would I see if I move at the speed of light?” wondered Albert Einstein. With that ever-present thought, after more than 10 years of study, he managed to change our view of the universe.

Einstein liked to do thought experiments that he then put into equations, one of which was the famous train car experiment. This consisted of the following:

First Postulate of General Relativity

You have two subjects. Noah* is standing next to some train tracks while Sophia* sits inside the train car. The train moves at 100 km/h. Sophia, inside the train car, decides to throw a ball forward with a speed of 15 km/h.

Noah, outside the train, would see how Sophia throws the ball. He observes a speed of 115 km/h. The speed of the ball (15 km/h) added to the speed of the train (100 km/h) creates this illusion. Obviously, Sophia has not thrown the ball at that speed. She feels only the speed of the throw. Just as if Noah threw the ball, she perceives a speed of 15 km/h.

Each one experiences the laws of physics from their own perspective, Sophia from the train car and Noah from the ground, Sophia does not feel that the train is moving since it is moving at a constant speed, in the same way that Noah does not feel the earth is turning.

This is the first postulate of general relativity: The laws of physics are the same in each reference frame as long as its speed is constant.

Second Postulate of Relativity

Next, suppose that instead of a ball Sophia uses a laser. She points it forward. Remember here that the speed of light is 300,000 km/s.

Based on the above we know that Sophia perceives only the speed of the laser. She will perceive that the speed of her laser is 300,000 km/s. Our intuition tells us that Noah, still outside the train, will measure the speed of the laser (300,000 km/h) plus the speed of the train (100 km/h). Instead, however, Noah will measure exactly the same speed as Sophia, 300,000 km/h. But why?

This is the second postulate of general relativity. The speed of light is an invariant constant. Yes, regardless of the reference frame from which it is measured.

There are several experiments currently and throughout history that demonstrate this fact. The speed of light is always the same. It does not matter if you are at rest or movement.

Space and Time are Inconstant

If Noah measures the speed of the train that passes him, he will measure a speed of 100 km/h. Speed is the distance an object travels during a certain time V=d/t, so Speed equals the distance between time.

But what if Noah decides to move in the same direction as the train? The distance between the train and him is shortened. Therefore the speed of the train decreases in view of him. So, he will measure a lower speed.

BUT what about the light? When measuring the speed of light at rest and in motion, the distances between us and the train change, but the speed remains constant. So, if speed remained constant, what changed was time.

Known as time dilation, time passes slower the faster you move in space, and vice versa. The slower you move in space the faster you move in time, but only with speeds close to light.

This is why it’s said that “Time is relative.” As a result of this discovery we understand that time is not a universal measure. There can be no single clock for all things in the Universe.

This shows that time is actually something personal. Each being, every particle, measures time differently. This happens because time is closely related to the object’s movements in space.

The Fourth Dimension

Thanks to this, we understand time is not universal but rather an actual dimension that can be traversed. Seeing as it alters with the movement of space, it’s also widely understood in the scientific community that time is actually a dimension coupled to the other three spatial dimensions, a fourth dimension called spacetime.

Conclusion

General relativity tells us that:

• Time and space are not absolute.
• Every object in the universe has its own time.
• Time changes with the movement of space.
• Time is one more dimension, the fourth dimension called spacetime.
• Remember that if you want to know more about these fantastic topics you can read more posts like these on our website.

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