Pop-up Planetarium
Folding of Time
From the Big Bang to the present day, the universe has undergone billions of years of evolution, giving rise to galaxies, stars, and planets.
Time Dilation
According to Einstein's theory of relativity, time can be stretched or compressed near massive objects, leading to phenomena like gravitational time dilation.
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​In the world of classical physics, an inch is an inch, and there's no debate about that. In the world of relativity, however, this concept has changed. The same length measured in a stationary coordinate system is the longest, while in a relative motion coordinate system the object becomes shorter.
Imagine that a teacher holds a 10 cm long ruler that has no relative motion with the teacher, so the length of the ruler appears to a stationary observer to be 10 cm. But if a student runs past the teacher with velocity v, the ruler appears to the student to be shorter than 10 cm. Putting it into the formula, l is the length of the ruler as seen by the teacher, and l' is the length of the ruler as seen by the student. v is the velocity of B's motion. v is the velocity of B's motion, and c represents the speed of light.
Similarly, time dilation means that time goes faster in a stationary coordinate system. For example, if an astronaut tosses a pencil in the space station, with the entire toss taking time t. If my relative velocity to yours is v, then the time taken for the toss is t' from my stationary perspective. . Then t' must be greater than your observed t. That is, it seems to me that your pencil is flying very slowly. But note that here you and I are also relative to each other. So when I toss a pencil, you will also think my coin flies very slowly. Nevertheless, this whole process will happen simultaneously only when you and I are at the same spatial point. As long as we have a distance and a relative speed in between, we will have a different view of the sequence of events.
Therefore, when an object moves close to the speed of light, time around the object will rapidly slow down and space will rapidly shrink. When the speed of motion of the object is equal to the speed of light, time will stop and space will shrink to a singular point. When the speed of motion goes over the speed of light, time, in theory, starts to go backward. However, only objects without mass, such as photons (light particles) can reach the speed of light, so you don’t have to worry about shrinking down to nothing anytime soon.
Time Travel and the Twin Paradox
Imagine that you wake up in a spacecraft traveling at a subvelocity of light. A few years ago, on the eve of your 32th birthday, your wealthy twin brother planned this gift for you. You were ecstatic and couldn't wait to see the stunning scenery of space and enjoy the feeling of weightlessness. So on your 32th birthday, you started this amazing journey. And now, the ship you are on is approaching Earth at 0.99 times the speed of light. The atomic clock on the spacecraft is accurately recording the precise time. You look up through the window and see that the stars become more dazzling or clear. The visual impact is simply wonderful. Looking at the azure planet coming closer to you, your heart pounded so fast. You can't wait to meet your family after such a long trip. The ship landed safely. When you opened the hatch, you had mixed feelings in your heart. After inspections and evaluations from Space Z, you return home. An elderly man embraces you with tears in his eyes, crying: "My dear brother, welcome back!" After several years of near light-speed flight, you are still a young man. But your twin brother has turned into a gray-haired old man. You found out that you traveled to the future.
