Name: Einstein’s Special Relativity Theory | Does Time really Slow down
Uploaded: 2024-11-25T21:00:39.000Z
Duration: 13 min 15 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

To us, it appears that time doesn't stop or slow down for anyone in this universe.

It's always running forward without waiting for anyone or anything.

Adverbs of degree

Special relativity theory tells us that time is really not like this.

Time is not the same for all across the whole universe.

Different observers feel different times at different places.

Einstein's special relativity theory starts with two basic ideas.

The first postulates that the laws of physics are the same in all frames of reference.

Imagine your friend is standing at the platform of a train station and you are in a train traveling at a constant velocity.

Constant velocity means an object is moving in a fixed speed and fixed direction.

This motion doesn't have any acceleration, deceleration, and does not change direction.

From his perspective, he is at rest and you are heading forward.

But from your perspective, who is traveling at a constant speed, does not feel any motion.

The rest of the world can appear to be running towards you.

If the train windows are closed, there is no outside reference point.

As all the laws of physics will apply to both your friend and you, both of you can be considered to be at rest.

To explain more simply, let's take a look at a mosquito in a flying plane.

Now that the mosquito is sitting on the back seat, it wants to fly to the pilot room.

If the laws of physics is not the same for a moving plane, it must travel above the flight speed.

But there is nothing like that and the mosquito feels at rest on a moving plane.

So the normal effort is enough to reach the pilot's room.

Now let's come back to the train scenario.

You are in a moving train and your friend is on the platform.

Whatever your friend is doing on the platform, you can also do in the moving train.

However, the speed is not the same for all observers.

Let's say the train is moving at a constant speed of 100 km per hour and you throw a football at the speed of 20 km per hour.

From your perspective, the ball is traveling 20 km per hour because you are at rest inside the train.

But from your friend's perspective who is on the platform watching the train, he feels the ball is traveling at a whopping 120 km per hour.

Let's say you have another friend who is running on the track towards your train at 10 km per hour.

Now if he measured the ball speed, he would judge it to be 130 km per hour.

How can the same moving object be showing different speeds?

Because speed is relative, it can vary from one to another.

Different observers can measure different speeds on the same moving object.

Special Relativity's other postulate is that the speed of light is the same for all observers.

The speed of light is constant and it always shows the same value for all frames of reference.

You and your friend are standing opposite each other.

He has a flashlight and your friend flashes the light.

If you could measure the speed of light quick enough, you would measure it at around 300,000 km per second.

No surprises yet, as you are both at rest.

But now you are running towards your friend at the phenomenal speed of 10,000 km per second and he flashes the light.

You would measure it still showing the same value of 300,000 km per second instead of showing 310,000 km per second.

This is due to the fact that the speed of light is constant.

It's the same for all the observers, no matter where that observer is at rest or in motion.

In order to maintain the constant nature of the speed of light, space and time experience weird features.

Even today we still don't know exactly what time is, but we do have some basic understanding.

We've all agreed that all the clocks in our world tick at the same rate.

We've all agreed on how long some things take to happen and what things happen at the same moment.

These basic things of time we experience in our everyday life.

But the constant nature of light says that it's all wrong.

Imagine there are two nations, nation A and nation B, and they want to sign an agreement.

The problem is that they both want to sign the agreement at the same moment, and they found the procedure too.

The procedure is that the leader of both nations need to sit at the opposite sides of the table, with a light bulb fixed over the middle of the table.

Turn on the light bulb, and when the light reaches both leaders' eyes, they can both sign simultaneously, because they already know that light always travels at the same speed.

Both nations' leaders decide to sit exactly the same distance from the bulb.

They turn on the light, and the light reaches both leaders at the same moment.

The agreement is signed simultaneously by leaders, and everyone is happy with the result.

A few months later, they decide to sign another agreement, but this time, both leaders want to do it differently.

Even though they have many different options, both leaders have an extreme love of trains.

So they set up the same scenario in a moving train.

The light flashes and goes to each leader, reaching them simultaneously.

They sign, and again everybody in the train is happy with the result.

But the people standing on the platform started fighting, because the people from nation B complained that nation A leader signed the agreement first.

How did they come up with that conclusion?

So they saw that the flash first reached leader A, because he was moving towards the flash.

They already know that light always travels at constant speed.

Leader B was moving away from the flash, so it took a longer time to reach leader B compared with leader A.

From the perspective of those watching the event from the platform, both leaders didn't sign at the same moment.

But from the leader's perspective, who were in the moving train at constant velocity, claimed that they signed at the same moment.

We already know that when an object travels at a constant speed from the perspective or frame of reference, it can claim it is at rest.

Now let's move on from the platform perspective.

We will draw a line when the flash takes place, and draw a line when the flash reaches leader A versus leader B.

So from this line, we can clearly see that the flash reaching leader B needs to travel a longer distance compared to leader A.

If it has to travel further, it takes a longer time.

The leaders inside the train or the people watching from outside?

The constant nature of speed of light means that the events take place at the same time even though from the perspective of one group of people, the events don't take place at the same time as from the perspective of other groups.

One group of people don't realize any time difference and the other group of people witness a time difference on the same event.

The thing that happened from one person's perspective does not happen at the same moment as from the other person's perspective if they are in relative motion.