# A Simple Time Travel Question

## Main Question or Discussion Point

Good Afternoon,

After watching a TV program "Into the Universe" "Time Travel " I have some doubts left.

I know Time Dilation has been proven by experiments (Atomic clocks on jet plains) and we see it every day that GPS system needs time correction (Due to Time Dilation perdicted in SR). But the question is, Can this be a instrumental error? Do we need right type of clocks?

I read every where, that to stay younger just hitch a ride on a very fast moving object/train/ship etc.

According to the TV show, if scientists could build a train track around the globe, And that train could travel close to the speed of light. After travling for a week on this train the passangers will travel thousands of years in future. WOW! But I have a problem with this.

Now suppose I am also on this train and the track runs through my city in BC. And I don't have any clock on me to track time, but I will be counting days just by looking at the Sun set when train passes through my city. Now I don't care if someone's wrist watch could not keep up with the time on Earth, because of the fast motion. For me seven days have passed, and the same is true for the people on the Earth. So I did not travel in the future. No Time Dilation.

I don't think running around at the speed of light will make Sun rise/set slower or faster.

Can somebody please explain what I'm missing. Thank you.

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I know Time Dilation has been proven by experiments (Atomic clocks on jet plains) and we see it every day that GPS system needs time correction (Due to Time Dilation perdicted in SR). But the question is, Can this be a instrumental error? Do we need right type of clocks?
No, it cannot be instrumental error, because its is consistent between many different versions of many different types of instruments and methods of measuring the effects. It is just about as certain as the world being round.

Now suppose I am also on this train and the track runs through my city in BC. And I don't have any clock on me to track time, but I will be counting days just by looking at the Sun set when train passes through my city. Now I don't care if someone's wrist watch could not keep up with the time on Earth, because of the fast motion. For me seven days have passed, and the same is true for the people on the Earth. So I did not travel in the future. No Time Dilation.
Your turning the question into a semantic one, instead of a physical one. If by 'time' you mean the number of sunrises that pass, then no---time is the same in both reference frames. If instead, you mean how much have your cells aged, how much time have you experienced, how many radioactive nuclei will have decayed, how many messages could you have sent to someone, etc etc then the observers experience entirely different amounts of time.

Does that make sense?

Dale
Mentor
I know Time Dilation has been proven by experiments (Atomic clocks on jet plains) and we see it every day that GPS system needs time correction (Due to Time Dilation perdicted in SR). But the question is, Can this be a instrumental error? Do we need right type of clocks?
We currently know of 4 fundamental forces: electromagnetism, the strong force, the weak force, and gravitation. All 4 of them exhibit time dilation, so no, it is not an instrumental error, it is a symmetry of nature. Unless some new force is found which does not have the same symmetry or unless the symmetry breaks down at some very high energy then time dilation is not a question of bad instrumentation.

For me seven days have passed, and the same is true for the people on the Earth. So I did not travel in the future. No Time Dilation.
This is incorrect, the same is not true for the people on Earth. This exact experiment has been done with muons in a containment ring instead of people on a train.

Btw, sunrise and sunset is a bad method since the sun will rise and set every time you go around the earth. At near light speed the sun will rise and set literally hundreds of times in the blink of an eye.

It is just about as certain as the world being round.

True, It appears so.

If by 'time' you mean the number of sunrises that pass, then no---time is the same in both reference frames.
If time is the same in both reference frames, Then what happened to the Time Dilation?

If instead, you mean how much have your cells aged, how much time have you experienced, how many radioactive nuclei will have decayed, how many messages could you have sent to someone, etc etc then the observers experience entirely different amounts of time.

Does that make sense?
Sorry I don't undersatand this, So can we say that I will breath less, age less, send less messages in these 12 hrs?

If by 'time' you mean the number of sunrises that pass, then no---time is the same in both reference frames.
If time is the same in both reference frames, Then what happened to the Time Dilation?

Sorry I don't undersatand this, So can we say that I will breath less, age less, send less messages in these 12 hrs?
What we perceive as time IS dilated---it IS different in those two reference frames. Yes, you will breathe less, age less, etc.
But you are correct that you will observe the same number of sunrises/sunsets---but for you those sunrises will occur at a much different rate than the 'stationary' people on earth.

I was trying to say that you were bringing up a question of how we define 'time.'

We currently know of 4 fundamental forces: electromagnetism, the strong force, the weak force, and gravitation. All 4 of them exhibit time dilation, so no, it is not an instrumental error, it is a symmetry of nature. Unless some new force is found which does not have the same symmetry or unless the symmetry breaks down at some very high energy then time dilation is not a question of bad instrumentation.

This is incorrect, the same is not true for the people on Earth. This exact experiment has been done with muons in a containment ring instead of people on a train.
Let us try this with the Hadron Collider, I'm in the collider spinning at near light speed, And there is a glass window so that I can see the sky and the passage on the Sun, Now according to you, I shall see Sun passing in the sky rapidly?

Btw, sunrise and sunset is a bad method since the sun will rise and set every time you go around the earth. At near light speed the sun will rise and set literally hundreds of times in the blink of an eye.
I know this, but we will stop the train and get off, after seeing seventh sunset in my city,

I think I'm getting what are trying to say, that All of this will happen very fast to the passangers on the train.? So the time dilation is indeed there. Thank you so much for clearing the doubts.

Dale
Mentor
Let us try this with the Hadron Collider, I'm in the collider spinning at near light speed, And there is a glass window so that I can see the sky and the passage on the Sun, Now according to you, I shall see Sun passing in the sky rapidly?
Yes.

I know this, but we will stop the train and get off, after seeing seventh sunset in my city,

I think I'm getting what are trying to say, that All of this will happen very fast to the passangers on the train.? So the time dilation is indeed there. Thank you so much for clearing the doubts.
Exactly. Although both the train and the city will agree that there were only 7 sunsets in the city from the passengers on the train will have only breathed in and out a couple of times and they will not have gotten hungry during those 7 sunsets in the city, whereas the people in the city will have breathed about 100,000 times and have eaten and slept several times.

What we perceive as time IS dilated---it IS different in those two reference frames. Yes, you will breathe less, age less, etc.
But you are correct that you will observe the same number of sunrises/sunsets---but for you those sunrises will occur at a much different rate than the 'stationary' people on earth.
Thank you, Now I understand, how this time dilation works.

I was trying to say that you were bringing up a question of how we define 'time.'
Defining time is easy, there is no proper time in this universe, because every thing is in motion relative to each other, except in the middle of the universe, where it all began.

Thank you again for your help.