# B Time Dilation dialogue

#### DavidMCR

Summary
Understanding the Time Dilation concept
I'll confess, I joined this forum solely to understand the reasoning behind the concept of time dilation, and to see if it is correct or not. If anyone could help me understand it better, and converse with me, I'd greatly appreciate it.

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#### phinds

Gold Member
I'll confess, I joined this forum solely to understand the reasoning behind the concept of time dilation, and to see if it is correct or not. If anyone could help me understand it better, and converse with me, I'd greatly appreciate it.
Time dilation is a very simple consequence of the fact that the speed of light is the same in all inertial frames of reference. It is something that you observe in objects moving relative to you, never something that you "feel" yourself or that happens to you directly.

What readying have you done and what is it that you don't understand?

#### Nugatory

Mentor
There are three related phenomena here: time dilation, length contraction, and relativity of simultaneity. All three are involved any time that two observers are moving relative to one another (for example, a spaceship flying away from earth) - it's a package deal.

My preference is to start with relativity of simultaneity; it's easier to explain time dilation and length contraction after you understand RoS than the other way around.

Do not confuse yourself by taking on the twin paradox (leave earth on a fast-moving spaceship; turn around and come back; on return you are younger than the twin you left behind) or time dilation caused by gravity. Those are different phenomena than basic time dilation and you don't want to take them on yet.

#### lomidrevo

I agree with the above replies. My suggestion would be, to read this Feynman lecture first. Whatever questions you might have after, I am sure people here would be happy to help you.

#### DavidMCR

Time dilation is a very simple consequence of the fact that the speed of light is the same in all inertial frames of reference. It is something that you observe in objects moving relative to you, never something that you "feel" yourself or that happens to you directly.

What readying have you done and what is it that you don't understand?
So, basically I don't understand why "time" is meant to slow down. Honestly I've not read a lot on it, so you'll have to excuse my ignorance.

What has the speed light travels at got to do with time?

#### DavidMCR

There are three related phenomena here: time dilation, length contraction, and relativity of simultaneity. All three are involved any time that two observers are moving relative to one another (for example, a spaceship flying away from earth) - it's a package deal.

My preference is to start with relativity of simultaneity; it's easier to explain time dilation and length contraction after you understand RoS than the other way around.

Do not confuse yourself by taking on the twin paradox (leave earth on a fast-moving spaceship; turn around and come back; on return you are younger than the twin you left behind) or time dilation caused by gravity. Those are different phenomena than basic time dilation and you don't want to take them on yet.
Ok thanks,

See, when I hear the idea that someone ends up younger upon returning from their spaceship journey compared to those on earth, I can't but help think it's lies.

The way I look at it (and admittedly from a non-physicist pov) is everything happens concurrently; therefore the only way someone becomes "younger" is if they were to be "removed" from this physical universe, temporarily.

#### DavidMCR

I agree with the above replies. My suggestion would be, to read this Feynman lecture first. Whatever questions you might have after, I am sure people here would be happy to help you.
Cheers

#### Orodruin

Staff Emeritus
Homework Helper
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2018 Award
The way I look at it (and admittedly from a non-physicist pov) is everything happens concurrently; therefore the only way someone becomes "younger" is if they were to be "removed" from this physical universe, temporarily.
You certainly need to stop thinking like this. It is blocking your progress in understanding how the Universe actually works. The bottom line is that the Universe does not care how you look at things or what you find intuitive, in particular when what you find intuitive is not how the Universe works.

See, when I hear the idea that someone ends up younger upon returning from their spaceship journey compared to those on earth, I can't but help think it's lies.
That would be because you do not have an understanding of what time is and how it relates to the relativistic spacetime. The view that "everything happens concurrently" is obsolete and will unfortunately not get you anywhere. In relativity, as has already been stated here, you have to get rid of "simultaneous" or "concurrently" as a physically meaningful concept.

#### Dale

Mentor
to see if it is correct or not
For this, the place that you should start is the experiments that demonstrate time dilation. See section 4 here:

Time dilation is an experimentally validated fact of nature. It is best to start your studies with that understanding. That way you are more willing to confront your own misconceptions rather than assume deception.

#### DavidMCR

You certainly need to stop thinking like this. It is blocking your progress in understanding how the Universe actually works. The bottom line is that the Universe does not care how you look at things or what you find intuitive, in particular when what you find intuitive is not how the Universe works.

That would be because you do not have an understanding of what time is and how it relates to the relativistic spacetime. The view that "everything happens concurrently" is obsolete and will unfortunately not get you anywhere. In relativity, as has already been stated here, you have to get rid of "simultaneous" or "concurrently" as a physically meaningful concept.
Why is that?

Is it fair to say then that you don't recommend delving into it before learning those topics recommended above?

#### Orodruin

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Why is that?
Which part? It is difficult to tell exactly what you are asking about when you quote the entire post.

#### DavidMCR

Which part? It is difficult to tell exactly what you are asking about when you quote the entire post.

Fair point

"In relativity, as has already been stated here, you have to get rid of "simultaneous" or "concurrently" as a physically meaningful concept."

#### DavidMCR

For this, the place that you should start is the experiments that demonstrate time dilation. See section 4 here:

Time dilation is an experimentally validated fact of nature. It is best to start your studies with that understanding. That way you are more willing to confront your own misconceptions rather than assume deception.
Thanks for the link. Look I know this might infuriate some as it looks like pig-headness, but take I can't but help think there is something off with the concept. Maybe these experiments will help me to change my mind. So again, excuse my contention, and I'll look at the link with an open mind.

Thanks again.

#### Orodruin

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2018 Award
Fair point

"In relativity, as has already been stated here, you have to get rid of "simultaneous" or "concurrently" as a physically meaningful concept."
Essentially because the concept of simultaneity is observer (or, more accurately, convention) dependent. What is simultaneous for one observer is not necessarily the same as for another observer. Einstein's classic example with a train and some lightning rods gives you the first hints as to why this is so. It is a direct consequence of the experimental fact that the speed of light is invariant. No, it is typically not intuitive for most people, but most people's intuition is based on everyday situations where speeds are much smaller than the speed of light.

#### Nugatory

Mentor
See, when I hear the idea that someone ends up younger upon returning from their spaceship journey compared to those on earth, I can't but help think it's lies.
You are mistaken - it is true. The experiment has been done (with airplanes instead of spaceships) and the effect is very small but measurable. The principle is further tested with the clocks on the GPS satellites every day - if we did not allow for time dilation effects the GPS system would not work.

You might also want to take a look at the sticky thread at the top of this subforum: https://www.physicsforums.com/threads/faq-experimental-basis-of-special-relativity.22903
The way I look at it (and admittedly from a non-physicist pov) is everything happens concurrently;
And this is one of the reasons that I suggested you start with relativity of simultaneity. Google for "Einstein train simultaneity" and see what you find.
....therefore the only way someone becomes "younger" is if they were to be "removed" from this physical universe, temporarily.
You cannot become younger than you were - that would be time going backwards and that's not possible. You can, however, age less quickly than someone else if less time passes on your path through spacetime than the other person's. But as I said, this is a different phenomenon than time dilation - you have to understand the basics first.

#### DavidMCR

You are mistaken - it is true. The experiment has been done (with airplanes instead of spaceships) and the effect is very small but measurable. The principle is further tested with the clocks on the GPS satellites every day - if we did not allow for time dilation effects the GPS system would not work.

You might also want to take a look at the sticky thread at the top of this subforum: https://www.physicsforums.com/threads/faq-experimental-basis-of-special-relativity.22903

And this is one of the reasons that I suggested you start with relativity of simultaneity. Google for "Einstein train simultaneity" and see what you find.
You cannot become younger than you were - that would be time going backwards and that's not possible. You can, however, age less quickly than someone else if less time passes on your path through spacetime than the other person's. But as I said, this is a different phenomenon than time dilation - you have to understand the basics first.
Cheers, I am going to try to understand this, it just might take some time.

#### Nugatory

Mentor
This might be a good time to mention the Physics Forums mission statement:
Our goal is to provide a community for people (whether students, professional scientists, or hobbyists) to learn and discuss science as it is currently generally understood and practiced by the professional scientific community.
The emphasis is mine.

This thread is now closed, but in keeping with the mission statement above, please start new threads as you come across questions in your study. And if you search some of our older threads, you will find many well-written explanations of the parts that most often trip people up, and also some pretty decent textbook recommendations (some online and free).

#### russ_watters

Mentor
See, when I hear the idea that someone ends up younger upon returning from their spaceship journey compared to those on earth, I can't but help think it's lies.

The way I look at it (and admittedly from a non-physicist pov) is everything happens concurrently...
Prior to civilizations' becoming seafaring, they lived on a flat Earth with a Sun of unknown nature traveling across the sky once a day (perhaps on a chariott?). They weren't altogether wrong about the Earth being flat on the scale they were viewing it.

We humans have a very limited set of day-to-day experiences that prejudice us toward viewing the universe in those terms. In our every-day experience, time and space are basically absolutes. You'll need to accept that there's a bigger reality out there and our every-day experiences don't show us all of it, and let go of those prejudices.

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