# The Naked Scientists Forum

### Author Topic: Are there different ways of looking at time dilation?  (Read 1180 times)

#### Bill S

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##### Are there different ways of looking at time dilation?
« on: 21/10/2015 03:41:26 »
I’m quite happy to accept the label “crackpot”, but my crackpottery does not extend to denying time dilation.  However, I like to try to look at things from a range of perspectives.

Let’s establish two spacetime events and call them X and Y.  We also have two people, call them Alice and Bob, (just to be different).

Alice and Bob are present at both X and Y, at which points time is synchronous for both, because they are both in the same reference frame.

Assume that between X and Y, Alice had taken a return space flight, at 0.8c, to a planet that is 10 light years from Earth. (Ignore acceleration).  Bob has remained stationary on Earth.

In Bob’s reference frame (RF) there were 25 years between X and Y; while for Alice there were 15 years.

There are two ways of looking at this.

1. For two people who are in the same reference frames at X and Y, a different amount of time has passed between those two events.

2. For the two people, the same amount of time has passed, but Alice has been able to achieve/experience less in the course of that time because change has happened more slowly.

How might we distinguish?  How does Alice know that only fifteen years have passed?  We all have an innate sense of the passage of time, but we know that that is distinctly unreliable, so let’s discount it.

Alice’s clock shows the passage of only fifteen years.  That’s much more reliable, but how can she be sure it’s not just that her clock was running slow?

It might be argued that every “accurate” clock runs slow, relative to a clock designated as stationary relative to some reference point, by exactly the same amount, at 0.8c, but this may tell us only that acceleration influences the rate of change in a consistent way.

How can we be sure that anything actually happens to time?

#### Colin2B

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##### Re: Are there different ways of looking at time dilation?
« Reply #1 on: 21/10/2015 08:32:06 »
Bill, I believe this is why we always talk about clocks, including biological and atomic clocks, running slow rather than saying time runs slow. If you look at the space time diagram for events like this neither time nor space change in any way.
Just a minor point, I understand it is speed not acceleration that causes this effect.

#### Bill S

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##### Re: Are there different ways of looking at time dilation?
« Reply #2 on: 21/10/2015 13:14:32 »
Quote from: Colin
Just a minor point, I understand it is speed not acceleration that causes this effect.

You are right, of course.   I was distracted by thoughts about the twins.  Trying to think of two things at once, at my age, is not a good practice.

From the relevant time dilation equation:
t = 25 years and t' = 15years.  At no time has Alice exceeded 0.8c, so she cannot have travelled 20 ly.

15x0.8=12, so in her RF the journey was only 12 light years.  Earth and the other planet must be only 6 light years apart.  What do we really know about space?

#### wolfekeeper

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##### Re: Are there different ways of looking at time dilation?
« Reply #3 on: 24/10/2015 23:18:49 »
Alice has accelerated four times, whereas Bob hasn't accelerated.

You ask what we know about space. Well, we know that in the frame of reference that Alice was in when she was travelling, the distance was shortened due to lorentz contraction, which explains how she made the distance so quickly in her travelling frame of reference.

With relativity, it's critical to always consider all three effects:
1) lorentz contraction
2) time dilation
3) lack of simultaneity

If you get into trouble, you've probably forgotten to apply one of them, as here.

#### PmbPhy

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##### Re: Are there different ways of looking at time dilation?
« Reply #4 on: 25/10/2015 09:09:10 »
Quote from: Colin2B726
Bill, I believe this is why we always talk about clocks, including biological and atomic clocks, running slow rather than saying time runs slow.
Where did you get that impression from?

#### Colin2B

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##### Re: Are there different ways of looking at time dilation?
« Reply #5 on: 25/10/2015 12:04:06 »
Quote from: Colin2B726
Bill, I believe this is why we always talk about clocks, including biological and atomic clocks, running slow rather than saying time runs slow.
Where did you get that impression from?
I thought it was you!

Edit: you quite reasonably posted in answer to someone that it is not logical to talk of time slowing.
I am however, of the view that in the case that Bill is talking about it is more to do with distance travelled. The traveller sees the distance to the destination shortened, contracted, but the observer in the rest frame does not measure the distance as shorter but only perceives that the traveller has taken less time than would appear to be needed for the speed.
It seems to me that time dilation and distance contraction are the same effect depending on who is observing.
« Last Edit: 25/10/2015 14:55:26 by Colin2B »

#### The Naked Scientists Forum

##### Re: Are there different ways of looking at time dilation?
« Reply #5 on: 25/10/2015 12:04:06 »