# The Naked Scientists Forum

### Author Topic: How does time dilation work from each perspective?  (Read 3186 times)

#### Chris Siragusa

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##### How does time dilation work from each perspective?
« on: 06/12/2010 09:30:02 »
Chris Siragusa  asked the Naked Scientists:

Hi guys, love the show.

I have a question about relativity and time. I understand that if you move faster, time slows for you.  The example I always hear is that you can fly off in a spaceship at near light speed for what seems like an hour and return , 100 years may have passed on Earth.  My question is this: when I am flying away at near light speed, doesn't it look to me like I am stationary and the Earth is flying away from me?  Why do I slow down if, from my perspective, the Earth is the one moving and I am stationary?

Thx,

Chris in new York

What do you think?
« Last Edit: 06/12/2010 09:30:02 by _system »

#### yor_on

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##### How does time dilation work from each perspective?
« Reply #1 on: 05/12/2010 15:17:57 »
Nice one :)

There is a recent theorem defining motion, relative stars being so far away from us that we can use them as 'fixed points'. A simpler definition would be using the 'history' defining you as you travel. You 'know' which one is moving relative the other as you remember leaving Earth.

That won't hold in a black box scenario though where you have been locked in by the evil Sagiturians kidnapping you. the only thing you might have noticed there would be an acceleration, and assuming that it was at a constant one gravity, same as Earth, you wouldn't even be sure of an acceleration, except at takeoff which would need to differ , at least slightly..

But reaching near light speed relative whatever you define as your point of distant 'rest' (Earth) the hard radiation created by your motion relative the radiation outside your ship should tell you that you're moving, those few milliseconds of existence left :)

I'm not sure of that one in fact, in a 'shielded' black box scenario it seems impossible to define. So the only way I can see is to assume that you will be able to communicate with your 'outside', and if so you will find all sorts of relative effects bending the light etc, informing you that you're in motion relative SpaceTime.
« Last Edit: 05/12/2010 15:19:42 by yor_on »

#### Bill S

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##### How does time dilation work from each perspective?
« Reply #2 on: 05/12/2010 16:17:25 »
The argument seems to be that because you are experiencing acceleration, whereas the Earth is not, you will experience the time dilation, whether you are aware of the acceleration, or not.
I'm not entirely happy with that explanation, so it will be interesting to see where this goes.

#### jartza

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##### How does time dilation work from each perspective?
« Reply #3 on: 05/12/2010 16:44:03 »

You are allowed to think that Earth is moving away from you during the flying away part of the trip.

BUT then you are required to think that Earth is moving away from you also during the return trip. SO during the return trip you must be moving really fast.

#### yor_on

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##### How does time dilation work from each perspective?
« Reply #4 on: 05/12/2010 18:10:09 »
Yes, you're correct Bill, but it can't only be acceleration. Assume that you go ten light-years. You accelerate one of those light-years, coast eight, then decelerate another light-year. You get a time dilation of fifty, as a number.

Now assume the same journey but instead of accelerating for one light-year, all seen from the pilots frame of reference, you need only a light-day to come up 'to speed' and then coast for the rest of the time. The deceleration takes only a light-day too. To have the same overall time-frame for the journey, as from the pilots 'frame of reference', I would then need to have a lower overall velocity.

If it now was only acceleration that makes the difference, then the time dilation should be 50 again, right?

What do you think?
Does that make sense?

(changed it to light-days and light-years as I was meaning the distance it takes for the light to propagate that 'distance', one gets blind reading what one writes at times:)

The worst thing is that it do make a sort of sense, as I feel intuitively that it shouldn't? :)
I mean, why only acceleration if so?
Expending energy?

But it can't be the same amount of energy 'burned' can it?
(JP, Lightarrow et al, I got one for you :) Bring your pocket calculators)

(I need to look at this tomorrow I think, fresh eyes and all that:)
==

But, add to it, that if it was that way then the time coasting doesn't seem to 'count' for a time-dilation? Which makes it rather absurd as I could coast 'forever' if I like, near light speed, after accelerating and then only have the acceleration to count as a 'time-dilation'?

(The only way I make sense of it is if all types of acceleration, as long as the journey from the pilots frame of reference took the same time, would be equivalent, am I right there? And, all the pilots 'time-equivalence' stated for different accelerations, should then be equivalent to the ones measured by those 'at rest' back on Earth? And all those accelerations would need the same amount of energy 'burned' too, for the equivalence to hold, as it seems to me? Why didn't I see this coming? Da*n :)

Which means that depending on acceleration I could accelerate one year, and see the universe die, or one hour, one second, and if I move uniformly at any moment, breaking my acceleration, I would join some universal 'time-frame' for all free falling objects of the same mass, no matter my measured 'velocity' relative my origin (Earth). Also it should mean that my 'relative mass/momentum' only exist in the acceleration? As it otherwise should add to a time-dilation, possibly?? Or turn it around, then it means that Relative mass and momentum can't add to a time dilation, only proper mass and acceleration can do that relative other 'frames of reference'. that way we can keep a relative mass and momentum for 'uniform motion', which makes more sense to me, if I am right in this?

So, not only would all uniform motions be equivalent from a perspective of 'velocity', they would also make you join a equivalent 'arrow of time', the exact same from all perspectives (Frames of reference), only differing slightly depending on the 'proper mass' of your rocket? As your relative mass won't matter :) in a 'free fall' following the geodesics?

And the muon example cited by me and so many others would be soO0Oo wroo0O0ong :)
As they can't be accelerating, as they only are following a geodesic.
That is, being in a 'free fall', 'weight-less', even if as measured from earth 'accelerating' due to its 'gravity'. If you believe in the idea of geodesics.

..Simplistic, Go get them :)

Also it seems to say that the Lorentz contraction I observe only would exist in a acceleration which have to be wrong.

Otherwise they are not equivalent, as I have presumed, as you then will have a Lorentz contraction both at the acceleration and at the later uniform motion, but a time dilation only at the acceleration? And what about the deceleration, will that act on time too if so?

Lots and lots of them..

(But those ones still makes sense I think)
(Well, maybe, make sense?)
« Last Edit: 05/12/2010 21:40:11 by yor_on »

#### granpa

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##### How does time dilation work from each perspective?
« Reply #5 on: 05/12/2010 22:40:23 »
Chris Siragusa  asked the Naked Scientists:

Hi guys, love the show.

I have a question about relativity and time. I understand that if you move faster, time slows for you.  The example I always hear is that you can fly off in a spaceship at near light speed for what seems like an hour and return , 100 years may have passed on Earth.  My question is this: when I am flying away at near light speed, doesn't it look to me like I am stationary and the Earth is flying away from me?  Why do I slow down if, from my perspective, the Earth is the one moving and I am stationary?

Thx,

Chris in new York

What do you think?

the simple answer is that it has to do with relativity of simultaneity.

#### yor_on

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##### How does time dilation work from each perspective?
« Reply #6 on: 06/12/2010 06:56:10 »
Okay, woke up, looked, went back to sleep :)
N00 I'm here, agai..

Ah well.

To make sense of it. Nope, acceleration is not the time dilation. But neither is uniform motion. What is time dilation is your SpaceTime geometry shifting.

Would you agree to that light will 'bend' when traveling close to light-speed?
That one we have tested several times and it is true as far as I know. If that light starts to bend in your frame of reference but as seen from Earth still behaves as usual then you're in a 'twisted SpaceTime geometry'.

A twisted SpaceTime geometry consist of 'four' properties.
Three dimensions called the 'room', length, width and height.
And one called 'time'.

Those together creates your 'real room' wherever you are.

Twisting that 'room' will contract the 'distance' of it, and introduce a 'time dilation. But to 'twist' it you need to introduce a 'change'

That brings us to the difference between 'uniform motion' and 'acceleration'. a uniform motion is the same for all uniform motions as you are following a geodesic, expending no energy. That also means that there is an equivalence to all 'speeds' or 'velocity's'. But does it mean that this 'velocity/speed' doesn't exist?

Nope, as we could see thinking of that 'light bending' before, depending on your 'speed/velocity' light will 'bend' for you, telling you that you are 'moving'. The equivalence of uniform motion is to how it 'moves' inside SpaceTime, always following a geodesic, never expending 'energy'.

So what happens when you accelerate? Well, you're no longer inside a geodesic, 'coasting along'. Suddenly you're expending energy, forcing a change to the 'room time geometry'. After closing your engines, coasting along again, would you expect that newly twisted 'room time geometry' to 'snap back' to what it was before? Why would it do that? It's not a rubber band is it? Well, maybe it is, but if so it change length with every acceleration, as with 'mass'

We have a universe of equilibrium, but it's a relative equilibrium adapted to your frame of reference, and mine, and his... Did your velocity disappear as you stopped the engines? Take a look at your 'place of origin' Earth and check. Nope, you're still moving at a respectable 'velocity' relative your home planet. So if that didn't 'snap back', why would the 'room time geometry' do it?

You introduced a change in SpaceTime, relative yourself, as you easily can check watching the twisted 'room times' light bend, so you're still inside the 'new room' your acceleration created. So yes, only acceleration can create a 'time dilation' (and 'mass'), but no, it doesn't disappear while 'coasting' after it. Well, as I see it.
==

So why can't I notice my 'time' go 'slow' when traveling near light? Imagine yourself noticing it, and, then tell me how you did it? Checking your watch? But that's in the same 'room time geometry' as you? Calling mum at Earth asking her time? But that's importing another 'frame of reference' isn't it? another persons 'room time geometry' too. and you can easily prove her time 'wrong' as you check your's :) That will give you a lively discussion informing her.

From your 'frame of reference' time will always 'tick as usual'. SpaceTime is all about 'room time geometry's' also referred to as 'frames of reference' and even thought we on Earth like to think we are in the same 'frame of reference' that's no absolute truth, only a relative one as we all will change the geometry sitting, walking, climbing, etc.

You can only notice the 'length contraction' from your own frame of reference, never a time dilation. so does this invalidate a certain 'universal clock' ticking the same for all?

That depends on how you define your 'times arrow'. To me I will always have the same 'arrow of time' as measured in my own 'room' of if you like 'frame of reference'. And so I can state that all of our 'personal frames of reference' are equivalent in that motto. Or you can play an 'outside observer', a little like we expect a God to be, creating our bubble of a universe. When he looks at us he will see the 'relations' joining all our different 'room times' but he will also be able, as he is 'outside' our times and rooms' to see that we carry different 'clocks' versus each other.

To think of SpaceTime this way I use to refer to as the 'mind space'. It's a perfectly logical space, just as our normal one, but it doesn't exist for you, except conceptually. The biggest mistake you can do is to assume that this 'mind space' is your 'reality'. It's not, in your 'reality' clocks always tick the same, not caring if you're on a spaceship near light, on the event horizon falling in to a black hole, or on Earth. Your clock will tick as usual.

Ir's about how you define reality, as an conceptual 'outside observer' or as you are, inside your own 'room geometry'. To assume you are 'outside' inserts all kind of strange ideas. As far as i know our 'SpaceTime' is a 'closed one' in terms of energy. We may have an expanding universe but it's still a universe where nothing gets to waste. So it's closed.

Remember that your room geometry is not a 'defined' one, every 'frame of reference' relative yours will have its own opinion about your 'distances' and 'clocks'. So discussing an 'open-ended SpaceTime expanding' having no limits may be true from one perspective but from my point of view something of a joke, as there is no such thing as a 'fixed distance' to me.

Phieeewww, that one got me worried :)
Well, a little..

« Last Edit: 06/12/2010 07:42:23 by yor_on »

#### yor_on

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##### How does time dilation work from each perspective?
« Reply #7 on: 06/12/2010 09:07:40 »
The point being here that I define it from my 'room time geometry'. From your point of view looking at me accelerating the 'distances' haven't changed, instead you find my 'time' to have slowed down relative yours. But if you were on my ship we would see the exact same, more or less, and that is distances 'shrinking'. To that one can add that from your point of view time will be 'as always' to  you when looking at me moving in slow motion, so nowhere can we say that time actually 'slowed down', except as an 'relation' to what you would expect normally when it comes to watching someone move. The twin experiment is about the twins on earth, one traveling near light speed and then coming back to his origin. And that is the only way to prove that time dilation, comparing what you know to have had, almost, the same frame of reference from the beginning, and then compare them after the travel.

If you travel very fast you will find the universe dying , but still doing it inside your normal 'frame of time' with your clock 'ticking as usual'. Which makes me realize that I might have to change the statement slightly that you can't see a 'time dilation', well you can, but not your own. The 'relations' you observe will give you an time-accelerated 'SpaceTime' instead of your 'clock' slowing down, with one exception, how do you expect to prove it having 'aged faster'?  What you should notice here is that the equivalence between the energy spent by you, and the accelerated universe is totally out of bounds as I sees it, as it in fact are your action that have accelerated it, well, as long you didn't spend infinite energy accelerating yourself? That used to confuse me, a lot :) But there is an explanation if we accept that we all are in a unique 'information space', then what change is 'information' and the 'room time' you get it from. and then there are some new exciting theories about what distances is like the Bekenstein bound as well as some very weird new constants.

Which brings me to one last point. To me it's all about relations, defined from one point, my 'room time geometry'. Or your's, or his, or that stones, no not the rolling stones, well them too. Everything existing have it's own unique 'room time geometry' as I expect it to be. And all that keeps us together is the 'relations' and the 'information' light, in all its equivalences, will bring us. You live inside a 'information space'.
« Last Edit: 06/12/2010 09:40:57 by yor_on »

#### jartza

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##### How does time dilation work from each perspective?
« Reply #8 on: 07/12/2010 16:49:33 »
You are allowed to think that Earth is moving away from you during the flying away part of the trip.

BUT then you are required to think that Earth is moving away from you also during the return trip. SO during the return trip you must be moving really fast.
You must move fast to catch the fleeing Earth.

Quite odd that the age old problem is solved in this simple way: [8D]
No acceleration of Earth and no "frame jumping" allowed -> you are younger, Earth is older

#### The Naked Scientists Forum

##### How does time dilation work from each perspective?
« Reply #8 on: 07/12/2010 16:49:33 »