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Offline DoctorBeaver

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Is time expanding?
« on: 20/04/2008 17:59:05 »
As a follow-up to http://www.thenakedscientists.com/forum/index.php?topic=13906.0, if spacetime is expanding, does that mean time is expanding as well as space?

If so, would there be any way to measure it? We would still see a minute as a minute even if it lasts what is now 2 minutes as our perception would inevitably also slow down along with our measuring devices.


 

Offline graham.d

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Is time expanding?
« Reply #1 on: 20/04/2008 18:46:44 »
It is hard to know what is meant by the words "is time expanding?" The concept of space-time expanding is understandable in that it means the 4D volume enclosed is enlarging. But everything within our local environment would not be perceived as changing. Even if any change occurred, how would we know? Our rulers would change in the same way. It would be the same with time. All our means of measuring time, including our own perceptions based on any internal biological clock would also change in the same way.

Another question would be, if space-time is expanding from an initial point, and a point at which time "began" as has been postulated by some, is there ANY local measuement that could be made that could reveal whether this was true? I don't think there is, at least I don't think anyone has suggested that there could be. The concept has an elegance though.
 

lyner

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Is time expanding?
« Reply #2 on: 20/04/2008 21:44:02 »
It all hangs on the speed of light, surely. If that is going to be constant then the time units would have to be 'constant'. I think???
 

Offline science_guy

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Is time expanding?
« Reply #3 on: 21/04/2008 00:19:02 »
if you think of time as a dimention, then it begins with space, in the same point.

so if space is expanding, and time wasn't then time wouldn't be in same place as the space dimentions, and I wouldn't have any time to post this statement

err... pun intended?
 

another_someone

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Is time expanding?
« Reply #4 on: 21/04/2008 01:25:09 »
if you think of time as a dimention, then it begins with space, in the same point.

so if space is expanding, and time wasn't then time wouldn't be in same place as the space dimentions,

Sorry, I don't understand what you are saying.

You seem to be suggesting that if you have a cube (which is a 3 dimensional object), you cannot stretch that object in one dimension without also stretching it in the other two dimensions.  The whole point about different dimensions is that they are independent of each other, and so can be modified independently.

In any case, I would have thought the converse argument holds true.  If you are inside the cube, being an intimate part of the cube itself (so if the cube expands, you expand with it), you would have no way of telling if all three dimensions expanded together, since everything would still seem relatively the same to you; but if one or two dimensions were to be stretched or shrunk relative to the others, then you would immediately see the change in shape, rather than the change in size, of the structure, and so can tell that some dimensions have changed relative to others.
 

Offline science_guy

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Is time expanding?
« Reply #5 on: 21/04/2008 05:23:26 »
what Im trying to say is that with the spacial dimentions expanding, we go along with it in the relative "area" of the dimentional "cube"

however, should time not expand along with the spacial dimentions, then it would not exist in any of the "areas" except for the point it started at, in the center of the universe.
 

Offline DoctorBeaver

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Is time expanding?
« Reply #6 on: 21/04/2008 07:44:07 »
It all hangs on the speed of light, surely. If that is going to be constant then the time units would have to be 'constant'. I think???

Not necessarily. We already know that time dilation is fact.
 

Offline JP

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Is time expanding?
« Reply #7 on: 21/04/2008 15:29:21 »
what Im trying to say is that with the spacial dimentions expanding, we go along with it in the relative "area" of the dimentional "cube"

however, should time not expand along with the spacial dimentions, then it would not exist in any of the "areas" except for the point it started at, in the center of the universe.

The problem with this argument is that it assumes that when time expands it does so in space.  Think of plotting something on an x-y grid (and that you're drawing this all on something stretchy).  Expanding space would correspond to stretching the paper on which you plotted the grid,  so that whatever you drew expands, but so do all the axes.  Time is a dimension like space, so you could also plot x vs. t on a grid.  Expanding time would just be stretching this whole plot in the time direction.  The stretch in time has nothing at all to do with what you're doing in space--time expansion takes place along the time-axis of your grid, and doesn't have to interact with the space axes. 

In particular, working in only x and t dimensions, you can draw a square, and then stretch along only t to make a rectangle.  You can also stretch along only x to make a rectangle.  Or, if you want, you can stretch equally along both to make a bigger square.  But even if you stretch only along x, the time axis still exists for every x point and you have a perfectly valid rectangle.

*Disclaimer:  However, there are all sorts of rules that govern just HOW space-time can expand and not break the rules of relativity.  These rules do tie time and space together, but not in the geometric way you argued above.
 

Offline JP

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Is time expanding?
« Reply #8 on: 21/04/2008 15:34:12 »
And to answer the original poster, I think that a good way to look at this is to think of black holes.  It's generally accepted that if you get close to a black hole, the fabric of space-time is stretched.  You end up being stretched out in space, but time slows down for you (relative to your buddy who's safely away from the black hole).  Since time is "stretching" here in response to a "stretching" of space-time, it will likely stretch in response to any distortion/expansion of space-time (the same equations for space-time behavior should govern the expansion of the universe and black holes).  This is just my intuition though.  I don't know the details of the expanding universe model.
 

Offline DoctorBeaver

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Is time expanding?
« Reply #9 on: 21/04/2008 15:40:55 »
Is there maybe a way to test it by observing very distant objects? From them we can tell that space is expanding; so if we observe a process at that distance that takes longer than it would here, can we say that time has expanded "between" us?
 

Offline LeeE

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Is time expanding?
« Reply #10 on: 21/04/2008 17:07:52 »
I think there are two different aspects of this that need to be identified: extension and expansion.

Space-time has certainly extended since the singularity of the BB, when/where all the dimensional values were zero.  Perhaps this is most obvious when you think of the time dimension and how, by now, it has extended from it's starting value of zero to it's current value of around 14 billion years.  Extension then, is about the increase in the number of units.  Expansion on the other hand, is about the size of those units and how that size can change.

If your frame of reference only exists within those units you won't be able to detect any change in their size because your reference will change alongside what you're trying to measure.

On this basis, the phenomenon of time dilation suggests that time expands or contracts, depending on whether one is the traveller or the observer.  Over the course of the journey the traveller will experience fewer units of time compared with the observer who will experience a greater number.  At the same 'time' though,each unit of time for the traveller will take longer to pass than those of the observer.  Once the traveller returns home and meets the observer, they will disagree about the number of units of time that have passed since they parted but they'll have to agree that, as they were before the journey, they are both now at the same point in time.

Now if the universe is getting bigger, is it extending or is it expanding?  With the inflating spotty balloon example of universal expansion, while the spots get further away from each other, they also get bigger and the ratios between their positions don't change.  Meanwhile, as the balloon is inflated, and even though it is getting bigger no new material is being added to the balloon the number of molecules in the balloon remains unchanged, which is equivalent to there being no extension, or increase in the number of units.

So if the universe is expanding, without there also being extension, we'd not only be unaware of the universe expanding but we'd also be very surprised when it suddenly popped:)
 

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Is time expanding?
« Reply #10 on: 21/04/2008 17:07:52 »

 

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