This poll was inspired by Asprung's thread. To me it seems like a no-brainer, but I could be wrong so let's vote. It's a vote on semantic preference, what you prefer the word simultaneous to mean.

 

View A: Simultaneous is observer-dependent. Two events are simultaneous (for the designated observer) if he experiences them as happening at the same time according to his personal clock. In the prevailing well-tested (GR) theory, how an observer's personal clock runs depends on his history--where and how fast he travels etc.

 

View B: Simultaneous is an absolute, though practically speaking rather imprecise idea. Imprecise, because universe time (global time, cosmology time) is only approximately measurable. But in any case it is not observer-dependent. Roughly speaking, two events are simultaneous if they happen when the Background has the same temperature.

 

Which view is preferable? Do we have to allow both? Can you suggest a third alternative?

 

To explain a bit more, in View B, we suppose the background temp is the same all over (after allowing for some small corrections) and gradually declining. At the present epoch it is 2.728 kelvin and after a long long time it will be 2.727 kelvin, and so on.

 

So this is a kind of absolute clock that can be read anywhere in the universe.

 

We could make up a word like "contemperate". Two events are "contemperate" if they happened at the same background temperature, to whatever (say one part per million) accuracy that can be determined.

 

Or maybe that is what simultaneous ought to mean.

Edited May 4, 200916 yr by Martin

This poll was inspired by Asprung's thread. To me it seems like a no-brainer, but I could be wrong so let's vote. It's a vote on semantic preference, what you prefer the word simultaneous to mean.

 

...

 

I am not sure if what I prefer the word "simultaneous" to mean is important. I would prefer that both events we observe as simultaneous also agreed with universal time but it does not usually do that in the big scale of the cosmos. I will not go any further as I do not want to influence any one's choice.

I think of simultaneous as being observer dependent but taking into account travel time. As in, if light reaches me that has been travelling for ten years (as measured in my own reference frame, i.e. the distance it has travelled in my reference frame divided by C), I consider what I'm seeing to be simultaneous to events that occured at my location ten years ago in my current reference frame.

 

It's the only way I can do it that feels intuitive, although I realize there are various reasons it falls apart.

For me, background temperature.

 

Similarly, I think where it (the background temperature) is the same in all directions is a preferred reference frame, although maybe it is more of a continuum of frames.

 

Edit: I picked this one in a long term sense. I would be pretty messed up if I was traveling at relativistic speed relative to my above preferred reference frame and used that description of simultaneity for my daily routine even if I could use it precisely. Similarly I generally use the Earth or perhaps the vehicle I'm in as my reference frame.

Edited May 4, 200916 yr by J.C.MacSwell

Doesn't motion relative to the CMB change its measured temperature, dependent on the angle of viewing?

Doesn't motion relative to the CMB change its measured temperature, dependent on the angle of viewing?

 

It certainly does! It creates a doppler dipole (hot spot ahead, cold spot astern) which we've discussed here at times. Something to average out or take out of the data.

 

So we should add some small print to that effect. And background has to be smoothed out to define it's temperature, get rid of dependence on direction.

I was refering to a single event viewed from diffrent refrence planes. Simultaneous= same time. No if time refers to clock time. Yes time refers to universal time. " The word means exactly what I say it means,nothing more,nothing less" {Mad Hatter"

I was refering to a single event viewed from diffrent refrence planes. Simultaneous= same time. No if time refers to clock time. Yes time refers to universal time. " The word means exactly what I say it means,nothing more,nothing less" {Mad Hatter"

 

 

If it is seen as simultaneous from different frames, it is most likely not simultaneous in universal time.

Probably because there's no such thing.

Probably because there's no such thing.

I see universal time as mans attempt to synchronize clocks in different frames, such as the GPS satellite clocks. Not to be confused with absolute time that does not exist. I am starting to see why swansont prefers "master time".

Lol. Me, as well. Sorry for my role in the mix-up, and thanks for clarifying.

So, if a supernova happened one light year away from Earth one year ago, and another supernova happened two light years away two years ago, you see them happen from Earth's perspective at the same time, so should they be considered simultaneous?

 

No, I don't think they should.

 

If, however, you saw the one-light-year away supernova a year before seeing the two-light-year away supernova, then I would consider those to be simultaneous, even though you don't personally experience them as such, from Earth's perspective.

 

Is that what you're asking? I'm not familiar with using background temperature as a reference.

I see universal time as mans attempt to synchronize clocks in different frames, such as the GPS satellite clocks. Not to be confused with absolute time that does not exist. I am starting to see why swansont prefers "master time".

 

That's it. To me, universal/absolute time implies that such a thing exists. A master clock, OTOH, is an agreed-upon reference, much like we agree to measure longitude starting at Greenwich. You can start anywhere and get consistent answers, but the problem arises when you try to talk to someone else.

I think we need a better definition of time to start with.

I vote for the one that suggests something like absolute time. The problem of considering something simultaneous when you observe it to be such is that other observers will not agree. Alternately, perhaps things separated in spacetime can't be simultaneous, though you at one place may observe them simultaneously. Just like you probably won't consider two places separated in spacetime to be the same place.

 

If I see events a distance away, I would probably consider the spacetime coordinates of the events even if only in my own reference frame. I might keep "simultaneous" for events happening at the same place.

That's it. To me, universal/absolute time implies that such a thing exists. A master clock, OTOH, is an agreed-upon reference, much like we agree to measure longitude starting at Greenwich. You can start anywhere and get consistent answers, but the problem arises when you try to talk to someone else.

You make an excellent point and I have been converted. If there is some reason to use the words "universal time" in the future, I will define it.

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Merged post follows:

Consecutive posts merged

Is that what you're asking? I'm not familiar with using background temperature as a reference.

 

A consistent background temperature in all directions will put you at rest with the CMB. Gravity will also have to be considered but it will help eliminate time dilation and length contraction issues when looking at simultaneous events. At Least that is the way I see it. I like your examples.

 

Edit - I started to elaborate but seem to be brain dead at the moment.

Edited May 8, 200916 yr by NowThatWeKnow
Consecutive posts merged.

We have not tried to define time itself, but have only tried to classify it.

We have not tried to define time itself, but have only tried to classify it.

 

Time has already been defined

http://en.wikipedia.org/wiki/Time

Everyone must use the same definition of time or we have mass confusion, and we do not need any more confusion.

WIKIPEDIA seems to be a discussion rather than a definition.