# Is There A Universal Now?

Unfortunately, the clock synch convention was conceived about 1900. It could only have been implemented for no more than 120 yr. The method would have synchronized clocks within a 60 ly radius. Beyond that, distances are speculations, dependent on theories.

If that experiment had actually been carried out over the last 120 years (in a region of space where gravitational fields are negligible), it would verify all the results of special relativity within a region of 60 ly in dimension. Do you seriously doubt that special relativity doesn't hold beyond that region (assuming again that there are no significant gravitational fields beyond that region?

All this talk about the beginning of the universe, or the end of the universe, ISN'T special relativity. At best, it's general relativity, and at worst, it's just philosophy. Special relativity is a theory about a static, infinite (in time and space) universe, in which there are no gravitational fields. (Such a universe has never existed, but our present existing universe, in some vast regions very far away from any galaxies, is a good approximation of it.)

Special relativity follows from a single assumption: that (in the absence of gravity) the speed of light, as measured by ANY given inertial (non-accelerating) observer, is always the same number: 186,000 miles per second. (That assumption is definitely NOT true in Newtonian physics.) If that assumption is true in our universe (and there is quite a bit of evidence that it IS true), then all the results of special relativity (such as the time dilation equation (TDE) and the length contraction equation(LCE), and the Lorentz equations) follow.

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Special relativity is a theory about a static, infinite (in time and space) universe, in which there are no gravitational fields.
Can you clarify that contradictory statement?
IMO, if there is/was such a thing as a static universe, then there can/could not have been Special Relativity of any kind.
(Such a universe has never existed, but our present existing universe, in some vast regions very far away from any galaxies, is a good approximation of it.)

Where does the notion that our spacetime geometry is infinite large (but expanding) come from? How can we measure its size if it is already infinitely large?

AFAIK, spacetime (our universe) had a "beginning" and is measurably expanding.

Are you suggesting that the BB theory is wrong altogether?
Are you suggesting that spacetime and the universe are two different things?
Are you suggesting that the universe already existed before the BB and the "beginning" of spacetime, some 13.8 billion years ago?

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I have nothing to add or correct in my previous post.
Except that the scenario plays out from observers "inside" spacetime. Bob and Alice both reside inside spacetime.

And that does not resolve the question if spacetime is infinitely big and infinitely old.

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Mike;
#241
If that experiment had actually been carried out over the last 120 years (in a region of space where gravitational fields are negligible), it would verify all the results of special relativity within a region of 60 ly in dimension. Do you seriously doubt that special relativity doesn't hold beyond that region (assuming again that there are no significant gravitational fields beyond that region?
The point is NOT about the range of application, but measuring astronomical distances of objects using light, we would only know them for objects within a 60 ly radius. That's a tiny range for a 14 billion ly universe. Not what Neddy envisions in #213. We can't know all the distances. If the purpose was synchronization, it wouldn't last because galaxies rotate, a departure from constant velocity.
SR has volumes of experimental evidence to support it.
#242
Special relativity follows from a single assumption: that (in the absence of gravity) the speed of light, as measured by ANY given inertial (non-accelerating) observer, is always the same number: 186,000 miles per second. (That assumption is definitely NOT true in Newtonian physics.) If that assumption is true in our universe (and there is quite a bit of evidence that it IS true), then all the results of special relativity (such as the time dilation equation (TDE) and the length contraction equation(LCE), and the Lorentz equations) follow.
There are two postulates.
Postulate 1, is physics is the same in all inertial frames.
Postulate 2, constant, independent velocity of light.

The point is NOT about the range of application, but measuring astronomical distances of objects using light, we would only know them for objects within a 60 ly radius. That's a tiny range for a 14 billion ly universe. Not what Neddy envisions in #213.

I never said anything about a 60 lightyear radius. Nothing in quote #213 requires that. And the radius certainly does not have to be equal to the entire universe. It is enough to show that there is no "universal now" in whatever size or volume of space you choose.

SR has volumes of experimental evidence to support it.

I am glad to know that you are not a denier of SR.

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Here is a thought experiment which might help

It also fits in with thread Does time exist

Thought experiment

Question

You are at home watching your cat play at the other side of the room

Would you consider you are in your now and the cat is in its own now?

A YES answer would be incorrect

An alternative answer (which I would deem correct) would be Cat and I are in the same now

Not because I am ignoring the close proximity of the cat (which could be used as a reason) but because there is only a singular NOW

Continuing

You look out your window and observe your neighbour back his car out and drive off. As the car gets further away do you consider you to be in your now and neighbour and car in a different now?

If your answer is yes I would question why? The only difference between between the cat situation and neighbour and car situation is distance. Extra clarification question, Do you consider distance to be the deciding factor about being in a different now?

I could continue increasing the distance ie asking about happenings on moon, planets, other galaxies etc

All those happenings happen at / in the same singular now, your knowledge about them is limited / restricted about them due (for extensive distances)
• finite speed of light
• degradation of the information and
• our ability to gather the degraded information which arrives and possibly
• interpretation

What The Universe Is Doing RIGHT NOW
September 14, 202212:10 AM ET

Illustration of the expansion of the Universe. The Cosmos began 13.7 billion years ago (left). Immediately it began expanding and cooling (stage 1). Its expansion slowed about 10 billion years ago (stage 2). We are now at stage 4. The expansion shows no signs of stopping and is in fact accelerating. The orange arrows indicate the force of gravity, which slows but does not stop the expansion.
MARK GARLICK/SCIENCE PHOTO LIBRA/Getty Images/Science Photo Libra
A century ago, astronomers were locked in a debate about the scope of our universe. Were we it?

The answer is no. There are other galaxies beyond the Milky Way, and they are speeding away from us. Answering that question left astronomers with an even bigger puzzle. Why is everything sprinting away from us and what does that mean for the center of the universe? Today, Scientist in Residence Regina G. Barber brings back astronomer Dr. Vicky Scowcroft for the final episode in our series on cosmic distances and humanity's place in the universe. It's a big one: The mystery of our expanding universe.

More...https://www.npr.org/2022/09/13/1122804873/what-the-universe-is-doing-right-now

I never said anything about a 60 lightyear radius. Nothing in quote #213 requires that. And the radius certainly does not have to be equal to the entire universe. It is enough to show that there is no "universal now" in whatever size or volume of space you choose.

I am glad to know that you are not a denier of SR.
The 60 ly range is a result of measuring distances for 120 yrs. If you needed to know the distance of a star 1000 ly distant, could you wait 2000 yrs for an answer?
It showed the impossibility of universal clock synchronization.
SR is well established, and I make an effort to eliminate the fiction and fantasy of misinterpretations.

You look out your window and observe your neighbour back his car out and drive off. As the car gets further away do you consider you to be in your now and neighbour and car in a different now?

If your answer is yes I would question why? The only difference between between the cat situation and neighbour and car situation is distance. Extra clarification question, Do you consider distance to be the deciding factor about being in a different now?

The reason my answer would be yes is not because of the distance, but rather it is because the neighbour is in motion relative to me. Likewise, he could say that I am in motion relative to him. For that reason, the specific details of what I consider to be "now" are different than the specific details of what he considers to be "now".

The driving neighbour can have two clock in his car, one in the front seat, and one in the back seat. He can synchronise those clocks using Einstein's synchronisation method, and he would then consider both of his clocks to be displaying the same clock readout simultaneously. His "now" is whatever is happening when both clocks display the same "now" time.

I would say that the clock in his back seat was set to be slightly ahead of the clock in his front seat, because I would correctly say that his synch method did not work properly in my reference frame, because he was moving relative to me when he synchronised his clocks. So my "now" is whatever is happening when his rear clock displays a time that is slightly ahead of his front clock.

He would argue that his synch method worked fine in the reference frame of the car, because he was driving at a constant speed and so considered the car frame to be an inertial frame, and he would also be correct.

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Michael345;
All those happenings happen at / in the same singular now,

"now" is the present, for any duration of time, hearing a sound for a second, to what is in vogue for a decade. We are considering brief short duration events.
You and your cat may be considered in the same ref. frame (house), with all your clocks synchronized. You and your cat have individual 'nows', which may be recorded as the same 'time', with no significant difference (nanosec).
You and your neighbor may be considered in the same ref. frame (time zone), with all your clocks synchronized. You and your neighbor have individual 'nows', which may be recorded as the same 'time', with no significant differences (millisec).
The communication time increases with distance. You can compensate for the delay IF you know the distance, which requires a measurement. The coordinate transformations work IF you know the velocity of the distant clock, which requires two separate measurements. Each 'now' has a different time stamp for each event for different observers, depending on their motion, which alters their 'time'. Thus 'time' is subjective.

A. Einstein, 1905 paper, par.1
[Thus with the help of certain imaginary physical experiments we have settled what is to be understood by synchronous stationary clocks located at different places, and have evidently obtained a definition of simultaneous,'' or synchronous,'' and of time.'']

The red calibration curve in the graphic represents a constant unit of time. A is a clock with an equivalent velocity as the U clock, thus at rest. The velocity profiles to either side of A will be perceived by U and A as running slower, even when synchronized.
A is the only clock that will have the same rate as the U clock when synchronized.

The astronomical distances and relative motions prevent any large scale clock synchronization

ignore

"now" is the present, for any duration of time, hearing a sound for a second, to what is in vogue for a decade
Who discovered the fundimential second of time and what properties does it have?

Who discovered the fundimential second of time and what properties does it have?

I like the simplicity but I think you have a few things mixed about…. Perhaps you should have worded it more along the lines of “Who discovered time and it’s fundamental properties” and the answer would contain the history of time. A couple names… what they did to further devolop our idea of time. All the different ways in which we can measure time
What The Universe Is Doing RIGHT NOW
September 14, 202212:10 AM ET

Illustration of the expansion of the Universe. The Cosmos began 13.7 billion years ago (left). Immediately it began expanding and cooling (stage 1). Its expansion slowed about 10 billion years ago (stage 2). We are now at stage 4. The expansion shows no signs of stopping and is in fact accelerating. The orange arrows indicate the force of gravity, which slows but does not stop the expansion.
MARK GARLICK/SCIENCE PHOTO LIBRA/Getty Images/Science Photo Libra

More...https://www.npr.org/2022/09/13/1122804873/what-the-universe-is-doing-right-now
Ahhhhh!!!! The galaxies are getting closer! Ahhhhh it’s the end!!!

I mean they have to right? The universe is accelerating faster than gravity is pulling it in. Right?

plus if you look at the pictures and extrapolate them using toothpicks; at some point you’ll be hit in the face by extremely large galaxies. And the universe would probably continue accelerating.

lol perhaps I am insane.

plus if you look at the pictures and extrapolate them using toothpicks; at some point you’ll be hit in the face by extremely large galaxies. And the universe would probably continue accelerating.
Looking at the picture from that POV and extrapolate; you'll be hit by a 2D slice of universal NOW regardless if the universe is expanding or contracting.

The astronomical distances and relative motions prevent any large scale clock synchronization

So ... is there a universal now?

Ahhhhh!!!! The galaxies are getting closer! Ahhhhh it’s the end!!!

Chillax

So ... is there a universal now?
Of course there is.

On a 2 dimensional temporal plane all points have the same NOW.
When we place an alarm clock on every point on that plane and set the alarm at the same time on all clocks, there will come a "time" when all the alarm clocks will ring at the same time @ NOW over the entire plane's temporal dimension.

But as soon as you introduce an observer you introduce a spatial dimension and the problem of 3D relativity from the point of the observer. IOW when all the alarm clocks sound the alarm at the same NOW over the entire 2D temporal dimension, to the spatial observer it becomes a relatively infinite continuation of sound as the sound from each clock has to travel a longer distance and reaches the observer later than all the closer clocks.

Therefore, the introduction of an observer creates a subjectively relative reality, that can only be mediated by differential synchronization of clocks to account for the different spatial distances to the POV.

Keep the observer out of the equation and you get objective temporal synchronicity, a universal NOW.

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The astronomical distances and relative motions prevent any large scale clock synchronization

Correct

But synchronisation is just that synchronisation

So ... is there a universal now?

Yes - synchronisation not required for universal now due to a universal now being, by default, universal

Synchronisation, or an allowance / adjustment made for a lack of synchronisation in tasks like Global Positioning

How much allowance / adjustment made will affect the accuracy of the location output