# The speed of time

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1 minute ago, swansont said:

Both of the ones I quoted in my previous post, both of which asked if time was constant. (the third statement wasn’t actually a question)

I stated no such thing. Once you know that time depends on certain factors, it answers the question of whether time is constant. Whether there are more variables is a completely different question.

The first question was whether GR is premised on time being constant

The second question was if there is any basis that time is constant.

Both questions are about time being constant over time, which I believe you have either misunderstood or ignored, hence your answer about time being dependent on your motion relative to another observer, and on your gravitational potential.

With respect, you are repeatedly selecting only some of my questions, and then providing answers that do not relate to those questions.

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3 minutes ago, AbstractDreamer said:

The first question was whether GR is premised on time being constant

The second question was if there is any basis that time is constant.

Both questions are about time being constant over time, which I believe you have either misunderstood or ignored, hence your answer about time being dependent on your motion relative to another observer, and on your gravitational potential.

With respect, you are repeatedly selecting only some of my questions, and then providing answers that do not relate to those questions.

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2 minutes ago, swansont said:

Thank you for this, but I'm not talking about motion or potential. Or either of them changing over time.

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2 hours ago, AbstractDreamer said:

Thank you for this, but I'm not talking about motion or potential. Or either of them changing over time.

No, you’re talking about time, and time depends on these things. Which you are ignoring for reasons I can’t comprehend.

You asked a very broad question. The answer is no (or “the question is nonsensical”). You might have given specific conditions under which the answer is yes, but it would only be yes under those specific conditions.

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Absent other variables which influence it, how would you be able to tell if time is constant over time ?

GR, a geometric theory of gravity, tells us that gravity is mostly due to time axis curvature.
IOW, time progresses differently in flat space-time, as opposed to curved space-time, and is definitely not constant.

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On 9/10/2022 at 9:30 PM, MigL said:

Absent other variables which influence it, how would you be able to tell if time is constant over time ?

GR, a geometric theory of gravity, tells us that gravity is mostly due to time axis curvature.
IOW, time progresses differently in flat space-time, as opposed to curved space-time, and is definitely not constant.

I don't think you could tell experimentally.  But my question is really: If we assume that time is constant over time, WHY do we assume that?  What evidence is there?  I'm guessing there is no evidence, hence my questions here.  As physics is based on evidence, why is this one assumption exempt?  What are the theories in the scientific community that considers time over time as non constant?

I'll posit again that motion or potential changing the experience of time is missing the point.   In a thought experiment, take two volumes of spacetime, both identical in motion and gravitation potential, and identical all other properties EXCEPT in the property of time.  One volume is close to t=0, and the other volume is close to t=infinity.

Theoretically or mathematically, but not experimentally, comparing the progress of time in each volume relative to the other volume, why do we assume they are the same?

Edited by AbstractDreamer
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1 hour ago, AbstractDreamer said:

I don't think you could tell experimentally.

Then why do you think it would make a difference ?

A Einstein based GR on the assumptions of homogeneity ( same in all locations oruniformity of structure ) and isotropy ( same in all directions or uniformity of properties ) of the universe.

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1 hour ago, AbstractDreamer said:

I don't think you could tell experimentally.  But my question is really: If we assume that time is constant over time, WHY do we assume that?  What evidence is there?  I'm guessing there is no evidence, hence my questions here.  As physics is based on evidence, why is this one assumption exempt?

There’s no evidence that it’s not, for any clock relative to another clock in the same frame of reference. People have looked for deviations from the predictions of relativity and haven’t found any.

1 hour ago, AbstractDreamer said:

What are the theories in the scientific community that considers time over time as non constant?

AFAIK there are none. Certainly none with evidence to support them.

1 hour ago, AbstractDreamer said:

I'll posit again that motion or potential changing the experience of time is missing the point.   In a thought experiment, take two volumes of spacetime, both identical in motion and gravitation potential, and identical all other properties EXCEPT in the property of time.  One volume is close to t=0, and the other volume is close to t=infinity.

How would we not notice this? That there is a time rate difference other than what we know to account for? If frequencies differ from expected, it would show up., and violate Einstein’s equivalence principle.

1 hour ago, AbstractDreamer said:

Theoretically or mathematically, but not experimentally, comparing the progress of time in each volume relative to the other volume, why do we assume they are the same?

Einstein’s equivalence principle

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2 hours ago, MigL said:

Then why do you think it would make a difference ?

A Einstein based GR on the assumptions of homogeneity ( same in all locations oruniformity of structure ) and isotropy ( same in all directions or uniformity of properties ) of the universe.

Because is it possible that a "non-constant time function over time" could model observed space expansion?   It would remain experimentally untestable, unless your experiment spanned a volume of space time that contained such expansion, yet somehow remained unaffected by the expansion.

I cant really grasp the scale that expansion works on, but I would wildly guess that it appears to me to break homogeneity at least on a scale reasonable local to expansion.  Of course the argument for homogeneity is to choose a larger scale.

If spacetime is a continuum, then homogeneity in space is also homogeneity in time.  How do we can reconcile homogeneity in time, when we have a universe that apparently expands locally or globally, or at least inflated globally?  It's appearance and properties shortly after t=0 is much different that today.

2 hours ago, swansont said:

AFAIK there are none. Certainly none with evidence to support them.

In the same way that GR has no evidence to support that time is constant over time?

2 hours ago, swansont said:

Einstein’s equivalence principle

Not sure how this relates at all.

What is the principle in GR that assumes time is constant over time?  I don't think there is one.  Indeed it doesn't need one... experimentally.

However it does assume this, right?

On a similar notion, what evidence do we have that any unit is constant over itself?  Take distance for example.   At a large enough scale that experiences significant expansion, something that measures D units using measurements within that expansion, might measure D+E relative to observations from without that expansion.

What is the principle of realism that states non-theoretical units are uniform from zero to infinity?

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26 minutes ago, AbstractDreamer said:

In the same way that GR has no evidence to support that time is constant over time?

No, because GR has evidence to support that it works.

28 minutes ago, AbstractDreamer said:

Not sure how this relates at all.

What is the principle in GR that assumes time is constant over time?  I don't think there is one.  Indeed it doesn't need one... experimentally.

However it does assume this, right?

But GR works. Tests of the equivalence principle don’t show any problems. Clocks that are supposed to agree, agree.

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32 minutes ago, swansont said:

No, because GR has evidence to support that it works.

But GR works. Tests of the equivalence principle don’t show any problems. Clocks that are supposed to agree, agree.

There is no evidence that "non-constant function of time over time" doesn't work.

GR, experimentally, works with "non-constant function of time over time" too.

Would you agree with the above statements?

Edited by AbstractDreamer
technical ambiguity
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53 minutes ago, AbstractDreamer said:

There is no evidence that "non-constant function of time over time" doesn't work.

GR, experimentally, works with "non-constant function of time over time" too.

Would you agree with the above statements?

How would you test for non-constant time?

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1 hour ago, swansont said:

How would you test for non-constant time?

Everyone in this thread (except AbstractDreamer) seems to be using the following: 1. Time is what clocks measure, by definition. 2. The only way to measure or reason about the rate of time, is to compare time measured by one clock to that of another clock. 3. A clock will always tick at 1 s/s compared to itself, there's simply no room for a clock to disagree with itself. AbstractDreamer, I can't make sense of what you're saying without knowing what definitions you're using or where you disagree with these 3 things.

I suggested this before but it was split off and buried: To test for a changing rate of time you would compare observations of the rate of a clock from earlier in its history, relative to the clock now. You would need to account for any effects not attributable to time, and then any deviation that's left over would have to be due to changing rate of time. Or, compare a clock now to one in an environment that is identical to the environment in the first clock's past. Or, you could compare a clock at two different times, against a reference clock whose rate can be considered constant. I'm sure this is possible to reason about, because you could say "A clock speeds up as it climbs out of a gravitational well," and it makes implicit sense, I think, using any of these 3 methods.

I think this is what AbstractDreamer is trying to ask about, specifically about expansion of space or other possible unknown effects in the history of the universe.

What I still don't get is whether GR can and/or does predict that expansion would have an effect on a clock like this, or whether the rate will still be constant because there is always something else other than time to account for any observations (eg. redshift in older images of a clock would be due to expansion of space, not an increase in the rate of time), or if it still makes no sense because there is no reference clock that can be defined "outside the universe" to escape the effects of expansion, or no way to compare a space that's undergone expansion to one that hasn't, etc.

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13 minutes ago, md65536 said:

I suggested this before but it was split off and buried: To test for a changing rate of time you would compare observations of the rate of a clock from earlier in its history, relative to the clock now.

Why can’t it be with another clock at another location, but in the past?

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9 minutes ago, swansont said:

Why can’t it be with another clock at another location, but in the past?

It could, but I think you'd have to remove any effects that depend on location, if you're trying to measure changes over time alone and not over space.

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12 hours ago, swansont said:

How would you test for non-constant time?

How would you test for constant time?

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1 hour ago, AbstractDreamer said:

How would you test for constant time?

You’re the one pushing non-constant time. If you have no way to test it, then it can’t have any measurable effect.

8 hours ago, md65536 said:

It could, but I think you'd have to remove any effects that depend on location, if you're trying to measure changes over time alone and not over space.

Same is true for comparing a clock with itself.

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11 hours ago, md65536 said:

1. Time is what clocks measure, by definition.
2. The only way to measure or reason about the rate of time, is to compare time measured by one clock to that of another clock.
3. A clock will always tick at 1 s/s compared to itself, there's simply no room for a clock to disagree with itself.

AbstractDreamer, I can't make sense of what you're saying without knowing what definitions you're using or where you disagree with these 3 things.

Ha!  I honestly struggle making sense of it myself, definitions in words is very difficult, and my maths is school boy level so I cant model anything.

But I think overall we are grasping at the same concept.

Let's say that this notion of "time over time" is NON-constant.  What I mean by this is that the "rate that time propagates" is different at two different values of time, relative to a theoretical frame of reference that is "outside" or "independent" of time.  Now clearly there is no such frame of reference in reality.  Everything in our spacetime is intrinsically bound within time.  Hence it is a theoretical, or imaginary frame of reference.  But that the frame is suitably "static" relative to both events in time.

So let's say that one second, today at time t=13.8B years, is defined by caesium frequency of caesium atoms that exist at time t=13.8B years.  Let's imagine at time t=1 year that caesium atoms exist, and 1 second is again defined by caesium frequency of caesium atoms that exist at time t=1 year.

So then you measure the speed of light in units of "distance per caesium frequency".   Assuming distance is constant (this is another problem), then both today and at t=1 year, both measurements would arrive at the same result - relative to the caesium frequency of that time

But if somehow you compared the two caesium frequencies, using a measure and a frame of reference independent of both time today and time 1 year, then a "non-constant time over time" would allow the two frequences to differ -  and yet nothing need be violated for either of the two events, within their respective time.

I'm really not sure how to respond to those three points and how they might relate and where I might disagree.  Those three points are total valid, but there are practical limitations implied within them.  Point 2 is about measuring time.  My point accepts that measuring is currently experimentally impossible.   Point 3 is about using the clock as its own frame of reference.  This is different to a frame of reference independent of time.

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On 8/24/2022 at 12:31 PM, AbstractDreamer said:

Whether Time is linear or cyclical or something else, is the speed of time constant?

What basis do we have to extrapolate time back to t=0 (or very close to 0) and assume it's speed is the same as it today?

I read descriptions of the early universe describing things like "A few millionths of a second (after the big bang), quarks aggregated to produce protons and neutrons"

How would an observer in the early universe measure time when there are only quarks, protons and neutrons?

Is it possible the speed of time is variable?  Could 1 millionth of a second in the early universe be equivalent to a few million years today, relative to that early universe?

If time is accelerating universally, would this not affect any empirical evidence of any experiments performed to date, unless we can time travel?

If time is accelerating, could that explain dark energy and space expansion?

Your question is nonesense because speed is distance over time d/t ! Your question reads , what is the d/t of time which makes no sense !

Perhaps you mean , what is the rate of time ?

Our present measured rate of time is 1 second of history per 1 second passed measure !

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10 minutes ago, swansont said:

If you have no way to test it, then it can’t have any measurable effect.

While this is a sensible statement rooted in logic, it's not a universal law or principle afaik.  Is there any way to test Hawking Radiation at the event horizon on the black hole?  Does it have a measurable effect?   Is there any way to test space expansion at the edge of our observable universe?  Does it have a measureable effect?

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If spacetime is a continuum, and space has been observed to expand, why is it not described as time contraction?  Why must the constancy and consistency of time be preserved?  What makes the immutability of time more sacred than space?

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5 hours ago, AbstractDreamer said:

While this is a sensible statement rooted in logic, it's not a universal law or principle afaik.  Is there any way to test Hawking Radiation at the event horizon on the black hole?  Does it have a measurable effect?   Is there any way to test space expansion at the edge of our observable universe?  Does it have a measureable effect?

Is it possible to measure Hawking radiation? Yes.

https://arxiv.org/abs/1401.6612 (the journal version is paywalled)

Not sure why you have to be at the event horizon to do this; that’s an unreasonable restriction, as is requiring that one be at the edge of the observable universe.

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8 hours ago, swansont said:

Same is true for comparing a clock with itself.

Does this mean it could be done (compare a clock with itself in the past while accounting for any effects related to the location of everything in the universe relative to the clock)? The implication is that any expected difference in the rate of the clock over time can be attributed to a change in the universe relative to the clock, leaving nothing that could be attributed to a change in time itself, in accepted theory?

8 hours ago, AbstractDreamer said:

What I mean by this is that the "rate that time propagates" is different at two different values of time, relative to a theoretical frame of reference that is "outside" or "independent" of time.  Now clearly there is no such frame of reference in reality.

If it's made up and can't be measured, how can you possibly draw a useful conclusion from it? What's stopping you from imagining it to be something where the rate of time varies wildly relative to what we measure, and someone else imagining it to be something where the two measures are the same?

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1 hour ago, md65536 said:

Does this mean it could be done (compare a clock with itself in the past while accounting for any effects related to the location of everything in the universe relative to the clock)? The implication is that any expected difference in the rate of the clock over time can be attributed to a change in the universe relative to the clock, leaving nothing that could be attributed to a change in time itself, in accepted theory?

In a limited fashion. You can send a clock signal into a long optical fiber and there will be a time delay which you can compare with the current output of the clock.

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On 9/13/2022 at 8:53 PM, md65536 said:

If it's made up and can't be measured, how can you possibly draw a useful conclusion from it? What's stopping you from imagining it to be something where the rate of time varies wildly relative to what we measure, and someone else imagining it to be something where the two measures are the same?

Its made up in the same way that we assume time is constant over time.   Any observations or experimental evidence that is itself "trapped" in time can neither support or disprove either that time is constant or non-constant.     And yet we are happy, without evidence or measurement, to assume it is constant; and draw as a useful conclusion, the FLRW metric to describe a Universe that changes over time - it "looks different at different moments in time".  Is this not in contradiction with its OWN premise of homogeneity of the Universe?

Edited by AbstractDreamer

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