# Can you explain how time is not constant?

So I read that time is actually relative, and that if I had a clock in orbit around earth and a clock on earth, that the one in orbit would move slower. Surely this is a problem with the clock, is it not? How could time possibly be slower, that makes no sense to me?

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Nugatory
Mentor
So I read that time is actually relative, and that if I had a clock in orbit around earth and a clock on earth, that the one in orbit would move slower. Surely this is a problem with the clock, is it not? How could time possibly be slower, that makes no sense to me?
Take a look at the muon lifetime measurements; there's a pointer in the FAQ on experimental support for relativity.

And then I'll turn your question around... WHY can't it make sense for time to pass at different rates for observers in different circumstances? It's certainly not consistent with our experiences dealing with relative speeds that are small compared with the speed of light, but that doesn't mean that it cannot make sense.

jtbell
Mentor
How could time possibly be slower, that makes no sense to me?
Can you cite or construct a specific example that doesn't make sense to you? Then people here can address it.

It is kind of funny... clocks in motion are observed to run slow and measuring rods in motion observed to contract. Typically, other measuring devices that misbehave like that in extreme conditions are considered a poor choice, but what else is there?

Well, NASA ran a test where they had clocks in space synced with a few clocks on earth, and they found that those in orbit of earth moved more slowly, can this be explained? Time always seemed to me as constant, the idea that slowing down time is possible seems science fiction.

It is kind of funny... clocks in motion are observed to run slow and measuring rods in motion observed to contract. Typically, other measuring devices that misbehave like that in extreme conditions are considered a poor choice, but what else is there?
Well, there'd be a difference of opinion, since observers traveling with those clocks & rods don't agree that there's any contraction. Also, any of these decisions about clock/rod contraction are dependent not simply upon observations from one observer's POV but also on other POV which the observer deems to be simultaneous / synchronized. But are they? Perhaps "simultaneity" is the misbehaving device...

pervect
Staff Emeritus
There is nothing that absolutely requires that two clocks, taking different paths through space-time, have to show the same time when they re-unite.

A simple analogy - there isn't any problem if two cars take different paths if you compare odometer readings - you don't expect them to be the same when they arrive, and in fact you expect the lowest reading to be on the car that travelled in a straight line.

A clock is something like an odometer applied to a space-time diagram.

I peronsally wouldn't attribute this to "time not being constant". But relativity IS incompatible with the notion of a "universal now". YOu'll have to modify your notion of "now" to realize that "now" depends on the observer, if you want to understand relativity.

I'm not quite sure if this lack of a universal now is the same as what you mean when you say "time isn't constant". So I thought I'd try to clarify the issue as much as I possibly could. I do hope this helps.

phinds
Gold Member
2019 Award
... makes no sense to me?
The universe doesn't care what makes sense to us humans. If you see something that seems to be taken as gospel by scientists in general (and time dilation certainly falls in that category since it has been proven numerous times in numerous ways), it is not helpful to start off saying "that makes no sense" but rather to say "ok, apparently this is true so why is it that it makes no sense to me". Try to articulate what you find puzzling, not just say that the concept in general is puzzling. WHY is it puzzling?

Time dilation is WAY easier to get your head around than some of the concepts in quantum mechanics. I mean they REALLY don't "make sense", they just are true.

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So I read that time is actually relative, and that if I had a clock in orbit around earth and a clock on earth, that the one in orbit would move slower. Surely this is a problem with the clock, is it not? How could time possibly be slower, that makes no sense to me?
The issue is purely a pragmatic one: how would you define "time" in physics, if not by means of some kind of clocks? Even the solar system which served as basis for time reckoning is a gigantic clock. In physics, when it is said that "time" slows down in a certain situation (e.g. a clock in a gravitational field, or a fast circulating clock), what is meant that any clock will slow down as measured with an unaffected reference clock. Another expression for the same is "clock retardation"; however as it equally refers to all physical processes, "time dilation" is more commonly used.

Well, NASA ran a test where they had clocks in space synced with a few clocks on earth, and they found that those in orbit of earth moved more slowly, can this be explained? Time always seemed to me as constant, the idea that slowing down time is possible seems science fiction.
The concept of "curvature" may be helpful. We've been brought up to know that the Earth is spherical. But what if you were brought up to think of the world is flat? How would you react to the suggestion that one can walk for a mile east and end up where you started (walking near the poles)? It wouldn't make any sense. Space-time is curved in a similar way which we never notice normally. The curvature of the Earth is just science-fiction if you live your life in a small part of a seemingly flat Earth. Also, remember that for the orbiting observer, one's *own* clock appears to be running normally. Relativity never takes that away from you. It's always the other guy's clock that seems to be running in an odd way.

That makes sense, just something new to get used to I suppose. Thanks

Another expression for the same is "clock retardation"; however as it equally refers to all physical processes, "time dilation" is more commonly used.
just so I'm clear here - when a physicist says "time slows down" they are not talking about a fourth dimension?
(sorry ...I'm lurking)

phinds
Gold Member
2019 Award
just so I'm clear here - when a physicist says "time slows down" they are not talking about a fourth dimension?
(sorry ...I'm lurking)
Yes, they are talking about time, which is the 4th dimension in the 4-dimensional space-time construct in which we live. I think you mean it in some other, undefined, sense, so probably the answer to your question as you intend it is no.

To add to phinds: in spacetime, there are 3 spatial dimensions and 1 time dimension. Anyone that talks about more than 4 dimensions is a crackpot or is a string theorist which is still very early and has not been confirmed experimentally.

To add to phinds: in spacetime, there are 3 spatial dimensions and 1 time dimension. Anyone that talks about more than 4 dimensions is a crackpot or is a string theorist which is still very early and has not been confirmed experimentally.
Or a mathematician.

Well, NASA ran a test where they had clocks in space synced with a few clocks on earth, and they found that those in orbit of earth moved more slowly, can this be explained? Time always seemed to me as constant, the idea that slowing down time is possible seems science fiction.

I have read, "scientists never understand theories, they just get used to them."

Nugatory
Mentor
Mathematically, there's no problem with four dimensional space: A one-dimensional space is a set of points that can be identified with one number (a line); a two-dimensional space is a set of points that can be identified with two numbers (x and y coordinates on a piece of graph paper, latitude and longitude on the surface of the earth); a three-dimensional space requires three numbers (to identify the location of an airplane in three dimensional space we need latitude, longitude, and altitude); and the same mathematical logic works just fine for spaces in which four numbers are needed to identify a point.

If there are more than three dimensions, we can't visualize the space as easily as we can the lower-dimensional spaces, but there's nothing wrong with the math.

It also turns out that this four-dimension mathematical structure is useful for describing some of the laws of physics. It takes four numbers to describe a moving particle: one to say WHEN it is somewhere and three to say WHERE in our three-dimensional world that somewhere is. It's a matter of personal taste whether you choose to describe this situation as "Time is the fourth dimension" or not; but you don't have to accept that statement as literally true to put the four-dimensional math of relativity to good use. It's a mathematical tool, and like any tool you can use it as it long as it works and discard it in favor of a different tool if it stops working.

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That should depend on how they're talking about the excess dimensions.

Mathematically, there's no problem with four dimensional space:
Oh I agree totally with all you say.
I just think the term "time" is thrown around far too easily without
us having a specific definition of what is being alluded to.
A lot of effort has gone into understanding gravity and the higgs because it is
fundamental to understanding issues - and yet this year it was announced that the higgs
had been "found." This seems to be the universal statement.
Please correct me if I missunderstood something but my understanding is that the higgs
was not found. My understanding is that the probability of it existing and where it is
likely to exist has been identified to a very high degree of certainty - but that nobody has
as yet detected and actual higgs boson. Am I wrong in that? If so appologies and
appreciation for the correction.

However I think the same effort in defining exactly what time is is long overdue.
Is it in fact some sort of dimension - if so prove it by experiment - if not what are we
using - is it just a sequence of events - if so what is causing the "appearance" of time dilation in experiments -or is it just a convenent mathematical construct that happens to coincide with certain experiments.

The entire notion of "cosmological constants" bugs me. And ""time" seems to be used that way.

yoyo....that time runs at different rates is not in our everyday observations...our senses are not attuned to it...nor do we observe everyday that space [distance] also varies among observers...it takes some time to get used to those ideas. We did not evolve such senses, I guess, because we could survive and evolve without doing so.

Space and time are not constant as they appear, nor is it apparent that the speed of light IS constant in all inertial frames. Different observers make different readings. A way to possibly think about 'change in observed length' is to note the example of a distant friend standing alongside a house....both appear 'small' to you....as you appear to that friend from his distant position. Yet you both agree you are some fixed distance apart...it's just the vertical scale that 'looks strange'... He says "this house is 22 feet tall; you say, "it only looks a few inches tall from here"....how do you resolve that 'paradox'....only by comparing heights from a common position...either his, yours, or something in between....then you both 'see' [measure] things the same way.

Keep in mind that right now, YOU are moving at near the speed of light in many reference frames...and so from those, say way out near the Hubble radius in the distant universe, observers would detect YOUR clock as running way slower than their local clock.

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Dale
Mentor
However I think the same effort in defining exactly what time is is long overdue.
Time is well defined. We already discussed this in the Gravitational Time Dilation thread. The fact that you don't like the definition doesn't invalidate it.

Is it in fact some sort of dimension - if so prove it by experiment -.
Done: http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

All of the referenced experiments are consistent with the Minkowski model of 4D spacetime, so it is "proven by experiment" to the maximum degree that such a statement can even be said to make sense.

Keep in mind that right now, YOU are moving at near the speed of light in many reference frames...and so from those, say way out near the Hubble radius in the distant universe, observers would detect YOUR clock as running way slower than their local clock.
Hmm. I wonder if "detect" doesn't stretch it a bit... I tend to prefer "calculate" or "determine" or "decide"...

Nugatory
Mentor
Hmm. I wonder if "detect" doesn't stretch it a bit... I tend to prefer "calculate" or "determine" or "decide"...
It's some of both. we can't say anything about a remote clock unless we have some observation about it, but we usually do some calculation as well. For example....

When my clock reads zero someone passes me at .5c, and as he passes I notice that his clock also reads zero. I keep watching his clock as it moves away, so after 6 seconds I see the light that left his clock when he had been traveling for 4 seconds and was 2 light-seconds away. I see this clock reading when my clock reads 6 seconds, and I say to myself "aha - that's what his clock said when my clock said 4, because it took the light 2 seconds to get to me and my clock says 6 now; and this clock reading that I see is less than 4 so I've just seen time dilation".

I've both "detected" his clock readings and "calculated" some dilation. (I've also badly abused the word "seen" in boldface above, but we've all committed worse sins than that).

You should have noticed that DaleSpam also worked in the critically important phrase "to the maximum degree that such a statement can even be said to make sense".

I see the light that left his clock when he had been traveling for 4 seconds and was 2 light-seconds away. I see this clock reading when my clock reads 6 seconds, and I say to myself "aha - that's what his clock said when my clock said 4, because it took the light 2 seconds to get to me and my clock says 6 now; and this clock reading that I see is less than 4 so I've just seen time dilation".

I've both "detected" his clock readings and "calculated" some dilation. (I've also badly abused the word "seen" in boldface above, but we've all committed worse sins than that).

You should have noticed that DaleSpam also worked in the critically important phrase "to the maximum degree that such a statement can even be said to make sense".
Just this: All of the physics you can do is consistent and makes sense if you adopt the traveler's IRF. The decision to use your own rest frame is a decision and not a measurement.

Nugatory
Mentor
this year it was announced that the higgs
had been "found." This seems to be the universal statement.
No, it is the near-universal oversimplification that you'll read in the popular press. the actual publications are much more precisely phrased and pretty much consistent with:
My understanding is that the probability of it existing and where it is
likely to exist has been identified to a very high degree of certainty - but that nobody has
as yet detected and actual higgs boson. Am I wrong in that?

And no matter how long we work on it, that's as "found" as it's ever going to be. Over time the probabilities will approach 100%, but it will never be "found" in the way that you're demanding.

A lot of science is that way. For example... the existence of gravity has not been conclusively proven. We know that every single person who has jumped from the top of a tall building has experienced a force that pulls them towards the ground (usually with catastrophic results for the jumper), we have never found any reason to doubt that the next jumper will have the same unpleasant experience, the theories of gravity (Einstein's, Newton's, and even Aristotle's) predict that the next jumper will have that unpleasant experience.... but we cannot prove that the next person to jump from the top of a tall building will have the same experience. Nonetheless, it would be perverse and irrational to suggest that the next jumper won't fall.

Nugatory
Mentor
I just think the term "time" is thrown around far too easily without us having a specific definition of what is being alluded to.... The entire notion of "cosmological constants" bugs me. And "time" seems to be used that way.
The cosmological constant is the quantity $\Lambda$ in $G_{uv}+\Lambda{g_{uv}}=8\pi{T}_{uv}$ and its correct value will ultimately be established by experiment and observation. Some theories suggest that it may have one value or another, but these theories will be accepted or rejected based on whether they correctly describe how the world really works, based on observation and experiment.

I don't see a lot of similarity between that notion and how "time" is used in modern physics. You've already rejected Einstein's definition of time ("time is what a clock measures") but it's not clear from your follow-up posts that you have read and understood Einstein's discussion of exactly what that means.

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