Clocks running slower is an illusion

[YellowTaxi]

I read this somewhere on a physics forum recently

Clocks running slower for moving observers is an illusion in any case and is observer dependent, which means it has no true physical significance. SR is based on the invariance of the proper-interval, i.e. all observers will agree on the elapsed time on a clock, when they coincide spatially with the clock.

What is meant by this statement? I spent a short while thinking about it but can't follow what the person is going on about. Maybe I misunderstood him, but he seems to be claiming that time dilation is actually an illusion.

 

[Vanadium 50]

Wouldn't it make more sense to ask him directly rather than asking some third parties what he might have meant?

 

[HallsofIvy]

The first statement is simply wrong- time dilation is NOT an "illusion". The second statement is trivial- two people at the same spot and moving at the same velocity (so they stay at the "same spot") will, of course, both observe a clock as running at the same speed.

 

[Dale]

Time dilation is a measurable but coordinate-dependent effect. The person you are quoting is simply of the opinion that coordinate-dependent effects are "illusion". It is a reasonable opinion, but far from universal.

 

[YellowTaxi]

Wouldn't it make more sense to ask him directly rather than asking some third parties what he might have meant?

It was posted by Mentz on here recently, I wanted to ask what he meant but doubted that I'd be able to attract his attention. So I put it up here for debate - hoping that you guys could shed some light on it for us. It seems an intersesting statement but I can't make head or tail of it myself. (sorry if that's a violation of the rules here , but anyway it seems a pretty good topic for discussion in its own right anyway...)

 

[YellowTaxi]

Time dilation is a measurable but coordinate-dependent effect. The person you are quoting is simply of the opinion that coordinate-dependent effects are "illusion". It is a reasonable opinion, but far from universal.

So he's saying that SR is an illusion ?

 

[Dale]

So he's saying that SR is an illusion ?

No, not everything predicted by SR is coordinate-dependent.

Personally, I don't like the word "illusion" any more than the word "real". It is a semantic argument waiting to happen. Time dilation is measurable and coordinate-dependent. That is sufficient for me.

 

[YellowTaxi]

I'm not sure what you mean by 'coordinate-dependent'
Do you mean frame dependent ?

 

[Dale]

Yes. "Reference frame" is simply another term for "coordinate system".

 

[yuiop]

Where two frames are moving relative to each other and remain strictly inertial (no acceleration) then perhaps in some sense the mutual time dilation is illusional because there is no absolute way to determine which clock is running slower than the other. When acceleration enters the picture then the time dilation is very real. For example a clock lowered down into a gravitational well and raised up again will show less elapsed time than a clock that remained higher up. In the twins paradox less time passes for the traveling twin in an absolute real sense. If a long rocket with synchronised clocks is accelerated using Born rigid acceleration, it will be found that the clocks on the rocket will be out of sync when the rocket stops accelerating and resumes inertial motion again. This is an indication of real physical time dilation that can be observed by the observers onboard the rocket. Basically, if two observers feel different accelerations due to their positions in accelerating rocket or due to being stationary at different heights in a gravitational field, then the clocks of the two observers will be running at different rates in a predictable, measurable and physically real sense.

 

[Mentz114]

I own up to making that statement, and I stand by it ( HallsOfIvy note ).

The term 'time-dilation' to me always meant 'moving clocks run slowly' which refers to the apparently paradoxical situation when two observers see that each others clocks are running slowly. If that is not an illusion, it must be a paradox.

Maybe illusion is the wrong word, but real it is not.

The only thing in SR that is not an illusion is the proper interval, which some people call 'time-dilation' which causes confusion and misunderstanding.

 

[Bob S]

Time dilation is not an illusion. Here is a quantitative measurement. Radioactive particles like muons have a decay time constant (in vacuum) of about τ=2.2 microseconds. Experimenters at Brookhaven National Laboratory stored muons in a magnetic synchrotron ring at a γ of 29.4. The measured lifetime in the lab was about 64.7 microseconds. The distance traveled during this time was βγcτ.
Bob S

 

[Mentz114]

Time dilation is not an illusion. Here is a quantitative measurement. Radioactive particles like muons have a decay time constant (in vacuum) of about τ=2.2 microseconds. Experimenters at Brookhaven National Laboratory stored muons in a magnetic synchrotron ring at a γ of 29.4. The measured lifetime in the lab was about 64.7 microseconds. The distance traveled during this time was βγcτ.
Bob S

There's a case for saying that this is a twins type scenario. How were the detectors arranged ? Aren't we just comparing clocks between a muon that has gone on a journey and one which stayed put ? In that case, naturally the traveling muon appears to live longer in the lab frame, and this is not what I'm calling time dilation.

I repeat:
The term 'time-dilation' to me always meant 'moving clocks run slowly' which refers to the apparently paradoxical situation when two observers see that each others clocks are running slowly. If that is not an illusion, it must be a paradox.

 

[edpell]

Brian Greene in his book The Elegant Universe has a nice analogy of a dragster with a speed of c running either straight down the track or running at an angle to straight. If it runs at an angle it will make less progress down the track in a unit of time. If we think of straight as the time direction and orthogonal to straight as a spatial direction then if we stand still spacially we will make maximum progress in time. If we move about left and right (in general orthogonal to the time direction) we will spend most of our motion spacially and have little left over to make progress in the time direction.

In orders words every thing in the universe is ALWAYS traveling at the speed of light it is just a question of how much is devoted to the time direction and how much is devoted to the spatial directions.

 

[bcrowell]

To me this sounds like an argument over the meaning of words like "illusion," which aren't really precisely defined, so the argument can never be resolved.

When I teach SR, I find that one of the most common things students have a hard time with is whether effects like time dilation "really happen" or whether it "just seems that way." AFAICT, the "just seems that way" interpretation usually implies that they think there is some correct picture that is hard to obtain, and observers get a distorted version of that picture because of their state of motion. They need to be led to understanding that there is no correct picture in the sense that what they have in mind is a preferred frame of reference in which simultaneity is well defined. Usually what seems to work in explaining this is to have them confront cases where they end up realizing that you can't get a snapshot of what's going on at one moment.

My usual summary of the above is that SR effects are not illusions, since an illusion is usually a situation where there's something true, and then some distorted version of the truth. But I'd also be perfectly happy to agree with Mentz114 that SR effects are illusions -- it all depends on what you mean by "illusions."

I'm less inclined to agree with Mentz114 that "The only thing in SR that is not an illusion is the proper interval." You can certainly divide up all the quantities we talk about into a class that is invariant and a class that is not invariant. But if you then try to do any physics while forbidding yourself to refer to the non-invariant quantities, you won't succeed. Something similar happens with gauge transformations. We know that it's the fields that are observable, not the potentials, and yet the Aharanov-Bohm effect shows that you can't just consign the potentials to the dust-bin of history.

 

[danielatha4]

I can understand where Mentz is coming from.

Imagine a scenario in which an object is traveling .9999c m/s relative to an outside observer.

Imagine that this outside observer is at rest relative to yet another outside observer (outside observer B)

And relative to outside observer b there is not only one "person" or "object" that's at rest but an infinite amount of peoples or objects that each have a different velocity. For example, the observer at rest, an object traveling .1c, an object traveling .2c, etc. and even sub-dividing those velocities even further creating an infinite amount of observers.

Relative to the infinite observers the initial object traveling .9999c m/s is now seen by infinite inertial reference frames to have infinite rates of time dilation. If the object traveling at .9999c came to rest would it really be scientific to say that the clock could show any difference in time depending on which observer looks at it?

 

[Al68]

The term 'time-dilation' to me always meant 'moving clocks run slowly' which refers to the apparently paradoxical situation when two observers see that each others clocks are running slowly. If that is not an illusion, it must be a paradox.

Would you say that if one observer sees the sun rise on his right and another sees the sun rise on his left, then either there is a paradox or at least one of the observations is an illusion?

Is any observation that depends on frame of reference either an illusion or a paradox?

 

[edpell]

Two observers move away from each other each see the other as getting smaller while their own size remains unchanged. No paradox. Now if they were brought back together and one really was smaller that would be a paradox.

Likewise for two observers that each accelerate equally away from each other they both see the others clock as slower. No paradox. Now if they were symmetrically brought back together and one really have less time on their clock that would be a paradox.

 

[Mentz114]

Would you say that if one observer sees the sun rise on his right and another sees the sun rise on his left, then either there is a paradox or at least one of the observations is an illusion?

There is obviously no illusion or paradox here. Both observers will agree that the sun rose.

Is any observation that depends on frame of reference either an illusion or a paradox?

No, only measurements.

SR is a local theory. Statements about spatially separated clocks must be treated with caution, because of velocity dependent effects.

If two spatially separated observers measure each others clock rates, and they know from an independent measurement what their relative velocity is, then a back claculation will tell them that both clocks are still operating as before, and there is no 'time-dilation'.

Therefore I assert that any dilation based on this type of calculation is an illusion. It isn't real, the clocks are still running at the same rate.

The word 'illusion' has obviously upset people. I don't mean illusion as in 'hallucination', but more like an optical-illusion. Spear fishing is an example. The fish is not where it appears to be. It's apparent position is an optical-illusion, and you won't catch anything by stabbing at the apparent location.

 

[Mentz114]

I've been looking at dictionary definitions of 'illusion' and 'optical illusion'.

Illusion

3. (Physiol.) A sensation originated by some external object, but so modified as in any way to lead to an erroneous perception; as when the rolling of a wagon is mistaken for thunder. [1913 Webster]

Note: Some modern writers distinguish between an illusion and hallucination, regarding the former as originating with some external object, and the latter as having no objective occasion whatever. [1913 Webster]

an erroneous mental representation

a false perception

something many people believe that is false; "they have the illusion that I am very wealthy"

Optical Illusion
an optical phenomenon that results in a false or deceptive visual impression
wordnetweb.princeton.edu/perl/webwn

An optical illusion (also called a visual illusion) is characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a percept that does not tally with a physical measurement of the stimulus source. ...
en.wikipedia.org/wiki/Optical_illusion

Not what I expected. So I have been misusing the word because there's no element of 'false perception' or 'erroneous mental representation' in the time-dilation scenarios.

So I have to find another term to use.

Special relativity fixes the problems that come up if Maxwell's equations are combined with Gallilean relativity. This is obviously very important and necessary, but as a result of the postulates, especially the finite speed of light, some non-physical side effects are observed.

One thing that's always bothered me is how much time is spent discussing these NPSE's on this forum. Some learners ( such as the OP in the thread from which my original remark is taken) jump on these as if they are important, while apparently not appreciating what SR is for.

 

[Al68]

I've been looking at dictionary definitions of 'illusion' and 'optical illusion'.

Not what I expected. So I have been misusing the word because there's no element of 'false perception' or 'erroneous mental representation' in the time-dilation scenarios.

So I have to find another term to use.

Since I'm so creative and original, I'll coin a term to use for that purpose: Frame dependent.

 

[Dale]

Since I'm so creative and original, I'll coin a term to use for that purpose: Frame dependent.

Hmm, since I am not original I will just copy your term.

 

[Polyrhythmic]

There's a case for saying that this is a twins type scenario. How were the detectors arranged ? Aren't we just comparing clocks between a muon that has gone on a journey and one which stayed put ? In that case, naturally the traveling muon appears to live longer in the lab frame, and this is not what I'm calling time dilation.

I repeat:
The term 'time-dilation' to me always meant 'moving clocks run slowly' which refers to the apparently paradoxical situation when two observers see that each others clocks are running slowly. If that is not an illusion, it must be a paradox.

The fact that the traveling muon lives longer in the lab frame is due to time dilation, no matter what you call it. With respect to the reference frame, the traveling muon's 'clock runs slower' than the one at rest, and therefore its decay is delayed. Just because something is counter-intuitive, it isn't necessarily a paradox or an illusion.

 

[Mentz114]

The fact that the traveling muon lives longer in the lab frame is due to time dilation, no matter what you call it. With respect to the reference frame, the traveling muon's 'clock runs slower' than the one at rest, and therefore its decay is delayed.

The traveling muon's clock does not run slower, it just appears to in the lab frame.

Just because something is counter-intuitive, it isn't necessarily a paradox or an illusion.

Well, you don't say, I never thought of that.

 

[Polyrhythmic]

It does run slower with respect to the lab frame. The point is that there is no absolute truth, each statement regarding the clock is true within the reference frame it is made in. Therefore words like "seems" and "illusion" are meaningless.

 

[edpell]

The traveling muon's clock does not run slower, it just appears to in the lab frame.

We could say the half life of a muon with mass 106 Mev is 2.2 microseconds and the half life of a muon with mass 1060 Mev is 22. microseconds.

 

[Rasalhague]

Brian Greene in his book The Elegant Universe has a nice analogy of a dragster with a speed of c running either straight down the track or running at an angle to straight. If it runs at an angle it will make less progress down the track in a unit of time. If we think of straight as the time direction and orthogonal to straight as a spatial direction then if we stand still spacially we will make maximum progress in time. If we move about left and right (in general orthogonal to the time direction) we will spend most of our motion spacially and have little left over to make progress in the time direction.

In orders words every thing in the universe is ALWAYS traveling at the speed of light it is just a question of how much is devoted to the time direction and how much is devoted to the spatial directions.

Except that, counterintuitively, in spacetime it's the other way round. Since the invariant interval is given by the equation $(\mathrm{d} \tau)^2 = (\mathrm{d} t)^2 - (\mathrm{d} x)^2 - (\mathrm{d} y)^2 - (\mathrm{d} z)^2$ (or similar, depending on which convention you use to define it), that sign difference between time and space components means that the dragster will have to make more progress through time the more progress it makes through space. When we stand still spatially, we make minimum progress through time; the proper time between two events is smaller than the coordinate time in any frame where they happen in different places.

 

[Bob S]

We could say the half life of a muon with mass 106 Mev is 2.2 microseconds and the half life of a muon with mass 1060 Mev is 22. microseconds.

According to the muon, its mass is always 105.658 MeV, and its mean life is always 2.197 microseconds.
Bob S

 

[yuiop]

Let's take a look at the assumption that time dilation (and length contraction) in SR is not an illusion. Under this assumption, we note that moving clocks run slower and moving rulers are shorter. Clocks and rulers are measuring devices, so we should treat with caution the measurements we make with those devices and it is a logical conclusion that what we measure is not always an absolute depiction of reality. In this sense, there are certain circumstances where what is measured is best described as an illusion.

For example let us take a look at some simple observations analysed from the Lorentz Ether Theory point of view (that gives identical predictions to Special Relativity). Imagine observer A is at rest with the Lorentz Ether. He observs B moving with respect to the LE and notices that B's clock is running slower relative to his own clock, due to the motion of observer B's clock relative to the ether. Observer B on the other hand claims that A's clock is running slow. Observer A responds that B is under an illusion because B is making measurements with a slow clocks, short rulers and clocks that are out of sync as far as A is concerned. B can counter claim that A is under an illusion for exactly the same reasons if B considers himself stationary with respect to the ether! Although we can not detect the ether, we can reason that if two observers are moving with respect to each other, at least one of the observers must be moving relative to the ether and at least one of the observers must be under an illusion (because the instruments they are making measurements with are affected by their absolute motion). Analysed in terms of LET it is very clear and very logical that the mutually observed time dilation of two observers with inertial motion relative to each other, is the result of an illusion of at least one of the observers. Sure, the end result is that both observers measure the clocks of the other observer to be running slower than their own.

 

[A.T.]

Brian Greene in his book The Elegant Universe has a nice analogy of a dragster with a speed of c running either straight down the track or running at an angle to straight. If it runs at an angle it will make less progress down the track in a unit of time.

Except that, counterintuitively, in spacetime it's the other way round. Since the invariant interval is given by the equation $(\mathrm{d} \tau)^2 = (\mathrm{d} t)^2 - (\mathrm{d} x)^2 - (\mathrm{d} y)^2 - (\mathrm{d} z)^2$ (or similar, depending on which convention you use to define it), that sign difference between time and space components means that the dragster will have to make more progress through time the more progress it makes through space.

You are thinking in terms of Minkowski space-time. Greene uses a different geometrical interpretation, where proper-time is the temporal dimension on equal footing with the spatial dimensions, while coordinate-time is the path length:

$(\mathrm{d} t)^2 = (\mathrm{d} \tau)^2 + (\mathrm{d} x)^2 + (\mathrm{d} y)^2 + (\mathrm{d} z)^2$

Here a visualization of what Greene means:
http://www.adamtoons.de/physics/relativity.swf

 

[Rasalhague]

Oh, I see. I'll have to read up more on Epstein diagrams. Just found another link here:

http://www.relativity.li/en/epstein2/read/

I've met the idea of 4-velocity, but haven't really looked at alternative visualisations to Minkowski's yet.

 

[YellowTaxi]

Where two frames are moving relative to each other and remain strictly inertial (no acceleration) then perhaps in some sense the mutual time dilation is illusional because there is no absolute way to determine which clock is running slower than the other.

I don't think that's true kev. You could video tape (or digitally record) the action going on inside a close to c spaceship then examine it later when the ship is back on earth. Or transmit the digital data as a short burst of information (to avoid doppler complication). The ticking of time observed would be absolute, and should according to SR be equal to 1s/s - ie the same as on earth.

The ticking of time when observed through an Earth based stationary telephoto camera tracking the spaceship occupants throgh a side window should be time dilated acording to SR. So theoretically it could be tested - just not with todays technology, since we can't get spaceships close enough to c that's all.