# Clocks running slower is an illusion

1. Dec 28, 2009

### YellowTaxi

I read this somewhere on a physics forum recently

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.

2. Dec 28, 2009

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

3. Dec 28, 2009

### 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.

4. Dec 28, 2009

### Staff: Mentor

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.

5. Dec 28, 2009

### YellowTaxi

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...)

6. Dec 28, 2009

### YellowTaxi

So he's saying that SR is an illusion ?

7. Dec 28, 2009

### Staff: Mentor

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.

8. Dec 28, 2009

### YellowTaxi

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

9. Dec 28, 2009

### Staff: Mentor

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

10. Dec 29, 2009

### 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 travelling 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.

Last edited: Dec 29, 2009
11. Dec 29, 2009

### 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.

12. Dec 29, 2009

### 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

13. Dec 29, 2009

### Mentz114

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 travelling 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.

Last edited: Dec 29, 2009
14. Dec 29, 2009

### 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 spacial 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 spacial directions.

15. Dec 29, 2009

### bcrowell

Staff Emeritus
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.

16. Dec 29, 2009

### 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?

17. Dec 29, 2009

### Al68

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?

18. Dec 29, 2009

### 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.

19. Dec 30, 2009

### Mentz114

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

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.

Last edited: Dec 30, 2009
20. Dec 30, 2009

### Mentz114

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.

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.