23 Nov 2007 17:51:54 UTC

Topic 193338

(moderation:

If I understand this right there are 2 kinds of 'slowing of time' that occurs when you observe a fast moving object. One is just an elusion based on the increasing distance and relative speed but one seams to be something that actually happens and not just an observational effect.

Now regarding only the real physical slowing of time. Wee will assume in this scenario that there are a clock on the rocket that you can observe as the rocket fly away. If you observe a rocket that fly away from you and accelerates to 0.9 c and then turn off the engine before you start to measure the time. Is the slowing of time that occurs caused by the fact that the object have been accelerated prior to the speed or is it caused just by the relative speed or a combination of both?

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## Is there a physical slowing of time that is only caused by the s

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I guess this Wikipedia article might be useful: http://en.wikipedia.org/wiki/Time_dilation

CU

Bikeman

## Thank you Bikeman. If I

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Thank you Bikeman.

If I understand this right the 'real' slowing of time is caused only by acceleration or gravitation and not by the relative speed.

So I assume that when the rocket turn off the engine and stop accelerating the time will move at the same rate as on Earth.

In the 'Time dilation and space flight' example in the page you linked to it also looks like a acceleration of 1g makes the time to move slower than a gravity of 1g.

## Actually I understand it the

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Actually I understand it the opposite way :-), that velocity does play a role. I guess we need a physicist here .. E.g. read the rpart about relativistic effects influencing the precision of GPS or the experimental test by putting atomic clocks aboard airliners.

CU

H-B

## My conclusion Bikeman was

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My conclusion Bikeman was based on this taken from the page you linked to

"

In special relativity, the time dilation effect is reciprocal: as observed from the point of view of any two clocks which are in motion with respect to each other, it will be the other party's clock that is time dilated. (This presumes that the relative motion of both parties is uniform; that is, they do not accelerate with respect to one another during the course of the observations.)

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To me this sounds as that the time dilation you observe when you look at a clock that are in a non accelerated relative movement is just an observational effect.

## RE: To me this sounds as

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An analogy thatâ€™s often made is to the apparent narrowing of a flat object, e.g. a road sign, as the viewing angle deviates from normal: the more nearly edge-on the view, the smaller the angle it subtends. In a four-dimensional view the â€œworld linesâ€? of objects with a non-zero relative velocity diverge at a corresponding angle; the length-contraction and time-dilation effects may be regarded as the results of a kind of foreshortening. The speed of light in vacuo determines the maximum possible angle of divergence, at which solid objects would seem quite two-dimensionalâ€”if only they could go that fast.

## Thank you Odysseus! I take

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Thank you Odysseus!

I take that as a 'Yes' to may assumption that the slowing of time that are observed on a non accelerated relative moving clock is just an observational effect.

## RE: Thank you Odysseus! I

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Well, it is and it isn't. First why it isn't just an "observational effect":

Firstly, the slowing of time is NOT dependent on the actual location of the observer, so it's not an "observational effect" in the sense of it being just an illusion because light can't travel fast enough to tell me what the correct time on the other ship is as I'm watching it. As you correctly noticed in your original post, there are two different types of effects that you need to consider in special relativity:

1. There are those that are just an illusion because an observer sees the world lagged a bit, because light has to travel to him first to inform him of what's going on out there (and light is "slow" :). These effects are dependent on the location of the observer and are not so often studied (though they are fun to think about sometimes :) - see the following problem for an example: "http://www.jyu.fi/tdk/kastdk/olympiads/2006/IPhO-37_Th2_final.pdf"),

2. Then there are those effects that are "real" or "physical" in some sense of the word. These are independent on the location of the observer, and once you account for the observational effects of the 1. kind (slowness of light) they are always still there, anywhere in your reference frame! These effects are therefore dependent only on the choice of the reference frame (i.e. relative velocity), and NOT on your precise location in this reference frame (minus the effects of the 1. kind). The slowing of time is an effect of this 2. kind, and is therefore "real" (so is length contraction for that matter)! These effects of the 2. kind just do not go away, no matter what you do in your reference frame (for example, fast moving particles DO actually take longer to decay in the laboratory that they would if they were stationary, and in this extra time they can move further than they could if time hadn't slowed for them! - this effect is exploited to great benefit in particle accelerators for example to study short lived particles)!

Now, onto the reason for which it could be called an "observational effect" (or maybe more correctly a "frame effect") in some sense.

As you correctly pointed out from the Wikipedia article, the slowing of time is reciprocal. If two ships are moving past each other, observers on both ships will see that the other ship's clock is moving slower (once they already take into account that light is not infinitely fast!). Now comes the big surprise of special relativity - this result is completely sensible! (and in no way paradoxical; even though it's called the "twin paradox" :). This is because the only way that this "slowing of time" is to have any meaning is if the two observers from both ships can sit down together in one common reference frame and compare their clocks - only then can they both say they agree on who's clock actually ran slower.

But to achieve this at least one of the two observers must "jump frames" so to speak - he must accelerate from his own reference frame to the other observer's reference frame. But since (only) the reference frame is what determines which events happen at which time (or how fast), the observer that jumps frames will have to adjust own his sense of time to match that of the inertial observer (he will "see" the inertial observer age really fast as he jumps frames). In the end the observer that wasn't accelerating (the "inertial observer") will be the one that will be declared right about the slowing of time (he will be older than the observer that accelerated!), and the one that accelerated will have to concede that he was wrong.

To recap, if two observers want to agree on how much time had passed (how much they had aged), they must first meet in one common reference frame. It always turns out that the reference frame in which they meet is the one that determines who aged faster than the other! This is also the reason why I like to call the slowing of time a "frame effect" (and not an "observational effect", as is explained in the first part of the post).

## Thank you Devilogic! I am

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Thank you Devilogic! I am very grateful for this information.

I will take my time to carefully study your explanation and maybe come back later for some clarifications.

## An observe in a rocket fly

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An observe in a rocket fly away from you and accelerates to a relative speed of 0.9 c in 1 second (the observer on the rocket is made by the exotic stuff and can take the punch) and then turn off the engine before he start to measure the time.

Now you both start to observe a periodic variable star located 1 billion light year away from you both and perpendicular to the line between you and the observer in the rocket, the relative speed between you and the star and the observer in the rocket and the star is close to zero.

Will the time you and the observer in the rocket measure between 2 pulses from that star depend on the relative speed between you and the observer in the rocket?

Will you and the observer in the rocket measure different time between 2 pulses?

## RE: Will the time you and

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No, as long as neither of us is a participant in the otherâ€™s observation (or trying to use a reference frame in which the other is stationary).

No, as long as the sourceâ€™s radial velocity is the same WRT each of us.