michel123456

However, that is what is shown in animation 1. Look at B's clock. You yourself looking at your screen are this observer. Which is wrong ( I mean the animation shows something unphysical) Say that you are this observer: you are located some LHours away, your distance to Earth is D and to planet X is also D (your position is at the summit of an isosceles triangle), your clock is synchronized to E and X, and you are at rest with E and X. For you would B (clock T) be time dilated & length contracted? Knowing that your distance to B remains roughly the same.

Because we cannot even agree on simple things. Like : if your clock looks like running slower for me, then my clock must also look like running slower from you. If I read 60 minutes on my clock & 45 minutes on your clock, then reversely you will read 60 minutes on your clock & 45 minutes on my clock. Not even that.

.edited because of cross posting

Lenght contraction applies instantly for Earth leaving clock T. But for 1 long hour, for clock T the distance to planet X will not be contracted. Planet X will appear resting at 1LH away. And the signal of departure will appear to come from 1LH away (just like the missile example, in which you had no problem at all to write down that it will be recorded traveling 1 hour & 15 minutes). You are all very intelligent people here. Why don't you take a rest & take a new look to this thought experiment. There is no danger, and Relativity will not change.

I firmly believe that the question IS the thought experiment, and ONLY the thought experiment. Not to say that this thought experiment is bad for Relativity, it suggests some weird effect that I think does not take place. Relativity is OK, no question about that. In the frame at rest. In this case, T is at rest. The Earth goes away (say westward) and planet X arrives (say from eastward).

"Tell us e.g. what is wrong with Janus' animations? If nothing, where do you differ in our interpretations of them?" You don't want me to do that. But since you asked: almost everything is wrong. More precisely animation 1: (I'll need some time to adjust my answer, please be patient) This is the view of some "exterior observer" since it does correspond to no FOR (not the Earth, not Planet X, not clock T) a.From Earth, when clock T starts it is 12.00 on Earth and 11.00 on planet X b.From Planet X, when clock T starts, it is 13.00 on planet X You cannot represent facts a & b on this kind of animation. The only good thing is that clocks on Earth & on planet X are synchronized. But you cannot extract any information from it, expect that clock T ticks slower than the clocks on Earth & planet X, which means that from T, the clocks on Earth & planet X are ticking FASTER (and where is time dilation then?) c. There is no length contraction in this animation, which as I stated before is WRONG. If you want to represent time dilation, you must represent length contraction too. By length contraction I do not mean the oval shape of T solely, but also its path 0,6 LH long. Where is it? IOW this animation 1 above is pure garbage. Now animation 2. In this 2nd animation the Earth is leaving at 12:00, so far so good. But planet X should be applied the delay: clock T does not get the start signal instantly. So, this animation is wrong. And in this "view from other observer" (that corresponds to animation 1) the clocks on Earth & on planet X are not synchronized anymore.

No. In its FOR, T is at rest. There is NO REASON why it would give a different result. Cool down. I am the one who has been insulted, not you. My understanding is different (let me be wrong). I thought that Relativity gives you the result of what happen in the other FOR, not what happens in your own FOR. IOW for the Earth ,T is time dilated, and for Earth, T is length contracted. In its own FOR, nothing happens to T. (I am trying to avoid the words "see" & "observe" that annoy you so much). In the FOR of T : 1. The departure time of the Earth is recorded at 12:00. 2. The departure time of planet X is recorded at 13:00 (1 hour after its actual departure) 3.The arrival time of planet X is recorded at 13:15 (15 minutes after the arrival of the signal from the departure). In the FOR of T,standing at rest, the travel was 1h15min. It is not possible that T records only 45 minutes. That would mean that planet X is arriving before the signal.

Quino. R.I.P.

1:15 It’s no different than the trip in the other direction. We all agree that the missile took 1hour and 15 minutes. Now, replace the missile by planet X itself. Simply change the label M (missile) with Planet X. How much time does it take, as seen by clock T (at rest) to see planet X come. The answer is the same: Earth Clock T will only see that the missile Planet X started from X after one hour, the time light takes to reach Earth clock T. But then, oh shock, the missile planet X arrives only 15 minutes later. So it looks as if the missile planet X took only 15 minutes. But the earth clock observer is not so stupid. He knows that he got the signal of the missile's planet x start after 1 hour. So he concludes that the trip took 1 hour and 15 minutes. So it all fits. That is exactly what T clock observes in its FOR. 1:15. that is the time interval between the departure of the Earth & the arrival of planet X. And not 45 minutes.

Thank you for the animations, very enlightening. So let me see if I understand correctly: If all three are at rest (clock T standing on Earth & planet X at rest) and a missile M is sent from planet X to Earth at 0.8c, how long will it take (as seen from T & Earth) to reach the Earth (and T)?

If you read the example from Janus, you will see that time dilation is included while length contraction is forgotten: here below in bold where it is argued that the clock reads 45 min. when arriving. Let's see the question from the FOR of the traveling clock: the clock is constantly at rest, the Earth & planet X are moving. At 12:00, the clock T is at rest. For some mysterious reason the Earth goes away at velocity 0,8c. At the same instant, at 12:00 on planet X (that is synchronized with Earth), planet X starts toward T, also at velocity 0,8c. Regarding the Earth: clock T sees Earth going away immediately, there is motion, and thus there is time dilation & length contraction. BUT regarding planet X, there is a delay: clock T continues to see planet X at rest, at distance 1LH. There is no motion yet, there is no length contraction yet, there is no time dilation yet. Motion of planet X will appear after the delay has passed, that is to say after 1 hour. IOW after 1 hour Earth has left, clock T will see planet X start its travel. If you agree with the above then you may also re-estimate the 45 minutes mentioned by Janus: 45 minutes is less than 1 hour, so that would mean that planet X would have arrived at T before the signal transmitted by light. This is impossible: the 45 min are wrong. In fact, after waiting 1 hour, clock T will see planet X rush and crash on clock T after 15 minutes. The dial on clock T will show 1h & 15 minutes for the trip, or 75 minutes.

The clock is observing itself at rest. The "length of the trip" is the distance between 2 moving objects (the Earth & planet X) ????????????? I am the one? who says that the clock traveled 1LH in 45 minutes? I said that in Earths frame, the clock traveled 45 minutes until 0,6 LH, that it has to travel another 30 minutes to reach planet X , total trip 75 minutes. I say that you cannot apply time dilation alone, or length contraction alone, you must use both, otherwise you don't get the 0,8c.

Yes exactly.

In T' s FOR, the Earth is traveling. And Planet X is traveling.

Why such a difference? Earth & T should be reversible. Why is there length contraction on the other & no time dilation? Who made the choice?

That is exactly what I say: from Earth we are taking count of time dilation (looking at the clock) without taking count of length contraction (that shows nowhere). It is a mistake. As seen from the Earth, we should look at both length contraction & time dilation, in such a way that when the clock ticks 45 minutes, the distance traveled is 0,6 LH.

I have no much problem with all of that. You wrote: So what about what I wrote: In the twins paradox, that is exactly what happens: it is argued that, as seen from Earth, the twin traveled 1LH in 45 minutes. So the traveling twin is younger than his brother. There is a mistake somewhere.

And no length contraction? An observer never observes a particle flatten? Or to say it otherwise: As viewed by the traveling clock, what is the distance between Earth & Planet X? You wrote: So, I understand that the traveling clock is observing length contraction.

Let's say it differently With 1LH of distance and 75min of travel you get 0,8c (which was the assumption) With 0,6LH of distance and 45min of travel you get 0,8c (which was the assumption) And everything is fine. BUT When you combine 1LH of distance and 45 min of travel, it is wrong.

I should have said : When length contraction is applied, Earth see the length traveled by the clock <in 45 minutes> as contracted by a factor of 0.6.

That is the question. IMHO if the clock reached destination, as seen by Earth it has traveled 1LH. If it traveled 0,6 LH (as seen by Earth) it has not reached destination. Because in Earth's FOR, the destination did not moved. As seen by Earth, at 0,6 LH away, there is no planet X.

In your opening statement you wrote that planet X is 1 light hour away. From Earth. And from planet X, the Earth is 1 LH away. So it is 1 LH in all cases.

The lack of comments gives a mixed feeling... ... length contracted. But not the length traveled. I hope that I am misreading your comment and that you don't suffer from the same syndrome with Swansont in this post: My mistake. The questions are still focusing on irrelevant details, given the fundamental misunderstanding of relativity. The Earth sees the traveling clock length contracted. No matter if there is a rod between the clock & Earth, contraction is observed in all cases. It is a geometric effect, it does not act on material things only, it acts on everything. My answers in bold below for clarity (not shouting). All from the Earth's viewpoint: Is the traveling clock moving? Yes the clock is moving Is a ruler attached to the traveling clock moving? No, it does not matter. Is a ruler attached to the Earth moving? No, it does not matter Do you know which are length contracted? Yes. From the Earth's viewpoint the traveling clock is contracted, the distance that it travels is contracted, its time is dilated. Also from the Earth's viewpoint, the destination (planet X at coord y on the diagram) is not moving. So the distance from Earth to planet X is still 1 LY: this distance is not length contracted. The question is: after 45 minutes (as read by the Earth on the traveling clock), did the traveler reach destination? (as observed by Earth) My answer is No: after 45 minutes (as read by the Earth on the traveling clock), the traveler has moved 0,6 LY (as seen by the Earth). It misses destination by 30 minutes (as read by the Earth on the traveling clock), and 0,4 LY (as seen by the Earth).

Yes, OK. The earth sees the travelling clock running slower by a factor of 0.6. And what about length contraction? The earth sees the traveling clock ....<insert your comment here>

Chicken? educational-chicken.pdf For anyone interested, everything is explained in this .pdf