#### NotEinstein

**Valued Senior Member**

I never claimed that it would. My point is: what does Charlie measure about his time at stand-still, now that he has been travelling again and received the information about what Alice's clock was doing at the time.I'm not sure what you mean. The whole purpose of the ground DVR recording was to show that letting the information travel a known distance at the speed of light does not change the end result.

Yes, obviously. But what does Charlie now measure "really" happened back there, now that he's moved on?The time information received by the ground DVR (t=40 x=0) matches Charlie's calculation.

That's exactly what I meant. I don't see how what I said can be interpreted in another way? At what other point earlier than that does "the information about Alice's clock reach[es] him?" I know the second half of the scenario is your blind spot, so I'm glad you managed to remember it this time!So that's why I am not sure what you mean. Perhaps you mean that after Charlie jumps on the train, and he waits for the time information to arrive to himself as he rides on the train, then the information would be t=10, x=0 for the moment he landed on the train?

And what would you call that, the fact that Charlie changing frames changed what DVR is relevant to him, and his conclusions about Alice's clock? I'm still open to a better word that "overwrite".Yes, that is true, and that is what the train DVR records. But that pertains to the moment he landed on the train, not the moment he landed on the ground.

The calculations, yes. But Alice's clock never ticked backwards;Yes, I agree. The t=40 x=0 calculation is specifically for the moment he is on the ground. Once he has jumped back on the train, his calculation produces t=10, x=0 which is the only calculation that applies to him then. That is the whole point isn't it? First it was t=40 then it was t=10.

__that__was the whole point.

Yes, obviously. But you've dodged the whole point: is Charlie now justified in saying Alice travelled back in time? My answer: no.1. If someone asks Charlie what time (t) it was at x=0 just before he jumped off the train, he would have to answer t=10.

2. If someone asks Charlie what time (t) it was at x=0 just after he jumped off the train, he would have to answer t=40.

3. If someone asks Charlie what time (t) it was at x=0 just after he jumped back on the train, he would have to answer t=10.

Even if the answer to #3 "overwrites" the answer to #2, those are still the answers he has to give to those questions.

No, I don't find ignorance offensive.I honestly don't see the conflict. Sorry you find that so offensive.

Go back a couple of sentence in your own post just now. You have, in order:If I could see it, I would say so.

1) t=10

2) t=40

3) t=10

Any object or clock going from t=40 to t=10 is travelling back in time. However, everybody knows that currently, we haven't got any evidence travelling back in time is possible. So, it can't be travelling back in time.

Thus, Charlie must not be able to draw the conclusion that Alice's clock ticked backwards, even though the (correct) calculations suggest it. How do you address this?

Same here.I've been doing SR calculations a long time,

Well, I've learned that the best way to really get to know a theory like SR, is to play with it, and address any questionable answer you get. For example, obviously the travelling backwards in time cannot be real, so I try to answer that question. I feel sorry for you that you don't experience such journeys anymore.so maybe I'm just used to it being this way.

I on the other hand have an internal drive to understand. Any questionable answer in a theory so obviously self-consistent and worked-out as SR must have a resolution, so I go looking for it. I really do feel sorry for you that you've lost that curiosity.I guess it does seem strange sometimes, but I just accept that's how it is.

Do you think travelling backwards in time is possible through this method?But you say none of that happens, even though Charlie's answers to the 3 questions are t=10, t=40, t=10 respectively, right?

Erm, you've just stated that Charlie calculates t=10, then t=40, and then t=10, and that those calculations are correct, but in the very next line you say that's not possible. Don't you see the problem?So you have already explained to yourself that none of that happens. So why is it still a problem?

If travelling backwards in time isn't possible, then how it t=10, followed by t=40, and then followed by t=10 possible?

I'm not sure if it does; it doesn't "feel" right, but I can't come up with a better one at the moment. "Invalidates" would be better, but that sounds like the old calculation was wrong in some way. "Supersedes" perhaps?Well if the word "overwrites" prevents all of those problems, then I guess it is a good word to use.

In science, it's actually quite important to use the right words.I didn't really realise there were all those problem to begin with, so I didn't realise how important it was to use the right words.

Right, and neither of those show Alice travelling backwards in time at any moment.Measured reality is what is on the two DVR recordings.

Correct.They corroborate Charlie's answers to the 3 questions t=10, t=40, and t=10 respectively.

You keep ignoring the second half. How can t=10 be followed by t=40 be followed by t=10, if travelling backwards in time isn't allowed?That's all there is to this, right?

Charlie's measurements also show that Alice at no point in time travelled backwards in time. However, t=10 followed by t=40 followed by t=10 suggests otherwise. It's really weird... You were just a few posts ago making all kinds of silly statements like me not understand that 40>10, but here you are, doing that exact thing. If time (t) starts at 10, then takes a value of 40, and then a value of 10, how did time not decreasing in that last step? And since it's the time on a clock; how is that not travelling backwards in time?It's not intentional. I honestly don't understand what else there is to this besides Charlie's answers to the 3 questions t=10, t=40, and t=10 respectively.

Really, you say there's a clock going from t=40 to t=10, but it's not ticking (or jumping) backwards?

No, I didn't ask about Charlie or his calculations. Show on Alice's worldline where she does it. Show me where her worldline changes direction, and starts flowing downwards.I thought I already did that. On the Minkowski diagram, there are two different planes of simultaneity for the two frames. One intersects the t axis at t=40 x=0 and the other intersects the t axis at t=10 x=0. When Charlie jumps from the ground to the train, his plane of simultaneity changes from the former to the latter.

Nothing more that I've already explained to you multiple times. It's just that you often ignoring half of it.I honestly don't know what else you think there is to this.