Companiero

Quote me if I m wrong.

Ok, i'll quote you, but i dont know if u are wrong.

thats why I'm asking. Here's my dilemma more concretely.

Let's say something (point A) travels with near c velocity relative to your position. When it gets to point B (which stands still relative to your position, namely C), you measure the distance, from C to B, and you measure 10km, for example.

But if observing the event with frame of reference in point A (the traveling object), and if you measure the distance between B and C, when point A overlaps with B (thus actually measuring A to C distance), then there's length contraction, and you get, say 6km.

And both frames have to be equally valid under Einsteins postulates. In this case they dont seem to me they are.

What I'm asking is which is the right way to measure the distance between two points, of which one is traveling with near c velocity.

Quote me if I m wrong.

Ok, i'll quote you, but i dont know if u are wrong.

thats why I'm asking. Here's my dilemma more concretely.

Let's say something (point A) travels with near c velocity relative to your position. When it gets to point B (which stands still relative to your position, namely C), you measure the distance, from C to B, and you measure 10km, for example.

But if observing the event with frame of reference in point A (the traveling object), and if you measure the distance between B and C, when point A overlaps with B (thus actually measuring A to C distance), then there's length contraction, and you get, say 6km.

And both frames have to be equally valid under Einsteins postulates. In this case they dont seem to me they are.

What I'm asking is which is the right way to measure the distance between two points, of which one is traveling with near c velocity.

How do you measure a distance between two points, if one of them is static (has 0 velocity relative to you), and the other one moves with near c velocity relative to you? Is there length conraction in this case?

How do you measure a distance between two points, if one of them is static (has 0 velocity relative to you), and the other one moves with near c velocity relative to you? Is there length conraction in this case?

Janus, I drew the diagrams you suggested, sort of like a final test for you to see if I’ve completed the course in relativity. Lol. I hope they are fine. (the colour of the lines indicates the position of the traveling twin) EDIT: there seems to be a mistake in pic 1 - the pole length is obviously 20, not 40 as it's written in the parameters.

Now, the problem i had wasn’t with the diagrams, but i was wondering if some of the twins actually aged more than the other. What you have said in the first sentence, about both twins in the same frame, opened me new view. Therefore, i figured myself (could be wrong though) that for the term aging only to have sense, we have to put the twins in the same frame, because if they are in diff frames moving with relative velocity to one another, than we cannot measure aging, because events don’t happen at the same time. I tend to think in absolute terms, and that’s why i had difficulties grasping that aging can only be determined relative to one frame. I think i’m right this time, cause if i’m not, i’ll probably have to rethink my whole concept of relativity.

I beg you for one answer more. It’s short:

How do you measure a distance between two points, if one of them is static (has 0 velocity relative to you), and the other one moves with near c velocity relative to you? Is there length conraction in this case?

I hope this answer can help me a lot in the final demystification of SR.

You're right. How could i not notice that the two events happen on the same place.

For now, i'll suffice with understanding SR, without the accelerations and GR.

One final question, janus. (i hope i dont make a fool out of myself this time too)

in the example with the traveling twin from the sun to the earth (no acceleration), you logically concluded that in the end, the "travelinng" one would measure less time than the earth twin, for which they would both agree. Consequently, he'll be younger than his earth brother. where's the acceleration here, which we said is needed in order one of the twins to realistically age? If there's only relative motion between them (no accel), then I would expect none of them to grow older than the other.

(i thought about what's the difference that makes the two ref frames not look symetrical, and i concluded it's the fact that the distance Sun-Earth is fixed with the earth twin time frame. However, i cant incorporate this fact in the theory, and just how exactly it makes the twins age differently, if at all, or its really just another one of my brainfarts)

i admire ur willingness and persistance to work with slowminded guys like me.

Yeah, I got that already, but..

Janus, you wrote earlier:

The traveling twin starts his stop watch as he passes the sun and stops it as he passes the Earth. since the distance between the two is 1/2 as measured by him, it will take 4.62 mins according to his stop watch.

The earth twin also sees that his brother starts his watch as he passes the sun and stops it when he passes the Earth and , due to time dilation sees his clock as running at half speed and thus it measures 4.62 mins.

The travelling twin starts his watch when he passes the sun, thus the two events are simultanious in his frame. However, the Earth twin cannot see his brother starting his watch as he passes the sun (as you wrote), because from his frame (which is moving at some speed relative to his brother's ref frame) the two events cannot happen at the same time. Which is what I asked you, why this effect wasnt taken into account by you.

And I aslo asked you about the acceleration.

"Assuming the cars are in outer space, and there are no observers, and their relative speed changes, how can you know which one actually accelerated?

I think it’s similar like the free fall. Imagine a ball moving with constant acceleration (like free falling), while you’re standing in the air on a plane platform. I cant see a way to tell which of the two (the plane or the ball) actually does the accelerating. Isnt that why there even is a General Theory of Relativity, so that it expands the concept of relativity to accelerated movements?"

thanx again.

I see Janus, but just one more thing. Shouldnt then the Earth twin observe the same Simultanity effects, since the two events (starting the earth clock, passing the sun) are also seperated by a distance? This is not in your calculations. I wonder why?

And you didnt respond to the question I asked you above about determening the acceleration. If you could please explain that too, I'll be forever grateful.

I know I'm a pain in the ass, if that helps.

Thank you Janus for explaining that to me. It was obvious, but I guess I didnt notice it.

lightSword, your site doesnt show up.

However, I believe the original purpose of this thread has been exhausted, and can now be closed/deleted (whatever you guys do with useless threads).

Thank you Janus for the lovely response.

I anticipated your reply would be the way you wrote.

However, what I don’t get here is why is it important for us to consider the light path all the way until it reaches back the source. Why not just from the source to the mirror (from point A to point B) in which case, we get two different times (t1 and t2, in your example) for the cases with opposite light direction, as opposed to the time we measure from the mirror (“static”) reference frame (to/2, in your example)? Thank you again for your help.

I don´t know how the others feel about your note you put on the end of your posts but to me it sounds pretty arrogant (are you an ongoing physicist, perhaps?).

Sorry if that came across as rude. But to tell you the truth, I consider arrogant being someone who acts like they know it all and try to act like “too wise to even be understood”. I cannot tell you the amount of wrong information I have read on this forum, that can only mislead people who want to learn.

I’m not proffesional physicist, but I happen to be a student who is state champion in physics for the past three years, and I consider myself to understand this subject pretty much, except for this god forsaken relativity. I never pretend I know something unless I really do, and prefer to ask questions than give answers. But I do recognize an informed person when I see one. (i think its better if this paragraph isnt quoted for the sake of staying on topic )

Janus, I noticed from the past threads that you posses a lot of knowledge on this subject, and I cant tell you how grateful I am for helping me out here. Thanx pal.

Thanks to all the others who have put their inputs also.

The one which changed velocities is the one that felt the forces of acceleration. If I'm driving down the road, and I hit the brakes, I will see the car next to me seemingly accelerate forward. However, anyone can tell which of the two of us actually changed velocities by watching to see which of us is either thrown forward into his seat belt or pushed back into his seat.

Yes, everyone can tell by watching, as you say, which implies that the acceleration is viewed from a particular reference frame (i.e. the observer’s). Assuming the cars are in outer space, and there are no observers, and their relative speed changes, how can you know which one actually accelerated?

I think it’s similar like the free fall. Imagine a ball moving with constant acceleration (like free falling), while you’re standing in the air on a plane platform. I cant see a way to tell which of the two (the plane or the ball) actually does the accelerating. Isnt that why there even is a General Theory of Relativity, so that it expands the concept of relativity to accelerated movements?

This also means that he will see a different time differential, then the Earth observer would.

So the Earth and ship reference frames are not equaly valid. But you’re talking about a changing acceleration rate in your example (as one keeps moving away from Earth). Mind you, if acceleration is constant during the entire experiment, there wouldn’t be difference between the two viewpoints. Which brings us back to the previous question.

Now your twin brother could not travel at c, but he could travel at .866c. In which case, you would would see him age at half speed and only age 4 min during the trip. He on the other hand, would measure the distance between Sun and Earth as only half that that you did (length contraction) and for him the trip would then take only 4 min. Thus both of you agree that he aged only 4 min during the trip.

Are you talking about the Twin Paradox (acceleration taken into account), or just two twins moving with diff velocities with relation to each other?

Sounds to me like it’s the second example (without acceleration), in which case I must admit the explanation sounds logical, but having never considered this particular example, I find some contradicting things with my present understanding. In that case, the “travelling” twin brother would measure only 2 minutes for his brother on Earth, no? (because each observer always measures the other’s time rate as slower, in this case by half.) And the Earth twin would measure entire 8 minutes on his watch. Now, this refutes itself a bit doesn’t it?

In his frame, that deceleration looks like a force (so he would feel like he has weight in the direction of motion). The other twin (who doesn't move) feels no force.

I dont think so. The reason why we feel weight pressure is because the ground we’re standing on. Gravitational force still works on an astronaut in orbit, but he doesn’t “feel” it, exactly like if he was hypotetically outside any force field.

Hi! My second post apparently, also in the form of a question. (if u havent noticed the thread sister of this one)

This time about time dilation. It's a common experiment to prove time dilation with the light reflecting a mirror, observed from a reference frame moving with velosity near c (vertical light beam path vs. horizontal movement), thus creating the right angled triangle. And I understand this.

However, what would be the effects on time dilation if the movement of the observer relative to the mirrors was in the same direction as the light beam path? And what if it was the opposite direction?

I forward the notion, that this way time dilation cant be proven. In fact in the two cases, we get two different values for the time: that it passes faster and in the second case slower than time in the reference frame. Consequently, time dilation depends on the direction of the beam of light and the observer, which is absurd (it is supposed to be solely dependent on the relative velocity between the two). Where am I mistaken?

Any help would be more than welcomed. Also, links to articles about this problem could be very helpful.

NOte: I prefer to get only reasonable replies, and not the Matrix-Oracle type of prophesies, which not even the author understands what they mean.

This is my first post, but I read most threads in the Relativity forum, and I still havent found a clear and explicit answer to the following question, concerning the Twin Paradox.

When one of the twins accelerates and travels near c and then gets back, it's said that he appears younger because he has undergone some "heavy acceleration", which has made the time rate for him to pass slower.

However, arent all reference frames equally valid? I know, you'd say it's not the same with acceleration and GR, but why wouldnt it be? Basically acceleration is defined as change of velocity vector in a period of time. We know that velocity is relative to the observer, so consequently change of velocity would be also. There's no way we can know which one of the two bodies has changed its velocity, dont you think?

Any help would be more than welcomed. Also, links to articles about this problem could be very helpful.

NOte: I prefer to get only reasonable replies, and not the Matrix-Oracle type of prophesies, which not even the author understands what they mean.