Moving at the speed of light

Is it true that if you move at the speed of light from earth in one direction for 12 hours and then come back to earth at the speed of light that you will have only aged 24 hours yet the earth is 1000 years older? Does anyone know how light speed travel would work?

Its a serious question, I'm trying to imagine how theoretical light speed travel would work so I can check the reality of scifi books etc.

Yep its true. In real I'm a medieval king but thanks to my spaceship I'm currently a indie game developer.

Thats hilarious. But doesn't traveling at light speed theoretically have some effect on time? Anyone have a clue or am I just asking the wrong forum

Some effect on time? Time is relative for everyone. Time is not one thing, there is not one timeline or so. So yes it affects time in that a moving entity progresses slower in time than one who stands still in respect to the moving entity.

But we all move with the rotation of the earth, rotation around the sun, our sun through the galaxy, and the galaxy through the universe with unimaginable speeds. The funny and mindboiling thing is that everything is relative. Whichever way you move (faster than our current speed, or "slower" when you move the other way) you always end up moving slower through time compared to your starting point.

http://en.wikipedia.org/wiki/Time_dilation

One thing which always bothers me is the example you use and everyone else states. If you say that you move at a considerable fraction relative to earth, thus your clock moves slower, I (the one on earth) say that I am moving at a considerable fraction relative to you, thus my clock moves slower.

Anyone knows the solution?

If I recall correctly the equation is:

t1 = t0/(1-(v^2/c^))^.5 (simplified for frame)

Traveling at c produces multiple singularities in the equations. Traveling at v << 0 (the only possible situation for an individual) then t1 = t0, i.e. no difference.

Special Relativity

?

I guess I forgot one part, couldn't remember what it was at first, but the time dilation effect is applied to the observer undergoing an acceleration. If the traveller moves away, and then back, the traveller becomes older younger as it underwent all accellerations.

That's correct (although the one traveling is the younger one in the end). The thing is that the one traveling away and then back changes his inertial system (e.g. by accelerating) which in the end cuts down his time consumption. The acceleration itself doesn't have an effect, though, as you're accelerating and decelerating. If you could change your inertial system without acceleration this would have the same effect.

This would allow us to build a time machine. *want*

Yes it is true..it is a serious answer
All the clocks on board of the shuttles are few seconds slower than the ones on the earth
What people often misunderstand is the following :
From the point of view of the pilot just 24 hours have elapsed
In other words you can travel in the future of the earth but you can not expect to live longer

vec_set( player, vector(x,y,z) )?
Since "inertial system" is just a fancy word for "we take this point as the origin of space", I could argue that space is just a made-up concept and our entire universe is defined by accelerations, thereby acceleration is the effect. But you could just as easily argue back your point of view .

What interests me is when you take the cosmic background radiation as inertial system. Then the space in our universe shrinks inwards instead of extending outwards!

Well, inertial systems have a velocity too, not only an origin. So it's the change from one velocity to another which causes the dilation.
http://en.wikipedia.org/wiki/Twin_paradox

Yes I know that, but isn't "the change from one velocity to another" defined as "accelerating"? I think this is just a matter of definitions, a conceptual thing, but physically we mean to describe the same. That's what I try to say.

Then again maybe teleporting isn't describable by the true definition of acceleration, unless teleporting without acceleration is not physically possible.

Yes I agree change in velocity is a more general description than acceleration, and thus be the preferred term to use.

Joey's question hasn't been answered yet, has it? Cause I always wondered the same thing... My Physics teacher tried to explain it to me, and I never got it...

All Movement is relative, as far as we know. So if I move away at the speed of light, I could also say YOU'RE moving away at the speed of light, so how does the universe "know" which one will have to age slower?

I was under the belief that it was the "relative speed" that caused the dilation, not "change of velocity".

Someone mentioned that the space-shuttle clocks ned adjusting after a flight...
I have also 'heard' that the clocks in the GPS-satellites also need adjusting occasionally,
and they are not 'changing velocity' to my eye, but they are 'travelling' faster
than us due to their greater distance from the center of the earth's axis.

That's exactly the problem. Relativity itself does only tell you that if you're moving, your clock is slower. Then you tell me that I am moving, so my clock is slower. Of course both are right.
The thing which causes a permanent "shift" of time, if you so will, is changing the inertial system (however that is done). That effect is hard to explain in few words but in principle that's the answer. That question is called "twin paradox" (see my link above).

edit: @evilsob: the problem with the gps satellites is not their greater speed but their constant acceleration; this is not a question of special relativity but of general relativity and much much harder.

The simplest and shortcoming answer is that the one who undergoes accelerating forces move slower through time. It's the acceleration (or as stated above, change in velocity) that causes time dilation, not the velocity itself.

Joozey, I Don't get it. In my eyes, what you said doesn't answer my problem.
If I drive away in a car, you'll say I'm accelerating. But then I tell you that relative to me, you're accelerating in the opposite direction. And, apart from the direction, your acceleration is exactly the same as mine. So we should both age slower, and both age "the same amount" slower.

Joey, I might find some time to look at the link, right now I'm in a rush...

space shuttle clocks need adjusting not because of the movement but because of the fact that time moves slower in space. and while this has something todo with movement, its not the movement of the clock, but of the earth. one ages slower in space. in fact, if you COULD travel at near light speed for ten years away from earth and then return for another ten years, you aged 20 years while on earth roughly 400 years passed (or something).

HOWEVER if you love on a planet bigger than earth, you age faster...

The standard reply you'll get when you ask people is that since you had to return to get back to earth, at that point you had to accelerate, and thus special relativity isn't valid anymore, and the problem has to be solved with general relativity.
This, however, is not correct. The truth is that the equation for time dilation you are using is only valid for the above, simplified case of constant movement with no acceleration. So here's the short reply: When the twin is returning home, the inertial system you've been using CHANGES. It suddenly moves in a different direction - that's not something inertial systems generally do ;), and is the cause of the problem: In your calculation, you've been assuming that you're calculating stuff in a inertial system, but you're not. That false assumption leads to the wrong result.


EDIT:



Nebolous statements like that really don't help much, don't ya think?
Relativity predicts TWO effects that change the "flow of time", if you will. One is described by special relativity - what is usually referred to as time dilation. It can be summarized as "Moving clocks go slower".

The second effect is due to general relativity. Masses curve spacetime, in other words, it changes the geometry of it. The time you're measuring (the "proper time") is nothing but the length of your path through spacetime (disregarding units). Now, obviously, if masses curve spacetime, it only makes sense that the lengths of your paths change, too (and thus your measured proper time).

Whats fascinating is that for satellites, bothe effects actually work in different ways: They're moving quiet fast relative to the earth, so that makes their clocks go SLOWER, yet at the same time, the mass of earth makes their clocks go FASTER compared to the one of someone on earth (the closer you are to masses, the slower your clocks go).

This is the most common misconception ( assuming I understood what you mean)
The key point to grasp the theory of relativity is the concept of symmetry

Newton and his maid(his word) gave for granted the existance of a hierachy of observers
A ship is in motion relatively to the earth
The earth is in motion relatively to the sun
The sun...

Newton thought that this chain must have a beginning
An entity at absolute rest (God)

This intuitive but false assumption made classic mechanics a bloody mess, despite of its success, which Newton himself and the others after hime , were aware of

Einstein cleaned it up claiming that all observers are equivalent thus he wrote a set of equations which are valid for any observers

Einstein's logical reasoning entails a non intuitive but true conclusion : There is not an absolute time

Coming to your doubts

The twin on the earth claims that the time on board flows slower
However also the pilot claims that the time on earth flows slower than the time on board
Otherwise you would break the principle odf symmetry

That's wrong. Acceleration can be measured, speed can't. So you can say very explicitly who is accelerating and who isn't. That's the definition of inertial system: coordinate frame and velocity is given, NO acceleration or whatsoever. Thus, the person who stays behind is always in the same inertial system, while you (the traveler) changes them by accelerating.


That has nothing to do with the velocity but with gravity and is again an effect of general relativity. "Time moves slower in space" is not correct (where did you get that from?). It doesn't matter if I'm in space or not, what matters is the curvature of the fourdimensional manifold - and now again we're getting too detailed for this forum. Sorry.


That's the problem we're discussing. It has nothing to do with speed itself, though (see our discussion).

Read Error's answer, it is quite a good one I think ^^.

@Alberto: It's true what you say but I don't get if you're trying to resolve the problem, you're merely restating it - or did I miss something?

well first of all I dont know whether everyboy here is really convinced :

The twin paradox is not a paradox but the plain truth
The pilot returning on the earth is really younger than his twin brother

Are you asking why is it ?

Nobody can really grasp this claim being something beyond our experience

It is not possible to provide an intuitive explanation just a logical explanation is possible , namely :

The symmetry of the observers

At a first glance all of us assume that an observer on the earth is somehow different than an observer on a train

However if you figure out two starships in the deep space there is no way to distinguish them
You can just claim that they are in a relative motion

If the two starships observes the same event, for example a super nova then they can meeasure a differnt duration of the event
However their measures must be symmetrical otherwise you could distinguish the two observers

There is one only way to make the symmetry of the observers compatible with the speed of light, being a costant :

The time on board of the two starships must flow in a different but symmetrical way
The identity is of course a special case off symmetry

In other words

The first pilot claims : the time on board of the other ship flow slower than my time
The second pilot claims : the time on board of the other ship flow slower than my time

It is not something that we can understand it is something that we must accept

Alberto, the twin-paradox actually consists of two paradoxes.

The first is the one you've stated, and formulated as "How can two twins age differently?". This one can quickly be understood -- it follows from the fact that the speed of light is a constant that has the same value to every observer. If you accept that, all the equations follow and this paradox is nothing but a different "line-length" in spacetime.

The second is trickier to solve, and is the one we're talking about. It is as such: "If all observers are equal, how come one ages MORE than the other"?
Even from your last statement:



that paradox is not resolved. Quite the opposite, in fact! The quote implies that BOTH twins still have the same age -- only that they will (somehow) seem younger to the other guy. But that is *NOT* what happens: Twin A is OLDER than Twin B. But how can that be if we've stated that every observer is equal, or everything is symmetrical, as you've stated it?

The reason for that is that in order for the two twins to meet again, Twin B had to turn around, and thus, he "changed inertial systems". Due to that, the problem is very easy to calculate for Twin A (on earth), yet difficult [but not impossible] for Twin B (in spaceship). But hearing "all observers are the same" suggests that the calculation should be the same, though it isn't for the reason given above.



I hope that cleared that up?

Haha,
I belive there is a way to move as fast as light.
But when you have a ship that flys as fast as light, it won`t take long till you crash something
But it could be that the ship would just fly throw the planits (and so on), becose it`s so fast.
Jest like the light that flys throw us all the time, I meen that dosn`t hurt.

You mean neutrinos? They're slower than light...

You still dont grasp the principle of symmetry

B is younger than A according to the A time
A is younger than B according to the B time

In other words
Should the twin on earth (A) reaches his brother (B) on board on the star ship then he would find that his brother is older than him

Anyway I was right to suppose that some people assume that the twin paradx is a... paradox
Nope it is not

The ultimate confirmation of the theory of relativity came from the spatial missions
Before that , a minority of great scientists still believed that the relativism of time was, so to speak, a sort of mathematical trick

I quote by heart

A shuttle remained in the space about 1 year time, travelling at an average speed v = 40.000 km/hour
On return on the earth the clocks on board were t = 3.6 sec late , in compliance with the theory
So the astronauts (if any) were 3.6 sec younger according to the earth time

I would suggest to read the Bertrand Russel's book
It was written before the spatial mssions
Russel said that only the future spatial mission can tell the last word on this question

The last word was : Time is actually relative

This is just avoiding the issues. "B is younger in A time"? The paradox is explicitly stated in a way everybody can understand as to avoid using additional parameters that make comparisions impossible [of course, only because it is possible to state problems that way doesn't alway mean that you can answer them as such -- but in this case, it should be possible].

The question is: Assuming both twins die after they lived the same proper time, who is closer to death when they meet again [in other words: Assume both do exactly the same things afterwards, and die a natural cause (=old age), who dies first]? A physically better way to say this is: What twin had more proper time pass (which equals age)?

Thats a clear question, and it has a clear answer. It is: Twin A stayed in a inertial frame the whole time. Things are easy to calculate in his system, and we find that Twin B will be younger than Twin A (since more proper time passed). Twin B is not staying in an inertial frame the whole time - things are difficult to calculate in his frame. But there really is no need to, since we already got our answer earlier.
The actual, easy, straight answer to all of this is: The twins age differently since their pathes are different in spacetime, and thus have different lengths.


Saying "A time" or "B time" isn't wrong, but it isn't helpful, either. It's good to establish that there is no such thing as absolute time, but we cannot really say anything about A's age in B-time. In other words, your statement:



is correct if Twin B would be moving in one direction at the same velocity forever. But the essence of the twin-paradox is that he's not, he went back, accelerated and thus "changed inertial frames".


EDIT: I read your post again, and now I'm not sure what you're arguing, to be honest. It seems you're stating that there is a paradox* sometimes, yet that there isn't at others.


* - for the sake of the argument, I've called it "paradox", even though we've already established that it isn't one. But even that seems weird, because, as I've stated, the "twin paradox" actually refers to two things. Which are we arguing about? Who can even tell...

The twin on the earth dies first
This is what happen everyday in particle accelerator

I don't get Alberto's arguments either.


That's wrong in general and depends on how A reaches B.

I've another riddle for you (rather easy):
Suppose you fly in a spaceship at a substantial fraction of c towards a human colony on planet Blorb. Blorb has a moon. On the moon, there live the evil Zarg. The constellation is as follows:
(Blorb) ---- (Moon) ---------------- <You|= ----
Your sensors detect an energy outburst on the moon and then you detect an explosion on Blorb.
Have the Zarg caused the explosion? Do you attack them?

Summing up

There are two twins A and , 20 years old
A remains on the earth
B leaves with his stracraft, travelling at 90 % of the light speed

B is back on the earth, 20 years later, according to the A calender

A is now 40 years old
According to the special relativity
B is only 20 + 8.71 = 28.71 years old

Therefore A will likely die first

You say (If I understood ) :
Nope B is also 40 years old because also the acceleration and deceleration must be also taken in consideration
In other words , in your opinion, there is a sort of compensation

It is right that you should consider also the acceleration for an accurate calculation of B age but the result is :

Actually B would be even younger than 28.7 years

If you have a look at the B calendar, the one hang on the starcraft , you will see that 8.71 years ( or less) have elapsed from the departure (The B time)

It seems unbelievable but it is the plain truth

The decay time ( the life spam ) of atomic particles in particle accelerator
comply with the theory
If this is true for a single particle there is no reason to doubt that it must be true also for a human being

I think you didn't understand.


special relativity has nothing to do with the outcome of the experiment.


no there is no compensation from acceleration. In contrast, acceleration is what makes B age slower in first place.


Nope, different effect. This is plain old special relativity (which has no effect on the twin paradox problem => it IS a paradox if you only look at special relativity. That's what we've discussed all the time).

You should seriously read through the twin paradox article on wikipedia.

because you didnt read and he said that both are 20 when they start.

thus one is 40 the other ne is 28 something.

and why is that? because time is relative to the gravitational force of its suroundings. end of story.

Ah sorry for that ^^. I changed my post. His explanation is still not valid, though. Yours is neither, but yeah, lets skip that discussion. I'm tired of it...

it is valid though. time is relative. thats as valid as it gets...

Are you joking , I suppose ?
The relativism of time and space has been introduced with the special relativity
The super famous equation :

dT = dT'/SQRT( 1- (v/c)^2))

It has been announced in the year 1905
The general relativity 1915 further expanded the principle of equivalence
assimilating inertial and gravitational forces

The surrounding mass / energy further curve the space /time but the relativism of time / space by itself does not depend only on the presence of mass / energy

"You must study a little bit more , young lad "

Albert(o) Einstein

Alberto:


... Uh, no, I didn't. Never did, I'm afraid!

I've tried to explain to you what the problem was, and then posted the solution to it. So with your new post, we've both settled on the same result. No more conflict. Isn't that nice?

~ ~ ~
Joey:


No, it isn't.
It's just a lot harder to derive than simply using t' = t/\gamma and x' = \gamma*x .

But it still ISN'T a paradox. Special relativity says: In inertial frames, THESE formulas work. But Twin B isn't moving in an inertial frame, and thus, those formulas won't work.

~ ~ ~
Spike:


Have you read my first post in this topic? You should. It might interest you.

If it's too long to your liking, here's the relevant part quoted for your convenience.




In other words: The twin paradox is not because of masses. it works teh same way if you remove earth from the equation and have A stay somewhere in vacuum, while B travels away and comes back (all in vacuum, with no masses anywhere). Their age difference is due to special relativity and NOT because of masses.

Masses DO curve spacetime and thus can make clocks go faster/slower as well, but that is an effect described by general relativity, and one that is not relevant here. It could be added to this scenario, but you don't have to to get the twin-paradox.

I'll skip these discussions from now on. I really don't need that.
@Error: Good luck.

I did not realize that,in principle, we were telling the same stuff

About above claim I am not that sure

Lets see

The special relativity plays of course a key role, in the twin paradox (Which is not pardox ) as well as in the particle accelerator experiments

However it is not exhaustive
Since the pilot comes back to the earth , the paradox entails an acceleration
The special relativity on the contrary assume a constant speed

The natural question is

What does it happen during the acceleration ?

In my opinion the explanation must be found in the Einstein's principle of equivalence , replacing the old Galileo's principle , i.e

Inertial forces == gravitational forces

The acceleration , regardless of the direction, is therefore equivalent to the presence of a mass ,which causes a further dilation of time, thus confirming the hyphotesi that the twin paradox is not a pardox but the plain truth

Dont get offended ...please
Read your posts instead
You have alwayes a "top down " attitude
" It is wrong...full stop " without even spending a words of explanation
Not only in this occasion

It is not the way to behave in a forum where nobody knows each other so nobody should play the role of the professor

btw
My sentence "You must study a little bit more, young lad ..." was simply a little bit ironic
It is the answer given by Einstein in a seminar to a young guy who was arguing with him
Since my name is Alberto was just joking

It is true that the formulas in special relativity for both time-dilation and length-contraction require to be solved in an inertial frame. However, that does not mean that accelerated paths cannot be solved in there - after all, we were able to calculate the proper time that passed for Twin B -- we only had to do it in an ienrtial frame (Twin A's system).

In that level, the paradox is solved simply by saying that it is not valid to simply assume the same formulas are valid for both for the WHOLE duration, since there is no inertial frame comoving with Twin B.


It may be derived without using Twin A's system [obviously], though of course it has to be an inertial frame, so "Twin B's system" is out of the question [no inertial frame].


~ ~ ~ ~ ~ ~ ~ ~ ~

I always wonder why in those science-threads, the tone often gets aggressive?
I do agree with Joey, your last comment can easily come across as arrogant. The fact that we're all speaking in different languages than our native one makes it all the more difficult to judge the intention behind comments.
Not sure how to resolve that, I'm just trying to explain why Joey might have reacted the way he did.

You might be correct about the tone of Joey's replies - for me, it feels different and less hostile (though maybe at times, there is a hint of arrogance), but again, I do believe that language (and maybe customs in those) plays a big part of it [Joey and I both are native germans].


I, for one, stick with my tone of careful idiocy

I'm sorry if my tone comes across in an arrogant way. In general I think I'm quite good at writing and understanding English, but I have no clue how my tone sounds like. I apologize.
Still I find it quite bothering that people (Alberto) keep telling me that I have no clue, that I should "study more" or "take my old physics books from the shelf - too long time has elapsed". Speaking of arrogant, stuff like "at 14 years old my main interest was soccer" is very helpful to keep arguments on an objective level.
I always try to give explanations, but what might sound clear to me might not be clear at all to others. Then ask. But I often have a feeling that posts are not read carefully and stuff is said only that something is said. I mean, we've already resolved the twin paradox like twenty posts ago and suddenly the discussion starts all over. In threads like these that's annoying. I spend quite a bit of time writing my posts, looking up stuff etc., but I don't see any sense in it if you're not interested. I could as well do other things.

So much for that. To repeat my arguments referring to the twin paradox let's look at it in a different way.

• Horizontal lines mean "at the same time" for A
• Red and blue lines mean "at the same time" for B
• Points mean proper time
• During flight, A thinks that B is aging slower while B thinks that A is aging slower (check that in the diagram, when B reaches point B he has passed 4 years proper time during which A has aged about 3.2 years - and vice versa)
• When B turns around - however that is acheived - A ages rapidly when seen from frame B
• This is what distinguishes both frames: while B ages slower all the time when viewed from A, A ages slower in frame B only while B is flying
• Both frames are clearly distinguishable
• There is no question whom of both will be older at the end
• No gravity involved whatsoever
• This effect is only due to the change of inertial systems of B

To calculate it with a cheap trick which completely obscures the physics behind it imagine that B travels for, say, ten years as seen from frame A and then beams back to earth. Of course his proper time is shorter than the time on earth (t/\gamma). As seen from frame B this picture is complete nonsense, though.

"While it might sound impressive for people who do not know much about it, I don't like to discuss with someone who can't express a clear idea."

Did you recognize who wrote above sentence ?
It doesnt sound quite nice , does it?
Anyway let's put aside such childish ( from both parties ) disputes

Now you provided an explanation
However unless I completely misunderstood your words,in my opinion it is a wrong explanation

You said

"During flight, A thinks that B is aging slower while B thinks that A is aging slower "

and then

•When B turns around - however that is acheived - A ages rapidly when seen from frame B"

for " turns around " do you mean that B has reached the farthest waypoint and he is returning to earth ?

If so
First of all I dont get what you mean for " When B is flying "
B is alwayes flying
except of course at the departure and at the arrival
Consequently the former sentence is alwaye true , the latter alwayes false

However to get rid of the complication due to the "inertial frame " and similar stuff, put it this way
Suppose that the duration of the journey ,at costant speed, is very long , so long to make negligible the departure / arrival / change of direction times

If so , the simple formula of the theory of the special relativity is applicable with a great degree of accuracy, you get:

A proper time higher than B proper time

It seems unbielivable but it is like that

This thread might now hold a record for the most solutions to the Twin-paradox.


Anybody wants to ask another question? As you can see, they're answered by EXPERTS (who will then proceed to argue about both content and semantics for four more pages).

It is not a question of sematics

The relativity of of space / time is a fact that it is explained by the theory of special reativity

Let's see the following examples

Bilions muons pass through our body every seconds

Muons are generated by the collision of cosmic rays with the atmosphere
You can generate muons also in lab and you can measure their decay time

If you multiply the decay time x the speed of light you get some hundreds meters

How can the muons reach us , the atmosphere being some km depth ?

The explanation is given by the theory of special relativity

muons travels at a speed close to the speed of light
If you multiply the muon decay time by the cofficient of time dilation than you find out that the muon can cross the atmosphere

We can therfore state that the speed by itself slow down the decay process
For the same reason it is reasonable to assume that the speed slow down also the aging process of the twin pilot

This is however valid only for us , living in the earth spatial/temporal dimension

From the muon viewpoint the decay time remains constant but the atmosphere is only some hundreds meter thick

It is the time / space symmetry

P.S.

Just to clear up
The key concept is the so called :

Time / space interval

Common sense assume that space and time are unrelated measures

The theory of special relativity on the contrary demostrate that all the observers share the same " space / time interval" which is a combinattion of local time and local space

The twin of the earth "consumes", so to speak, only "time"
The twin on the srarcraft "consumes" both time and space thus his time consuption is lower ,the "space/time interval " being a constant for both twins

Useless to say that a/m explanation is non intuitive but it is confirmed by the experience :

a) Spatial missions
b) decay time of particle in particle accelerators
c) Behviour of particles coming from the space
d) Spectrum of far stars

You know... I was only trying to end this discussion, since there's actually nothing elft to discuss (though we've reached that state at about page 3).

Error014

I definitely agree with you that a thread sooner or later must come to an end

However I don agree with you that a solution has been reached at page 3

If you go through the thread the term "inertial system" is recursive

Sorry , it has nothing to do with the twin paradox

The twin paradox is an immediate and exclusive consequence of the special relativity

The relative speed only generate the permanent shift of time
You dont need anything else

The poblem of acceleration and deceleration is a false problem

I ask you a question :
Suppose that the duration of the journey at constant speed is very long,so long to make negligible the departure the arrival phases

Do you mean that the twins on arrival are equally aged ?

PS

if joey is not yet upset with me I would appreciate also his comment

I'm not upset.
I don't get the question though. The duration of the journey for B is very long and acceleration at the beginning and deceleration at the end should be neglected. That's ok, we could assume (*) that B doesn't have to accelerate/decelerate at all, for example he has already some speed, flies by the earth and in that moment synchronizes clocks and compares them at the end of his journey. (Just to avoid that we're again talking about different things - is this what you mean?)

What I don't get though is what you mean with constant speed. Do you mean the modulus should be constant? Or how is turning around managed for B? Since, when the direction of his travel changes, he also changes his inertial system and his internal clock (or better: the relation internal clock - exterior clock) changes in a non-trivial way.
Having said that, if you would calculate it like that anyway - wrong, though - and just take the time to be the same for B before and after he has turned around (neglecting the time which passed in between) (assume for example that B flies away from earth and then, instantaneously, has turned around and now flies back to earth, neglecting all physical effects this action comes with) and plug in special relativity, you would get a paradox. Namely, for B, A has aged slower. At the same time, in the rest frame of A, B has aged slower. According to special relativity, both are correct. When they come to compare their clocks, clock A shows less than clock B in the B fame and vice versa. Since both clocks are at the same place (on earth) when they compare them this result is nonsense.
=> something has to happen during turning back.

Note the "=>", since this is actually a valid proof - we can describe physics during flight with special relativity and we haven't cheated at the beginning or at the end when we assume the experiment conditions to be as stated at the beginning (*). You can check it, or ask Error if you trust him more.

So I can't answer your question. Could you restate it?

So let me ask you a question: How - with which formulas, which theory - would you describe the change of flight direction from "away from earth" to "towards earth", so that the outcome of the experiment unambiguously gives the result "B is younger than A"?

Thanks for the answer
Actually my example was a little bit obscure

Consider the following cases :

#a departure - acceleration - turn - deceleration - arrival
#b departure - acceleration - cruise - turn - cruise - deceleration - arrival

I think that the permanent shift of time in case b) is higher than in case a) due to the two cruise time intervals at constant speed ( module) , assuming of course that the acceleration , deceleration and turn phases are the same both for a) and b)

b) is a pratical case of a shuttle which remains in the space for a long period
I mentioned in my previous post that a starcraft travel at about v = 40.000 km/hour
Suppose it remains in the space for 1 year
If you put these data in the simple equation of the special relativity for the time dilation, you get a time shift t = 3 - 4 secs

Well this is exactly what has been experienced in the spatial missions
The clock on board were found 3-4 sec late on the arrival

Summing up

I did not mean that all the discussion about the change of " inertial frame " etc are wrong , I mean they are redundant

The "essence" of the twin paradocx lies the time / space symmetry
whereas the twin at rest "moves" only in time , the pilot "moves" both in "time" and in "space"
The time shift is a immediate consequence of the fact that time and space are related items , even though it is hard for everybody to really grasp this claim
For common sense time and space are unrelated measures

According to you, at which point does the relative aging process (making A older than B) occur? During flight? The cruise time?

Alberto, it seems that you still haven't noticed that we've mostly been talking about the other part of the twin paradox. We weren't wondering about the fact THAT the twins have different ages - I believe that this is true has been established early. The fact is merely that people say (**NOT ME**, by the way. I add this disclaimer since that went wrong last time :P) that since everything is relative, you should be able to do the same for Twin B, giving you the opposite age difference. That this is false (because of the fact that twin B is not moving in an inertial frame, see below) has been established on page three.


However, I'm getting really tired of discussing this here, since - again - there is absolutely nothing LEFT to discuss.
I really feel that I've answered this "paradox" about ten times already. I really don't feel like doing it yet again.


But fine, one last time: Here's my answer regarding everything of the twin paradox: Calculate it in Twin A's system, which is an inertial frame. You'll get the result that we've seen above several times.
Now, since that it a completely tensorial equation, it'll be true in all coordinate systems.

Yes, different paths trough spacetime will have a different "length" (referring to the Minkowski-metric here, which, as we know, is no real metric), so the age-difference will depend on parameters that define the path, including the relative percentage of constant movement, acceleration and "turning around".



[The term "inertial frame" has definately something to do with the twin paradox: the formulas are only valid in inertial frames. Twin B's system is *NOT* an inertial frame. It follows that the equations cannot be used in his system - and applying them in that system was the essence of the twin paradox (well, it's second part)].

How would stars look like when you travel at the speed of light and faster?

i also wonder about this.

is the effect on the enterprise windshield realistic? or those points/lines star systems?

The aging process occur during the whole flight




Your scenario is exactly what I meant but, in my opinion, you draw a false conclusion

Before explaining my point of view,let's make a simple consideration :

a) a mission to mars : departure - 1 year flight to go - turning back - 1 year flight back - landing
b) a mission to juppiter : departure - 4 year flight to go - turning back - 4 year flight back - landing

Following your reasoning the time shift ahould be the same but in case b) it is higher than in a)

Having said that, my point of view :

The two twins have been living for a period of time in different " time / space domains" which caused the de syncronization of their clocks
The logical but non intuitive explanation of this effect has been already discussed
Once the twin is back, his clock remains desyncronized
This effect is described in the special relativity whereas both twins are in a inertial system ( exactly the same system according to the Galileo's definition of inertial system)
Therefore there is no reason to emphasize the concept of "inertial " system

Someone claimed that the scenario of the special theory is not realistic due to the fact that in the reality the pilot must reverse his velocity

Ok but such kind of abstractions are normal in pyisics !
You can figure out a realistic scenario whereas the effect of turning acceleration / deceleration are negligible
The velocity is , in the equation, in term of : V^2 thus the sign does not play any role



You must apply the theory of general relativity
I agree with you and Error00014
But in my opinion you and Erro014 missed, so to speak, "The big target"
In other words
To explain the time shift in a real situation you dont need to evoke "inertial system" and all that stuff which are redundant or even deceiving
The " essence" of the twins paradox lies in the special relativity

Puh.


It doesn't, as my example proved. Anyway, if you don't believe me, believe Wikipedia or any text book on that issue.



no, I didn't say that. The inertial systems that B changes are not the same in scenario a) and b).


Ok, one last question on this, maybe then you'll see that you are wrong on that.
Following your reasoning, which is - again to make sure we're talking about the same thing - turning around has nothing to do with it but ONLY the free (no acceleration/turning/whatever) motion of B in space. How do you explain that B is the older one in the end if NOTHING distinguishes their relative motion?


It is, because without it the twin paradox is a paradox.


Tell us that scenario, I am quite sure that in this case you can't.


What does the sign of v have to do with that?


Maybe you can do such a calculation for us. The steps B moves away from earth, B turns around and B moves towards earth should be done separately. You may neglect the middle step if you think it is not important.

Mathematically this is whole simple. Just take the speed, multiplied by gamma and you get the speed of the ship it has when you take Einstein's formulae. Like that, the travel is for B much shorter than for A. This should be common knowledge for everyone seriously into game programming as it is mathematically trivial.

I suppose I made myself understood on this point
I said that both the free motion and the turning around cause a permanent time shift
The former can be calculated using the simple equation of the special relativity
The latter need the much more complicated general relativity theory

I said also that in case of a long journay the former definitely prevail over the latter which can be therefore ignored
The special relativiy only is therefore sufficent to explain the time shift which occur in spatial missions without the need of further complication




This question has been asked also by Error014

The twins are identical at the beginning but their Hystories are differnt

a spend the whole life on the earth
b part on the earth and part in the space

It is b who returns to earth ,iit is not a

We are talking about a " principle of equivalence "

Equivalence entails "symmetry" , it does not entail "identity"

There is thereforo no logical contraddiction in the claim " A is older than B"

It is not , of course, an intuitive claim

These are the only serious questions, and you got my answer
The others are only issues...sorry

If you say that the turning point can be neglected then there is no such thing as a different history for A and B, as both - according to your symmetry principle, or better special relativity - are moving relative to each other. With constant speed. Thus, A is older in B's frame, B is older in A's frame. Contradiction.

As relative motion does not distinguish A and B, what does? (I give you the answer: B turns around.)

@AdrenalinMob: Maybe you can make such a trivial calculation for us. I'd really like to see it.

@Joey: no time

Now this is an interesting question, deserving of a thorough discussion!


One of the effects predicted by Special Relativity is that moving things (imagine a one-meter long pole) appear shorter in the direction they're moving in for an observer at rest. I.e., if I throw a 1-m-pole past you:



Then it'll appear shorter than 1m to you. Instead, it'll seem to be 1m/\Gamma for you.


So if you're moving at a noticable fraction of c (let's ignore velocities bigger than c for now -- on the grounds that since the energy needed to get you there approaches infinity, but we can try and discuss that theoretical thing later if you want), in a system that has you at rest (your rest frame) will see the stars moving in a noticable fraction of c. In turn, they'd appear shorter.

But here's the thing: This is all regarding the pure length of the object. We're interested in what we see, right? If that is the case, then we'll find the complete opposite to be true.

This is known as Terrell Rotation (or, depending on who you ask, Penrose-Terrell rotation).
Here's a nice article on it: Link!

Now, if you do the math, you'll find:
objects passing you appear rotated (see above)
objects moving away from you appear contracted
objects moving closer to you appear elongated.


So yes, you might actually see those kind of lines.

Joey, Alberto -- does anyone of you know more about this Perrell-Tenrose-effect that you'd like to add?

There is no condradiction at all
It is just our common sense which refuse to accept this claim

Clocks are alwayes non syncronized even when B is on flight

Suppose that B is travelling at 50 % of speed light
A and B can comunicate via radio msg's

A says

" You must reach waypoint P(x,y,z) at time T "
if B take the figures as they are he will miss the target
He must turn the figures into his " B's time / space frame "
Not only
If A says
"You must reach waypoint P(x,y,z)"

To us it makes sense but not for the pilot
B would not know where to go to

A must comunicate also the Time !

In the theory of relativity time and space can not be split

The concept of "contemporaneity" has been banned
It does not make any sense to claim

" If the time on earth is...then the time on board..."

The famous equation

Tb = K Ta

with k = 1/sqrt((1 - (v/c)^2)

it does not give the time on board (tb) given the time on the earth(Ta), as many people assume
Tb is the time on board in the " A's space/time frame "

The only thing in common between A and B is the so called "Interval" which is a mix of space and time

I = SQRT(T^2 -L^2) the formula contains also the light speed "c"

If you still think that "turning" is the solution, borrow 3 syncronyzed atomic clocks
One in Berlin, one In Hamburg, one for you
Buy a one way ticket
You will see that your own atomic clock will be some milli seconds late

P.S

Many years ago I made an animation in visula basic 6.0 , using the simple equations of the special relativity, to show how the two observers, in relative motion, describe the same event : an explosion of a super nova
It is rather easy to create but it was quite interesting , at least for those strange guys who are interested in such stuff

I read an article written by Penrose himself , a couple of years ago, but I dont remember the details
I must read it again

A-choo!


Would be great if you could find more details about it, since I do find the question interesting, though it appears to be "little more" (not saying that its trivial) than relativistic addition of velocities applied to light emitted from several points of an object.
Sadly, I don't have any time right now to read such things

Got what. I understand both problem and solution and I'm sure that I'm right, because countless books cover this topic and all agree with me, not you.
I UNDERSTAND special relativity, my common sense sees no contradiction in my sentence. A ages slower in B's frame, B ages slower in A's frame is totally true during flight. However, if both are back to earth ONE of them MUST be the older one. There is absolutely no way that this could still hold when they're both at the same position.


I don't understand you. It might be me but I really have a hard time following your arguments, they sound wishy-washy to me.


Don't make the experiment more complicated than it is. Radio messages travel at the speed of light. What's the problem with the concept of a REST FRAME?


Not really. A spatial coordinate can be Lorentz-transformed without knowledge of time. Of course it has to be communicated if he has to meet the target in a given time span, and of course he has to transform it then, but that's nothing new.
You always make it sound like we'd be doubting relativity. We're not. I'm quite convinced Error has no problem with it, either.


It does make sense if you give the rest frame as information, as I always do. Funny enough, YOU don't do that. I cite: "Clocks are alwayes non syncronized even when B is on flight". In which frame? Or later: "One in Berlin, one In Hamburg". How are they synchronized?
You are missing so much information in your arguments that your explanations simply do not satisfy me.


great you've noticed.


that's a 4-vector contracted to itself. what has that to do with them? Why do A and B have that in common?


http://en.wikipedia.org/wiki/Twin_paradox
http://physics123.net/2009/02/the-twin-paradox-explained/
http://www.einsteins-theory-of-relativity-4engineers.com/twin-paradox-graphical-solution.html
http://www.iep.utm.edu/time-sup/#H17
...
they ALL agree with ME. Find me only ONE article which supports your idea, until then, I will see this discussion as finished.

Joey.

PS: I don't want to sound rude, but honestly, have you ever discussed like that with your physics professor?

Joey, you might have missed (it was easy in the rush of posts a few pages back) an interesting question Joozey asked:



I consider that an interesting new topic, and certainly a discussion that is now more fruitful to discuss then to continue to beat the twin-paradox-horse

If you find the time, I'd be interested in your opinion on that as well.
Though I do assume that it might already be pretty much solved with Terrell-rotation. Still, you're knowledgable on the subject, so I'd like to hear what you think and if maybe I missed something.

Yup, I have no "problems" with relativity ~ I may not be able to fully calculate whatever I want in general relativity, but I do think that I have grasped the fundamentals.

Nope that's why I removed it...you were faster than light

But I think that also joey should remove something, dont you agree ?

And then, something mysterious happened... causing me to keep my promise.

EDIT: Joey? Pardon my language, but I guess he was just a bit pissed off when he wrote the reply - given the length of it and the time you edited, I suppose he saw the earlier version. It might not be the way a gentleman handles things, but I suppose one can understand why he was pissed in the first place. But now that you've corrected your post, it'd only be fair if he now, in turn, removes the little stitches, too...

But then, there's a good reason I'm not a moderator. I guess in the end, we all should just really start to grow up, get a thicker skin, and not get angry at things written by people on the internet.

EDIT EDIT: I'd hate for this to seem like I've been picking sides. Because I don't want this, because I don't feel like I can do this in a fair way. The reason for this simply being the different context and native languages we all have - Joey and me are both german, but I suppose you aren't, Alberto? Since customs and way of speaking can vary alot in different languages, I might just be more sensitive for one type of "insult" over the other. So in essence, I'd really prefer to not say anything definite.

[ Surprisingly, we haven't been asked yet to do someones physics- and/or math-homework yet. Is that a sign of maturity of the schoolkids these days... or just a sign that we're coming across like we have no clue!? ]

What's going on? Who has edited which posts?
I don't want to change my post, that's not the way I write in these forums. Don't take the "great, you've noticed" too serious, though, as I said. I'm sure you know what you talk about. I just don't understand your point. Still, if you could give me a reference, an article or whatever, then maybe I'll understand.

Anyway, Error, it seems as if you're even more knowledgeable on that topic than me. Terrell-rotation? Never heard of that ^^. Have you read that article? That's some weird stuff. I also found that:
http://www.youtube.com/watch?v=JQnHTKZBTI4
sounds quite interesting, although the term "rotation" somehow seems misleading if you ask me. When the sign bends its tip is farther away from your position (obviously), so light will travel longer to your eye from that position. As the middle part is nearer to you, it also seems as if you've passed that part a bit more than the tip. I'm not sure, though, if that accounts for the whole effect.

I'd rather not know what they think of me

Edit: And btw I think we're far calmer than the average forum these days. What's going on in these hiring threads?

Do not discuss about things which you do not know nothing about. I see when someone speaks angry, because then he makes mistakes. And there are many mistakes in your posts (in this guy's there are more, though - reread it and you'll see).

Concerning your discussion, I have read some books on this topic and I have the following conclusion. Consider moving fast.

The red color, being on the lower half of the spectrum then is slower to your eyes. Question: in which direction does the observer move in these pictures?

Not convinced? Imagine your eye being a telescope. Then

the light always moves 'inside' your eyeball, like that. Since your eyeball is not a flat surface (how that would look like - ahahaha) there is great distortion - this also explanes the Terell rotation effect (in parts).

AdrenalinMod

@Error: I'm so jealous of you. How can you stay so settled when the crap meter goes crazy all around you?

Haha, I wish I was always able to do that!
I guess I felt too embarrassed in older posts of mine that one day I finally learnt my lesson and started to not write posts angrily (or, if I must, only allos myself those kind of posts when they're written very, very cynically, so as to make sure they at least provide some kind of comical value [to me, if my humor isn't your kind of thing]).


As far as relativity goes, I had to give a small (bit more than an hour) presentation about general relativity back in November (with a ~10-page handout to go with it), so I had to learn all that stuff back then.
Personally, I never really "got" special relativity the way I did when I correctly did the basics for general relativity. For anyone who's looking for a great book on the subject thats easy to follow (yet still a "science"-book, so there's lots of formulas and math-talk), I can highly recommend Sean Carrolls book "Spacetime and Geometry". Maybe the library of your local university has it, if you want to take a look.
It's easy to follow, complete and well-written (he actually only claims that things are trivial when they are! Anyone who read a mathbook will know what I'm trying to say here).
By the way - the twin paradox is in there, too. And fills about half a page...


EDIT:



However, THATS a thing I've noticed, too!

It seems that the intention of going into game development has changed, and quite a lot, too. A few years ago, it wasn't so much creating commercial games and having commercial success, but rather creating the "dream game", or trying out new fun stuff.
Nowadays, everything seems to be connected to a business-scheme. It's almost impossible to find free help, too, contrary to how that used to work.

So... maybe unlike the "general" indie-community, this one shifts to a more... market-, or money-orientated place...
Not trying to judge here... really only making the observation.


...

Oh, screw this. I'm going to judge: I find that sad. It seems so much less pioneer-spirity, less friendly and way too businesslike.
But I guess quite a lot of the active members here weren't there back then... and some of those had those other targets in mind back then as well. Which, again, is fine, just different.

http://faraday.physics.utoronto.ca/PVB/Harrison/SpecRel/Flash/ContractInvisible.htm

An animation explaining the Penrose Tarrel effect
The relativistic contraction is true but it would not be visible

There is an other amazing paradox

Andromeda is about d = 2.5 mio light/years far away from the earth

We are in the "stars wars" universe

Two people are walking on the earth to opposite direction
According to the theory of special relativity it is:

Ta = (Te + vd/c^2)/sqrt(( 1 - (v/c)^2)

where
Te is the time on earth which is about the same for the two observers the relative speed v being negligible
Ta is the supposed time of a simultaneous event on Andromeda
Ta is not the same for the two obeservers because of the factor : vd/c^2
If the two observers walk at a normal speed the difference is about 1 day

The first observer says :
The Andromeda council of war must still decide whether to attack the earth
The second observers says :
No a decision has been already taken

Both observers are right according to the relativity

Should we assume that the future is , so to speak , already pre programmed ?

Error014

Last time I figured out the following example but I preferred to quit since the discussion had become too hot
Summing up

In may opinion the twin paradox is a direct consequence of the special relativity only but Joey posted some articles whereas it seems the time shift entails a change of speed
I must confess I had some doubt
I would appreciate to get your comments as well as Joey's and any others having some interest in this stuff

The example is :

When the starcraft reaches the cruise speed V = 0.95 C the instrumentation on board start transmitting the pilot's heart beats
On the earth the frequency is f = 60 beats / minute
The twin brother, a doctor, doesnt know the theory of relativity therefore he would expect to receive the beats at a time interval T = 1 + 0.95 = 1.95 secs but he gets
T = 1 + 3.20 = 4.20 secs
He makes the false but consistent assumption that his brother's heart, on board, beats at a frequency f = 18.75 beats / minute
Same consideration on his return
Conclusion : The doctor would not be surprised if his brother twin looks younger than him

What's your comment ?

@AlbertoT: If the brother that stays on Earth would transmit his own heartbeat to the brother on the ship, the brother on the ship would also think that the heartbeat of the other brother is slower than it is as measured on Earth.

That is the case for all inertial systems in special relativity. Moving inertial systems always perceive the other as if time was moving slower there. It seems strange and illogical, but it's true.

Therefore, you need general relativity which explains it by the acceleration that is inevitable if the brother on the ships ever wants to come back.

This is a very common misconception.
The problem is that the brother's reference frame is not an inertial frame at the time where he turns around. Thus, you can't work in his frame. You don't need general relativity to show how acceleration and forces work - General relativity is really more concerned about masses and how it curves spacetime. Electromagnetism, in the form of the electro-magnetic field strength tensor, is one of the nicest examples to show the advantages of "relativistic notation", so to speak, and would be more than capable of providing acceleration. Of course, do that, and the brother's frame won't be an inertial frame anymore.

But special relativity has proven to be very capable of solving this "problem" - it's been done dozens of times in this thread already. The trick is: Solve the equation in an inertial frame, and that would be the one of the brother back on earth.


It may seem difficult who and who isn't in an inertial frame (i.e., to see who of the two got accelerated). But it isn't impossible to find out - unless that acceleration is due to gravity. This, the strong equivalence principle, is what lead Einstein to formulate general relativity.
To illustrate that point, imagine being in a box that is freely falling in the vicinity of a massive object. You'd note that you're accelerating, since it seems to you that forces act on small test particles you throw around. But you cannot tell if you are in a gravitational field, or if you're otherwise getting accelerated.
Now consider the situation not in a gravitational field, but an electromagnetic one. Again, you could measure the effects by using (this time charged) particles. But this time, you'd see that the particles behave DIFFERENTLY if they're charged differently. The reason is that this force is *NOT* proportional to mass, but rather to (electric) charge. Therefore, this force seems "different". (Of course, the situation may be more DIFFICULT to approach in a more "realistic" situation, but there's no physical reason that would make it impossible)




I can't remember those articles in question, or Joey claiming that, but it's been a while. Do you have a link ready? I don't want to go through this thread again.
I agree though that the twin paradox can be completely solved in special relativity.

This post should get a huge disclaimer, because I've done these calculations quickly, and thus may have done mistakes.



I'm not sure I got the problem completely. I think there are three ways to look at this problem:

ASSUME c IS CONSTANT FOR ALL OBSERVERS, BUT IGNORE TIME DILATION
(which is nonsense in a way - after all, time dilation and length contraction can be DERIVED from this assumption - but let us assume this doctor is careless, perhaps, and dangerously misinformed)

0.95 seconds corresponds to what is in essence how long light takes to travel the additional distance, right?

Therefore, he used: v*1s/c
However, at this point, he already realized that c is the same for all observers - after all, he did not assume that the light only travels with speed (1-0.95)c, as you'd do if you naively add the velocities (as we'd do in classical mechanics, if we were to calculate, for instance, the speed of a ball thrown inside a train observed from outside - just adding velocities).



NO RELATIVITY AT ALL
If he actually didn't know about relativity, he'd expect:



the first term being the actual time difference due to the heartbeat (here, we may use an "absolute time and space", since we're ignoring relativity on purpose :)), the second the time our now "slow light" needs to travel the additional distance the second heartbeat has to travel more (which is 0.95*c*1s).
Note please that the first lightray now also takes much longer, but since we're calculating DELTA t, the term vanishes.

Of course, he does not measure 20 seconds, but 6.24s (I get the 3.20 seconds if the doctor corrects those 6.24 seconds UNDER THE ASSUMPTION that c is constant for all observers, but we don't do that here :)). He assumes that the error lies in a different time difference between two heartbeats, that is to say, he assumes a formula of this type:



Solving for t_<3 gives us:

\Delta t_<3 = 0.312 s
Which implies 192.31 heartbeats per minute.
So this particular doctor would be very surprised indeed to find his brother didn't age at all!

... Also, perhaps, that his brother is still alive.

SPECIAL RELATIVITY

Time dilation tells us that the time intervall between two heart beats, observed from the lab frame (=earth, here) would be

\gamma * 1s = 3.20 seconds.

In the lab frame, the ship travels an additional distance of v*c*3.20s in that time, and light travels (for us, and everyone else) at c. So all told, we'd expect a time difference of



Where the first term is just the time between two heartbeats in our reference frame, whereas the second is an additional time difference we get since the second heartbeat has to travel additional distance. This time, it travels at "c" (not 0.05c, as the naive doctor assumed). So we end up with 6.24 seconds.



If our doctor was uneducated (didn't know about relativity at all), he'd get the wrong result - and would assume that his brother would either be dead (due to his crazy heartbeat), or, at the very least, be much older (if he also assumes that, uhm, life speed grows if heartbeat frequency rises).

If our doctor was careless (knew about c=const., but not the rest of it), he'd find from the measured 6.24 seconds a heartbeat-time-difference of 3.20 seconds (which is actually completely right in his rest-frame. A lucky "coincidence", if you will - it is not really a coincidence, granted - this thing is "constructed" to work out like this, but from his wrong assumption, the right thing followed). From there, he'd find the new heartbeat that you've given above.

Another thing to consider is that while time transforms with \gamma, frequency does not (it goes with 1/time, after all). A more complicated transformation may be needed, I do not know right now.




So yeah, that was my slightly longish take on it.
This may actually be a standard problem (the nice result of 20 seconds if done classically, and the still nice result of 3.20 seconds if done in special relativity implies someone chose those numbers with great care ;)), but I didn't know it before, and I didn't google results. What fun is life if there is no risk to embarrass oneself in physics-replies

Lol, kinda hard to make the dream game without electricity or a home;)p

Exactly, simmetry is the essence of relativity
If the doctor reaches the pilot on board then the doctor would be younger

BTW I got the inspiration for this version of the twin paradox from one of most amazing evidence of the special relativity

Remote stars travel at a speed close to the light speed
The spectrum of their light is shifted to the red to such extent that can it not be explained by the Doppler effect only
there are two alternatives :

#1 Time is absolute,
Remote stars are made of a sort of "hybernated" unknown elements
#2 Time is relative
Remote stars are made of the same elements as our sun

Of course the latter is true but also the former is consistent with the measures
It is the twin paradox

Error 014

Here are the articles quoted by Joey

http://physics123.net/2009/02/the-twin-paradox-explained/
http://www.einsteins-theory-of-relativity-4engineers.com/twin-paradox-graphical-solution.html
http://www.iep.utm.edu/time-sup/#H17

I thought you agreed with Joey

BTW of course it is T = 3.2 + 0.95 * 3,2 = 6.24 sec
Anyway in principle , I suppose that my example is valid

The twin on the earth measures a time interval between two next heart beats which is longer than expected

The doctor reasonably assumes that his brother was in a hybernated state caused by the speed
Once on the earth he should therefore look younger

The expectation about the apparent aging of his brother is right even though the interpretation of the measures is wrong

Actually is brother does not "look" younger , he "is" younger , time being relative
The pilot will claim to have alwaeys been conscious

That depends.

If you say: he doesn't know about special relativity, then he'd expect to measure a time intervall of 20 seconds, but measures 6.24 seconds. This means what he measures is shorter than expected.

If you say: he knows that c is constant, but is careless, and thus ignores time dilation, THEN he'd measure something that is longer than expected. That, however, is not the true pre-relativistic interpretation.*


Thanks for the links! I'll check them out soonish.



I think we all agreed with Joey (yes, you too :P). The isse was, if I remember correctly (and a quick look through this thread suggests the same), that Joey was referring to the, if you will, "second" part of this paradox - that is to say he explained why you cannot invoke symmetry of the observers NAIVELY (Travelling twin is not at rest in any inertial system for the whole duration of flight, including turning around). He never showed any signs of not understanding this "paradox", I think.
If you disagree, that's fair enough. But could you then, perhaps, state in clear words where exactly you believe our opinions differ?


* That is to ignore the fact that c=constant can be "seen" from the Maxwell-equations, but let us assume our doctor is a medical doctor, and perhaps not so well-versed in the physics

I dont follow you
The doctor doesn't know the theory of relativity thus he assumes that the time on board flows in the same way as on the earth
He assumes also that his brother's heart still beats at 60 beats / minute same as on the earth
If the starcraft is moving away then he expects to receive a pulse every T = 1 + 0.95 = 1.95 sec
Of course the theory of relativity is valid regardless of the doctor's knowledge thus he will actually receive two next signals at a time interval T = 6.24 sec

What is the most natural explanation ?

The pilot's heart frequency on board is lower than on the earth as well as any other metabolic process
Thus the pilot will look younger on his return

No sorry the topic was clear and nett
I claimed that special relativity by itself entails the twin paradox
Joey did not agree

Again a little bit of hystori of physics

The twin paradox had been proposed by Einstein himself before the general relativity
Someone argued that the special relativity may be true , yet the paradox may be false
The reason being that the special relativity is valid only during the cruise phase
What does it happen during acceleration - turning - deceleration ?
In principle the acceleration phase may compensate the cruise phase so that on his return the pilot may have the same age as his twin brother
Nobody knew the answer at the time

This is the key difference between Joey's opinion and mine

I claimed that the paradox is true " Despite " the acceleration - turning - deceleration phases
Joey claimed ( at least this is what I understood ) " Thanks" to the acceleration - turning - deceleration phases

What Joey said is valid to explain why the pilot is younger than his brother on the earth
Why not the other way round ?

The different aging of two twin brothers is however a direct consequence of the relativism of time
If the time flows in a different way on the earth and on the starcraft, why should the two brother alwayes have the same age ?

In other words

Everybody knows that the special relativity claims that it does not make any sense to claim that two events are alwayes simultaneous
or that the same event has alwayes the same duration regardless of the observer
If so
Why should the twin brothers have alwayes the same age ?

NO.

He wouldn't.

If he doesn't know about relativity, he'd expect light to travel with c-v = 0.05c.
That means he expects 20 seconds.
I've written the rest before.
I have tried to be as clear as possible in my post before, but perhaps it wasn't clear enough. Just a "I don't follow you" is hardly helpful to narrow down where things weren't clear. Could you be more precise?

~~~



The problem is that "the twin paradox" itself is ill-defined. What are we talking about?

• The fact that one twin ages more than the other?
  (Everyone agreed on that this is true)
• The idea that there is no "symmetry"?
  (I.e.: One twin ages more than the other, which is because only one stays in an inertial system).

We all agreed on those points and how to solve them, I believe.



Yes, but I don't think anyone disagreed on the fact that the twins are of different age.

We can agree on these points, I believe?

A) Twin A (on earth) is in an inertial system, and what we find calculating in "his" rest frame is valid
B) Twin B (on spaceship) is *NOT* in an inertial system (due to turnaround), and thus we cannot calculate things the same way as we did for Twin A.

The latter point is, I believe, the essence of what gave trouble for MOST of the thread.



~ ~

To summarize, I'm just not sure what we're arguing here.

Why do you assume that my twin studied classic mechanics only ?

He does not know the theory of relativity but he is aware of the Michelson's experiment
My twin stidied also classic electromagnetism
Even Maxwell knew in 19th century that the speed of electromagnetic waves is an invariant

Uhm, I assumed that because you said he didn't know about the theory of relativity.

The fact that c = constant is widely accepted to be one of its postulates. If you don't believe me, then see wikipedia.



Yes, the Maxwell-equations are not invariant under Galilei-transformations, and they provide very strong evidence for why this postulate is a good idea.
Still, going that one step further and realizing that this is actually a fundamental part of nature is what we usually consider a genius move. Something Einstein himself did. Not something just anyone would do.



Setting c=constant and then not doing the rest of special relativity is some sort of "mish-mash" of classical mechanics and the theory of relativity. Sometimes, this mix will provide good results, and sometimes it won't - which can be said for every mix of "correct and wrong" theories to various degrees.

Same mish mash as great scientists such as Lorentz and Poincare did

The twin assumes that his brother is " asleep at the wheel"
He is not
The interpretation of the measures is wrong but the prediction "my brother will look younger than me " is true

Also Lorentz and Poincare and even Maxwell, as you said yourself, kwew that c is a costant but they tried to explain the "light paradox " assuming that Time is absolute

The speed of ligh being a constant entails the relativism of time, ok that's true, but it is not a so straightforward conclusion