# twin paradox?? (lightspeed + time)

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#### riflemannl

##### Guest
can someone explain it ?? <img src="/images/icons/blush.gif" />

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#### Saiph

##### Guest
in short: If someone leaves earth at near the speed of light, the earth people will say "look! he's moving, and thus aging slower."<br /><br />According to relativity however, the spaceguy can say, "Look! Earth is moving, they'll age slower!"<br /><br />At this point, they're both right, and will continue to be so, until they come together agian and compare clocks, when they'll find only one of them aged slower.<br /><br />The reason is: Only 1 turns around. And in doing so, some people feel an acceleration, and others don't. You can now tell who moved (basically) and "reciprocity" is broken. The spaceguy (we'll assume we didnt' turn the earth around) turned around, and knows it.<br /><br />When he returns, he'll have aged less.<br /><br /><br /><br />Now, general relativity, IIRC put all the time dilation that occurs, and stays, in that acceleration point. The straight aways cancel out. GR does a better job of handling it, but the math is more complicated.<br /><br />If you use the little trick above, you can calculate the same answer from SR (which doesn't address acceleration any more than that). <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### alokmohan

##### Guest
Two twins of age 20.One decides to go to alpha centuri at speed of nearly light.His age remains almost same.He is around 20 still then.But in earth 30 years have lapsed .So his twin is 50.That is the paradox.

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#### lewcos

##### Guest
I don't believe it. The guy moving fast will be 50 also - just because a clock slows down doesn't convince me that a person will age any slower.

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#### Saiph

##### Guest
well, it does work that way actually.<br /><br />Look up relativity and "muons". Basically muons created by cosmic rays entering our atmosphere are detected on the ground. The problem is we observe far to many. Almost all of them should decay before hitting the ground (they are unstable particles), because they just don't live that long. They can't get to the ground fast enough.<br /><br />The explaination: Time dilation. While we measure a second go by, the muon experiences only a fraction of it, and thus only ages by that fraction, and that's the time used for the "decay period". So it has longer to get to the ground.<br /><br />The catch to this paradox is that the muon doesn't think it's gone as far as we say it has (the length contraction part of relativity).<br /><br />The reason is that it isn't just the "clock" that slowing down. The clock runs just fine. The person in the craft will look at it and say, yep, seems like a second.<br /><br />The reason is <i>time</i> is slower, and thus so are all the processes that depend upon it (any possible thing that requires time and can measure time).<br /><br />So the clock doesn't go slower, it still measures a "second" of time. However the people on earth, and the spacegoers, no longer agree on how long a second is (despite using the same definition!) <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### jurgens

##### Guest
believe it or not, that's the way it works.

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#### i_think

##### Guest
Perhaps it's easier to believe if you don't think of it as a paradox. The 20 year year old experienced 20 years of life in his frame of reference, and the 50 year old experienced 50 years of life in his frame of reference. No paradox. Neither twin cheated time, nor was cheated of time, time was conserved in a sense. If you believe the clock slows (as already proven) but not the twin, then you are believing that the twin will age 30 years in perhaps a matter of minutes by his "slow" clock and be cheated of time, an alternative which is even harder to believe IMO.

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#### jatslo

##### Guest
Paradox?<br /><br />Speed up, the earth slows, speed up some more, the earth is even slower, speed up even more, the earth stops, at some point, your constant acceleration will send the earth backwards in time causing a paradox.<br /><br />However, I think it is more likely that you would enter a new dimension. Space is made up of the following three dimensions: x, y, and z, as in space-time, in which time is the forth dimension.<br /><br />The only problem with my dimension hypothesis is that, if my space traveler crosses over to another dimension, then my space travelers opposite will enter our universe at exactly the same time. My space traveler’s opposite my not be able to exist in this universe, and visa versa, and even if they could survive, I doubt that either of them would understand anything they are observing.<br /><br />Paradox?

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#### jurgens

##### Guest
jatslo, wtf are you talking about? Certainly not General Relativity... <br /><br />Also Saiph is right here.

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#### riflemannl

##### Guest
k then.. let's get biologics in it..<br /><br />getting 'older', is just the 'stretching' of your bodycells. Time isn't the thing that controls our body, it's just the mitose of our cells.. Maybe the 'time' on earth might have advanced a lot of years, or 'slowed', whatever you like, but the two bodies will still be the same..

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#### jurgens

##### Guest
RifleMan wtf are you talking about?<br /><br />Whats with these crazy theories that defy general relativity. Look, the person in the spaceship that is travelling very fast experiences less time then the person staying on the earth. EVERYTHING in the spaceship experiences less time too, not just the "biology" of the person. For example you have a particle with a half life of 20 years and you travel such that the dilation between your proper time and the earth's time is .5, within 20 years, you will have more of the original amount of the radioactive particle then you should have.<br /><br />Also you don't age because your cells get stretched. Aging has many many different causes...

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#### jatslo

##### Guest
Evidence:<br /><br /><font color="yellow">"The possibility of paradoxes<br />The Novikov self-consistency principle and recent calculations by Kip S. Thorne indicate that simple masses passing through time travel wormholes could never engender paradoxes—there are no initial conditions that lead to paradox once time travel is introduced. If his results can be generalized they would suggest, curiously, that none of the supposed paradoxes formulated in time travel stories can actually be formulated at a precise physical level: that is, that any situation you can set up in a time travel story turns out to permit many consistent solutions. The circumstances might, however, turn out to be almost unbelievably strange.<br /><br />Parallel universes might provide a way out of paradoxes. Everett's many-worlds interpretation of quantum mechanics suggests that all possible quantum events can occur in mutually exclusive histories. These alternate, or parallel histories would form a branching tree symbolizing all possible outcomes of any interaction.<br /><br />Since all possibilities exist, any paradoxes can be explained by having the paradoxical events happening in a different universe. This concept is most often used in science-fiction. However, in actuality, physicists believe that such interaction or interference between these histories is not possible (see Chronology protection conjecture) (Wikipedia)."<br /><br />http://en.wikipedia.org/wiki/Time_travel<br /></font>/safety_wrapper>

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#### alokmohan

##### Guest
Possibly time dialation will affect biology also.

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##### Guest
So the red stars in the far distance, whose clocks are very slow are just burning very slowly giving red heat & not red shifted by Doppler effect.

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#### riflemannl

##### Guest
so time travel and the eternal youth do excist... in a certain way (theoretic)

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#### Saiph

##### Guest
Biologics are affected by time dilation as well.<br /><br />You correctly describe physical aging as a change or deterioration of the body's component parts (mostly cells and DNA).<br /><br />However, you must also realize that those changes take time. If you change the rate of time, those reactions will occur faster, or slower. If these chemical reactions and changes occur slower, the phsyical changes occur slower, and the person ages less.<br /><br />All physical processes are time dependent to one degree or another. Change the rate of time, and the process changes at a different rate. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### Saiph

##### Guest
Jatslo:<br /><br />You cannot accelerate to speeds higher than C, and thus cannot go backwards in time using velocity as the mechanism.<br /><br />Kip S. Thorne also showed that wormholes won't do it, becuase the mere act of using one of those, causes it to collapse before you go anywhere. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### alokmohan

##### Guest
Thats what I wanted to say.

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#### Saiph

##### Guest
and you did (it's a good point to make), I just wanted to elaborate a bit. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### jatslo

##### Guest
<font color="yellow">You cannot accelerate to speeds higher than C, and thus cannot go backwards in time using velocity as the mechanism.</font><br /><br />Yes you can. I will prove it to you when I finish building my quantum computer. I don't think that a particle that has a mass, and/or charge can go back in time, but if I encrypted information onto a photon, I could send the photon back in time, then I could utilize the information to manufacuture masses from the future.<br /><br />I am going to corner the stock market, and rule the world, ah!! ha! ha! ha! ha!!!

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#### i_think

##### Guest
<font color="yellow"> in short: If someone leaves earth at near the speed of light, the earth people will say "look! he's moving, and thus aging slower." <br /><br />According to relativity however, the spaceguy can say, "Look! Earth is moving, they'll age slower!" <br /><br />At this point, they're both right, and will continue to be so, until they come together agian and compare clocks, when they'll find only one of them aged slower. </font><br /><br />Thank you Saiph. You made an important distinction that I should have realized before but didn't. We have no fixed reference point to know who or what is increasing or decreasing in speed relative to light. I have been thinking about this for a while, and while I learned something new I find myself more confused by our mysterious universe.<br /><br />Let's say we send two spaceguys in opposite directions away from Earth and "accelerating" towards two distant stars. Each star appears to be receding away from us if I understand correctly. I will assume the speed of Earth must be between zero and c. One spaceguy will be increasing in speed relative to light, and the other will be decreasing in speed, but they won't know which is which until they return to Earth and compare their clocks. How will the spaceguy who is decreasing in speed relative to light ever reach his destination? Can he get there from here? Is half the universe beyond our reach because if we try to go in that direction we will be slowing down relative to light speed?

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#### nexium

##### Guest
Main stream theory is relativity. That means nothing unusual happens to the people or equipment in a spacecraft traveling 0.9999c because their speed is approximately zero with respect to the space craft. Earth is travelling that speed with respect to the most distant parts of the visable universe. The speed becomes important only if we collide with something, or return home and discover more time has elaped back home than in the space craft. Neil

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#### i_think

##### Guest
Thanks for the reply, but I still can't reconcile how if Earth is travelling between zero and the speed of light, and two spaceguys are accellerating in opposite directions away from Earth, one spaceguy may eventually get close to the speed of light and the other will eventually reach what, speed zero? How will he get anywhere?<br /><br />Thanks to a link from maxtheknife in the thread about the speed of gravity, I found something which makes more sense to me but I'm still digesting it. Lorentzian relativity makes more sense to me than Einstein's special relativity.<br /><br /><font color="yellow">Although LR has no intrinsic speed limit, it recognizes the innate difficulty of material bodies composed in part of electrons, while propagating in luminiferous ether, being able to exceed the wave speed of that ether, the speed of light. LR treats this as analogous to a propeller-driven aircraft exceeding the speed of sound without any outside assists, such as from gravity. A force that cannot itself propagate faster than light cannot propel material bodies faster than light....<br /><br />In LR, one reference frame (the local gravity field) is preferred; and speed cannot affect time, but only the rate of ticking of mechanical, electromagnetic, or biological clocks. However, just as we do not assume that time has been affected when the temperature rises and causes a pendulum clock to slow down, LR says that changes in clock rates are changes in the rates of physical processes, and do not affect space or time....<br /><br />Aside from these practical difficulties with the use of SR in the GPS, Einstein's special relativity is also under challenge in a more serious way from the "speed of gravity" issue, because the proven existence of anything propagating faster than light in forward time (as all experiments indicate is the case for gravity) would falsify SR outright. So it is entirely possible that reality is Lorentzian, not Einsteinian, with respect to the relativity of motion.</font>

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#### Saiph

##### Guest
It's a frame of reference again. <br /><br />Moving relative to <i>what</i> is always the question.<br /><br />You have an object leave earth, and "decelerate". <br /><br />Deceleration is merely acceleration, same thing. The only reason to give it a name, is that it would have an acceleration opposite your motion (relative to something anyway). To clear things up, lets call it what it is, acceleration.<br /><br />So you have two craft leave earth in opposite directions, each accelerating to higher speeds (relative to earth). For simplicities sake, we'll have earth stationary (velocity = 0) though it could be any other velocity betwee 0 and C.<br /><br />Now, everyone, both craft and earth, will measure the speed of light to be the same thing, going 3x10^8 m/s relative to them. No matter how they move, they'll always measure this.<br /><br />And they'll always measure the speed of the other guys to be less than C due to the way velocities add in relativity. .8c +.8c does not give 1.6c. It gives something like .9c.<br /><br /> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### kmarinas86

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