# Younger?

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#### atticus808

##### Guest
if you travelled far into space and came back after a long time<br />would everyone you know look a lot older while you still look about the same as when you left?

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#### rfoshaug

##### Guest
But who is to decide whether you have travelled fast, or if the Earth has travelled fast, when there is no universal grid or reference? Seen from your spaceship, you'd think that you were stationary and that the Earth was the one moving.<br /><br />So if you went away for 10 years at 0.866c, people on earth would see that you were 5 years yonger than those born the same year as you, because they would have seen you travel very fast. But you would see that it was the people on Earth that had aged only 5 years in the 10 years you were away, because as seen from your ship, it was Earth that moved very fast. Or maybe the other way around but still the one in the spaceship would have a different opinion than the earthlings as to who aged normally and who didn't.<br /><br />So who would be right? Would the universe explode because of this paradox? Not likely as the same thing occurs at walking speed, only to a much smaller extent, and the universe doesn't explode...<br /><br />The possibilities:<br /><br />1. The universe in fact does have a global reference or "grid" for speed, so there's no doubt about who was moving fast - this also means that it is possible to become truly stationary relative to space itself. Earthlings see that the astronaut is only 5 years older than he was when he left 10 years ago, while the astronaut can see that everyone on Earth is 10 years older even though he - and all his clocks in the spacecraft - only think that he left 5 years ago). The earthlings are correct and the astronaut is the one that has gained 5 years.<br /><br />2. Parallel realities are forked out at that moment (and in fact at any moment when anything in the universe moves relative to anything else) and both you in the spaceship and the ones on Earth are correct in their observations, and everyone agree about the observations, although the conclusion is different in the different realities (ie. on Earth they see a person coming out of the spacecraft that is only 5 years older than he o <div class="Discussion_UserSignature"> <p><font color="#ff9900">----------------------------------</font></p><p><font color="#ff9900">My minds have many opinions</font></p> </div>

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#### serak_the_preparer

##### Guest
The first possibility is closest to the truth.<br /><br />The universal frame of reference is none other than the speed of light which is everywhere the same. Relative to that, Earth is not moving so fast, and the starship moving at 0.866 <i>c</i> is where the effects of relativity would be most apparent, such as time dilation.

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#### ag30476

##### Guest
> But who is to decide whether you have travelled fast, <br /> /> or if the Earth has travelled fast, when there is no <br /> /> universal grid or reference? Seen from your <br /> /> spaceship, you'd think that you were stationary and <br /> /> that the Earth was the one moving.<br />It's a paradox because while travelling at constant speed there is complete symmetry between the traveller ad the non-traveller. But there is a difference between the two. This place has a good explanation http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_intro.html<br /><br /> /> Einstein and other scientists got it wrong.<br />Hardly. The theory could be wrong. But so far it has been verified by such things as the flying Cesium clock experiments, decay rates of subatomic particles travelling at near light speed, etc.

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#### atticus808

##### Guest
i'm not smart enough to understand<br />what does c mean?<br />this would be the scenario<br />a space ship is built and is controlled on Earth<br />the space ship will travel the speed of light<br />someone goes into the space ship and is launched into space<br />the person that controls the space ship on Earth will let the spaceship travel away from Earth for 5 years<br />once that 5 years is up the person controlling it will turn the spaceship back towards Earth and that would take 5 years<br />so 10 years later<br />would the person in the spaceship look the same as when they left or would they look like the aged the same as us?

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#### dragon04

##### Guest
At light speed (c), time essentially stops for the traveler on the spaceship. 10 years go by for you, but not for him. <div class="Discussion_UserSignature"> <em>"2012.. Year of the Dragon!! Get on the Dragon Wagon!".</em> </div>

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#### newtonian

##### Guest
Atticus808 - c=speed of light.<br /><br />In the formula E=mc^2:<br /><br />E= Energy<br />m = mass<br />c= speed of light<br />^2 = squared, or multiplied once by itself [2^2= 2x2=4; etc.]<br /><br />Your original post is correct if you traveled near the speed of light, and it wouldn't just be looked older - they would be older! <br /><br />Btw (= by the way) c (the speed of light) is 186,000 miles per second, approximately (another definition of the symbol c (circa, about)).

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#### dougum3882

##### Guest
"But who is to decide whether you have travelled fast, or if the Earth has travelled fast, when there is no universal grid or reference?"<br /><br />I think that to determine whether it is the rocket or the Earth which is travelling faster, you need a reference which is neither on Earth nor on the rocket. There is are infinitely many points in space which could be used as this reference point, so that doesn't contradict the no universal reference hypothesis.

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#### harmonicaman

##### Guest
<i>"But who is to decide whether you have travelled fast, or if the Earth has travelled fast, when there is no universal grid or reference?"</i><br /><br />The reference by which we are limited is indicated by the photons which travel in the space all around us. They travel at "c" and this is the velocity which cannot be exceeded -- you cannot travel faster than the universe is expanding! <br /><br />The reference you are seeking is the expansion of space and time itself...<br /><br />Note that all the "m" (mass) and "E" (energy) in the universe is constant and unchanged since the Big Bang event. You cannot get "m" up to "c" velocities because to do so would upset the balance of the universe as proscribed by our laws of physics -- mass and energy can never leave the universe by exceeding "c".<br /><br />Our universe is the expansion of time and space within an infinitely small singularity and the edge of this singularity surrounds every point in space, but it's at "c" velocity away from you (it's right there in front of your nose but you can never reach it)! Every point in space shares this same relative viewpoint.<br /><br />And the opposite is also true; the photon, which is travelling at "c" velocities, does not experience the passage of time. From its perspective the universe is a one dimensional point.

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#### rfoshaug

##### Guest
<blockquote><font class="small">In reply to:</font><hr /><p>It's a paradox because while travelling at constant speed there is complete symmetry between the traveller ad the non-traveller. But there is a difference between the two. This place has a good explanation http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_intro.html<br /><p><hr /></p></p></blockquote><br /><br />That's a very good explanation! Thanks! <img src="/images/icons/smile.gif" /> It's all about who has to accelerate that decides whether the spaceship or the Earth has been moving very fast.<br /><br />But then that leads to another question. The example included the twins Stella and Terence, so let's stick with that.<br /><br />The example was that Stella travels for 7 years (seen from Terence on Earth) at about 0.99 c, then turns around and travels back for another 7 years. Terence knows that Stella has been gone for 14 years, but Stella's clocks say she's been gone only 2 years. <br /><br />But if this spacecraft travelled to a star 7 lightyears away, wouldn't that mean that as seen from Stella, she's travelled 7 lightyears in only 1 year - as in faster than light? Or would space seem to "compress" as seen from her, so that the distance to the star suddenly seemed to be less than a lightyear? <div class="Discussion_UserSignature"> <p><font color="#ff9900">----------------------------------</font></p><p><font color="#ff9900">My minds have many opinions</font></p> </div>

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