Everyone knows the twin paradox. Two twins, one stays on earth, other goes in a space ship at high speed and leaves earth and comes back and the twin in the ship comes back younger.
Well, everyone tells me that theres no ether in space. And Space and time are relative. So relative to the space ship the earth moves away from it and then comes back. But relative to earth the spaceship moves away and comes back. Well sense they both experience the same thing relative to each other how come the twin in the space ship comes back younger? It seems to me like you could just say that the Earth was a big fat spaceship and it flew away from the small one?
Can someone explain how and why that is so in laymen terms?
[This message was edited by Dave on 12-21-03 at 09:25 PM.]
Posts: 1 | Location: Fairfax, VA, USA | Registered: 12-21-03
I'm going to try best I can, and someone who is smarter, please correct me.
The gravitational pull of the earth flexes space and time around it. Kind of like the way that a rubber band can bend into almost a figure eight--just doesn't meet in the middle. As time gets closer to the earth's pull, it moves faster, because its being pulled down.
This means, as the space traveling twin leaves the earth's atmosphere and travels away from the earth's gravitational pull, the time around the ship is growing slower and slower--the further away, the slower it is. However, relative to the space traveling twin, time is moving normally--just like it will look like a car is not moving on the highway when you are both going the same speed.
The earth bound twin is experiencing the faster time that is being pulled by gravity, relative to the slower time further out.
Thus, when the space traveling twin returns to earth, he will not have experienced as much time passage as the earth bound twin, and will be physically younger (though mentally the same age).
The earth is in fact a big fat space ship, just like the little space ship. The difference is though that the earth is SUCH a big fat space ship that its gravitational pull is strong enough to bend time, whereas the ship is far too small.
Posts: 3065 | Location: A place with palm trees and sunshine! | Registered: 03-17-03
First, you need to realize that, as far as relativity is concerned, acceleration and gravitational fields are equivalent. Think of standing in an elevator as it accelerates upwards. Doesn't it just feel like gravity has temporarily increased? Dave said that all frames of reference are equivalent. This is almost true. All frames that aren't feelign an acceleration (either due to gravity or due to actual motion) are equivalent. Acelerating frames or frames with gravity are not equivalent to others.
Mx2 is right that time dilation causes time to be different for the earth bound and travelling twins, but the time dilation due to the stronger field on the surface of the earth actually makes time pass more slowly for the earth bound twin, which would make him younger at the end of the story, not older.
But remember, acceleration and gravity are equivalent. The travelling twin must accelerate in order to turn around and re-approach the earth. It is this acceleration that dilates time and makes it pass more slowly for him. this is why the travelling twin ages less. ************************************************** 12-23-03, 12:44 AM gerry There have been scores of explanations as to why it is the travelling twin who ages less. I agree with Methos that it is in fact due to the acceleration of the traveller who must turn around to return to earth. Otherwise, there would be no way to make the comparison as to who aged less, as each would see other aging more slowly, and it could not be confirmed until the travelling twin returned, who would have aged less both physically AND mentally.
12-28-03, 08:16 AM MommyTimesTwo Hrm.
I must have misunderstood the other day, I was watching a show on the Science Channel on this very topic (with the same example of two twins, too) and it said pretty clearly that the one on Earth would be older.
I'm going to have to find those old Physics texts from the class I didn't bother to attend in college and look it up! Wink
01-04-04, 09:35 PM methos Mx2, I may not have been clear.
Yes, the earthbound twin would be older in the paradox, but your explanation would make the earthbound twin younger, therefore it must not be the correct explanation.
The primary answer is a resounding YES! the space traveling twin can be thought of as being stock still and Earth the moving entity -- and Relativity will still dictate that the astronaut twin comes home younger. This is because, although it is up for grabs which entity is *moving*, it is unambiguous which entity changed course: that being the astronaut twin. Some very simple straight-forward arithmetic is included at my cited page, but it is nothing overbearing, I assure you. Check it out.
05-08-04, 12:30 AM methos In your example, observe the path of the clock's motion whether it is a physical clock moving from hand to hand or timekeeping passed from one physical clock to another physical clock. At the moment that Stella and Alf pass, the clock undergoes instantaneous deceleration from Stella's velocity / acceleration to Alf's velocity.
Strictly speaking, your example doesn't actually eliminate acceleration, it merely hides it.
Calculation-wise, it's no different than Stella undergoing an instantaneous reverse at the point she meets Alf.
That said, it's a fascinating example.
05-08-04, 01:48 AM Professor I agree with Methos.
I think the essential feature of the Twin Paradox -- the reason it's a paradox -- is the apparent symmetry between the twins, prompting Dave's original question about why we can't just reverse roles of spaceship and Earth. Where is symmetry broken?
And the answer (already given) is that, in order to appear for their joint photo-op back on Earth, one twin but not the other will have accelerated. kj's novel scenario boils it down to the bare essentials, but symmetry is still broken because one of the twins uses another traveler to carry a clock, while the other keeps her own clock the entire time.
In any case the "paradox" actually occurs, and does not represent a logical flaw in Relativity. Mountains of experimental evidence have confirmed it.
05-09-04, 11:48 PM methos Perhaps a clearer way of saying it:
Acceleration is a change in velocity. Velocity is speed in a given direction. Your reference frame follows Stella until she meets Alf, at which point it switches directions. Therefore, your reference frame accelerates at the point where Stella and Alf pass.
05-10-04, 01:49 AM Professor kj: The "jabbering nonsequitor" (I take that as a compliment!) was directed to the original questioner, who seems to harbor doubts about the validity of relativity. There's a lot of that going around.
I meant to say "clock reading" instead of "clock." It's clear that Stella and Alf can pass arbitrarily close to exchange information almost instantaneously.
Who are the "naysayers" to whom you refer? I liked your site and thought I implied that.
Have you tried switching to decaf?
01-28-06, 10:15 AM socratus My parrot and quantum of light.
Gentlemen. I have only two questions: First - Hadn,t my parrot flown to you? Second - Why does everyone speak, that the relative movement is real only, if the speed of quantum of light is absolute and it can occur only in absolute space (Vacuum)? P.S. You can easily find out my parrot, it has learned only two sentences: “there is no absolute movement’, “ there is no absolute reference system”.
01-28-06, 03:25 PM kj
quote: Originally posted by Professor: kj: The "jabbering nonsequitor" (I take that as a compliment!) was directed to the original questioner, who seems to... Have you tried switching to decaf?
Ok, I've recinded the post
01-28-06, 03:30 PM kj
quote: Originally posted by socratus: My parrot and quantum of light. Gentlemen. ...
Look at it this way Gerry said it right, and add that the certainty of the acceleration doesn't signify any certainty of motion.
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