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Twin Paradox aging question

  1. Feb 28, 2010 #1
    I'm kind of a noob who's posted a question or two here before to settle arguments and such. Please don't kill me if this has been covered before.

    I was reading some stuff about space travel trying to get some creative ideas for a sci/fi book idea I had. Nothing serious and really nothing to do with the twin paradox, but I stumbled accross that and had a thought.

    I was reading about it on wikipedia and the example used there is one of twin "A" making a trip of 4.45 light years at a speed of .866c. Of course for simplicity they assume that the velocity is reached instantly. Upon twin "A"s return, twin "B" would have aged 10.28 earth years, while twin A would have only aged 5.14. Aside from the complicated (for me) math that they used to generate these results I understand the idea. BUT...

    What would the twins look like physically? If my thoughts are correct, shouldn't they still be the same height/weight etc? What I'm thinking is the human body ages based on it's own biological clock right? It doesn't trigger growth hormones or the aging process based on it's location on earth right? So the 10 earth years/5 earth years thing doesn't really matter here.

    I started thinking of it like this: If it were possible for twins to be born at the same instant on opposite sides of the galaxy, and then relocated instantly to a neutral location, wouldn't they be the same? The earth years number is just an arbitrary number that WE use because it's how we understand time. The human body does it's own thing.

    How far off am I?
  2. jcsd
  3. Mar 1, 2010 #2
    If you reach 86.6% of the speed of light you will be travelling at 259620268 m/s if you were to travel 4.45 light years away it would take exactly 5.14 years to reach the destination and another 5.14 years to get back. So I think earth a and b are 4.45 light years apart.
  4. Mar 1, 2010 #3
    Um...I'm confused.

    Did I not ask the question clear enough? What does restating the provided information in the OP accomplish?
  5. Mar 1, 2010 #4
    They say growth is accelerated in space because of the zero gravity. And that it changes the way a body structure is formed. Look up Dr. Holton this could explain your confusion.
  6. Mar 1, 2010 #5


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    This is not about the aging process, and not abhout different conditions the twins have been living in.
    For one twin, 10 years passed, for the other 5. With all consequences.
    if this were possible, relativity would be wrong. A phrase like "at the same instant on opposite sides of the galaxy" has no well-defined meaning in relativity, and you can't check "instantly" different clocks at different positions.
    You're reading too much into these "earth years". That's just "years", used to measure time. One and the same time for clocks, nucleons, minds, and bodies, btw.
  7. Mar 1, 2010 #6


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    I don't really understand what you're asking. Their weights will of course depend on what they've been eating for all those years. People's heights also tend to change over time. (Kids get taller, old people get shorter).

    I don't think the question makes sense as it stands, because there's no theory that allows instant relocation. If they're going to meet at the neutral location, their ages when they meet will depend on how they moved to get there.
  8. Mar 1, 2010 #7

    I'm amused at the discussion so far. A lot of information that is new for me. But I'm confused about the above quote. Fredrik disqualifies the question due to the fact that there is no theory that supports instant travel, but does not object when I mentioned twins being born at the same time, on opposite sides of the galaxy. Of course NEITHER of those things can happen, but for the sake of argument assuming there WAS a way to instantly relocate (teleport...whatever) wouldn't the twins be the same age biologically?

    For the sake of further clarification let me adjust the hypothetical situation and state that in this case the twins are both female. They are genetically identical and perfect in every way. The hormones and genes that will trigger puberty are going to fire up at the exact same moment (measured in earth years). They will begin their menstrual cycle on the same day, same second....except as we know, twin A will begin her journey at .866c while twin B stays on earth. Shouldn't they both be then menstruating upon twin A's return to earth, even though she's only 5 years old? ( I know I know, twin B is 10 and most girls don't start that early either, but some do so lets assume that these twins do to).

    So essentially their bodies are the same physically even though the number that annotates their "time on earth" so to speak differs?

    How can approaching the speed of light affect the bodies aging process is what I don't get I guess?

    If I were to approach and then go BEYOND the speed of light (I know I know, but come along with me) would that be some sort of fountain of youth? As long as the body keeps traveling faster and faster toward infinity, it just stops growing, aging etc? Where can I learn more about this?
  9. Mar 1, 2010 #8
    Time runs slower. That means 5-earth year boy will still only be five. He will look like 10-earth year boy did 5 years ago. And 5 years ago, 5-earth year boy will look like a toddler(2.5years).

    Carl Sagan explains it in another way
    Last edited by a moderator: Sep 25, 2014
  10. Mar 1, 2010 #9


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    When your two twins are traveling relative to each other, they measure time and space differently. As strange as this might seem, this is the way the universe is put together.

    This means that if Our Earth twin were watching his brother in a telescope, he would see the his brother is aging more slowly (this is after he compensates for the time it takes the light reach him from his brother.) his heart beats slower, his hair grows slower, his movements are slower, etc. When he returns he will be 5.14 years older biologically while the stay at home twin will have aged 10.28 yrs biologically.

    The traveling twin notices nothing of this. As far as he is concerned, he ages normally. What he does notice is that the distance between the Earth and the point where he turns around is only 2.225 ly. (this same distance is 4.45 ly by the Earth twin's measure). Since it only take 5.14 yrs to out and back to a point 2.225 ly at 0.886c, this is how long the trip is by his clock (biological clock included).

    Thus according to the Earth twin his brother aged less because time ran slower for him, while the traveling twin says he aged less because the trip was shorter in distance.
  11. Mar 1, 2010 #10
    Thank you Janus! Exactly the explanation I needed. And Tcheq for the video link, I had forgotten about that portion of "Cosmos".

    It is still a hard thing for me to grasp, but I have fleeting moments where I feel I'm starting to understand it. Maybe thats the sign that I'll soon have it in full. Since I'm not studying physics in any way, I don't have the knowledge necessary to formulate the most educated of thoughts and so I appreciate everyones help.

    I didn't take much physics in school (radiation physics for x-ray tech school) I don't spend much time with it, but I'm becoming fascinated with all the fun little thoughts a person can have.

    When I asked earlier about the "fountain of youth" thing, it wasn't until later that I realized that I was really beginning to ask about time travel. If there was a magic speed at which aging simply froze, then there must be a theoretical speed at which aging would be reversed then right? Essentially time travel.
  12. Mar 1, 2010 #11


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    Identical twins born on planets at opposite sides of the galaxy, at events that have the same time coordinate in the inertial coordinate system in which Earth is not moving, is not in violation of the laws of physics. It's difficult to accomplish, but possible in principle. But to ask what happens if you instantly relocate someone, that's like asking if you would still be hungry if you ate yourself for dinner. It's logically impossible, at least in the theories that I was supposed to use to answer the rest of your question. You're asking what relativity would be saying if it had been a different theory which is logically inconsistent.

    Are you still talking about twins that start out far apart?

    If you mean "at events that are assigned the same time coordinate by the inertial coordinate system in which Earth is not moving", this isn't going to happen if one of the twins is on a journey towards the other. But maybe that's not what you meant. What's going to happen is that the clocks they carry with them will show approximately the same time at the two events where their "hormones and genes fire up".

    Human bodies can be considered "clocks". They're not as accurate as, let's say the clock on your cell phone, but they're still clocks, and a clock measures the proper time of the curve in spacetime that represents its motion.

    Again, a question doesn't make sense if it assumes something that contradicts the theory that's supposed to answer the question.

    You can go arbitrarily go arbitrarily close to the speed of light relative to some other object, like the Earth, but no matter how fast you go, it won't increase the amount of experiences you can have before you die.
  13. Mar 2, 2010 #12


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    Here is an explanation I gave some time ago on this topic.

    All biological processes can be considered to be (inaccurate) clocks. So if all clocks slow down then so do all biological processes, otherwise you could determine absolute speed by comparing the rate of biological processes to other clocks. This would violate the first postulate of relativity.
  14. Mar 2, 2010 #13


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    It seems like no one on this thread has actually used the phrase "relativity of simultaneity", might help to do so since then Jackslap could look it up (and find pages like this one and this one) if he wanted to learn a little more about it. Basically the relativity of simultaneity says that events that happen at the same time (simultaneously) in one frame happen at different times in other frames, and according to the first postulate of SR, no frame's perspective is physically privileged over any other frame's. So this is why the phrase "same instant" has no objective frame-independent meaning in relativity, and similarly why instantaneous travel doesn't make sense in SR (even if the event of an object dematerializing at one location happened simultaneously in some frame with the event of it rematerializing at another location, in other frames the rematerialization would happen some time after the dematerialization, and in still other frames the rematerialization would happen before the dematerialization).
  15. Mar 2, 2010 #14
    It's not relevant to the understanding of the implications of time dilation.

    I see no need to overword a discussion when a simple answer will do. I see it all too often, very recently, for example a 16 year old asked a simple question and people quoted GR equations in their replies. When I was 16 I barely understood what differential notation was, let alone double differential equations across a tensor in spherical coordinates.

    The implications of GR isn't a complicated thing to understand. The maths that make it so might be, but that is unimportant unless you are a theoretical physicist, cosmologist or otherwise a complete psychotic. One job of a good scientist is to make the astoundingly overwhelming seem easy (because it is).

    Bringing big words or concepts such as simultaneity into a simple discussion is like a GR dogmatist elbowing his way into a discussion on low-velocity physics, repeatedly poking a stick into the ribs of the group who can't remember in which case to use sin and cos, asking them where their gamma is.
  16. Mar 2, 2010 #15
    Smokes, now you guys are blowing my mind. It is obvious that my comprehension of such complicated topics is infantile compared to some of you folks. I am still a bit confused about certain terminologies you guys use, since I have not had any formal education in physics. For instance, you all seem to speak of "frames" frequently. I'm supposing frames to mean an area or a range, perhaps a perspective. But maybe I'm not on target with that.

    I obviously have much learning to do if I wish to carry on similar conversations with everyone. I feel a little more stupid now thank you very much.

    I can take everyones word for it that my instincts were wrong on this one, that indeed the body's biological clock will slow down as accelleration of the body increases. I hear you, but still don't quite understand the why.

    I can liken it my job in x-ray. Limited x-ray techs are button pushers, with little or no knowledge of radiation and how it's controlling factors can be used to manipulate radiographs. But fully licensed techs actually have a degree and understand the "why" radiation does what it does. I need to get to the "why" of all of this. I'm sure understanding comes with plenty of study and time.
  17. Mar 2, 2010 #16


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    TcheQ: In #7, Jackslap was talking about two girls, one of which was on Earth, and the other was traveling towards Earth in a spaceship at a relativistic speed, and he talked about how they would be having their first period "at the exact same moment (measured in earth years)". This clearly indicates that he thinks of simultaneity as something absolute.

    If you understand why that's a mistake, you also know that he has no chance of understanding what was wrong with that assumption until he has learned about simultaneity. So why the attitude? If you want to give him those "simple answers", then just do it. Save the agression for a time when its appropriate.
  18. Mar 2, 2010 #17


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    Frame is short for "reference frame", and when that term is used in the context of special relativity, it always means "inertial reference frame". So what you need to know is just what an inertial reference frame is. I prefer a mathematical definition myself, but I think that would be of little use to you right now, so I suggest that you think of an inertial frame as an imaginary grid of meter sticks and synchronized clocks, all moving with the same velocity, and extending to infinity in all directions. You can use this grid to assign coordinates to events. I'm saying "imaginary" because we're going to have to consider at least two such grids moving at different velocities, so they'd have to be able to pass through each other.

    The idea is that an inertial observer (someone or something that isn't accelerating) can use a grid that's co-moving with him to assign coordinates to events, and those assignments are what we're referring to when we talk about e.g. the length of an object "in his frame".

    That part isn't really easy to explain, so you're going to have to study some SR if you really want to understand it. The easiest way to learn the basics of SR is to learn about spacetime diagrams.

    Edit: After reading dacruick's response to this quote, I noticed that the quote gets it wrong. Time dilation isn't caused by acceleration. It only depends on the relative velocity of the clocks. (But be careful, a misinterpretation of that statement is what leads to the twin "paradox").

    I'll add one more thing: Motion is represented mathematically by curves in spacetime. A point particle is an object whose motion can be represented by only one curve. Ideal clocks are treated as point particles. One of the axioms of SR is that a clock measures a certain mathematical property of the curve in spacetime that represents its motion. The fact that two different observers will disagree about how much time has elapsed between two events can be traced back to that. The properties of the clocks aren't different. It's just that they measure coordinate independent properties of different curves.

    It does. SR isn't so hard to understand compared to many other things in physics. The math is very easy compared to GR or QM. Most of it can even be understood by drawing a bunch of straight lines on a piece of paper.
    Last edited: Mar 2, 2010
  19. Mar 2, 2010 #18
    No ones biological clock slows down in their frame of reference. Time is normal for both the twin on earth and the twin in the spaceship. Its just the concept that time is "elongated" at higher speeds. I mean this is all sketchy terminology and can be easily misinterpreted. You should look up anything that these fellows say as it will all be useful in helping you understand. But no ones clock slows down or speeds up, the clock stays the same. its just the value of time depends on your speed. I suggest you look up time dilation. There is an experiment that i remember really began to interest me in the concept of relativity. there are two mirrors travelling together with a ray of light bounces from one side to another.
  20. Mar 2, 2010 #19


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    The question was not solely about the implications of time dilation. As Fredrik said, this part of Jackslap's question seemed to require a response discussing how simultaneity isn't absolute in relativity:
  21. Mar 2, 2010 #20
    maybe to make it slightly more clear. in 1 second on earth, twin A's biological clock will carry out the same actions as twin B's biological clock will in a second. The only thing different is that if on earth 2 seconds pass, 1 second will pass at .866c.

    I think this is where your understanding breaks down. biological clock is dependent on time and time is dependent on speed. so at higher speeds, the biological is not affected by speed. say we want to define the biological processes of the brain(hormones etc) in a "reaction per second" unit. as you can see, no matter what speed there will be, there will still be the same reactions per seconds carried out by the brain. The only thing different between twin A and twin B is the amount of seconds that has passed, not the rate of biological processes.

    You're right, this topic is mind blowing, keep asking questions. more questions, more answers, more understanding.
    Last edited: Mar 2, 2010
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