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Thread: einstein's twin theory

  1. #1 einstein's twin theory 
    Forum Sophomore schiz0yd's Avatar
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    over the years i've heard about a theory connected to einstein that explains two twin brothers, one going into space and one staying on earth. the one going into space travels to a distant solar system and back, at the speed of light. that's all easy to understand.

    i begin to get lost when einstein says that it would take one year to accellerate to the speed of light, and then he would be at his destination, and then another year to accellerate again and be instantly back at earth. and in this time, he has aged two years, but his brother back on earth has been dead for thousands of years.

    i like to pride myself in knowing a lot about physics, but this is still one thing i'm struggling to wrap my mind around.

    the ultimate question: WHY!?


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    over the years i've heard about a theory connected to einstein that explains two twin brothers, one going into space and one staying on earth. the one going into space travels to a distant solar system and back, at the speed of light. that's all easy to understand.
    That would be Einstein's special relativity, one of the most important theories in physics, laying the cornerstone of general relativity. What you are talking about here refers to the twin paradox that special relativity implies. You can read more about it on Wikipedia.

    i begin to get lost when einstein says that it would take one year to accellerate to the speed of light, and then he would be at his destination, and then another year to accellerate again and be instantly back at earth. and in this time, he has aged two years, but his brother back on earth has been dead for thousands of years.
    This is actually incorrect. The real scenario is that the twin goes to space at a speed close to the speed of light, and spend some time there. On returning, he will actually have aged less than the twin who was back on Earth. There is no mention of him taking an year to accelerate to the speed of light (in any case, actually moving at the speed of light is impossible).

    It seems to me that you are unfamiliar with special relativity, so before I explain the apparent paradox to you, I will acquaint you with it. I will, however, only give you the bare essentials which you need to understand in order to understand the paradox. If you really wish to learn relativity, I suggest you read about it, or ask anyone here to help you understand it.

    Now, Einstein's theory has only two postulates:

    1. The laws of physics are the same in all reference frames. i.e. they will be the same if you are in motion or at rest.

    2. The speed of light will always be measured to be the same in all reference frames. i.e. you will always measure the speed of light to be constant no matter how fast you are moving. This is important because relative velocity implies that you would measure a different speed for light depending on your speed; Einstein raised light to a status above relative velocity.

    On the basis of these two postulates, Einstein deduced several things. One is that the faster you go, the slower time goes i.e. you will measure time to flow more slowly when you are in motion, and the exact slowing down of time depends on your speed. You can find out the amount by which time slows - or dilates - by a certain formula, which I will not mention here.

    This idea of time dilation is all you need to know to comprehend the paradox. The brother who is flying in space at a very high speed will measure time to flow more slowly for him, while the brother on Earth experiences normal time (we are assuming that he is at rest here). Obviously, since time flows more slowly for the brother in space, he will appear to have aged less than his brother on Earth when he returns. This is the paradox in its entirety: according to relativity, a person flying at a very high speed would appear to age less than his brother.

    As DrRocket was kind enough to inform me, this paradox actually represented a problem; in the words of Wikipedia, it suggested that "an absolute effect could be generated by relative motion", which contradicts normal experience, and, also, indirectly, special relativity itself.

    The problem was later resolved because it was assumed that this was a case of relative motion. However, it is easy to show that this would not be a form of relative motion because of acceleration that will obviously occur when the brother goes to space and when he has to turn around to reutrn to Earth; differentiating the two cases, in DrRocket's words. So, ultimately, there really is no problem.

    The paradox no longer poses a problem; it is however, correct in that a person travelling at a high speed will appear to age less than someone at rest.


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    thanks, though i actually have some understanding of einstein's theory's of relativity but the memory i have of the twin theory is obviously a little distorted, i heard about it in an elementary school science class.

    i wonder though, if the speed of light is constant regardless of relativity and the speed of light as a measurement, relevant to our galaxy as a system, would change over time if the expansion of the universe brought our galaxy closer to the speed of light and as you say, would that also slow down not only the measurement of light speed but the flow of time as we would experience it, would time eventually stop flowing for us?
    I prefer to use my right brain to study the universe rather than my left brain.
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    wonder though, if the speed of light is constant regardless of relativity and the speed of light as a measurement, relevant to our galaxy as a system, would change over time if the expansion of the universe brought our galaxy closer to the speed of light and as you say, would that also slow down not only the measurement of light speed but the flow of time as we would experience it, would time eventually stop flowing for us?
    No. That is because time will only appear to 'stop' for us were we to move at the speed of light, which is impossible for any object which has mass. Since it is impossible to even reach this speed, the question of time stopping its flow does not even arise.
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    Quote Originally Posted by schiz0yd
    thanks, though i actually have some understanding of einstein's theory's of relativity but the memory i have of the twin theory is obviously a little distorted, i heard about it in an elementary school science class.

    i wonder though, if the speed of light is constant regardless of relativity and the speed of light as a measurement, relevant to our galaxy as a system, would change over time if the expansion of the universe brought our galaxy closer to the speed of light and as you say, would that also slow down not only the measurement of light speed but the flow of time as we would experience it, would time eventually stop flowing for us?
    It is not a "twin theory" but is sometimes called the "twin paradox" which is not a paradox at all, but just misapplication of the special theory of relativity.

    http://en.wikipedia.org/wiki/Twin_paradox
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    Quote Originally Posted by Liongold
    in any case, actually moving at the speed of light is impossible.
    I wouldn't say impossible, infact faster is possible in some instances, such as:

    c = speed of light (originating from the Latin word celeritās (speed))

    Only zero-rest mass particles can travel at the speed of light. It is generally considered that it is impossible for any information or matter to travel faster than c, because it would travel backwards in time relative to some observers. however, there are many physical situations in which speeds greater than c are encountered.

    Some of these situations involve entities that actually travel faster than c in a particular reference frame but none involves either matter, energy, or information traveling faster than light.


    Wave velocities and synchronized events
    It is possible for the "group velocity" of light to exceed c and in an experiment in 2000 laser beams traveled for extremely short distances through caesium atoms with a group velocity of 300 times c. It is not, however, possible to use this technique to transfer information faster than c since the velocity of information transfer depends on the front velocity, which is always less than c.

    Exceeding the group velocity of light in this manner is comparable to exceeding the speed of sound by arranging people distantly spaced in a line, and asking them all to shout "I'm here!", one after another with short intervals, each one timing it by looking at their own wristwatch so they don't have to wait until they hear the previous person shouting. Another example can be seen when watching ocean waves washing up on shore. With a narrow enough angle between the wave and the shoreline, the breakers travel along the waves' length much faster than the waves' movement inland.

    If a laser is swept across a distant object, the spot of light can easily be made to move at a speed greater than c. Similarly, a shadow projected onto a distant object can be made to move faster than c. In neither case does any matter or information travel faster than light.


    Quantum mechanics
    In some interpretations of quantum mechanics, certain quantum effects may be transmitted at speeds greater than c. For example, the quantum states of two particles can be entangled. Until the particles are observed, they exist in a superposition of two quantum states. If the particles are separated and one of them is observed to determine its quantum state then the quantum state of the second particle is determined automatically and faster than a light signal could travel between the two particles. However, it is impossible to control which quantum state the first particle will take on when it is observed, so no information can be transmitted in this manner.

    Another prediction of faster-than-light speeds occurs for tunneling and is called the Hartman effect. However, no information can be sent using these effects. One ‘‘bit’’ of information is received when a detector has received a sufficient number of photons to be sufficiently sure that an on-bit rather than an off-bit was received.


    Curved space time
    Quantum field theory predicts an apparent superluminal propagation of photons due to vacuum polarization. This prediction raises the question of whether causality may be violated by quantum effects in curved spacetime. This matter is a subject of ongoing research.




    Quote Originally Posted by Liongold
    it is however, correct in that a person travelling at a high speed will appear to age less than someone at rest.
    Actually they will be younger, when clocks are set on space ships they actually go faster then the clocks in earth. Take GPS (Global Positioning System) for example,
    According to the theory of relativity, due to their constant movement and height relative to the Earth-centered, non-rotating approximately inertial reference frame, the clocks on the satellites are affected by their speed (special relativity) as well as their gravitational potential (general relativity). For the GPS satellites, general relativity predicts that the atomic clocks at GPS orbital altitudes will tick more rapidly, by about 45.9 microseconds (μs) per day, because they have a higher gravitational potential than atomic clocks on Earth's surface. Special relativity predicts that atomic clocks moving at GPS orbital speeds will tick more slowly than stationary ground clocks by about 7.2 μs per day. When combined, the discrepancy is about 38 microseconds per day; a difference of 4.465 parts in 1010.[58] To account for this, the frequency standard on board each satellite is given a rate offset prior to launch, making it run slightly slower than the desired frequency on Earth; specifically, at 10.22999999543 MHz instead of 10.23 MHz. Since the atomic clocks on board the GPS satellites are precisely tuned, it makes the system a practical engineering application of the scientific theory of relativity in a real-world environment. Placing atomic clocks on artificial satellites to test Einstein's general theory was first proposed by Friedwardt Winterberg in 1955.


    It's pretty insane to think about, I didn't think the whole time going at a different speed was a "sure thing" myself until I heard about the GPS situation, then I was like, woah!
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    Only zero-rest mass particles can travel at the speed of light. It is generally considered that it is impossible for any information or matter to travel faster than c, because it would travel backwards in time relative to some observers. however, there are many physical situations in which speeds greater than c are encountered.
    I'll have to disagree with you on that. While it is true that it is possible to go faster than light, this is only possible for tachyons, which have yet to be discovered, and also, they suffer from the major disadvantage of never being able to go slower than light. Also, any object having mass will require infinite energy to actually accelerate to the speed of light, which, as you know, is impossible.
    And even if they do, they will appear to have infinite mass at the speed of light.

    I should rephrase myself: it is actually impossible to break the speed of light barrier. If you normally go slower than c, then you may never go as fast as c. But if you normally go faster than c, then the same barrier says that you can never go slower than c.
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    Quote Originally Posted by deadcat
    It's pretty insane to think about, I didn't think the whole time going at a different speed was a "sure thing" myself until I heard about the GPS situation, then I was like, woah!
    Yeah, you can get some funny looks when you start about that in casual conversation...
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    Quote Originally Posted by Liongold
    Only zero-rest mass particles can travel at the speed of light. It is generally considered that it is impossible for any information or matter to travel faster than c, because it would travel backwards in time relative to some observers. however, there are many physical situations in which speeds greater than c are encountered.
    I'll have to disagree with you on that. While it is true that it is possible to go faster than light, this is only possible for tachyons, which have yet to be discovered, and also, they suffer from the major disadvantage of never being able to go slower than light. Also, any object having mass will require infinite energy to actually accelerate to the speed of light, which, as you know, is impossible.
    And even if they do, they will appear to have infinite mass at the speed of light.

    I should rephrase myself: it is actually impossible to break the speed of light barrier. If you normally go slower than c, then you may never go as fast as c. But if you normally go faster than c, then the same barrier says that you can never go slower than c.
    actually, you're not disagreeing with me you're disagreeing with wiki:
    http://en.wikipedia.org/wiki/Speed_o...nd_experiments

    as that statement was copied and pasted from there, it's been proven by lining up lasers to get a boost, click on the link or copy and paste it in your browser and you can read for yourself, then let me know if you still disagree. I wouldn't disagree with anyone on a science forum without a valid reference, that's not science, it's an opinion.

    I was not referring to tachyons, but that did cross my mind as well, however, like you said tachyons have not yet been proven/disproven yet and may never be.
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    All right. After reading the links, I agree with you; however, particles with mass cannot under any circunstances go faster than light or at c.
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    Quote Originally Posted by Liongold
    All right. After reading the links, I agree with you; however, particles with mass cannot under any circunstances go faster than light or at c.
    right, yeah, it was saying, information absolutely CANNOT go faster than light. I think the term I heard for it was "invisible mass" or something like that b/c if information was going faster than light it would potentially be going backwards in time relative to the objects around it. Pretty trippy to think about, b/c if that's the case and we can find out how to get mass to go along w/the faster than light experiments then we can potentially move mass backwards in time. On that show "the universe" there is a physicist that believes he is close to doing this. He says that things would only be able to travel back until the device was created b/c the device would be needed to travel, so he thinks if he gets it working he would get a bunch of messages right when started it. Is it possible? I don't know, wouldn't think so, but this guy knows way more than me about the subject, so I wouldn't argue with him about it. If you haven't seen that series you should, it's on the history channel, it's a sweet series. It's about everything in the universe, planets, black holes, quasars, etc... Kinda makes learning a form of entertainment.
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