Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Einstien's twins

  1. Aug 13, 2003 #1
    we all know abt twin paradox(except those poor souls who would rather read playboy than physics today!)
    now a very fundamental question abt it is;
    does time realy gets slow when we r moving with speed comparable to that of light or its just 'malefunctioning'(mark the word) of clocks?
    what i mean is that, our most realiable source of measuring time is light itself!
    consider a clock witch consists of a stick an a light source, we measure time by using lenth and velocity relation. now when we move this clock to a rather high velocity refrence frmae, length of stick changes , speed of light remains same, so we got defrent time rate.
    does this really have to do with real time?
    i mean time we can measure by biologicl processes like aging.
    i dont know abt it, do u?
    it may be that when twin come back from his journey he will find himself as agd as his brother, just his rolex is some years behind!
  2. jcsd
  3. Aug 13, 2003 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Doesn't work that way; anything that is capable of "keeping" time, such as light clocks, oscillating atoms, biological processes, and radioactive decay, are equally dilated. If this were not true, it would violate the principle of relativity because one could identify absolute velocity by comparing the speed of one's biological processes (say, heartbeat) to a light clock.

    Tests have indeed been done; people have flown high precision atomic clocks around in jets and found their rates changed just as predicted by special relativity. Physicists using particle accelerators routinely observe the half-lives of particles being dramatically extended when they are near travelling near the speed of light.
  4. Aug 14, 2003 #3


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    It's important to add that the moving twin experiences all this dilation/contraction only with respect to the other twin. She herself experiences nothing unusual. Indeed she can regard herself as being at rest and her twin as doing the moving. And relative to her the other twin experiences the dilation/contraction.

    The dilation/contractions are realbut relative, and every differently moving inertial observer will see different values of them.
  5. Aug 15, 2003 #4
    Basically we can determine who's moving by comparing the time as perceived by each of the twins.
  6. Aug 16, 2003 #5


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    If both twins remain inertial, i.e. neither is accelerated, then the first postulate of relativity says that each experiences normal physics within her own frame of reference. So each twin's local time is the same in her own frame of reference.

    Who are the "we" who going to examine the times? Just some other inertial observer with a velocity relation to each twin and therefore a pair of Lorentz transformations to render each twin's time, and other data, into "our" own frame.
  7. Aug 17, 2003 #6
    Well, as I see it, the POINT of the whole paradox is that one of the twins gets accelerated. And it must be the one who's time is flowing slower, because the acceleration is what makes it flow slower.
  8. Aug 17, 2003 #7


    User Avatar

    No, because then we use GR and apply the equivalence principle, and consider the accelerating twin to be simply under an uniform gravitational field. In which case, we still cannot determine an absolute velocity for either one.

    The point of the paradox is that simultaneity is not present if the relativistic postulates are true.
  9. Aug 17, 2003 #8


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    And indeed GR says time runs slower in a gravitational field! Did you think GR proved SR false, within its flat space environment?

    The key to the twins is this. Imagine a triangle in which one side, vertical, represents the worldline of the stayathome twin. Since she doesn't travel, but does age, we represent her world line as a line going up the time axis with no excursions into horizontal space.

    The other two sides of the triangle represent the travels of the other twin, out and back. One leg goes out, starting from the bottom of the time axis whre the twins part, and makeing some angle less than 45o with it. This is the outward journey. The second leg returns to the stayathome's timeline, joining it at the top of the triangle.

    Now relativity - Minkowskian geometry - says the proper time along the time side of the triangle is greater than the sum of the proper times along the other two sides. That is the core of the paradox, that proper time doesn't behave like Pythagorean length. And proper time is what each twin accumulates as she ages.
  10. Sep 5, 2003 #9
    Self adjoin is the only writer I've read who still recognizes that the "twin paradox" is still a paradox. ALL books about physics in Barnes and Noble that mention the subject try to explain it away with GR.( go look--in the indices--takes a half hour) Einstein very clearly stated that it was still a paradox notwithstanding the opinions of the physicists who assumed GR would "cure" the paradox.
    He specifically wrote of the thought-experiment in which both twins accellerated off but one reverse accellerated quickly and the other much later. They both experienced the same accellerations. He pointed out that nothing in GR dealt with clock slowing due to a past accelleration. His Special Theory doesn't predict either is younger.
    Existence of a paradox in a math dominated subject reminds us there is a mite of difference in the physical and the tools used to model it.
    Last edited by a moderator: Sep 6, 2003
  11. Sep 6, 2003 #10


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Are you tring to say that Einstein didn't understand his own Theory?.

    The magnitude of the accleration experienced by the twins is only one factor involved. You also have to consider the distance and direction of separation along the line of accleration at the time the accelerations are experienced.

    If the Twins accelerate off together, Then one twin decelerates, the distance between the Twins will be small. Thus the difference in measured time seen by the decelerating Twin, caused by his deceleration will be small. Once his deceleration stops, both will just see the SR time dilation, with each seeing the other's clock as running slow. When the second twin finally decelerates, the distance between the twins will be much greater. Thus the combined experienced acceleration he feels, and the distance and direction of his brother will cause him to measure his brother's time rate as moving much faster than his own. To the point that when He finally matches speeds with his brother he will show less time as having past on his clock.

    His brother, not having experienced acceleration at this time, will only see a gradually changing SR dilation as the velocity difference between them changes, So he will have seen the same time difference accumulate between clocks, and will also say that his other brother experienced less time passage.

    They both agree on each other's clock readings, so no conflict.
  12. Sep 6, 2003 #11
    He understood his theory to agree with your last sentence, that the clocks would read the same. He would agree also that at any time the two would perceive the other's clock slowed by an amount determined by their relative velocity. This relative velocity is the same for both of them; they each plug the same v into the Lorentz contraction. He would not agree with the sentences that would make the contraction depend on distance or prior accelerations.
    He would find it disturbing that you find no difference in the watch readings but find a difference in wrinkles of the ones wearing them. Janus would probably agree that those who imagine that a theory has no questionable points will find some points disturbing as have the writers of self-help Physics books sold at Barnes and Noble.
    Media writers read these books and digest the works of some physicists and report the products of their digestion. The experimental physicist doesn't accentuate the fact that his very limited experiment only tests a limited aspect of a theory leaving seeds for further quests. The media writer passes these seeds un-noted and therefore indigested.
  13. Sep 6, 2003 #12


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    not the same, But both twin would read the same difference. In the clocks.

    Not at any time, but only those times when the perceiver is not experiencing an acceleration.
    He most certainly would, as these statement are a direct result of and in complete concordance with his theory. And at no time do I depend on "prior accelerations", only on the effect present acceleration has on the observations of the one experiencing the accleleration at that time.

  14. Sep 6, 2003 #13
    I don't know much about Einstein's theories either. I do know that he said that when velocity of mass increases, its mass will increase. Since the atoms of a mass would increase in size as the velocity increased, would'nt that mean that the electrons would have a greater distance to travel in their orbits. Since the velocity of the electrons cannot increase, would'nt that mean that the electrons would simply take longer to make their orbits. A clock would naturally run slower if that happened. A humans biological clock should even slow down. Since the brain works by electrical pulses, thought processes would slow down because the distance between neurons would increase, yet the electrical pulses cannot increase their speed in proportion to the greater distance. They are already going as fast as nature will allow. Their sense of time passing would be altered.
    Last edited: Sep 7, 2003
  15. Sep 6, 2003 #14
    Here's an observation that I feel really support the idea that time dilation is real. It's from Tipler's book on Modern Physics.

    Particles called muons are created in the upper atmosphere by cosmic rays. Measured in an at rest frame of reference they have a mean lifetime of 2 millionths of a second. It takes about 30 millionths of a second to make the trip to the earth's surface. They are traveling a large fraction of the speed of light however and they travel the 9000 meters to the surface of the earth and are detected. So the conclusion is that time is slowed for the muons such that they survive to reach the surface of the earth. If the high speed 'slows' the decay of muons then I think it's real and not just a 'clock' thing.
  16. Sep 8, 2003 #15
    Dear Janus, Sorry I misinterpreted your statement that the watches would agree. Twenty years ago I went to library in Springfield MO to read stuff by Einstein for fun, not a textbook to pass a curse as I had decades before. One book held a short tract by him in which he disagreed with the folks who assumed that GR would remove the paradox. A fellow grad student in the mid'60's at Hopkins, George Murphy, who went on to get his doctorate in GR also assumed the paradox would be removed by GR. Love to find him now and give him his hero's opinion.
    Einstein specifically gave the example of both twins accelerating off with the same acceleration at the same time with acceleration time =one second(like Jules Vernes) One immediately decelerates(for one second). The other continues for years and years before he reverse accelerates and years and years later he returns to compare watches and wrinkles. Any GR effects of the acceleration is limited to a few seconds. Again, I'm simply quoting; the experiment will never be done so it wasn't worth his time arguing widely about it. I don't remember the date of the book in which I read this.
    In grad school most of us never even gazed at the pages of GR. They weren't in our textbooks. I assumed they were unapproachable by us common folks. I did have to stand at the board in front of undergraduates and starting from the two postulates, 1. observors in all inertial frames measure the same speed of light. 2.(I forget the second) derive the contraction equations for SR. The textbook I taught from was Sears and Zemansky. I looked last night and they don't mention the paradox. There were four authors and maybe one of them knew Einstein's opinion. I will check at Universities to see if later texts assume that GR explains why the frames are different in the long periods without acceleration

    Einstein didn't feel this challenged his theory; he was correcting his collegues who hadn't worked thru GR who imagined that GR would explain away the paradox. The "COMPANY" didn't like what he was saying about the bomb and Israel and Russia at the time(50's) and so his then current sayings weren't published popularly.
    Last year I bought and lost a small black book written by Einstein(not digestions of others) and took his word for it when he says(over and over) "it can be shown". I'd like to find that one again. at the end it gave his questins that future experiment will answer. It also pointed out how he used a formula from hydrodynamics as the starting place. Then he replaced pressure with Lambda. He created the theory for an infinite static
    universe. After Hubble let him look at his work he dropped Lanbda. Now it looks like folks are trying to put it back.
    Maybe he was wrong but I've never seen anybody actually point to how GR does this. I wonder if George ever got around to it.
    I'm envious of your ability to seperate out a quote for reply(blue lines) Teach me?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook