1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Simulteneity and other myths, maybe?

  1. Mar 17, 2004 #1
    [SOLVED] Simulteneity and other myths, maybe?

    In perusing Einstein's ‘Relativity’, I made a few notes for discussion. Using the moving train vs. the platform model some Einsteinian axioms get a tad oily. A mark on the train coincides with a mark on the platform just as two lights flash, both equal distances from the mark on the platform. The mark on the platform receives the light at the same instant. The mark on the train receives the light from the approaching light before the platform mark and then the light from the receding source arrives at the mark on the train.

    Question: Cannot the train measure times when the light arrive and conclude the train is moving? In addition, using Doppler information, cannot the exact process of the light radiation be measured and inferred and if so what has this to do with “simultaneity”?

    The train is motionless with respect to the platform. Then train and platform are then seen to move with respect to each other. The observers on the train notice a jerking force due to the train accelerating, yet they do not notice the persons on the platform moving in any manner. So, cannot the observers on the train know that they are moving because of the observed acceleration? May we not then determine the absolute motion between the train and platform?

    It seems to me that acceleration is extremely important in these kinds of events as the acceleration manifestly places one object in a higher energy state than an unaccelerated object.

    Similarly, an accelerated electron, for example, traveling at .9999 the speed of light has undergone massive accelerations. The mass increase can be explained by an increase in electron internal vibration states, huge vibration increases. Now if the electron at v << c can load up with energy efficiently such that relativity mass increases are not seen a until a certain critical velocity is seen cannot this be explained by the lowering of efficiency of energy transfer and storage process of the electron. In other words, as the vibration increases in scope it becomes ever more difficult to on load and store the energy with the same velocity increases seen at lower speeds?

    As nothing moves without acceleration the increased energy state is a constant process of nature, but the relativity ‘law’ is expressed as a simple speed limitation as (1 - v2)-1/2, which is an expression void of dynamic information and expresses a trivial and rather uninteresting oddity of nature

    Finally, we have a space ship moving at .9999c. We have prepared a platform of enormous length that has a number of computer data registers laid out – the data is the current time measured by the CS clock to one part in 1016 or so. As the space ship passes by it trips a switch on the platform and dumps the contents of the platform registers into registers aligned to those on board the ship. So now, we have two clocks that agree on the time. What will those on the spaceship see? And how will they react?
    Similarly, the platform has a bank of data registers that get similar data dumps from the space ship clicking every tick of the Cs clock that was just updated and dumped its data to the platform the next duty cycle of the Cs clock. Have we ‘violated any relativity protocols?
  2. jcsd
  3. Mar 17, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The actual set-up of this experiment goes like this:

    You have your lights equidistant from your observer on the track. They are turned on such that the light from them reaches this observer at the same time as our observer on the train is even with him. Thus both observers see the light come on at the same time. But sine the speed is a constant relative to any observer, the observer on the train concludes that the lights were not switched on simultaneously, as this is the only way that he could see the light form both arrive at the same instant. Thus events (the turning on of the lights) that are simultaneous in one frame (the Embankment observer) are not simultaneous for another (the Train observer).

    You can conclude nothing about the absolute motion of the train from this, since you get the exact same results if you consider the train as stationary and the embankment and lights as moving.

    No, for two reasons:
    1.The fact that the passengers feel an acceleration does not mean they are speeding up, they could be slowing down. The train and platform could have been intially both been traveling at a high speed, and then the train came to a stop while the platform kept moving. the observations by the passengers would be exactly the same. All we can tell at the end is that the train and platform now have different relative speeds, buit we can say nothing about their absolute speed

    2. The equivalence principle states that is impossible to locally determine between acceleration and gravitation. So the passengers in the train can associate the acceleration of the platform with it falling in a gravitational field, while the forces they feel are due to the train's engines holding them in place against the pull of the field.

    No, because as I stated above, you can have no knowledge of the objects absolute velocity before the acceleration, so the accleration could just a equally be considered a deceleration. Besides, there is nothing absolute about kinetic energy, it is purely relative.
  4. Mar 17, 2004 #3


    User Avatar
    Science Advisor

    This is true form the reference frame of someone on the platform. To someone on the train, the mark on the train receives both lights simultaneously, and the mark on the platform does not.
  5. Mar 21, 2004 #4
    As I understand Einstein as he wrote in his book "Relativity" he didn't go quite as far as both Janus and Lurch had indicated. His model had the lightning strike occuring at the instant the M' traing mark coincidded with the platform M mark. The platform observers at M would see the light from both sources simukltaneopusly, while th etraon observers woukld see th elight in the direction of motion before the light arrived from the rear.
    There is no problem here. Then we must consider all persons on the platform not located at M would also see the light arriving at different times.

    Janus stated earlier the following:
    This seems skewed.
    Let us assume that the instant the M and M' marks coincided, say within a wave length of light, both lights struck both detectors simultaneously both observers recorded light from both sources. If the train observer sees the both lights at the same time the platform observer sees the light how can he conclude the lights weren't switched on simultaneously? If the train observer wasn't aware of the relative motion the light measurement wouldn't provide information of that motion.
    So measuring light from the two sources is measured simultaneously at only one position in the universe.

    Lurch offered the following

    This cannot be Lurch. A stationary observer located located at the point of the M' mark on the train will see the lights coing in the samne sequence as the train observer. Those closer to the light in the direction of motion of the train will see both lights arriving at different instances. If the platform observer at M' receives the light simultaneously and the mere fact of observing the train he may rightfully conclude that the M' mark will see the light before he does, and that the train will see the light from the other source after he does.
    I see no physical difference between those persons stationary on the platform seeing two flickers of light and the train observers seeing two flickers of light, especially when their detectors are located within a wave length of each other and are within a wave length of light.

    Mhernan said in the original post that

    To which Janus replied:
    I may have erred in a prior thread. If the two platforms are moving with a conmstant velocity with respect to a third inertial platform, then deceleration could have th e effect of slowing on the observers with respect to the other as measured by the third inertial paltform. However, a decelerating electron wil neverthe less radiate when acclerated, or decelerated, so the motion of the acclerating or decelerating platform , can be measured.

    If the 'non-absolute' nature of energy is as stated, then the twin paradox is no more. An unfetteered look at the universe around us says that planets don't acclerate and move about, in general, and their sheer size prevents us from seriously making an attempt to move a planet just to prove a point. But the little things like burning gasses, meteors, and some one running a 9.9 100 meters is certainly strong evidence that RT needs some serious work.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Simulteneity and other myths, maybe?
  1. Urban myth (Replies: 2)

  2. Medal Detectors' myth (Replies: 20)

  3. 2012 myth (Replies: 3)

  4. Is Anti-Gravity a myth? (Replies: 12)