• The Twins Paradox in Relativity

    From Joe Gwinn@21:1/5 to All on Sat May 14 17:35:44 2022
    It turns out there is a long history, with many parallel explanations:

    .<https://en.wikipedia.org/wiki/Twin_paradox>

    Joe Gwinn

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  • From Anthony William Sloman@21:1/5 to Joe Gwinn on Sat May 14 20:50:17 2022
    On Sunday, May 15, 2022 at 7:35:56 AM UTC+10, Joe Gwinn wrote:
    It turns out there is a long history, with many parallel explanations:

    .<https://en.wikipedia.org/wiki/Twin_paradox>

    But no reference to the traveling clock experiment,

    https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experiment

    or the Global Positioning System satellites, where these effects are of practical importance.

    Experimental fact is very effective at disciplining theoretical speculation.

    --
    Bill Sloman, Sydney

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  • From Jeff Layman@21:1/5 to Joe Gwinn on Sun May 15 09:46:18 2022
    On 14/05/2022 22:35, Joe Gwinn wrote:
    It turns out there is a long history, with many parallel explanations:

    .<https://en.wikipedia.org/wiki/Twin_paradox>

    It can be expanded to the Triplets Paradox, for example <http://www.mysearch.org.uk/website1/html/251.Triplets.html>

    SRT is well above me, I'm afraid. Some of the explanation of the Twins
    Paradox refers to the twins' clocks transmitting their time to the other
    twin (the clock signal is transmitted at the speed of light). Even
    allowing for the travelling twin's speed when approaching the speed of
    light, and the relativistic effect it has on each clock's perceived
    time, as the travelling twin's speed doesn't exceed that speed, each
    twin will, eventually, receive the clock time of the other.

    But what happens with the Triplet Paradox where the moving triplets are accelerating away from each other? Once they've "exceeded" C in relation
    to each other, although they can receive the stationary triplet's clock
    reading (and he can receive theirs), can one moving triplet still
    receive the other moving triplet's clock signal? If there is such a
    moment when they can no longer receive each other's signal, when they
    finally stop moving away and start moving towards each other again, will
    there be a moment when they suddenly start receiving that "missing"
    clock signal as they catch up with it (or perhaps it catches up with
    them)? Will there be a specific moment when they not only receive a
    missing clock time, but coincidentally receive the "accurate" time as transmitted by the other moving triplet, so appear to be receiving two different clock readings at the same time?

    --

    Jeff

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  • From Dimiter_Popoff@21:1/5 to Joe Gwinn on Sun May 15 14:05:36 2022
    On 5/15/2022 0:35, Joe Gwinn wrote:
    It turns out there is a long history, with many parallel explanations:

    .<https://en.wikipedia.org/wiki/Twin_paradox>

    Joe Gwinn


    I am not a physicist but like many of us here I have been banging my
    head into various technical problems so I am used to see when some
    claim is somewhat questionable.
    It's been years since I have read Einstein's papers but I remember
    an example he gives, that with a train and a periodically flashing
    light on it.
    Obviously if the train is moving away from the observer because of the
    fixed speed of light the period will seem somewhat longer to the
    observer.
    What is not addressed by this simple example is the case when the train
    moves towards the observer - in which case obviously the period
    will seem shorter to the observer.
    A way to think of all that in terms obvious to most of us here
    is that our reality is a state machine clocked (IIRC there was some
    minimum time defined by Max Planck, could be the clock period) by
    some clock; what we perceive as time is the resulting change of
    states.
    While this is a simplified and probably naive model it does
    explain the train-flashing-light-period dependence on direction.
    And it explains why clocks at the equator and on the poles run
    the same without involving gravity.

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  • From Martin Brown@21:1/5 to All on Mon May 16 14:07:13 2022
    On 15/05/2022 12:05, Dimiter_Popoff wrote:
    On 5/15/2022 0:35, Joe Gwinn wrote:
    It turns out there is a long history, with many parallel explanations:

    .<https://en.wikipedia.org/wiki/Twin_paradox>

    Joe Gwinn

    And it explains why clocks at the equator and on the poles run
    the same without involving gravity.

    Actually they don't.

    One of Einstein's minor errors in his 1905 paper on special relativity
    was to predict how much slower a clock at the equator would tick when
    compared to one at the pole (due to the extra rotational speed of a
    clock at the equator). Every now and then someone points it out... eg

    https://physicstoday.scitation.org/doi/10.1063/1.1897562#

    They only run at the same speed when *both* the GR and SR corrections
    are applied simultaneously and only then at mean sea level.

    It is hard to get your head round but everybody's clock ticks at a
    different speed. Your head ages marginally more quickly than your feet.

    The best clocks in the world at NIST are now sensitive and stable enough
    to detect a vertical shift of about 30cm or a foot in old money.

    https://www.nist.gov/news-events/news/2010/09/nist-pair-aluminum-atomic-clocks-reveal-einsteins-relativity-personal-scale

    This isn't a bad introduction by Brian Cox for BBC science series.

    https://www.bbc.co.uk/iplayer/episode/m000x9v4/brian-coxs-adventures-in-space-and-time-series-1-4-what-is-time

    (you might have to spoof a UK address to see it)

    --
    Regards,
    Martin Brown

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  • From Dimiter_Popoff@21:1/5 to Martin Brown on Mon May 16 16:38:10 2022
    On 5/16/2022 16:07, Martin Brown wrote:
    On 15/05/2022 12:05, Dimiter_Popoff wrote:
    On 5/15/2022 0:35, Joe Gwinn wrote:
    It turns out there is a long history, with many parallel explanations:

    .<https://en.wikipedia.org/wiki/Twin_paradox>

    Joe Gwinn

    And it explains why clocks at the equator and on the poles run
    the same without involving gravity.

    Actually they don't.

    One of Einstein's minor errors in his 1905 paper on special relativity
    was to predict how much slower a clock at the equator would tick when compared to one at the pole (due to the extra rotational speed of a
    clock at the equator). Every now and then someone points it out... eg

    https://physicstoday.scitation.org/doi/10.1063/1.1897562#

    They only run at the same speed when *both* the GR and SR corrections
    are applied simultaneously and only then at mean sea level.

    It is hard to get your head round but everybody's clock ticks at a
    different speed. Your head ages marginally more quickly than your feet.

    The best clocks in the world at NIST are now sensitive and stable enough
    to detect a vertical shift of about 30cm or a foot in old money.

    https://www.nist.gov/news-events/news/2010/09/nist-pair-aluminum-atomic-clocks-reveal-einsteins-relativity-personal-scale


    This isn't a bad introduction by Brian Cox for BBC science series.

    https://www.bbc.co.uk/iplayer/episode/m000x9v4/brian-coxs-adventures-in-space-and-time-series-1-4-what-is-time


    (you might have to spoof a UK address to see it)


    I have been digging into physics just as much as it takes to do what I
    do so me being naive with that sort of thing is no surprise. I am
    vaguely aware of what your references say, I think I may have read some
    of these some time ago.
    What I don't get though is the flashing light on the train thing.
    Looks obvious to me that the observed period depends on the movement
    direction (assuming gravity is constant, i.e. it is no factor).
    In fact this should be easily measurable (not that I would go into
    it, just wondering if you or someone else has an explanation, I am
    not the "out there to challenge the science" type, more the "curious
    until things get clarified for me" sort).

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  • From Martin Brown@21:1/5 to All on Mon May 16 15:13:11 2022
    On 16/05/2022 14:38, Dimiter_Popoff wrote:

    I have been digging into physics just as much as it takes to do what I
    do so me being naive with that sort of thing is no surprise. I am
    vaguely aware of what your references say, I think I may have read some
    of these some time ago.

    What I don't get though is the flashing light on the train thing.
    Looks obvious to me that the observed period depends on the movement direction (assuming gravity is constant, i.e. it is no factor).

    The bit you are missing is that to be able to meaningfully compare times between two different moving objects they *have* to be at the same
    location. That means a round trip back to the stay at home.

    In fact this should be easily measurable (not that I would go into
    it, just wondering if you or someone else has an explanation, I am
    not the "out there to challenge the science" type, more the "curious
    until things get clarified for me" sort).

    One of the classic illustrations is to draw a world lines diagram for
    bleep who stays put and booster who goes off at 4c/5 (3,4,5 triangle).

    This illustration says it more clearly than words ever can. It was a
    diagram of this sort that convinced me to give up on common sense where relativity was concerned and trust the mathematics.

    <https://www.google.com/url?sa=i&url=https%3A%2F%2Faapt.scitation.org%2Fdoi%2F10.1119%2F1.4947152&psig=AOvVaw2kQVy1xGR-cIehmwjR9R5y&ust=1652796475815000&source=images&cd=vfe&ved=0CAkQjRxqFwoTCLialeiY5PcCFQAAAAAdAAAAABAE>

    It points to this article (behind a paywall:( ) https://aapt.scitation.org/doi/10.1119/1.4947152

    Be a miracle if that works so Google keywords
    "world lines signal twin paradox illustration"

    --
    Regards,
    Martin Brown

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  • From Sylvia Else@21:1/5 to Jeff Layman on Mon May 16 23:25:28 2022
    On 15-May-22 6:46 pm, Jeff Layman wrote:

    But what happens with the Triplet Paradox where the moving triplets are accelerating away from each other? Once they've "exceeded" C in relation
    to each other, <snip>

    That doesn't happen.

    Sylvia.

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  • From Phil Hobbs@21:1/5 to Jeff Layman on Mon May 16 10:18:05 2022
    Jeff Layman wrote:
    On 14/05/2022 22:35, Joe Gwinn wrote:
    It turns out there is a long history, with many parallel explanations:

    .<https://en.wikipedia.org/wiki/Twin_paradox>

    It can be expanded to the Triplets Paradox, for example <http://www.mysearch.org.uk/website1/html/251.Triplets.html>

    SRT is well above me, I'm afraid. Some of the explanation of the Twins Paradox refers to the twins' clocks transmitting their time to the other
    twin (the clock signal is transmitted at the speed of light). Even
    allowing for the travelling twin's speed when approaching the speed of
    light, and the relativistic effect it has on each clock's perceived
    time, as the travelling twin's speed doesn't exceed that speed, each
    twin will, eventually, receive the clock time of the other.

    But what happens with the Triplet Paradox where the moving triplets are accelerating away from each other? Once they've "exceeded" C in relation
    to each other, although they can receive the stationary triplet's clock reading (and he can receive theirs), can one moving triplet still
    receive the other moving triplet's clock signal? If there is such a
    moment when they can no longer receive each other's signal, when they
    finally stop moving away and start moving towards each other again, will there be a moment when they suddenly start receiving that "missing"
    clock signal as they catch up with it (or perhaps it catches up with
    them)? Will there be a specific moment when they not only receive a
    missing clock time, but coincidentally receive the "accurate" time as transmitted by the other moving triplet, so appear to be receiving two different clock readings at the same time?


    If you shine your laser pointer at two points 180 degrees apart in the
    sky, the relative speed of the light pulses in your frame of reference
    is 2c. No paradox is involved.

    Also, there's no simultaneity between separated objects moving at
    different speeds. The relativistic garage illustrates this.

    Say you have a 1927 Bugatti Type 41, which is 252 inches long. Your
    garage is the standard 20 feed (240 inches) long, and has a very fast automatically-controlled door at each end. The doors are designed to
    open and close automatically to allow the car to enter and leave.

    Because the Bugatti is so fast, you drive towards the open end of the
    garage at 0.5c. You measure the length of the garage as

    240 inches * sqrt(1-0.5**2) = 207.8 inches.

    The hood of the car passes through the open door, then the closed door
    opens before the back bumper has passed through the doorway. No
    collision occurs, because the second door opens before the first one closes.

    Your spouse, waiting for you to come home from your drive, measures the
    length of the car as

    252 inches * sqrt(1-0.5**2) = 218.2 inches.

    The car fits into the garage, so as it enters, the first door closes
    before the second door opens. Once again no collision occurs, because
    the car is shorter than the garage.

    The math works out fine in both English and metric, and no paradoxes are involved.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

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  • From corvid@21:1/5 to Phil Hobbs on Mon May 16 08:31:00 2022
    On 5/16/22 07:18, Phil Hobbs wrote:
    Jeff Layman wrote:
    On 14/05/2022 22:35, Joe Gwinn wrote:
    It turns out there is a long history, with many parallel
    explanations:

    .<https://en.wikipedia.org/wiki/Twin_paradox>

    It can be expanded to the Triplets Paradox, for example
    <http://www.mysearch.org.uk/website1/html/251.Triplets.html>

    SRT is well above me, I'm afraid. Some of the explanation of the
    Twins Paradox refers to the twins' clocks transmitting their time
    to the other twin (the clock signal is transmitted at the speed of
    light). Even allowing for the travelling twin's speed when
    approaching the speed of light, and the relativistic effect it has
    on each clock's perceived time, as the travelling twin's speed
    doesn't exceed that speed, each twin will, eventually, receive the
    clock time of the other.

    But what happens with the Triplet Paradox where the moving triplets
    are accelerating away from each other? Once they've "exceeded" C
    in relation to each other, although they can receive the stationary
    triplet's clock reading (and he can receive theirs), can one
    moving triplet still receive the other moving triplet's clock
    signal? If there is such a moment when they can no longer receive
    each other's signal, when they finally stop moving away and start
    moving towards each other again, will there be a moment when they
    suddenly start receiving that "missing" clock signal as they catch
    up with it (or perhaps it catches up with them)? Will there be a
    specific moment when they not only receive a missing clock time,
    but coincidentally receive the "accurate" time as transmitted by
    the other moving triplet, so appear to be receiving two different
    clock readings at the same time?


    If you shine your laser pointer at two points 180 degrees apart in
    the sky, the relative speed of the light pulses in your frame of
    reference is 2c. No paradox is involved.

    Also, there's no simultaneity between separated objects moving at
    different speeds. The relativistic garage illustrates this.

    Say you have a 1927 Bugatti Type 41, which is 252 inches long. Your
    garage is the standard 20 feed (240 inches) long, and has a very
    fast automatically-controlled door at each end. The doors are
    designed to open and close automatically to allow the car to enter
    and leave.

    Because the Bugatti is so fast, you drive towards the open end of the
    garage at 0.5c. You measure the length of the garage as

    240 inches * sqrt(1-0.5**2) = 207.8 inches.

    The hood of the car passes through the open door, then the closed
    door opens before the back bumper has passed through the doorway. No
    collision occurs, because the second door opens before the first one
    closes.

    Your spouse, waiting for you to come home from your drive, measures
    the length of the car as

    252 inches * sqrt(1-0.5**2) = 218.2 inches.

    The car fits into the garage, so as it enters, the first door closes
    before the second door opens. Once again no collision occurs,
    because the car is shorter than the garage.

    The math works out fine in both English and metric, and no paradoxes
    are involved.

    Cheers

    Phil Hobbs

    But, but... the bottom of the tires are in contact with the garage
    floor. Shouldn't that anchor the Bugatti and garage into the same frame?

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  • From Dimiter_Popoff@21:1/5 to Martin Brown on Mon May 16 19:10:59 2022
    On 5/16/2022 17:13, Martin Brown wrote:
    On 16/05/2022 14:38, Dimiter_Popoff wrote:

    I have been digging into physics just as much as it takes to do what I
    do so me being naive with that sort of thing is no surprise. I am
    vaguely aware of what your references say, I think I may have read some
    of these some time ago.

    What I don't get though is the flashing light on the train thing.
    Looks obvious to me that the observed period depends on the movement
    direction (assuming gravity is constant, i.e. it is no factor).

    The bit you are missing is that to be able to meaningfully compare times between two different moving objects they *have* to be at the same
    location. That means a round trip back to the stay at home.

    In fact this should be easily measurable (not that I would go into
    it, just wondering if you or someone else has an explanation, I am
    not the "out there to challenge the science" type, more the "curious
    until things get clarified for me" sort).

    One of the classic illustrations is to draw a world lines diagram for
    bleep who stays put and booster who goes off at  4c/5 (3,4,5 triangle).

    This illustration says it more clearly than words ever can. It was a
    diagram of this sort that convinced me to give up on common sense where relativity was concerned and trust the mathematics.

    <https://www.google.com/url?sa=i&url=https%3A%2F%2Faapt.scitation.org%2Fdoi%2F10.1119%2F1.4947152&psig=AOvVaw2kQVy1xGR-cIehmwjR9R5y&ust=1652796475815000&source=images&cd=vfe&ved=0CAkQjRxqFwoTCLialeiY5PcCFQAAAAAdAAAAABAE>


    It points to this article (behind a paywall:( ) https://aapt.scitation.org/doi/10.1119/1.4947152

    Be a miracle if that works so Google keywords
    "world lines signal twin paradox illustration"


    I am not talking about the "twin paradox", I had not heard the
    phrase until this post (or did not remember I had). It involves
    acceleration so things get more complex.
    I refer only to the blinking object moving towards us and away from
    us, no acceleration (I put that into more detail in my post to
    Phil).
    I'll try to switch to doing useful work now though. Thanks for
    the links and explanations, once I get bugged again I'll probably
    be heard of :-).

    --- SoupGate-Win32 v1.05
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  • From Dimiter_Popoff@21:1/5 to Phil Hobbs on Mon May 16 18:58:35 2022
    On 5/16/2022 17:18, Phil Hobbs wrote:
    Jeff Layman wrote:
    On 14/05/2022 22:35, Joe Gwinn wrote:
    It turns out there is a long history, with many parallel explanations:

    .<https://en.wikipedia.org/wiki/Twin_paradox>

    It can be expanded to the Triplets Paradox, for example
    <http://www.mysearch.org.uk/website1/html/251.Triplets.html>

    SRT is well above me, I'm afraid. Some of the explanation of the Twins
    Paradox refers to the twins' clocks transmitting their time to the
    other twin (the clock signal is transmitted at the speed of light).
    Even allowing for the travelling twin's speed when approaching the
    speed of light, and the relativistic effect it has on each clock's
    perceived time, as the travelling twin's speed doesn't exceed that
    speed, each twin will, eventually, receive the clock time of the other.

    But what happens with the Triplet Paradox where the moving triplets
    are accelerating away from each other? Once they've "exceeded" C in
    relation to each other, although they can receive the stationary
    triplet's clock reading (and he can receive theirs), can one moving
    triplet still receive the other moving triplet's clock signal? If
    there is such a moment when they can no longer receive each other's
    signal, when they finally stop moving away and start moving towards
    each other again, will there be a moment when they suddenly start
    receiving that "missing" clock signal as they catch up with it (or
    perhaps it catches up with them)? Will there be a specific moment when
    they not only receive a missing clock time, but coincidentally receive
    the "accurate" time as transmitted by the other moving triplet, so
    appear to be receiving two different clock readings at the same time?


    If you shine your laser pointer at two points 180 degrees apart in the
    sky, the relative speed of the light pulses in your frame of reference
    is 2c.  No paradox is involved.

    This is obvious enough. I refer to the case where the laser pointer is
    moving towards us; no RTT involved, we just measure the period at which
    it flashes. Since the light speed is always c and every next flash will
    have less distance to travel until it reaches us it seems obvious
    we will see a period shorter than it is for an observer moving together
    with the pointer. And vice versa, if the pointer moves away from us
    each next flash will have more distance to travel at c to reach us
    so the period we will see will be longer than at the pointer (the
    latter being the example Einstein gives in some of the papers, IIRC).
    Should not be too hard to test experimentally nowadays (the direction dependence, that is). At the moment I can't see how the period of an approaching pointer will be longer for the observer.
    I'll better switch to doing something useful, it is not that I don't
    have enough to do :-).

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  • From Phil Hobbs@21:1/5 to corvid on Mon May 16 16:26:15 2022
    corvid wrote:
    On 5/16/22 07:18, Phil Hobbs wrote:
    Jeff Layman wrote:
    On 14/05/2022 22:35, Joe Gwinn wrote:
    It turns out there is a long history, with many parallel
    explanations:

    .<https://en.wikipedia.org/wiki/Twin_paradox>

    It can be expanded to the Triplets Paradox, for example
    <http://www.mysearch.org.uk/website1/html/251.Triplets.html>

    SRT is well above me, I'm afraid. Some of the explanation of the
    Twins Paradox refers to the twins' clocks transmitting their
    time to the other twin (the clock signal is transmitted at the
    speed of light). Even allowing for the travelling twin's speed
    when approaching the speed of light, and the relativistic effect
    it has on each clock's perceived time, as the travelling twin's
    speed doesn't exceed that speed, each twin will, eventually,
    receive the clock time of the other.

    But what happens with the Triplet Paradox where the moving
    triplets are accelerating away from each other? Once they've
    "exceeded" C in relation to each other, although they can receive
    the stationary triplet's clock reading (and he can receive
    theirs), can one moving triplet still receive the other moving
    triplet's clock signal? If there is such a moment when they can
    no longer receive each other's signal, when they finally stop
    moving away and start moving towards each other again, will there
    be a moment when they suddenly start receiving that "missing"
    clock signal as they catch up with it (or perhaps it catches up
    with them)? Will there be a specific moment when they not only
    receive a missing clock time, but coincidentally receive the
    "accurate" time as transmitted by the other moving triplet, so
    appear to be receiving two different clock readings at the same
    time?


    If you shine your laser pointer at two points 180 degrees apart in
    the sky, the relative speed of the light pulses in your frame of
    reference is 2c. No paradox is involved.

    Also, there's no simultaneity between separated objects moving at
    different speeds. The relativistic garage illustrates this.

    Say you have a 1927 Bugatti Type 41, which is 252 inches long.
    Your garage is the standard 20 feed (240 inches) long, and has a
    very fast automatically-controlled door at each end. The doors
    are designed to open and close automatically to allow the car to
    enter and leave.

    Because the Bugatti is so fast, you drive towards the open end of
    the garage at 0.5c. You measure the length of the garage as

    240 inches * sqrt(1-0.5**2) = 207.8 inches.

    The hood of the car passes through the open door, then the closed
    door opens before the back bumper has passed through the doorway.
    No collision occurs, because the second door opens before the first
    one closes.

    Your spouse, waiting for you to come home from your drive,
    measures the length of the car as

    252 inches * sqrt(1-0.5**2) = 218.2 inches.

    The car fits into the garage, so as it enters, the first door
    closes before the second door opens. Once again no collision
    occurs, because the car is shorter than the garage.

    The math works out fine in both English and metric, and no
    paradoxes are involved.

    Cheers

    Phil Hobbs

    But, but... the bottom of the tires are in contact with the garage
    floor. Shouldn't that anchor the Bugatti and garage into the same
    frame?

    And the pistons are going up and down pretty good too. ;)

    Just stick with the front and rear bumpers for present purposes.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From corvid@21:1/5 to Phil Hobbs on Mon May 16 13:53:59 2022
    On 5/16/22 13:26, Phil Hobbs wrote:

    Also, there's no simultaneity between separated objects moving at
    different speeds. The relativistic garage illustrates this.

    Say you have a 1927 Bugatti Type 41, which is 252 inches long.
    Your garage is the standard 20 feed (240 inches) long, and has a
    very fast automatically-controlled door at each end. The doors
    are designed to open and close automatically to allow the car to
    enter and leave.

    Because the Bugatti is so fast, you drive towards the open end
    of the garage at 0.5c. You measure the length of the garage as

    240 inches * sqrt(1-0.5**2) = 207.8 inches.

    The hood of the car passes through the open door, then the closed
    door opens before the back bumper has passed through the
    doorway. No collision occurs, because the second door opens
    before the first one closes.

    Your spouse, waiting for you to come home from your drive,
    measures the length of the car as

    252 inches * sqrt(1-0.5**2) = 218.2 inches.

    The car fits into the garage, so as it enters, the first door
    closes before the second door opens. Once again no collision
    occurs, because the car is shorter than the garage.

    The math works out fine in both English and metric, and no
    paradoxes are involved.

    Cheers

    Phil Hobbs

    But, but... the bottom of the tires are in contact with the garage
    floor. Shouldn't that anchor the Bugatti and garage into the same
    frame?

    And the pistons are going up and down pretty good too. ;)

    Just stick with the front and rear bumpers for present purposes.

    Cheers

    Phil Hobbs

    Now I want to tweak the car and garage lengths, and the speed, so that
    the car goes thru unscathed in one frame but gets smashed in the other.
    Is it possible?

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Clifford Heath@21:1/5 to Phil Hobbs on Tue May 17 08:55:29 2022
    On 17/5/22 12:18 am, Phil Hobbs wrote:
    then the closed door opens before the back bumper has passed through the doorway.  No collision occurs

    Umm, sorry? "before"? Is that a slip? Did you mean "as the back bumper
    passes through the doorway?

    Very cool illustration BTW. I want to use it, but want to make sure I
    have it correct first.

    Clifford Heath.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to Clifford Heath on Mon May 16 19:42:29 2022
    Clifford Heath wrote:
    On 17/5/22 12:18 am, Phil Hobbs wrote:
    then the closed door opens before the back bumper has passed through
    the doorway.  No collision occurs

    Umm, sorry? "before"? Is that a slip? Did you mean "as the back bumper
    passes through the doorway?

    No, the point is that seen from a point in the car's reference frame,
    the events happen *in a different order* from what you'd see in the
    garage's frame.

    Very cool illustration BTW. I want to use it, but want to make sure I
    have it correct first.

    Clifford Heath.

    I picked the Bugatti because I'm a fan, and looked up the standard
    length of a garage on the net--the fact that it worked out well with a
    speed of c/2 was fortuitous. (I'm very far from the first to use that
    general sort of illustration, of course, but it's a fun one.)

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to corvid on Mon May 16 22:30:01 2022
    corvid wrote:
    On 5/16/22 13:26, Phil Hobbs wrote:

    Also, there's no simultaneity between separated objects moving at
     different speeds.  The relativistic garage illustrates this.

    Say you have a 1927 Bugatti Type 41, which is 252 inches long. Your
    garage is the standard 20 feed (240 inches) long, and has a very
    fast automatically-controlled door at each end.  The doors are
    designed to open and close automatically to allow the car to enter
    and leave.

    Because the Bugatti is so fast, you drive towards the open end
    of the garage at 0.5c.  You measure the length of the garage as

    240 inches * sqrt(1-0.5**2) = 207.8 inches.

    The hood of the car passes through the open door, then the closed
     door opens before the back bumper has passed through the
    doorway. No collision occurs, because the second door opens
    before the first one closes.

    Your spouse, waiting for you to come home from your drive, measures
    the length of the car as

    252 inches * sqrt(1-0.5**2) = 218.2 inches.

    The car fits into the garage, so as it enters, the first door closes
    before the second door opens.  Once again no collision occurs,
    because the car is shorter than the garage.

    The math works out fine in both English and metric, and no paradoxes
    are involved.



    But, but...  the bottom of the tires are in contact with the garage
     floor. Shouldn't that anchor the Bugatti and garage into the same
    frame?

    And the pistons are going up and down pretty good too. ;)

    Just stick with the front and rear bumpers for present purposes.


    Now I want to tweak the car and garage lengths, and the speed, so that
    the car goes thru unscathed in one frame but gets smashed in the other.
    Is it possible?

    Nope. When a collision occurs, it's because the two objects moving at different speeds are trying to be in the same place at once. If there's
    no difference in position, you can have simultaneity, same as if there's
    no difference in speed.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Clive Arthur@21:1/5 to All on Tue May 17 10:02:20 2022
    On 16/05/2022 14:38, Dimiter_Popoff wrote:

    <snipped>

    I have been digging into physics just as much as it takes to do what I
    do so me being naive with that sort of thing is no surprise. I am
    vaguely aware of what your references say, I think I may have read some
    of these some time ago.
    What I don't get though is the flashing light on the train thing.
    Looks obvious to me that the observed period depends on the movement direction (assuming gravity is constant, i.e. it is no factor).
    In fact this should be easily measurable (not that I would go into
    it, just wondering if you or someone else has an explanation, I am
    not the "out there to challenge the science" type, more the "curious
    until things get clarified for me" sort).

    One extra thing - the (very fast) train's time is dilated, so if both stationary you (ie, waiting in the station) and the driver are both
    flashing at 1Hz, he'll send fewer flashes than you.

    Is that right?

    --
    Cheers
    Clive

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Clifford Heath@21:1/5 to Phil Hobbs on Tue May 17 18:23:05 2022
    On 17/5/22 9:42 am, Phil Hobbs wrote:
    Clifford Heath wrote:
    On 17/5/22 12:18 am, Phil Hobbs wrote:
    then the closed door opens before the back bumper has passed through
    the doorway.  No collision occurs

    Umm, sorry? "before"? Is that a slip? Did you mean "as the back bumper
    passes through the doorway?

    No, the point is that seen from a point in the car's reference frame,
    the events happen *in a different order* from what you'd see in the
    garage's frame.

    Oh ok, I get it.

    Very cool illustration BTW. I want to use it, but want to make sure I
    have it correct first.

    Clifford Heath.

    I picked the Bugatti because I'm a fan,

    My grandfather had a Type 40 for a while - it's now restored and living
    in a garage 15km from here. But his really interesting car that I'd like
    to find more about was a Lea-Francis "Hyper" - the first supercharged
    British production car. He used to race that at the Albert Park track
    and the Philip Island track, which are both current or previous F1
    tracks. I don't think he ever entered F1, but I have a number of photos
    he took while flag marshalling at Phillip Island in 1933.

    Clifford Heath.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to Clifford Heath on Tue May 17 10:55:41 2022
    Clifford Heath wrote:
    On 17/5/22 9:42 am, Phil Hobbs wrote:
    Clifford Heath wrote:
    On 17/5/22 12:18 am, Phil Hobbs wrote:
    then the closed door opens before the back bumper has passed through
    the doorway.  No collision occurs

    Umm, sorry? "before"? Is that a slip? Did you mean "as the back
    bumper passes through the doorway?

    No, the point is that seen from a point in the car's reference frame,
    the events happen *in a different order* from what you'd see in the
    garage's frame.

    Oh ok, I get it.

    Very cool illustration BTW. I want to use it, but want to make sure I
    have it correct first.

    Clifford Heath.

    I picked the Bugatti because I'm a fan,

    My grandfather had a Type 40 for a while - it's now restored and living
    in a garage 15km from here. But his really interesting car that I'd like
    to find more about was a Lea-Francis "Hyper" - the first supercharged
    British production car. He used to race that at the Albert Park track
    and the Philip Island track, which are both current or previous F1
    tracks. I don't think he ever entered F1, but I have a number of photos
    he took while flag marshalling at Phillip Island in 1933.

    Clifford Heath.

    Fun!

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From "Kevin Aylward" @21:1/5 to All on Wed May 18 18:23:22 2022
    It turns out there is a long history, with many parallel explanations:

    Yep... pretty much all wrong....

    .<https://en.wikipedia.org/wiki/Twin_paradox>

    Joe Gwinn

    Well...... An actually correct account of the solution to the "Twins
    Paradox" is here:

    It explains the situation without accelerations, or frame switching. Yep.
    Trust me, this is the real deal...:-)


    https://www.kevinaylward.co.uk/gr/twinsparadox/twinsparadox.htm


    -- Kevin Aylward

    http://www.anasoft.co.uk/ SuperSpice http://www.kevinaylward.co.uk/ee/index.html

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From "Kevin Aylward" @21:1/5 to All on Wed May 18 18:25:55 2022
    "Martin Brown" wrote in message news:t5tia2$6m9$1@gioia.aioe.org...

    On 15/05/2022 12:05, Dimiter_Popoff wrote:
    On 5/15/2022 0:35, Joe Gwinn wrote:
    It turns out there is a long history, with many parallel explanations:

    .<https://en.wikipedia.org/wiki/Twin_paradox>

    Joe Gwinn

    And it explains why clocks at the equator and on the poles run
    the same without involving gravity.

    Actually they don't.

    One of Einstein's minor errors in his 1905 paper on special relativity was >>to predict how much slower a clock at the equator would tick when compared >>to one at the pole (due to the extra rotational speed of a clock at the >>equator). Every now and then someone points it out... eg

    https://physicstoday.scitation.org/doi/10.1063/1.1897562#

    They only run at the same speed when *both* the GR and SR corrections are >>applied simultaneously and only then at mean sea level.

    It is hard to get your head round but everybody's clock ticks at a >>different speed. Your head ages marginally more quickly than your feet.

    Often quoted, but wrong. Its not how SR works.

    In Special Relativity, clocks do not tick at different rates

    Its a fundamental axiom of special relativity that "the laws of physics are indepandant of inertial motion". This means, according to special
    relativity, clocks must always tick at the same rate.

    SR explains the apparent measurement of clock ticks reading slow by "time travel". One travels through time at different t rates. Its a subtitle, but important distinction.

    Special Relativity holds that for example, one can cover time at a rate of
    say, 100 secs/sec

    Consider Dr.Who in his Tardis. He is traveling into the future , his own
    ageing and clok ticks stay the same , for him, but he gets to the future
    before someone else. If Dr. Who sent pulses as he is traveling, as he is observe red to be traveling into the future, the received clock pulses would
    be received as if slower .

    The analogy is that there are many routes from London to Edinburgh. The odometer will read different distances, but it always clocks up distance at
    the same rate.

    Clocks actually slowing down is a feature of the Lorentz Ether Theory, known prior to the invention of SR and which Special Relativity claims to be
    superior to.

    The elephant in the room is that if the SR model is correct, then it leads
    to the "Block Universe", that is, intrinsic to SR is that the future already exists for everyone. This is in direct contradiction to Quantum Mechanics, which holds that the future is intrinsically non deterministic.

    Of note, is that QFT, is, essentially, and Ether theory in denial:

    Professor (UK head of department) of Physics at Cambridge, David Tong (Adams prize winner) has a YouTube general audience lecture on QFT:

    https://www.youtube.com/watch?v=zNVQfWC_evg


    0:31 - "...What are we made of...what are the fundamental building
    blocks of nature...?"

    19:30 - "... so there is spread something throughout this room, something
    we call the electron field..its like a fluid that fills ..the entire universe..and the ripples of this electron fluid..the waves of this fluid
    get tied into little bundles of energy, by the rules of quantum
    mechanics..and these bundles of energy are what we call the particle the electron....and the same is true for every kind of particle in the
    universe..."


    Kevin Aylward

    https://www.kevinaylward.co.uk/gr/index.html
    http://www.anasoft.co.uk/ SuperSpice http://www.kevinaylward.co.uk/ee/index.html

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From John Robertson@21:1/5 to All on Wed May 18 13:24:54 2022
    DQpPbiAyMDIyLzA1LzE2IDc6MTggYS5tLiwgUGhpbCBIb2JicyB3cm90ZToNCj4gSmVmZiBM YXltYW4gd3JvdGU6DQo+PiBPbiAxNC8wNS8yMDIyIDIyOjM1LCBKb2UgR3dpbm4gd3JvdGU6 DQo+Pj4gSXQgdHVybnMgb3V0IHRoZXJlIGlzIGEgbG9uZyBoaXN0b3J5LCB3aXRoIG1hbnkg cGFyYWxsZWwgZXhwbGFuYXRpb25zOg0KPj4+DQo+Pj4gLjxodHRwczovL2VuLndpa2lwZWRp YS5vcmcvd2lraS9Ud2luX3BhcmFkb3g+DQo+Pg0KPj4gSXQgY2FuIGJlIGV4cGFuZGVkIHRv IHRoZSBUcmlwbGV0cyBQYXJhZG94LCBmb3IgZXhhbXBsZQ0KPj4gPGh0dHA6Ly93d3cubXlz ZWFyY2gub3JnLnVrL3dlYnNpdGUxL2h0bWwvMjUxLlRyaXBsZXRzLmh0bWw+DQo+Pg0KPj4g U1JUIGlzIHdlbGwgYWJvdmUgbWUsIEknbSBhZnJhaWQuIFNvbWUgb2YgdGhlIGV4cGxhbmF0 aW9uIG9mIHRoZSBUd2lucyANCj4+IFBhcmFkb3ggcmVmZXJzIHRvIHRoZSB0d2lucycgY2xv Y2tzIHRyYW5zbWl0dGluZyB0aGVpciB0aW1lIHRvIHRoZSANCj4+IG90aGVyIHR3aW4gKHRo ZSBjbG9jayBzaWduYWwgaXMgdHJhbnNtaXR0ZWQgYXQgdGhlIHNwZWVkIG9mIGxpZ2h0KS4g DQo+PiBFdmVuIGFsbG93aW5nIGZvciB0aGUgdHJhdmVsbGluZyB0d2luJ3Mgc3BlZWQgd2hl biBhcHByb2FjaGluZyB0aGUgDQo+PiBzcGVlZCBvZiBsaWdodCwgYW5kIHRoZSByZWxhdGl2 aXN0aWMgZWZmZWN0IGl0IGhhcyBvbiBlYWNoIGNsb2NrJ3MgDQo+PiBwZXJjZWl2ZWQgdGlt ZSwgYXMgdGhlIHRyYXZlbGxpbmcgdHdpbidzIHNwZWVkIGRvZXNuJ3QgZXhjZWVkIHRoYXQg DQo+PiBzcGVlZCwgZWFjaCB0d2luIHdpbGwsIGV2ZW50dWFsbHksIHJlY2VpdmUgdGhlIGNs b2NrIHRpbWUgb2YgdGhlIG90aGVyLg0KPj4NCj4+IEJ1dCB3aGF0IGhhcHBlbnMgd2l0aCB0 aGUgVHJpcGxldCBQYXJhZG94IHdoZXJlIHRoZSBtb3ZpbmcgdHJpcGxldHMgDQo+PiBhcmUg YWNjZWxlcmF0aW5nIGF3YXkgZnJvbSBlYWNoIG90aGVyPyBPbmNlIHRoZXkndmUgImV4Y2Vl ZGVkIiBDIGluIA0KPj4gcmVsYXRpb24gdG8gZWFjaCBvdGhlciwgYWx0aG91Z2ggdGhleSBj YW4gcmVjZWl2ZSB0aGUgc3RhdGlvbmFyeSANCj4+IHRyaXBsZXQncyBjbG9jayByZWFkaW5n IChhbmQgaGUgY2FuIHJlY2VpdmUgdGhlaXJzKSwgY2FuIG9uZSBtb3ZpbmcgDQo+PiB0cmlw bGV0IHN0aWxsIHJlY2VpdmUgdGhlIG90aGVyIG1vdmluZyB0cmlwbGV0J3MgY2xvY2sgc2ln bmFsPyBJZiANCj4+IHRoZXJlIGlzIHN1Y2ggYSBtb21lbnQgd2hlbiB0aGV5IGNhbiBubyBs b25nZXIgcmVjZWl2ZSBlYWNoIG90aGVyJ3MgDQo+PiBzaWduYWwsIHdoZW4gdGhleSBmaW5h bGx5IHN0b3AgbW92aW5nIGF3YXkgYW5kIHN0YXJ0IG1vdmluZyB0b3dhcmRzIA0KPj4gZWFj aCBvdGhlciBhZ2Fpbiwgd2lsbCB0aGVyZSBiZSBhIG1vbWVudCB3aGVuIHRoZXkgc3VkZGVu bHkgc3RhcnQgDQo+PiByZWNlaXZpbmcgdGhhdCAibWlzc2luZyIgY2xvY2sgc2lnbmFsIGFz IHRoZXkgY2F0Y2ggdXAgd2l0aCBpdCAob3IgDQo+PiBwZXJoYXBzIGl0IGNhdGNoZXMgdXAg d2l0aCB0aGVtKT8gV2lsbCB0aGVyZSBiZSBhIHNwZWNpZmljIG1vbWVudCB3aGVuIA0KPj4g dGhleSBub3Qgb25seSByZWNlaXZlIGEgbWlzc2luZyBjbG9jayB0aW1lLCBidXQgY29pbmNp ZGVudGFsbHkgcmVjZWl2ZSANCj4+IHRoZSAiYWNjdXJhdGUiIHRpbWUgYXMgdHJhbnNtaXR0 ZWQgYnkgdGhlIG90aGVyIG1vdmluZyB0cmlwbGV0LCBzbyANCj4+IGFwcGVhciB0byBiZSBy ZWNlaXZpbmcgdHdvIGRpZmZlcmVudCBjbG9jayByZWFkaW5ncyBhdCB0aGUgc2FtZSB0aW1l Pw0KPj4NCj4gDQo+IElmIHlvdSBzaGluZSB5b3VyIGxhc2VyIHBvaW50ZXIgYXQgdHdvIHBv aW50cyAxODAgZGVncmVlcyBhcGFydCBpbiB0aGUgDQo+IHNreSwgdGhlIHJlbGF0aXZlIHNw ZWVkIG9mIHRoZSBsaWdodCBwdWxzZXMgaW4geW91ciBmcmFtZSBvZiByZWZlcmVuY2UgDQo+ IGlzIDJjLsKgIE5vIHBhcmFkb3ggaXMgaW52b2x2ZWQuDQo+IA0KPiBBbHNvLCB0aGVyZSdz IG5vIHNpbXVsdGFuZWl0eSBiZXR3ZWVuIHNlcGFyYXRlZCBvYmplY3RzIG1vdmluZyBhdCAN Cj4gZGlmZmVyZW50IHNwZWVkcy7CoCBUaGUgcmVsYXRpdmlzdGljIGdhcmFnZSBpbGx1c3Ry YXRlcyB0aGlzLg0KPiANCj4gU2F5IHlvdSBoYXZlIGEgMTkyNyBCdWdhdHRpIFR5cGUgNDEs IHdoaWNoIGlzIDI1MiBpbmNoZXMgbG9uZy7CoCBZb3VyIA0KPiBnYXJhZ2UgaXMgdGhlIHN0 YW5kYXJkIDIwIGZlZWQgKDI0MCBpbmNoZXMpIGxvbmcsIGFuZCBoYXMgYSB2ZXJ5IGZhc3Qg DQo+IGF1dG9tYXRpY2FsbHktY29udHJvbGxlZCBkb29yIGF0IGVhY2ggZW5kLsKgIFRoZSBk b29ycyBhcmUgZGVzaWduZWQgdG8gDQo+IG9wZW4gYW5kIGNsb3NlIGF1dG9tYXRpY2FsbHkg dG8gYWxsb3cgdGhlIGNhciB0byBlbnRlciBhbmQgbGVhdmUuDQo+IA0KPiBCZWNhdXNlIHRo ZSBCdWdhdHRpIGlzIHNvIGZhc3QsIHlvdSBkcml2ZSB0b3dhcmRzIHRoZSBvcGVuIGVuZCBv ZiB0aGUgDQo+IGdhcmFnZSBhdCAwLjVjLsKgIFlvdSBtZWFzdXJlIHRoZSBsZW5ndGggb2Yg dGhlIGdhcmFnZSBhcw0KPiANCj4gMjQwIGluY2hlcyAqIHNxcnQoMS0wLjUqKjIpID0gMjA3 LjggaW5jaGVzLg0KPiANCj4gVGhlIGhvb2Qgb2YgdGhlIGNhciBwYXNzZXMgdGhyb3VnaCB0 aGUgb3BlbiBkb29yLCB0aGVuIHRoZSBjbG9zZWQgZG9vciANCj4gb3BlbnMgYmVmb3JlIHRo ZSBiYWNrIGJ1bXBlciBoYXMgcGFzc2VkIHRocm91Z2ggdGhlIGRvb3J3YXkuwqAgTm8gDQo+ IGNvbGxpc2lvbiBvY2N1cnMsIGJlY2F1c2UgdGhlIHNlY29uZCBkb29yIG9wZW5zIGJlZm9y ZSB0aGUgZmlyc3Qgb25lIA0KPiBjbG9zZXMuDQo+IA0KPiBZb3VyIHNwb3VzZSwgd2FpdGlu ZyBmb3IgeW91IHRvIGNvbWUgaG9tZSBmcm9tIHlvdXIgZHJpdmUsIG1lYXN1cmVzIHRoZSAN Cj4gbGVuZ3RoIG9mIHRoZSBjYXIgYXMNCj4gDQo+IDI1MiBpbmNoZXMgKiBzcXJ0KDEtMC41 KioyKSA9IDIxOC4yIGluY2hlcy4NCj4gDQo+IFRoZSBjYXIgZml0cyBpbnRvIHRoZSBnYXJh Z2UsIHNvIGFzIGl0IGVudGVycywgdGhlIGZpcnN0IGRvb3IgY2xvc2VzIA0KPiBiZWZvcmUg dGhlIHNlY29uZCBkb29yIG9wZW5zLsKgIE9uY2UgYWdhaW4gbm8gY29sbGlzaW9uIG9jY3Vy cywgYmVjYXVzZSANCj4gdGhlIGNhciBpcyBzaG9ydGVyIHRoYW4gdGhlIGdhcmFnZS4NCj4g DQo+IFRoZSBtYXRoIHdvcmtzIG91dCBmaW5lIGluIGJvdGggRW5nbGlzaCBhbmQgbWV0cmlj LCBhbmQgbm8gcGFyYWRveGVzIGFyZSANCj4gaW52b2x2ZWQuDQo+IA0KPiBDaGVlcnMNCj4g DQo+IFBoaWwgSG9iYnMNCj4gDQoNCkhvdyBmYXN0IGRvIHRoZSBkb29ycyBoYXZlIHRvIHJp c2Ugb3IgY2xvc2UgdG8gY2xlYXIgdGhlIGNhci4uLnRoaXMgDQpzZWVtcyB0byBiZSBnZXR0 aW5nIGFubm95aW5nIGNsb3NlIHRvIHRoZSBzcGVlZCBvZiBsaWdodC4NCg0KUHJldHR5IHN1 cmUgbXkgZ2FyYWdlIGRvb3Igd291bGQgd2FycCBpZiBJIHJhbiBpdCB0aGF0IGZhc3QhIChk dWNraW5nKQ0KDQpKb2huIDstIykjDQo=

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Clive Arthur@21:1/5 to All on Wed May 18 23:06:38 2022
    Hi Kevin

    What is the speed of a photon from the photon's POV? I think it must be infinite, am I right?

    Likewise, a sufficiently fast spaceship would have a speedometer showing
    its speed as being >c? Is that right?

    --
    Cheers
    Clive

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to John Robertson on Wed May 18 21:53:20 2022
    John Robertson wrote:

    On 2022/05/16 7:18 a.m., Phil Hobbs wrote:
    Jeff Layman wrote:
    On 14/05/2022 22:35, Joe Gwinn wrote:
    It turns out there is a long history, with many parallel explanations: >>>>
    .<https://en.wikipedia.org/wiki/Twin_paradox>

    It can be expanded to the Triplets Paradox, for example
    <http://www.mysearch.org.uk/website1/html/251.Triplets.html>

    SRT is well above me, I'm afraid. Some of the explanation of the
    Twins Paradox refers to the twins' clocks transmitting their time to
    the other twin (the clock signal is transmitted at the speed of
    light). Even allowing for the travelling twin's speed when
    approaching the speed of light, and the relativistic effect it has on
    each clock's perceived time, as the travelling twin's speed doesn't
    exceed that speed, each twin will, eventually, receive the clock time
    of the other.

    But what happens with the Triplet Paradox where the moving triplets
    are accelerating away from each other? Once they've "exceeded" C in
    relation to each other, although they can receive the stationary
    triplet's clock reading (and he can receive theirs), can one moving
    triplet still receive the other moving triplet's clock signal? If
    there is such a moment when they can no longer receive each other's
    signal, when they finally stop moving away and start moving towards
    each other again, will there be a moment when they suddenly start
    receiving that "missing" clock signal as they catch up with it (or
    perhaps it catches up with them)? Will there be a specific moment
    when they not only receive a missing clock time, but coincidentally
    receive the "accurate" time as transmitted by the other moving
    triplet, so appear to be receiving two different clock readings at
    the same time?


    If you shine your laser pointer at two points 180 degrees apart in the
    sky, the relative speed of the light pulses in your frame of reference
    is 2c.  No paradox is involved.

    Also, there's no simultaneity between separated objects moving at
    different speeds.  The relativistic garage illustrates this.

    Say you have a 1927 Bugatti Type 41, which is 252 inches long.  Your
    garage is the standard 20 feed (240 inches) long, and has a very fast
    automatically-controlled door at each end.  The doors are designed to
    open and close automatically to allow the car to enter and leave.

    Because the Bugatti is so fast, you drive towards the open end of the
    garage at 0.5c.  You measure the length of the garage as

    240 inches * sqrt(1-0.5**2) = 207.8 inches.

    The hood of the car passes through the open door, then the closed door
    opens before the back bumper has passed through the doorway.  No
    collision occurs, because the second door opens before the first one
    closes.

    Your spouse, waiting for you to come home from your drive, measures
    the length of the car as

    252 inches * sqrt(1-0.5**2) = 218.2 inches.

    The car fits into the garage, so as it enters, the first door closes
    before the second door opens.  Once again no collision occurs, because
    the car is shorter than the garage.

    The math works out fine in both English and metric, and no paradoxes
    are involved.


    How fast do the doors have to rise or close to clear the car...this
    seems to be getting annoying close to the speed of light.

    Pretty sure my garage door would warp if I ran it that fast! (ducking)

    As I mentioned upthread, the pistons would be going up and down at
    impressive speeds too. ;)

    The garage thing can be crispened up so as to be practically measurable.
    The point of the doors is that if a collision occurs, it occurs in all reference frames.

    Cheers

    Phil Hobbs


    --
    Dr Philip C D Hobbs
    Principal Consultbant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

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  • From Phil Hobbs@21:1/5 to John Robertson on Thu May 19 02:54:43 2022
    John Robertson wrote:

    On 2022/05/16 7:18 a.m., Phil Hobbs wrote:
    Jeff Layman wrote:
    On 14/05/2022 22:35, Joe Gwinn wrote:
    It turns out there is a long history, with many parallel explanations: >>>>
    .<https://en.wikipedia.org/wiki/Twin_paradox>

    It can be expanded to the Triplets Paradox, for example
    <http://www.mysearch.org.uk/website1/html/251.Triplets.html>

    SRT is well above me, I'm afraid. Some of the explanation of the
    Twins Paradox refers to the twins' clocks transmitting their time to
    the other twin (the clock signal is transmitted at the speed of
    light). Even allowing for the travelling twin's speed when
    approaching the speed of light, and the relativistic effect it has on
    each clock's perceived time, as the travelling twin's speed doesn't
    exceed that speed, each twin will, eventually, receive the clock time
    of the other.

    But what happens with the Triplet Paradox where the moving triplets
    are accelerating away from each other? Once they've "exceeded" C in
    relation to each other, although they can receive the stationary
    triplet's clock reading (and he can receive theirs), can one moving
    triplet still receive the other moving triplet's clock signal? If
    there is such a moment when they can no longer receive each other's
    signal, when they finally stop moving away and start moving towards
    each other again, will there be a moment when they suddenly start
    receiving that "missing" clock signal as they catch up with it (or
    perhaps it catches up with them)? Will there be a specific moment
    when they not only receive a missing clock time, but coincidentally
    receive the "accurate" time as transmitted by the other moving
    triplet, so appear to be receiving two different clock readings at
    the same time?


    If you shine your laser pointer at two points 180 degrees apart in the
    sky, the relative speed of the light pulses in your frame of reference
    is 2c.  No paradox is involved.

    Also, there's no simultaneity between separated objects moving at
    different speeds.  The relativistic garage illustrates this.

    Say you have a 1927 Bugatti Type 41, which is 252 inches long.  Your
    garage is the standard 20 feed (240 inches) long, and has a very fast
    automatically-controlled door at each end.  The doors are designed to
    open and close automatically to allow the car to enter and leave.

    Because the Bugatti is so fast, you drive towards the open end of the
    garage at 0.5c.  You measure the length of the garage as

    240 inches * sqrt(1-0.5**2) = 207.8 inches.

    The hood of the car passes through the open door, then the closed door
    opens before the back bumper has passed through the doorway.  No
    collision occurs, because the second door opens before the first one
    closes.

    Your spouse, waiting for you to come home from your drive, measures
    the length of the car as

    252 inches * sqrt(1-0.5**2) = 218.2 inches.

    The car fits into the garage, so as it enters, the first door closes
    before the second door opens.  Once again no collision occurs, because
    the car is shorter than the garage.

    The math works out fine in both English and metric, and no paradoxes
    are involved.

    Cheers

    Phil Hobbs


    How fast do the doors have to rise or close to clear the car...this
    seems to be getting annoying close to the speed of light.

    Pretty sure my garage door would warp if I ran it that fast! (ducking)

    There are also a few other practical problems, e.g. that the kinetic
    energy of a 2000 kg car going at c/2 is

    m c**2 (gamma -1) =
    2000 kg * (299792458 m/s)**2 * ( 1 / sqrt(1-0.25) - 1 ) =
    2.87E19 J.

    That's 6653 megatons, at the usually quoted rate of 1 MT = 1e15 cal
    (4.18E15 J).

    The XKCD baseball is a mere 8 MT.
    <https://what-if.xkcd.com/1/>.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

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    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Martin Brown@21:1/5 to Jeff Layman on Thu May 19 09:12:24 2022
    On 15/05/2022 09:46, Jeff Layman wrote:
    On 14/05/2022 22:35, Joe Gwinn wrote:
    It turns out there is a long history, with many parallel explanations:

    .<https://en.wikipedia.org/wiki/Twin_paradox>

    It can be expanded to the Triplets Paradox, for example <http://www.mysearch.org.uk/website1/html/251.Triplets.html>

    SRT is well above me, I'm afraid. Some of the explanation of the Twins Paradox refers to the twins' clocks transmitting their time to the other
    twin (the clock signal is transmitted at the speed of light). Even
    allowing for the travelling twin's speed when approaching the speed of
    light, and the relativistic effect it has on each clock's perceived
    time, as the travelling twin's speed doesn't exceed that speed, each
    twin will, eventually, receive the clock time of the other.

    But what happens with the Triplet Paradox where the moving triplets are accelerating away from each other? Once they've "exceeded" C in relation
    to each other,

    That never happens. Their relative velocity is *always* less than c as
    viewed from one of their fast moving rest frames (doesn't matter which one).

    You are applying a Galilean/Newtonian addition of velocities as measured
    in the rest frame of the Earth in a situation where the full
    relativistic treatment for addition of velocities is required. see

    https://en.wikipedia.org/wiki/Velocity-addition_formula#Special_relativity

    u = (v + u')/(1+vu'/c^2)

    Mr stay at home with v=0 sees his two twins going away from him at +/-w

    u[stay at home] = w

    On either of the fast moving rockets they also see Mr Stay at home
    receding from them at velocity w and their other twin receding at

    u[other traveller] = (w+w)/(1+w^2/c^2) = 2w/(1+(w/c)^2)

    To see why set w = c*(1-e)

    = 2c*(1-e)/(1+(1-e)^2) = c*(2-2e)/(2-2e+e^2)

    Common sense doesn't work with relativity at all. You can really only
    trust the mathematics and the laws of physics always remaining self
    consistent. Everything else derives from that basic axiom.

    although they can receive the stationary triplet's clock
    reading (and he can receive theirs), can one moving triplet still
    receive the other moving triplet's clock signal? If there is such a
    moment when they can no longer receive each other's signal, when they
    finally stop moving away and start moving towards each other again, will there be a moment when they suddenly start receiving that "missing"
    clock signal as they catch up with it (or perhaps it catches up with
    them)? Will there be a specific moment when they not only receive a
    missing clock time, but coincidentally receive the "accurate" time as transmitted by the other moving triplet, so appear to be receiving two different clock readings at the same time?

    Spacetime itself can expand faster than the speed of light but that is a consequence of GR. There are parts of the (presumed infinite) universe
    that will remain forever inaccessible to us at any sub light speed.

    --
    Regards,
    Martin Brown

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  • From RichD@21:1/5 to corvid on Fri May 20 12:12:36 2022
    On May 16, corvid wrote:
    Also, there's no simultaneity between separated objects moving at
    different speeds. The relativistic garage illustrates this.
    Because the Bugatti is so fast, you drive towards the open end
    of the garage at 0.5c. You measure the length of the garage as
    240 inches * sqrt(1-0.5**2) = 207.8 inches.
    The hood of the car passes through the open door, then the closed
    door opens before the back bumper has passed through the
    doorway. No collision occurs, because the second door opens
    before the first one closes.
    Your spouse measures the length of the car as
    252 inches * sqrt(1-0.5**2) = 218.2 inches.
    The car fits into the garage, so as it enters, the first door
    closes before the second door opens. Once again no collision
    occurs, because the car is shorter than the garage.

    Now I want to tweak the car and garage lengths, and the speed, so that
    the car goes thru unscathed in one frame but gets smashed in the other.
    Is it possible?

    heh
    Einstein skeptics have been trying to do that for a hundred years.

    Playing around with the the numbers does make a nice exercise for the student.

    You can also gin up variations on this paradox. For instance, the garage becomes a two lane highway, with north and southbound lanes. Two cars approach, simultaneously, as seen from the garage. In the garage frame, it's much the same as before, both cars fit inside.

    Then work out the various door sequences, as seen from each vehicle, for some real brain twisting -

    --
    Rich

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  • From Phil Hobbs@21:1/5 to RichD on Fri May 20 16:55:49 2022
    RichD wrote:
    On May 16, corvid wrote:
    Also, there's no simultaneity between separated objects moving at
    different speeds. The relativistic garage illustrates this.
    Because the Bugatti is so fast, you drive towards the open end
    of the garage at 0.5c. You measure the length of the garage as
    240 inches * sqrt(1-0.5**2) = 207.8 inches.
    The hood of the car passes through the open door, then the closed
    door opens before the back bumper has passed through the
    doorway. No collision occurs, because the second door opens
    before the first one closes.
    Your spouse measures the length of the car as
    252 inches * sqrt(1-0.5**2) = 218.2 inches.
    The car fits into the garage, so as it enters, the first door
    closes before the second door opens. Once again no collision
    occurs, because the car is shorter than the garage.

    Now I want to tweak the car and garage lengths, and the speed, so that
    the car goes thru unscathed in one frame but gets smashed in the other.
    Is it possible?

    heh
    Einstein skeptics have been trying to do that for a hundred years.

    There's a class of things, called _Lorentz_scalars_, that are the same
    in all reference frames, i.e. hitting them with a Lorentz transform
    doesn't change the answer. On example is phase. (It's based on
    counting, not length measurements.) Whether a collision occurs is one
    of those.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

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    * Origin: fsxNet Usenet Gateway (21:1/5)