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<div class="moz-cite-prefix">On 22/08/2013 20:58, John Clark wrote:<br>
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<blockquote
cite="mid:CAJPayv0cm2wwza=Fx37JydJBQ-vMBQSEKJFf5n3GkhkO6UWoCA@mail.gmail.com"
type="cite">
<div dir="ltr">On Thu, Aug 22, 2013 at 7:29 AM, Gordon <span
dir="ltr"><<a moz-do-not-send="true"
href="mailto:gts_2000@yahoo.com" target="_blank">gts_2000@yahoo.com</a>></span>
wrote:<br>
<div class="gmail_extra">
<div class="gmail_quote"><br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
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<div style="font-size:12pt;font-family:times new
roman,new york,times,serif">
<div>> I am in a spaceship accelerating away from
earth and approaching the speed of light. I have a
flashlight. My instruments tell me that the light
emitted from my flashlight travels at c. This is
true no matter whether I shine it forward in the
direction of my travel or backward toward the rear
of the ship. </div>
<div
style="font-style:normal;font-size:16px;background-color:transparent;font-family:'times
new roman','new york',times,serif">I am accelerating
away from earth such that my ship's velocity
relative to earth compared to c is halved in each
time period. For example at time t, my ship is
travelling at 90% of c. At t2, my ship is travelling
at 95% of c. At t3, my ship is travelling at 97.5%
of c, and so on for an infinite amount of time as I
approach c.</div>
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<div
style="font-style:normal;font-size:16px;background-color:transparent;font-family:'times
new roman','new york',times,serif"><span
style="background-color:transparent"><br>
</span></div>
<div
style="font-style:normal;font-size:16px;background-color:transparent;font-family:'times
new roman','new york',times,serif"><span
style="background-color:transparent">It's OK for thought
experiments to be wildly impractical but they must be
physically possible, and the above experiment would not
only take an infinite amount of time to perform it would
also take an infinite amount of energy.<br>
</span></div>
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<br>
However, the conclusion about infinite time and energy are a
*result* of doing the Einstein thought experiment properly. You
cannot dismiss the lightspeed case straight away.<br>
<br>
Einstein had a nifty way of showing that there is something
problematic going on in this case, which I think was one of his
primary reasons for developing the full theory: suppose you run past
an electromagnetic wave while travelling at c. What do you see? It
ought to be static in your reference frame, but in that case it
breaks Maxwell's laws. So either electrodynamics is wrong, or the
velocity addition formula is wrong (and we already have some
suspicions since light seem to move at c regardless of speed). So
let's see what happens if we assume the velocity addition formula
has to be something else...<br>
<br>
<pre class="moz-signature" cols="72">--
Anders Sandberg,
Future of Humanity Institute
Oxford Martin School
Faculty of Philosophy
Oxford University </pre>
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