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  "events": [ {
    "tStartMs": 6036,
    "dDurationMs": 5200,
    "segs": [ {
      "utf8": "On their 20th birthday, identical twin \nastronauts volunteer for an experiment."
    } ]
  }, {
    "tStartMs": 11236,
    "dDurationMs": 4075,
    "segs": [ {
      "utf8": "Terra will remain on Earth, while Stella \nwill board a spaceship."
    } ]
  }, {
    "tStartMs": 15311,
    "dDurationMs": 3592,
    "segs": [ {
      "utf8": "Stella’s ship will travel at 86.6% the \nspeed of light"
    } ]
  }, {
    "tStartMs": 18903,
    "dDurationMs": 2750,
    "segs": [ {
      "utf8": "to visit a star that is 10 \nlight-years away,"
    } ]
  }, {
    "tStartMs": 21653,
    "dDurationMs": 2190,
    "segs": [ {
      "utf8": "then return to Earth at the same speed."
    } ]
  }, {
    "tStartMs": 23843,
    "dDurationMs": 1930,
    "segs": [ {
      "utf8": "As they prepare to part ways,"
    } ]
  }, {
    "tStartMs": 25773,
    "dDurationMs": 2940,
    "segs": [ {
      "utf8": "the twins wonder what will happen \nwhen they’re reunited."
    } ]
  }, {
    "tStartMs": 28713,
    "dDurationMs": 3640,
    "segs": [ {
      "utf8": "Since a light year is exactly the distance\nlight can travel in a year,"
    } ]
  }, {
    "tStartMs": 32353,
    "dDurationMs": 2870,
    "segs": [ {
      "utf8": "Stella’s journey should take 23 years."
    } ]
  }, {
    "tStartMs": 35223,
    "dDurationMs": 2460,
    "segs": [ {
      "utf8": "But from having studied \nspecial relativity,"
    } ]
  }, {
    "tStartMs": 37683,
    "dDurationMs": 2880,
    "segs": [ {
      "utf8": "the twins know it’s not that simple."
    } ]
  }, {
    "tStartMs": 40563,
    "dDurationMs": 3250,
    "segs": [ {
      "utf8": "First of all, the faster an object moves \nthrough space,"
    } ]
  }, {
    "tStartMs": 43813,
    "dDurationMs": 4780,
    "segs": [ {
      "utf8": "the slower it moves through time \ncompared to an unmoving observer."
    } ]
  }, {
    "tStartMs": 48593,
    "dDurationMs": 3670,
    "segs": [ {
      "utf8": "This relationship can be quantified with \nsomething called the Lorentz factor,"
    } ]
  }, {
    "tStartMs": 52263,
    "dDurationMs": 2240,
    "segs": [ {
      "utf8": "which is defined by this equation."
    } ]
  }, {
    "tStartMs": 54503,
    "dDurationMs": 4000,
    "segs": [ {
      "utf8": "And secondly, the length of a moving \nobject as measured by an observer at rest"
    } ]
  }, {
    "tStartMs": 58503,
    "dDurationMs": 2360,
    "segs": [ {
      "utf8": "will contract by the same factor."
    } ]
  }, {
    "tStartMs": 60863,
    "dDurationMs": 4390,
    "segs": [ {
      "utf8": "At 86.6% of the speed of light \nthe Lorentz factor is 2,"
    } ]
  }, {
    "tStartMs": 65253,
    "dDurationMs": 3700,
    "segs": [ {
      "utf8": "meaning time will pass twice as slowly \naboard the spaceship."
    } ]
  }, {
    "tStartMs": 68953,
    "dDurationMs": 3454,
    "segs": [ {
      "utf8": "Of course, Stella won’t notice \ntime slowing down."
    } ]
  }, {
    "tStartMs": 72407,
    "dDurationMs": 4000,
    "segs": [ {
      "utf8": "That’s because all time-based processes \nin the ship will slow down as well–"
    } ]
  }, {
    "tStartMs": 76407,
    "dDurationMs": 1870,
    "segs": [ {
      "utf8": "clocks and electrical devices;"
    } ]
  }, {
    "tStartMs": 78277,
    "dDurationMs": 3420,
    "segs": [ {
      "utf8": "Stella’s biological activities including \nher rate of aging"
    } ]
  }, {
    "tStartMs": 81697,
    "dDurationMs": 2650,
    "segs": [ {
      "utf8": "and her perception of time itself."
    } ]
  }, {
    "tStartMs": 84347,
    "dDurationMs": 3190,
    "segs": [ {
      "utf8": "The only people who could notice time\non the moving spaceship"
    } ]
  }, {
    "tStartMs": 87537,
    "dDurationMs": 1700,
    "segs": [ {
      "utf8": "passing slower for Stella"
    } ]
  }, {
    "tStartMs": 89237,
    "dDurationMs": 4570,
    "segs": [ {
      "utf8": "would be observers in an inertial, \nor non-accelerating, reference frame–"
    } ]
  }, {
    "tStartMs": 93807,
    "dDurationMs": 1987,
    "segs": [ {
      "utf8": "like Terra back on Earth."
    } ]
  }, {
    "tStartMs": 95794,
    "dDurationMs": 3280,
    "segs": [ {
      "utf8": "Thus, Terra concludes that when they meet\nback on Earth,"
    } ]
  }, {
    "tStartMs": 99074,
    "dDurationMs": 2160,
    "segs": [ {
      "utf8": "she’ll be older than Stella."
    } ]
  }, {
    "tStartMs": 101234,
    "dDurationMs": 2500,
    "segs": [ {
      "utf8": "But that’s just one way of \nlooking at things."
    } ]
  }, {
    "tStartMs": 103734,
    "dDurationMs": 1879,
    "segs": [ {
      "utf8": "Because all movement is relative,"
    } ]
  }, {
    "tStartMs": 105613,
    "dDurationMs": 5040,
    "segs": [ {
      "utf8": "Stella argues it would be just as valid to\nsay her spaceship will stand still"
    } ]
  }, {
    "tStartMs": 110653,
    "dDurationMs": 4479,
    "segs": [ {
      "utf8": "while the rest of the universe, \nincluding Terra, moves around her."
    } ]
  }, {
    "tStartMs": 115132,
    "dDurationMs": 4080,
    "segs": [ {
      "utf8": "And in that case, time will pass twice as \nslowly for Terra,"
    } ]
  }, {
    "tStartMs": 119212,
    "dDurationMs": 2794,
    "segs": [ {
      "utf8": "making Stella the older twin in the end."
    } ]
  }, {
    "tStartMs": 122006,
    "dDurationMs": 4000,
    "segs": [ {
      "utf8": "They can’t each be older than the other, \nso which one of them is right?"
    } ]
  }, {
    "tStartMs": 126006,
    "dDurationMs": 3950,
    "segs": [ {
      "utf8": "This apparent contradiction is known as\nthe “Twin Paradox.”"
    } ]
  }, {
    "tStartMs": 129956,
    "dDurationMs": 1660,
    "segs": [ {
      "utf8": "But it’s not really a paradox–"
    } ]
  }, {
    "tStartMs": 131616,
    "dDurationMs": 4990,
    "segs": [ {
      "utf8": "just an example of how special relativity \ncan be easily misunderstood."
    } ]
  }, {
    "tStartMs": 136606,
    "dDurationMs": 2310,
    "segs": [ {
      "utf8": "To test their theories in real-time,"
    } ]
  }, {
    "tStartMs": 138916,
    "dDurationMs": 3390,
    "segs": [ {
      "utf8": "each of the twins agrees to send \na burst of light to the other"
    } ]
  }, {
    "tStartMs": 142306,
    "dDurationMs": 2420,
    "segs": [ {
      "utf8": "every time a year has passed for them."
    } ]
  }, {
    "tStartMs": 144726,
    "dDurationMs": 3330,
    "segs": [ {
      "utf8": "Unlike other objects, the speed of light \nis always constant"
    } ]
  }, {
    "tStartMs": 148056,
    "dDurationMs": 2680,
    "segs": [ {
      "utf8": "regardless of an observer’s \nreference frame."
    } ]
  }, {
    "tStartMs": 150736,
    "dDurationMs": 3300,
    "segs": [ {
      "utf8": "A light burst sent from Earth will be \nmeasured at the same speed"
    } ]
  }, {
    "tStartMs": 154036,
    "dDurationMs": 2200,
    "segs": [ {
      "utf8": "as a light burst sent from the spaceship,"
    } ]
  }, {
    "tStartMs": 156236,
    "dDurationMs": 3710,
    "segs": [ {
      "utf8": "regardless of whether it’s on its \noutbound or return trip."
    } ]
  }, {
    "tStartMs": 159946,
    "dDurationMs": 2500,
    "segs": [ {
      "utf8": "So when one twin observes \na burst of light,"
    } ]
  }, {
    "tStartMs": 162446,
    "dDurationMs": 4270,
    "segs": [ {
      "utf8": "they’re measuring how long it took the \nother twin to experience a year passing,"
    } ]
  }, {
    "tStartMs": 166716,
    "dDurationMs": 3035,
    "segs": [ {
      "utf8": "plus how long it took for light \nto travel between them."
    } ]
  }, {
    "tStartMs": 169751,
    "dDurationMs": 2610,
    "segs": [ {
      "utf8": "We can track what’s happening on a graph."
    } ]
  }, {
    "tStartMs": 172361,
    "dDurationMs": 5340,
    "segs": [ {
      "utf8": "The X axis marks distance from Earth, \nand the Y axis tracks the passage of time."
    } ]
  }, {
    "tStartMs": 177701,
    "dDurationMs": 4079,
    "segs": [ {
      "utf8": "From Terra’s perspective, her path will \nsimply be a vertical line,"
    } ]
  }, {
    "tStartMs": 181780,
    "dDurationMs": 1640,
    "segs": [ {
      "utf8": "with distance equal to zero"
    } ]
  }, {
    "tStartMs": 183420,
    "dDurationMs": 4080,
    "segs": [ {
      "utf8": "and each tick on the line equivalent \nto a year as she perceives it."
    } ]
  }, {
    "tStartMs": 187500,
    "dDurationMs": 5294,
    "segs": [ {
      "utf8": "Stella’s path will stretch from the same \norigin to a point 11.5 years in time"
    } ]
  }, {
    "tStartMs": 192794,
    "dDurationMs": 2894,
    "segs": [ {
      "utf8": "and 10 light-years in distance from Terra…"
    } ]
  }, {
    "tStartMs": 195688,
    "dDurationMs": 4360,
    "segs": [ {
      "utf8": "before converging again at zero \ndistance and 23 years’ time."
    } ]
  }, {
    "tStartMs": 200048,
    "dDurationMs": 1929,
    "segs": [ {
      "utf8": "At her first one-year mark,"
    } ]
  }, {
    "tStartMs": 201977,
    "dDurationMs": 4260,
    "segs": [ {
      "utf8": "Terra will send a pulse of light from \nEarth towards Stella’s spaceship."
    } ]
  }, {
    "tStartMs": 206237,
    "dDurationMs": 2837,
    "segs": [ {
      "utf8": "Since light takes a year to travel \none light-year,"
    } ]
  }, {
    "tStartMs": 209074,
    "dDurationMs": 3280,
    "segs": [ {
      "utf8": "its path will be a 45-degree \ndiagonal line."
    } ]
  }, {
    "tStartMs": 212354,
    "dDurationMs": 2430,
    "segs": [ {
      "utf8": "And because Stella is \ntraveling away from it,"
    } ]
  }, {
    "tStartMs": 214784,
    "dDurationMs": 1940,
    "segs": [ {
      "utf8": "by the time the light catches up to her,"
    } ]
  }, {
    "tStartMs": 216724,
    "dDurationMs": 4810,
    "segs": [ {
      "utf8": "over 7 total years will have passed for\nTerra, and over 4 for Stella."
    } ]
  }, {
    "tStartMs": 221534,
    "dDurationMs": 3232,
    "segs": [ {
      "utf8": "By the time Stella observes \nTerra’s second burst,"
    } ]
  }, {
    "tStartMs": 224766,
    "dDurationMs": 2700,
    "segs": [ {
      "utf8": "she will already be on her return journey."
    } ]
  }, {
    "tStartMs": 227466,
    "dDurationMs": 3160,
    "segs": [ {
      "utf8": "But now, since she’s moving towards the \nsource of the light,"
    } ]
  }, {
    "tStartMs": 230626,
    "dDurationMs": 2080,
    "segs": [ {
      "utf8": "it will take less time to reach her,"
    } ]
  }, {
    "tStartMs": 232706,
    "dDurationMs": 2520,
    "segs": [ {
      "utf8": "and she’ll observe the bursts \nmore frequently."
    } ]
  }, {
    "tStartMs": 235226,
    "dDurationMs": 3040,
    "segs": [ {
      "utf8": "This means that Stella observes Terra \naging slowly"
    } ]
  }, {
    "tStartMs": 238266,
    "dDurationMs": 1620,
    "segs": [ {
      "utf8": "for the first half of her journey,"
    } ]
  }, {
    "tStartMs": 239886,
    "dDurationMs": 2940,
    "segs": [ {
      "utf8": "but aging rapidly during the return half."
    } ]
  }, {
    "tStartMs": 242826,
    "dDurationMs": 3588,
    "segs": [ {
      "utf8": "Meanwhile for Stella, it seems as though \nTerra, the destination star,"
    } ]
  }, {
    "tStartMs": 246414,
    "dDurationMs": 2530,
    "segs": [ {
      "utf8": "and the whole universe are \nmoving around her."
    } ]
  }, {
    "tStartMs": 248944,
    "dDurationMs": 1730,
    "segs": [ {
      "utf8": "And because of length contraction,"
    } ]
  }, {
    "tStartMs": 250674,
    "dDurationMs": 4430,
    "segs": [ {
      "utf8": "Stella observes the distance between \nthem shrinking by a factor of 2."
    } ]
  }, {
    "tStartMs": 255104,
    "dDurationMs": 4025,
    "segs": [ {
      "utf8": "This means each leg of the trip will only \ntake about six years"
    } ]
  }, {
    "tStartMs": 259129,
    "dDurationMs": 1640,
    "segs": [ {
      "utf8": "from Stella’s perspective."
    } ]
  }, {
    "tStartMs": 260769,
    "dDurationMs": 4110,
    "segs": [ {
      "utf8": "When she sends the first signal to Earth, \ntwo years will have passed for Terra."
    } ]
  }, {
    "tStartMs": 264879,
    "dDurationMs": 3713,
    "segs": [ {
      "utf8": "Stella will send four more light bursts \nduring her outbound journey,"
    } ]
  }, {
    "tStartMs": 268592,
    "dDurationMs": 1940,
    "segs": [ {
      "utf8": "each one from farther away."
    } ]
  }, {
    "tStartMs": 270532,
    "dDurationMs": 4420,
    "segs": [ {
      "utf8": "By the time Terra observes the first pulse\nfrom Stella's inbound journey,"
    } ]
  }, {
    "tStartMs": 274952,
    "dDurationMs": 3066,
    "segs": [ {
      "utf8": "over 21 years will have passed for her."
    } ]
  }, {
    "tStartMs": 278018,
    "dDurationMs": 2220,
    "segs": [ {
      "utf8": "For the rest of Stella's return home,"
    } ]
  }, {
    "tStartMs": 280238,
    "dDurationMs": 3520,
    "segs": [ {
      "utf8": "Terra receives multiple light \nbursts each year."
    } ]
  }, {
    "tStartMs": 283758,
    "dDurationMs": 6050,
    "segs": [ {
      "utf8": "Thus, Terra observes Stella aging slowly \nfor about 90% of their 23 years apart,"
    } ]
  }, {
    "tStartMs": 289808,
    "dDurationMs": 2969,
    "segs": [ {
      "utf8": "and aging rapidly during the last 10%."
    } ]
  }, {
    "tStartMs": 292777,
    "dDurationMs": 4090,
    "segs": [ {
      "utf8": "This asymmetry accounts for why the \nparadox isn’t really a paradox."
    } ]
  }, {
    "tStartMs": 296867,
    "dDurationMs": 2088,
    "segs": [ {
      "utf8": "Although each twin witnesses time"
    } ]
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      "utf8": "both speeding up and slowing \ndown for the other,"
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  }, {
    "tStartMs": 301795,
    "dDurationMs": 2090,
    "segs": [ {
      "utf8": "Stella sees an even split,"
    } ]
  }, {
    "tStartMs": 303885,
    "dDurationMs": 4200,
    "segs": [ {
      "utf8": "while Terra sees Stella aging slowly for \nmost of the time they’re apart."
    } ]
  }, {
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    "dDurationMs": 3757,
    "segs": [ {
      "utf8": "This is consistent with each twin’s \nmeasurement of the space voyage,"
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  }, {
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    "dDurationMs": 5640,
    "segs": [ {
      "utf8": "which takes 23 Earth years, but only \n11.5 as experienced aboard the ship."
    } ]
  }, {
    "tStartMs": 317482,
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    "segs": [ {
      "utf8": "When the twins are reunited, Terra will be\n43 years old, while Stella will be 31."
    } ]
  }, {
    "tStartMs": 323262,
    "dDurationMs": 1122,
    "segs": [ {
      "utf8": "Where Stella went wrong"
    } ]
  }, {
    "tStartMs": 324384,
    "dDurationMs": 5070,
    "segs": [ {
      "utf8": "was her assumption that she and Terra had\nequal claim to being inertial observers."
    } ]
  }, {
    "tStartMs": 329454,
    "dDurationMs": 4794,
    "segs": [ {
      "utf8": "To be an inertial observer, one has to \nmaintain a constant speed and direction"
    } ]
  }, {
    "tStartMs": 334248,
    "dDurationMs": 2446,
    "segs": [ {
      "utf8": "relative to the rest of the universe."
    } ]
  }, {
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    "segs": [ {
      "utf8": "Terra was at rest the entire time, \nso her velocity was a constant zero."
    } ]
  }, {
    "tStartMs": 341144,
    "dDurationMs": 2915,
    "segs": [ {
      "utf8": "But when Stella changed her direction \nfor the return journey,"
    } ]
  }, {
    "tStartMs": 344059,
    "dDurationMs": 3960,
    "segs": [ {
      "utf8": "she entered a different reference frame \nfrom the one she’d started in."
    } ]
  }, {
    "tStartMs": 348019,
    "dDurationMs": 4290,
    "segs": [ {
      "utf8": "Terra and Stella now both have a better\nunderstanding of how spacetime works."
    } ]
  }, {
    "tStartMs": 352309,
    "dDurationMs": 2990,
    "segs": [ {
      "utf8": "And as twins who are eleven \nyears apart in age,"
    } ]
  }, {
    "tStartMs": 355299,
    "dDurationMs": 3480,
    "segs": [ {
      "utf8": "they’re a perfect example \nof special relativity."
    } ]
  } ]
}