Special Relativity's spaceship is not recommended for humans: Weight and body dimensions of a returning twin Ramzi Suleiman University of Haifa & Al Quds University Abstract In the present note, we consider the relativistic effects, according to SR, on the "travelling" twin's body dimensions and weight. Keywords: Twin Paradox, Special Relativity, Time dilation, Lorentz contraction. Almost all applications of Einstein's Special Relativity (SR) theory [1] to the twin-paradox gedanken experiment, originally proposed by Langevin [2], have focused on the time dilation effect (e.g., [3-5]). This effect prescribes that the "traveling" twin will return to Earth younger that the "staying" twin. The ratio between the increase in age of the staying twin relative to the increase in age of the travelling twin is predicted to be equal to the Lorentz factor γ = 1 √1− (   ⁄) 2 . In the present note, we consider the relativistic effects, according to SR, on the "travelling" twin's body dimensions and weight. Consider the case in which Alice leaves her identical twin Bobbie and travels with very high constant velocity, β =   ⁄ = 0.99, to a distant star, and returns back to Earth. Upon leaving, both Alice and Bobbie are 180 cm. tall, 40 cm. hip-width, and 60 kg. weight. During the entire travel, Alice lies with her body stretched on a bed positioned parallel to the travel direction. Suppose the entire trip lasts one year according to Alice's time. According to SR, Alice should return to Earth about 7.1 years younger than Bobbie (and all other creatures on Earth). Moreover, due to distance contraction (l = 0  ⁄) , she will become approximately 25.4 cm. height, much shorter than Bobbie (180 cm), while her hip width will stay the same (40 cm.). If, on the other hand, Alice's bed is positioned perpendicular to the travel direction, she will return as tall as Bobbie (180 cm.), but her hip width will shrink to about 5.6 cm. Positioning the bed at various angles between 0 to  2 will produce the heights and hip widths depicted in equations 1 and 2 (see also Figure 1). For all angles 0 ≤ α ≤  2 , Alice's mass is predicted to increase by a factor of γ, causing her weight to increase from 60 kg. to 60 γ ≈ 425.3