Weekly links

Written on 5 October 2015, 02:43pm

Tagged with: , ,

Inaugurating a new post type – a collection of links to interesting articles/stories. Aimed to be updated weekly 🙂

“If a gravitational wave were to pass through you now, this ripple in spacetime would stretch you taller and thinner, then squash you shorter and fatter. The reason you wouldn’t notice is because your height would be altered by less than the width of a proton (a fraction of the size of an atom).”
Cosmos Magazine – Einstein’s gravitational waves remain elusive

Every picture ever made during the Apollo moon missions has been made available on Flickr through the Project Apollo Archive.
Washington Post – Over 9,000 Apollo moon mission pictures are now online

“All of these feature were built for one reason — a self driving future combined with an entire self-driving mobility platform. The Model X was built to be either the ultimate self-driving taxi, or the ultimate human/self-driving rental car — or both.”
Gavin Sheridan – Elon Musk’s sleight of hand

Time dilation

Written on 7 August 2015, 02:47pm

Tagged with: , ,

The passing of time can be altered by 2 factors, gravity and velocity.
Increase in both gravity or velocity will slow down time.
1. Gravity: Everything likes to live where it will age the most slowly, and gravity pulls it there (Kip Thorne – the Science of Interstellar)
2. Velocity: A space traveler approaching the speed of light will age slower than another one staying on the earth (Twin paradox).
What does this mean?

On the ISS, time runs slower because of the high speed

Altitude: about 400km
Orbital speed: 8km/s
Time dilation: 0.01 seconds slower every year
The smaller gravity pull from the Earth accelerates the time, but not enough to compensate for the velocity of 8km/s which slows down time. So in the end, an astronaut spending 6 months on the ISS will be about 0.005 seconds younger.

[On the ISS] Velocity has increased for the astronauts, slowing down their time, whereas gravity has decreased, speeding up time (the astronauts are experiencing less gravity than on Earth). Nevertheless, the ISS astronaut crew ultimately end up with “slower” time because the two opposing effects are not equally strong. The velocity time dilation is making a bigger difference, and slowing down time. The (time-speeding up) effects of low-gravity would not cancel out these (time-slowing down) effects of velocity unless the ISS orbited much farther from Earth.

On the GPS satellites, time runs faster because of the decrease of gravity

Altitude: 20.000km
Orbital speed: 4km/s
Time dilation due to velocity: 7 microseconds per day
Time dilation due to less gravity: 45 microseconds per day

The combination of these two relativistic effects means that the clocks on-board each satellite should tick faster than identical clocks on the ground by about 38 microseconds per day (45-7=38)! This sounds small, but the high-precision required of the GPS system requires nanosecond accuracy, and 38 microseconds is 38,000 nanoseconds. If these effects were not properly taken into account, a navigational fix based on the GPS constellation would be false after only 2 minutes.
The engineers who designed the GPS system included these relativistic effects when they designed and deployed the system.
GPS and relativity
[Contrary to popular belief, GPS satellites are not in geosynchronous or geostationary orbits). The satellite orbits are distributed so that at least 4 satellites are always visible from any point on the Earth at any given instant (with up to 12 visible at one time)]

Inspired by: The Science of ‘Interstellar’ Explained (Infographic)