This Day in Tech History

On This Day . . .

It’s All Relative

Albert Eintein in 1904, clerc at the Patent Office in Bern, Switzerland.

December 16, 1915: Einstein Publishes General Theory of Relativity

The theory of relativity transformed theoretical physics and astronomy during the 20th century. Einstein’s contemporaries did not all accept his new theories at once. However, the theory of relativity is now considered as a cornerstone of modern physics.


The genius of Einstein’s discoveries is that he looked at his experiments and assumed the findings were true. This was the exact opposite of what other physicists seemed to be doing. Instead of assuming the theory was correct and that the experiments failed, he assumed that the experiments were correct and the theory had failed.


General relativity has been confirmed many times, the classic experiments being the perihelion precession of Mercury’s orbit, the deflection of light by the Sun, and the gravitational redshift of light.


In GPS systems, the gravitational field of the earth has to be accounted for. GPS systems work with such precision because of the Theory of Relativity.


Einstein’s theory of special relativity created a fundamental link between space and time. The universe can be viewed as having three space dimensions — up/down, left/right, forward/backward — and one time dimension. This 4-dimensional space is referred to as the space-time continuum.

If you move fast enough through space, the observations that you make about space and time differ somewhat from the observations of other people, who are moving at different speeds.

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You can picture this for yourself, imagine that you’re on a spaceship and holding a laser so it shoots a beam of light directly up, striking a mirror you’ve placed on the ceiling. The light beam then comes back down and strikes a detector.


The spaceship is traveling at a constant speed, let’s say half the speed of light. According to Einstein, this makes no difference to you — you can’t even tell that you’re moving. However, if astronaut Amber were spying on you, as in the bottom of the figure, it would be a different story.

Amber would see your beam of light travel upward along a diagonal path, strike the mirror, and then travel downward along a diagonal path before striking the detector. In other words, you and Amber would see different paths for the light and, more importantly, those paths aren’t even the same length. This means that the time the beam takes to go from the laser to the mirror to the detector must also be different for you and Amber.

This phenomenon is known as time dilation.

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Some of the consequences of general relativity are:

  • Clocks run more slowly in deeper gravitational wells. This is called gravitational time dilation.
  • Orbits precess in a way unexpected in Newton’s theory of gravity. (This has been observed in the orbit of Mercury and in binary pulsars).
  • Rays of light bend in the presence of a gravitational field.
  • Rotating masses “drag along” the spacetime around them; a phenomenon termed “frame-dragging”.
  • The universe is expanding, and the far parts of it are moving away from us faster than the speed of light.


James Chin-Wen Chou of the National Institute of Standards and Technology explained that: “If you are experiencing stronger gravitational pull, then your time is going to go slower”, gravity actually slows down time.

Chou ran an experiment in which he had two clocks set to the same time. One was on a staircase and one was on the ground. They discovered that gravitational difference is enough that the higher clock started to tick slightly behind the lower one. Okay so not as impressive as a rocket to the moon. But still pretty cool, right. So next time you’re standing at the top of the stairs, stop for a moment and remember that the moment you just took is just a little bit longer than it was at the bottom of the steps.  (for details of the experiment)






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