Relativity does not mean that everything is relative. That is the short answer to the question this MinutePhysics video poses, and it is the most common misunderstanding of Einstein’s best-known theory. Relativity says that some measurements, such as how long something takes or how long an object is, depend on how the observer is moving. But it is built on things that do not change for anyone: the laws of physics and the speed of light.
About the video and MinutePhysics
MinutePhysics is a YouTube channel created in 2011 by Henry Reich, who studied mathematics and physics at Grinnell College and later earned a master’s degree in theoretical physics through the Perimeter Institute. Its videos are drawn as fast whiteboard animations, and the early ones ran about a minute, which gave the channel its name. Reich and his family later launched the sister channel MinuteEarth.
“Relativity Isn’t Relative” was shared widely in April 2014. Its title is a deliberate twist: the theory’s name suggests that nothing is fixed, while the physics is really about what stays the same when you change your point of view.
What the theory of relativity actually says
The starting point is the principle of relativity: the laws of physics take the same form for every observer moving at a steady speed in a straight line. No experiment done inside a smoothly moving train or spaceship can tell you whether you are “really” moving or at rest.
Einstein added a second rule in his 1905 paper on special relativity: light in a vacuum travels at the same speed for every observer, whatever the motion of the source or the observer. Combining those two rules forces something surprising. If everyone must measure the same speed of light, then time and distance cannot be the same for everyone. Moving clocks tick slower as seen by someone at rest, and moving objects are measured as shorter in their direction of motion.
General relativity, developed between 1907 and 1915 and published in its final form in 1916, extended the idea to gravity. It describes gravity not as a force pulling across empty space, but as the curvature of spacetime by mass and energy.
What changes between observers, and what does not
It helps to sort the quantities into two lists.
Depends on the observer
- How much time passes between two events
- The length of a moving object
- Whether two distant events happen “at the same time”
- An object’s speed, kinetic energy and momentum
The same for everyone
- The speed of light in a vacuum
- The laws of physics themselves
- The spacetime interval, a combination of distance and time between two events that every observer calculates to be the same value
- The order of events that can affect each other: if one event causes another, every observer agrees on which came first
That second list is the heart of the matter. Relativity gives a precise rule for translating one observer’s measurements into another’s, and the translation always preserves the invariants. Physicists usually find it more useful to think in terms of those fixed quantities than in terms of the changing ones.
These effects are not just thought experiments. Satellite navigation systems such as GPS have to correct their onboard clocks for both the speed of the satellites and their height in Earth’s weaker gravity; without those relativistic corrections, position fixes would drift quickly.
Where the misleading name came from
Einstein did not choose the name. In 1906 Max Planck described the new theory as a “relative theory” (Relativtheorie), stressing the principle of relativity, and in the discussion of the same paper Alfred Bucherer used the term “theory of relativity” (Relativitätstheorie), which stuck.
Einstein is often reported to have said later that a name such as “theory of invariants” would have described it better. Whatever his exact words, the point is fair: the theory is about which physical facts survive a change of viewpoint, not about everything being a matter of opinion.
Common myths about relativity
- “Einstein proved everything is relative.” He showed that certain measurements depend on motion, and that the laws behind them are the same for everyone. Moral or cultural relativism is a separate philosophical idea that does not follow from physics.
- “Time dilation is only an illusion.” Clocks that have travelled at high speed, or sat at different heights in a gravitational field, really do show different elapsed times when they are brought back together.
- “Relativity allows anything if you look at it the right way.” The theory is strict. It forbids information travelling faster than light and keeps energy and momentum conservation in a consistent form, which is why it rules out perpetual-motion and “unlimited free energy” claims rather than supporting them.
Getting the most out of the video
MinutePhysics videos are dense, so it is worth watching twice: once for the overall idea and once with pauses to follow each drawing. As you watch, try to keep the two lists above in mind and ask, for each example, which quantity is changing and which is staying fixed. If the video sparks questions, the channel has many follow-up episodes on spacetime, light and gravity that build on the same ideas.
Minute Physics on YouTube.
Sources: MinutePhysics (Wikipedia); Theory of relativity (Wikipedia); Laughing Squid, April 2014; MinutePhysics on YouTube.
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