Time dilation is not about clocks or light, it’s about time itself.

Measures of time are simply different for different observers in motion relative to each other.

Time dilation is often described by saying “moving clocks run slow”. Can you see the problem with this statement? It infers that there’s one clock that’s right, and the rest are all slow, which totally violates the principle of relativity!

For relativity to hold true, the observer in a fast plane would feel nothing usual is happening whatsoever! The observer in the plane doesn’t experience slow motion or anything else strange like that. In fact, the watch on her wrist still ticks by as it always has. She does not notice anything unusual in her reference frame.

Einstein is the man behind the Theory of Relativity. Relativity is the idea that everything we see and observe is relative to us, and can be seen differently relative to someone else. Two people can have two different frames of reference, which can alter how they observe the world. Physicists use the term “event” to describe something that happens at a certain place and time (like lightning striking a pole for example). Events are used to analyze situations where relativity comes into play.

We have observed relativity to be true, but there are some rules—which we call the postulates—of relativity.

1) The laws of physics are the same in all reference frames

The first rule just tells us that no matter what frame of reference one is in, the laws of physics still apply to them just the same as someone in a different reference frame. Force still equals mass times acceleration no matter what frame you are in.

2) The speed of light in a vacuum is the same for all observers, regardless of their motion

The second rules tells us that light will travel at the same speed (c = 3.00 x10⁸ m/s) no matter what frame of reference one is observing the light in. For example, if you’re moving in a space ship traveling at 0.999c, and someone shoots a laser beam in the direction of your travel, you will still see it traveling at 3.00 x10⁸ m/s.

As we can see, these simple rules can lead to a whole lot of really weird things. Objects flying very fast passed other objects will have different perceptions of lengths and time! Weird! However to see any noticeable effect, the object has to be traveling near the speed of light!

So how does this apply to us here on Earth? It’s not every day you’re going to see a car traveling near the speed of light down the highway. One use scientists have found for utilizing the theory of relativity is in the analysis of very small atomic and sub atomic particles. Some radioactive elements will decay VERY fast (fractions of milliseconds in their own reference frame). But, if we speed these particles up close to the speed of light, in our reference frame they will last much longer!

The best way to show how time dilation works is using something called a light clock. The clocks on our walls use gears that rotate the hands slightly every second, minute, and hour. A light clock uses a pulse of light fired at a mirror some distance away, and measures how long that pulse of light takes to get back to the source. So if a pulse of light is fired at a mirror 1.5×108 meters away, the pulse will return to the source one second later (as seen by the source).

But relativity is about objects moving relative to other objects, so instead of watching the light pulse while stationary, let’s imagine the light source and mirror moving passed us near the speed of light. What will happen?

Well, according to the Second Postulate of Relativity, the speed of light is constant no matter what frame of reference is chosen. So in the frame of the light source, the pulse will travel straight out to the mirror and come straight back one second later. BUT, if the light source and mirror are moving passed an observer, they will see the pulse traveling along a diagonal path to the mirror, and along a diagonal path back to the source. Since the light travels a farther distance at the same speed, it will take longer for the light clock to tick! Moving clocks tick slower!

The video describes the classic light clock example. Once you see moving light clocks in action, it’s not hard to understand why moving clocks seem to tick slower. But how much slower? Using some complex trigonometry, physicists can actually calculate what they call the time dilation factor (or the Greek letter gamma). This allows physicists to do calculations in situations where speeds are high enough to alter time, distances, even energies!

The simultaneity of events can get complicated when talking about relativity. Einstein tells us that due to relativity, an event observed by two different reference frames is not observed as simulations if one frame is moving.

But what does that mean? It means, for example, that if you are running very fast (near the speed of light) while holding a rod with two identical flashing strobe lights on the tips, you will always observe the lights flashing at the same time in your own frame of reference. However, if you run passed one of your friends, they will not see the lights flashing at the same instant due to relativity.

So in the speeding train example, we have a similar situation. However, the roles are reversed. What we call the stationary observer (on the platform) sees simultaneous flashes at either end. When the speeding train goes by, the moving train observer does NOT see the flashes as simultaneous!

Other weird things will also happen. Not only will the you and your friend view time differently, you will also view the length of the rod differently! Your friend will think the rod is actually a shorter distance than you think! Now things are getting really wacky and cool!

Albert Einstein was born in a small town in Germany way back in 1879. He was only 21 years old when he published his first paper! However, it was in 1905 when he was 26 that his work really flourished. He published four separate papers all within a few months. In these papers he discovered the basics of photos, an experiment to test for the existence of atoms, a connection between electromagnetic theory and motion (relativity!), and the relation between mass and energy in his famous equation E = mc2.

In 1916 Einstein completed his theory on general relativity and reached a new understanding of gravity. As time progressed, Einstein became more involved in the new theory of quantum physics and the behavior of atoms.

Einstein spent his final days in Princeton contemplating a new theory which would unite all of physics, both very big scales and very small scales. Even 100 years later, his work is seen as nothing short of genius. Scientists are still working to complete what he began in his late life; a unified physics theory.

Two parallel lines can intersect if you are in non-Euclidean geometry. It’s hard to imagine this one being true, but it is!

If you take out a sheet of paper and draw two parallel lines, you notice that they will never cross. But what happens if you use a bigger sheet of paper? Will those longer lines ever cross? What about a sheet of paper the size of the room?

What if the paper was as large as Europe? How would you draw an airplane’s flight path between France and Switzerland? Or Ohio and India? What if the paper was the size of the Earth?

When you get to these sizes, you have to take into account the curvature of the Earth (something that regular old Euclidean geometry doesn’t do).  Mapmakers have been working at this puzzle for years: trying to draw something round (the Earth, or large parts of it) on a flat sheet of paper.

When the Mars Rover landed on Mars, it was 11 light minutes from Earth (meaning that it would take a radio signal 11 minutes to get from Earth to Mars).

If NASA sent a signal to the Rover saying “go left at 5 mph”, the Rover would get that signal in 11 minutes from when we sent it.

If a rock crushed the Mars Rover, we wouldn’t know about it for 11 minutes here on Earth.

If we knew that 5 minutes from now, a rock was going to crush our Rover, is there anything we would do about it? No. it takes our signal too long to get there. No action on Earth can affect anything on Mars for 11 minutes.

When one thinks about events happening with reference frames moving near the speed of light, he or she can come up with some wierd paradoxes. A paradox is an event which causes a logical impossibility in another frame. Paradoxes in relativity can get complicated, but Einstein’s theory of special relativity gives logical explanations of them.

For example, image a loaf of bread one foot long laying on a conveyer belt moving near the speed of light. Now imagine a butcher with two knives standing alongside the conveyer belt. In his frame of reference, he chops both of his knives one foot apart at the same instant. What happens as the loaf passes by? Well, according to classical physics (not relativity), if he chops down right as the loaf is in between his knives, he wont cut the bread (he will be very close!).

But what if we include relativity? The butcher will see the loaf traveling very fast, and thus he will see it as having a shorter length than when it’s at rest. So when he chops down he will have more clearance, and definitely shouldn’t cut the bread. Simple enough? Try switching reference frames. Now the bread sees the butcher approaching fast. Since moving objects shrink, his knives are less than one foot apart! Wont he cut the bread? Think about it while watching this visualization.

Thanks for the video to the Rocker Spaniels!

This is a complicated topic, so the explanations are complicated as well, but bear with it!

Do you think he will cut the bread? Wouldn’t this cause a logical impossibility if so? In the butcher’s frame he wont cut the bread, but in the bread’s frame he will? There’s one thing we didn’t account for. The butcher thinks he’s chopping simultaneously, BUT as Einstein told us, events in two different reference frames are NOT seen as simultaneous if one is moving. The bread will see the butchers knives chopping at different times! Different enough to not be sliced in either reference frame!

Now that we’ve used light clocks to show how the perception of time changes in different reference frames, let’s look at some really cool applications. Well what’s cooler than time travel? Since a clock moving fast by Earth seems to tick a little slower than a stationary clock on Earth, what happens if the moving clock is moving really fast for a long time?

Well depending on how fast the moving clock is traveling, just 10 seconds on the moving clock could be 100 years on Earth! This clock also doesn’t have to be moving passed earth like a spaceship flying by. It could simply be flying in an orbit around Earth, and never have to leave home!

Today, physicists have successfully accelerated particles to over 99% the speed of light! However, these particles are unimaginably small and have nearly zero mass. It would take massive amounts of energy to accelerate a spaceship to these speeds. But, if a new propulsion technique is invented, humans could theoretically sit in a spaceship for a month, and come back to a completely new Earth 100’s or 1000’s of years later!

Albert Einstein also predicted the existence of something called gravitational waves. He did this in his theory of general relativity in 1916, and the study still continues today.

What is a gravitational wave? Gravitational waves are ripples in the curvature of space-time itself, which propagate as waves away from the source of gravity. These sources are large bodies of mass, like a neutron star or a black hole.

We have seen indirect evidence of gravitational waves, but we still have not directly observed gravitational waves. In March 2014 however, an image produced by the Harvard-Smithsonian Center for Astrophysics appears to show evidence of the waves existence around the time of the big bang! Some further analysis is required before this conclusion can be made however.

Gravitational waves can be very useful in astrophysics. They can be used to make observations and measurements of very large objects, like black holes. The nice thing about gravitational waves is they appear to be unaltered by matter in the path of propagation. This means there is much less noise in detected signals, compared to traditional methods of measurement.