The crazy stuff that happens when something moves at 299,792,458 m/s.
Time is the fourth dimension in the fabric of spacetime. It’s the only dimension that extends in one direction. It’s compelling to believe that the hands on the clock tick the same for all of us and that we move through time at the same pace, but this isn’t actually true.
This is where my little friend called Special Relativity comes into the picture and messes things up A LOT!
The universe hasn’t really been understood yet. With each passing day, we make a couple advances into understanding how the Universe actually works. The Universe is in fact, a really good secret keeper.
Everytime we think we’ve got the answer, we end up in knots and paradoxes as each theory contradicts the next. Nonetheless, humans have been trying to comprehend the laws of nature that govern the world around us.
If you’ve ever taken a high school physics course, then you know that someone named Sir Issac Newton invented calculus and the three laws of motion upon which all mechanics is based.
His work gained immense popularity and all of physics were dominated by his ideas for a long long looooong time.
He later formulated the law that dictates gravitational force, and how two objects having mass will exert a force on each other proportionate to the mass and inversely proportionate to the distance between them.
Finally, something explains the way our universe works.
But remember, our Universe is reaally good secret keeper. Turns out the Newtonian physics everyone came to love and adore, had several inconsistencies that couldn’t be explained using these rules. The parts of the world we interact with during our lives move at super slow or even negligible speeds. This situation turns upside down when things start getting fast. speed of light fast. That’s when things start getting pretty mind boggling weird.
Relative Speeds
All speeds are relative. Except the speed of anything moving at 670,616,629 miles per hour. That turns out to be the speed of light .
Note that light doesn’t violate the “all speeds are relative rule” just because it’s light or any intrinsic property it posesses, but because of the speed at which it propogates.
This speed is universe’s speed limit. Nothing can move faster, in any inertial reference frame.
This violates Newtonian physics because according to Newtonian physics, if you were to shoot a flashlight and have your friend Jim run at 5 mph in the same direction as the flashlight, Jim should observe light moving slower from his perspective because According to Newtonian physics the resultant speed of the light should be c-5 (c is the shorthand for speed of light) as both of them travel in the same direction.
Light however, has a fixed speed so Jim doesn’t see light moving at c-5 but instead at c. This is interesting because the laws of motion we were used to growing up, fail.
Consider the same scenario but now, Jim is running opposite to the direction of the light at the same 5 mph. When he looks back he notices that light isn’t moving at c+5 mph like he thought it would but it’s moving at c instead. This result would be the same if Jim travels at 100,000 mph or even 600,000,000 mph for that matter. Jim sees the light moving at the speed of c, no matter what speed he moves at.
That’s what’s so interesting about special relativity.
The speed of light is constant no matter what speed the reference frame is.
This is the fundamental rule that guides the Theory of Special Relativity.
It’s what clouds everything we’ve ever learnt so far and what one could only understand it if they loosen their perceptions of already well known facts.
Ready?
Another thought experiment about a really long rod coming right up.
Let’s take a rod that’s 186,000 miles long. Now, we attach two mirrors on either side. On one side we attach a light emitter and detector.
When a light particle is emitted from one end of the rod reaches the other end, it gets reflected back to the side it had been emitted. Without even knowing it, we made a clock! How?
Light takes 1 second to move across a distance of 186,000 miles. Since the light particle takes 2 trips across the rod (one for reaching point B from point A and another for reaching Point A from Point B) it takes the light particle 2 entire seconds to make a round trip.
Lets start moving this rod at a speed close to the speed of light. We now have two choices regarding the orientation of the rod during its motion.
- The Rod is perpendicular to the direction of motion.
- The Rod is parallel to the direction of motion.
Rod is perpendicular to the direction of motion
In scenario 1, when the rod is oriented perpendicular to the directon of motion the light particle still moves up and down the rod but since the rod is moving at a constant speed close to the speed of light, the light particle needs to catch up with the other end of the rod as it keeps fleeing forward.
In other words, the light particle needs to travel extra distance to reach the other end of the quickly moving rod on time.
Keep in mind that we are still looking at this rod, so according to the ‘speed of light is constant in all inertial frames of reference’ rule, we are the inertial frame of reference so, the light in the rod should move at a constant speed for us..
but light needs to travel an extra amount of distance in the same amount of time ( which should change it’s speed) because of the rod moving suuuper fast. We arrive at a conundrum.
We know that Speed = Distance/Time right?
1. Speed is constant. We know that for a fact.
2. The Distance that the light needs to travel has increased.
This can only mean one thing.
The time taken by light to travel that distance HAS TO INCREASE. This is what Einstein proposed.
It now takes much more than 2 seconds for a round trip of the light particle. This is called Time Dilation. Time Dilation refers to the Dilation or Expansion of time.
This is huge. What we just proved from our thought experiment is that light to an observer such as us, in an inertial reference frame slows down when it moves at a very high speed.
This isn’t just applicable to light in particular. It applies to everything moving in comparable speeds to light. You might think that light is ‘special’ and that’s why all this wierd stuff happens with light, but remember what I told you earlier.
Note that light doesn’t have any intrinsic property that differentiates it from other phenomena, but because of the speed at which it propogates.
But how can the Special Relativity Principle force this on everything else?
The light clock, I understand that all that wierd mumbo jumbo happens but why would any other object travelling at the speed of light follow the same rules? IT JUST DOESN’T MAKE SENSE?!
Special Relativity forces systematic adjustments in almost every branch of physics. These adjustments will have the effect of forcing every clock that to slow when it is set in motion.
For example, a mechanical clock keeping time by counting the oscillations of a mass bouncing on the end of a spring is also affected.
A digital watch that works by counting the oscillations of a crystal is also affected in the same way. The physics of all of these systems will be altered slightly by the principle of relativity and in a way that will slow these clocks by the same amount as a light clock.
Even if you were to move at 99.5% of the speed of light, time would slow down for you when compared to someone stationary on Earth. Your heart would beat slower, and each blink of the eye you take would take longer. The most interesting thing is that you wouldn’t notice that time slowed down for you.
Rod is Parallel to the direction of motion
In scenario 2, when the Rod is placed parallel to the direction of motion, we know that Time Dilation still occurs and time still slows down by the same amout it slowed down by when the rod was perpendicular.
This is because all we did was change the orientation of the rod. Right?
The light has to go from one end to the other of a 186,000 mile rod. The light moves at c, 186,000 miles per second. But the rod is also moving in the same direction at almost the speed of light, so the light has to chase after a quickly fleeing end and will need much more than a second to catch it. With a little arithmetic it turns out that the light will need much more time to reach the other end when compared to it’s paralell counterpart.
Does this mean that the paralell clock’s time move slower than the clock placed perpendiclar to the direction of motion? But this can’t be possible because there can only exist one time for bodies moving at the same velocity.
So, according to Speed = Distance/Time
Speed is constant. Time is constant. Distance is what changes. Length of the rod will get shortened so that light can make the other end of the rod without compromising on speed and time. Note that this contraction only occurs in the direction of motion. Wait what? The length of the rod gets shorter? I told you things get pretty intense. It looks like this:
Observers such as us from an inertial frame of reference observe the shortening of the rod, but for someone travelling with the rod, it would appear as though the rod hadn’t changed.
It’s all relative.
The funny thing is that when stuff moves at the speed of light all this strange stuff happens. Remember when I told you earlier that the Universe is a really good secret keeper? There are still a couple incosistencies and paradoxes that The Special Theory Of Relativity can’t explain but we’re only getting closer to our comprehension of the Universe.
