Most of us who are fans of the superhero Flash
would have wondered; how fast can he go? There is a very strict speed limit
that he has to adhere to and that is not just because he has signed some
contract with the production company. This speed limit comes from the inherent nature of reality itself and is none other than the speed of light. One may then be led to ask how did this speed of 3*10^8 m/s become
the speed of our wide wild world anyway? Many of the physics geeks may already
have guessed it: The Special theory of relativity. Before we plunge into this
weird and complex part of special relativity, we must dip our toes into the old
relativity. What is relativity? Relativity is sixes and
nines, allow me to explain. Suppose you write the number 6 on a piece of paper.
For you, it is the number six but to a person standing on the opposite side of
the paper, it will be nine. We also experience relativity every day with motion. When
you are on a train, you and the other people on the train
seem to not move whereas the world and the people outside seem to be moving
backwards. Whereas if you are a person on the platform, you would see all the
people in the train moving past you. Let’s complicate this just a little,
let’s say that a person on the train gets up and starts moving in the direction
of the train. To the person sitting in the train, it would look like he is just
walking at a normal walking speed whereas to the person on the ground, the
person walking in the train is travelling at his speed plus the speed of the
train. So you see? It’s all relative. Newton’s and Galileo’s most prominent
contribution to relativity was that nothing is absolute. We are used to
measuring speeds and direction with respect to the Earth but isn’t the Earth
also moving with respect to the sun and isn’t the sun also moving? Wouldn’t it
be a weird reply to the question “Do you know what speed you were travelling
at?"
"Relative to Earth, the Sun, or the Milky
Way?" This all seems to make sense, doesn’t it? But along came James Clerk
Maxwell announcing that the speed of light is constant for all observers. This
very proclamation shattered the physics community and thankfully Einstein came
along to fix it with all his time travel, length contraction, etc.
So how in The Special Theory of Relativity
does light become the speed limit of the universe? It all has to do with mass.
E=mc2? No, it has to do with the equation:
The m' (m prime) in the equation is mass you observe if the object is moving past you and the m is the mass that you measure the object has when it is stationary relative to you. What this equation means to say is
that the faster you go, the more your mass increases. Well, so you get heavier?
Not exactly.
What most people get wrong is that this equation is talking about
inertial mass. Inertial mass is also a phenomenon observed in our everyday
lives. What would be easier to get rolling a real boulder or a hollow plastic
one of the same size? The plastic one. And which one is easier to stop? Again,
the plastic one is. We can infer from this that the more the mass increases, the
harder it is to change its state of motion. That is inertial mass. You can make sense of the concept that inertial mass increases when an object is traveling faster if you carefully observe the equation. The relativistic mass as in the mass you observe an object to have if you move past it is on the left hand side and the stationary one on the right. The equation shows that the relativistic mass is equal to the stationary mass divided by something. Since it is such, shouldn't the relativistic mass be smaller than normal mass as division makes a number smaller. Not if you divide by a number smaller than one and greater than or equal to zero. Luckily for us the denominator is within that range as velocity can never be greater than the speed of light.
Again, why is that? I haven't explained it have I? If you plug
in the velocity to be the speed of light, one’s inertial mass becomes infinite. That is
to say that it would require infinite force and energy to slow down or speed up. And that
is why Flash has to adhere to this speed limit and also the reason why you
can’t slow down light no matter how much force you put against it.
Hi Aarit
ReplyDeleteYou have made a good attempt to explain a complex Physics topic and since you tried to explain a lot of things in one post, the post can be a bit confusing.
Great post, keep it up.
Reading your post, I understand that if (inertial) mass increases with speed and will tend to infinity at speed of light, then nothing (at least nothing with mass) can exceed speed of light).
I don't know how the scientists came up with inertial mass equation and it will be an interesting read because, as the post says, if all speeds are relative, we may be moving at a very high speed relative to something and in that case we should already feel our (inertial) mass to be very very large.