Originally posted by BLKMJK
... I still do not understand WHY or HOW mass increases as the velocity increases. The more I think about it the more tangled up in my head I get.
I hope this will make sense, but I'm gonna give it a shot...
The most basic side-effect of Special Relativity is actually something that a guy by the name of Lorentz came up with before Einstein ever got in on
the action: length contraction
. According to both Lorentz and Einstein, as an object speeds up, it shrinks in the direction of motion - that
is, effectively, it gets shorter.
Take a train, for instance. As the train speeds up along the track (and assuming the track is in a straight line), it gets shorter. Don't worry about
why the train gets shorter... this is merely an artifact of nature keeping the speed of light constant (you'll see why/how in a minute). At regular
train speeds, the shortening isn't measurable. But, if the train were to speed up to near the speed of light, its length would shrink dramatically,
until finally, at the speed of light, the train becomes 2-dimensional (it still has width and height, but its length has contracted to 0).
Now, all of this is according to an external observer. But, what about a passenger on the train? Well, according to them, it's not the train that's
moving at the speed of light - it's the stuff outside that's moving by them at the speed of light. So, according to someone inside the train,
everything outside has shrunk to a distance of 0 in the direction of motion.
Let's say, now, that the train is moving near, but not quite at, the speed of light. This means that the distance to its destination (wherever that
might be) hasn't quite shrunk to 0, but it is a lot closer now than it would be if the train were at rest,
To better illustrate this... say you're in a spaceship travelling to a star named Star (I used this example in the other thread I linked to in my
last post). And, let's say Star is 100 light-years away according to an observer on Earth. In your spaceship, you leave Earth and travel towards Star
at very near the speed of light. At this near-luminal velocity, if you were to measure the distance between the Earth and Star now, you might find
that it's 2 light-years, as opposed to the 100 light-years measured by someone at rest.
Because, according to you, the distance between the Earth and Star has decreased dramatically, it doesn't take you nearly as much time to get to
Star. According to an observer on Earth, it would still take you about 100 years to get to Star, travelling at very near the speed of light, but,
according to you, it only takes about 2 years.
This is time dilation
, and it is caused by length contraction (according to you, the distance between Earth and Star has contracted).
But, there is another side-effect of this. If, according to you, the distance between the Earth and Star has shrunk, you could also say that, instead,
you spaceship has gotten longer (this is the opposite of what an external observer would say - they would say that your spaceship is shorter, just as
the train from before got shorter - but it's the basis of relativity
... it's all relative). The density of your spaceship has stayed the
same, but it has gotten longer, so this requires an increase in the mass of the spaceship. This is mass dilation
If any part of that is unclear, feel free to ask me, and I will do my best to clarify what I can