MadSci Network: Physics

Re: Confusion about relativity and mass

Date: Tue Feb 22 17:46:34 2000
Posted By: Layne Johnson, Undergraduate
Area of science: Physics
ID: 951106420.Ph

Hello, Judson.

First question first - if an object and its observer were both moving at a 
velocity near enough to the speed of light that a noticable mass increase 
took place, would the observer detect the change?

No, he wouldn't.  To understand this, let's consider an example that is 
happening right now.  The universe is expanding at a very fast rate.  Parts 
of it are moving away from us at speeds close to the speed of light.  Let's 
consider that the point of reference is on one of those far off points, and 
it is the earth that is moving at 99%c compared to it.

From that distant observation post, time on earth has slowed down.  Mass 
has increased, and all of the other strange effects associated with 
near-light velocities are taking place on the earth right now.  But because 
we are at rest relative to our planet, we don't detect any of this.  
Instead, we see the far off observation post as shrinking, slowing, and 
behaving strangely.

Second question - why does acceleration become more difficult as an object 
approaches c, since the object being accelerated seems to be at rest to 

To understand this question it is necessary to understand a little about 
the nature of space and time.  They are not absolutes.  Both can be 
contracted by the presence of mass and velocity.  The more massive an 
object is, or the faster it is moving, the more time and space are warped 
by it.  This is over-simplified, but the main idea holds true.

It is not only the accelerating object that experiences change as it 
approaches the speed of light, the very fabric of space and time around 
it experiences it, too.  To an observer within the space and time being 
affected, the object does not seem harder to accelerate.  To him, it's 
standing still, and Newton's laws about motion apply.  It is only to us, on 
a observation point outside the space and time in question, that it seems 
harder to accelerate.

Think of it this way - if a portion of space shrinks so that what was 10 
meters now only takes up 5 meters, then an object accelerating through it 
at 1g (10 meters per second, every second) appears to an outside observer 
to only have accelerated at half that speed.  To us, the area it traveled 
across is only 5 meters.  But the object shrinks to the scale it enters 
into, so an observer on the object would think he had traveled a full 10 
meters.  Once again, everything in relativity is relative!  It all depends 
on where you are when you watch.

For a more in depth answer to these questions, you may find PBS's NOVA 
website helpful.  The section on Albert 
Einstein has the easiest to understand explaination of relativity I've 
ever found.  Another excellent explaination without all the math is the 
relativity channel.  

I hope this answers your questions.

Layne Johnson

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