cavediver writes:
Sorry Oni, missed this, so only a month late! But there was a few issues with the reply you received - sorry kbertsche
- so better late than never...
kbertsche was exactly right in saying that there is plenty of confusion over the term mass, and that the mass in which we are most interested is the rest-mass of an object.
I'm sure you're aware from SR that we have time dilation and length contraction. These are purely observational effects, resulting from observing 4d space-time from a 3d perspective. But these effects will distort the results of any mechanics we calculate for an observed object travelling at relativistic speeds. If we know the rest mass of the object, the distorted time and distance measurements will make us calculate an altered "relativistc" mass. But this is purely observational and is observer dependent - we can simply match speed with the object, and the relativistic mass disappears - it has been "transformed" away. Thus it is not "real" and has no gravitational impact.
Exercise 1: Can we imagine a scenario where the "relativistic" mass of something* cannot be transformed away? And so could have a gravitational effect?
* hint - requires the right sort of something
==============================================
OT comment for Oni: Being such a Bill Hicks fan, do you like Tool?
Your saying that those are purely observational effect shows the root of our disagreement. Your understanding of relativity is that one perspective is intrinsically real and the other is not. That is not so simple. Can you say that the ground under your feet is intrinsically curving yet apparently flat? Which is more of an impression here, flatness or curviness? I guess it's both real and apparent as a function of its distance from you as an observer. The closer you come to the earth the flatter, it grows in one perspective, yet that again would depend on your starting point.
For, in another perspective if you start from observing the earth as a sphere while gradually increasing the distance from it, it may grow flatter again first becoming a flat disk to merge eventually with the apparent flatness of the space serving it as the background and to shrink into a point before vanishing from your view completely.
So the earth could be then said to be once round and twice flat.
Which is which entirely depends on the arbitrary chosen perspective again.
What you can say legitimately is only that the earth is really not torus-shaped piece of green cheese.
The same goes for the length contraction, time dilation and mass transformation. Strictly speaking claiming that an object may have any intrinsic length, shape, mass or rate of ageing other than to itself comes only from the long habit of observing that object in close proximity or in the rest frame shared with the observer. Only while the magnitude of the effect is negligible for all practical effects and purposes. Rest is as relative as distance while being but uniform motion relative to something else assumed at rest again and all motion is but breaking of a uniformity of rest.
That's where your ludicrous claim that the universe has a real age you pretend to measure. It's age may again depend on you distance from it. What is your distance from the universe is a nonsensical question making any certain answer no wiser than was the question itself.