salvestrom
- 226
- 0
John232 said:... It would be as though each frame was saying that the other frame had accelerated, but it didn't.
... They can't have two equally valid proper times. If they had two equally valid proper times it would be like saying there are parrallel universes.
Special Relativity provides maths that demonstrates how proper time is always 1/1 for any observer within their frame of reference. It also shows how that observer will look at other observers and say that their time is less than 1/1. These observers will be looking back at him observing that his time is also less than 1/1. This apparent paradox exists as long as there is a relative velocity between them. Imagine this all occurring in a universe devoid of stars and planets so that the only point of reference these observers have is each other. Only afterward, when these observers regroup and compare their clocks will any sense of who was moving become clear. The difference in the clocks will depend on the duration of the relative velocity as well as the magnitude.
In the twin paradox, if, halfway thru the journey, the stay-at-home twin decides to go after his brother, while other twin decides to "stop", then when they meet their clocks will have measured almost the same total amount of time. The only difference will be that while the stay-at-home twin was at rest in a gravitational field, his brother's been waiting at rest in a far weaker field. But we aren't talking a lot here, not compared to two-thirds c. But let's put aside gravity. And let's put aside acceleration. It's been shown that if the stay-at-home twin makes a shorter journey, but experiences the precise same decceleration and acceleration as his brother has while doing so, the returning twin will still be significantly younger.
But why is one twin younger if they see each other's clocks as running slower ? Because while both have been at rest from their own point of view, they have had different velocities. Different velocities at rest. Now there's a contradictory statement. But you can't say much else about what those at rest velocities are. Only the difference between them, which can be mathematically applied to either and makes a pretty clear statement about how we percieve reality. Because you can't tell how fast you are already going. Think you're sitting still at your keyboard? Guess again. Mother Earth's got a pretty good 100m record: 0.0037s. 27km/s around the sun we go. The sun blazes a trail about the galaxy at 220km/s. Helios's chariot's got some serious rpm. And the galaxy is headed toward the Virgo Cluster at 600km/s.
We share these velocities, which work with and against each other over the years. During one part of our orbit we are headed roughly the same way as the sun is. Six months later we're going the opposite way. Ever casually noticed it? Nope. Noone has. Our velocity relative to the galactic core varies from 193-247km/s. Yet this makes no difference to how we record time and distance. Noone has declared that their rulers are shorter in March compared to in August. Because, even if they appear that way to someone observing us from the galactic core, they always seem the length that they should be to us.
So, you can treat one observer as not having any velocity at all, and just use the difference. But it rather tidly turns out you can arbitrarily chose either one to be at rest and still get accurate information about what they each percieve. In fact, in everything that Special Relativity has to say about the entire concept the only time it ever really matters who was moving is when you invoke the extreme and unlikely situation of the twins.
Last edited: