0 Members and 1 Guest are viewing this topic.
For example, if a rocket leaves the Earth and flies away into space for a year at 0.866 of the speed of light, then turns round and comes back at 0.866 of the speed of light, that trip will take two years from the point of view of the people in the rocket, but four years will have run through on the Earth before the rocket comes back. If we do our analysis from the "frame of reference" in which the Earth is considered to be stationary, then the rocket was moving and its clocks were ticking at half the normal rate throughout both legs of its voyage. However, if we use a different frame of reference instead, we could then imagine that it's the rocket that is stationary during the first leg of its voyage while the Earth is moving away from it at 0.866 of the speed of light, and that would mean that the clocks on the rocket can now be thought to be ticking twice as fast as the clocks on the Earth throughout this half of its trip. During the second half of the rocket's journey though, the rocket will be calculated to be chasing the Earth at 0.99 of the speed of light to catch up with it, and its clocks will be reckoned to be ticking about three and a half times as slowly as clocks on the Earth. The end result will still be that the whole journey will take two years for the rocket (as recorded by its clocks) while four years will still have gone by on the Earth (as recorded by clocks there). So, while we have accounts of events that contradict each other as to when the different clocks were running faster or slower than each other, the most important numbers about how long the whole trip takes will always agree at the end of the process when the two parties are reunited - all accounts determine that the rocket records two years while the Earth records four.
I repeat to anyone who wants to listen that acceleration is absolute, and that we thus shouldn't change reference frames when acceleration is involved, but @David Cooper does the contrary in his Relativity page, which probably means that many readers here think the same, so I thought it might be useful to discuss it.
I think that changing reference frames in this case simply adds a useless complexity to the problem. When we feel an acceleration, we know we are accelerating, and we know the direction, so changing reference frame is like refusing to admit that we are accelerating even if we can feel it.
The earthbound observer that starts moving away knows it is not accelerating, and the one that knows he is accelerating is not moving away: where does this happen in real observations?
One of them is that the system contracts during acceleration, one of the features of relativity.
I repeat to anyone who wants to listen that acceleration is absolute, and that we thus shouldn't change reference frames when acceleration is involved, but @David Cooper does the contrary in his Relativity page, which probably means that many readers here think the same, so I thought it might be useful to discuss it.[...]
Things contract due to speed, not acceleration. I can have two objects with identical speed but one accelerates 1000 times as much as the other, and they’ll both still have matching times on their clocks when compared. If your simulation doesn’t show this, it is wrong.
Who ever said the Earthbound observer accelerates? You’re reading it wrong.
Considering a situation from a different frame does not add complexity, but is instead essential to understanding relativity, that physics works the same in any frame.
Yes, it is true that in the frame of the spaceship going out, Earth clocks are the ones dilated to half their normal rate. That is straight-up relativity theory.
Quote from: HalcWho ever said the Earthbound observer accelerates? You’re reading it wrong.If he doesn't accelerate, then he cannot write down that he is the one that starts to move away from the other observer, otherwise he would be falsifying the observation.
That works fine as far as only constant motion is concerned, but when acceleration is involved, it only serves to universalize the reference frame principle.
Relativistic phenomenon are not due to reference frames, they are due to the speed of light being limited. I don't need that principle to build my simulations, only that the speed of light stays the same in any direction on the screen, and that the particles move with regard to the screen.
Quote from: HalcYes, it is true that in the frame of the spaceship going out, Earth clocks are the ones dilated to half their normal rate. That is straight-up relativity theory.It is true only if we sweep acceleration under the rug, otherwise the traveling twin knows he has to reverse the data,
and he also knows how much younger he will be when he gets back
He doesn’t get back. He is stationary for the first half (never left home), and then he accelerates hard for the second half to catch up to Earth that has been moving the whole time, and never comes back.
Reverse what data? He does no such thing.
That doesn’t sound like relativity. It sounds like your screen is the preferred frame.
That’s fine since David never said that Earth ‘starts to move’. In the frame of the other observer, Earth was always moving and never started to move. There is no acceleration implied.In that frame, Earth clocks are dilated to half the rate of the ship clock. Your simulation should be able to show this.
But that assumption is wrong: the traveling twin must conclude that the home twin's age instantaneously increases during the instantaneous turnaround.
Quote from: MikeFontenot on 27/10/2018 15:12:35But that assumption is wrong: the traveling twin must conclude that the home twin's age instantaneously increases during the instantaneous turnaround.Unfortunately, the relativity principle is based on the assumption that instantaneous stuff is not part of this world.
The instantaneous age change of the home twin (according to the traveling twin), caused by an instantaneous velocity change by the traveling twin, is qualitatively quite similar to the very rapid age change of the home twin (according to the traveling twin), caused by a short finite acceleration by the traveling twin. So the instantaneous velocity change scenarios are quite valuable as good approximations to what happens when the accelerations are finite.
I think that changing reference frames in this case simply adds a useless complexity to the problem.
When we feel an acceleration, we know we are accelerating, and we know the direction, so changing reference frame is like refusing to admit that we are accelerating even if we can feel it.
In my simulations on motion...
The apparent paradox in the twin "paradox" scenario arises because it would seem that each twin should conclude that the other twin is ageing more slowly, due to the well-known "time-dilation" result, during the entire trip, except for the single instant at the instantaneous turnaround ... "surely" nothing could happen to peoples' ages during a single instant. But that assumption is wrong: the traveling twin must conclude that the home twin's age instantaneously increases during the instantaneous turnaround.
you can't know whether the planet's moving or not, so you're making a mistake if you assume that it's stationary in the absolute frame and that a rocket that accelerates away from it must be contracting in length and have its clocks run slow, because if the planet's actually moving in the absolute frame, that acceleration of the rocket could actually be a deceleration which leads to it moving more slowly through space rather than faster, with the result that it length-extends rather than contracting (i.e. it loses some existing contraction) and its clocks would run faster.
It adds unnecessary complexity, but it works. You aren't forced to use that approach, but you ought to recognize that I don't use it either. I only change frame to reconsider the same set of events from the perspective of different frames - you can't know whether the planet's moving or not, so you're making a mistake if you assume that it's stationary in the absolute frame and that a rocket that accelerates away from it must be contracting in length and have its clocks run slow, because if the planet's actually moving in the absolute frame, that acceleration of the rocket could actually be a deceleration which leads to it moving more slowly through space rather than faster, with the result that it length-extends rather than contracting (i.e. it loses some existing contraction) and its clocks would run faster.
You can't tell whether you're accelerating or decelerating - they both feel the same.
I don't think you should draw attention to your simulations in the physics forum until you have some that remove length contraction when you slow your particles back down instead of adding more contraction - they need to be modified until they match up to the real universe. They also need to produce the right amount of contraction, but I think they're currently just producing an unrelated compression that's never released.
What matters to us is that at the moment the ships pass each other, the boys disagree about the age of their twin sister. The one who is still travelling away from her thinks she's moving at 0.866c away from him while he is stationary, so she has only reached 6 years old, whereas he is now 7. His twin brother has a different opinion, because he think's he's stationary and that his sister is moving towards him at 0.866c, and he has worked out that when they are reunited, she will be 13 and he will be 9, which means that she must currently be 12.
Two features need to be present for a difference in the elapsed time to be measurable: acceleration and roundtrip.
All the other features are useless so they only serve to defend the reference frame principle against the idea that rotation is absolute.
It's also because light makes a roundtrip in the device we are using that we can measure our rotational motion. There is no other way than rotation to measure our own motion through space.
Relativity tells us why we can't measure our motion with a two arms interferometer, but it can't explain the twins paradox or the Sagnac interferometer as easily as with LET. The problem is the reference frame principle, it is superfluous in a roundtrip and the relativists keep using it.
To me, considering that the earth might be moving away only serves to discuss with relativists,
We can't know which direction through space the system is moving, but we certainly know which observer is accelerating, so it might lead to future mistakes if we don't take it as a data. We're only at our first steps through that relativistic stuff, so we will probably make huge leaps in the future. Keeping using useless complexities in this case is like stubbornly drawing epicycles while we already know better.
We don't have to know if we are decelerating or accelerating with regard to the universe to know we are accelerating away from somebody else. We know because we can feel our own acceleration, so we also know that the whole redshift is ours, and if we want to discover how relativity works at the particles' scale, I think we have to stick to that principle.
You may be right on that one, but you are still neglecting the way my particles move.
They move with regard to light, which is an absolute reference.
There is nothing more precise than light for two particles to move with regard to one another. We can't know if we are decelerating or accelerating anyway, so how could the particles know?
And if they knew, how could they change the way light travels between them during that time? Even if my simulation would produce the right amount of contraction, for instance while accounting for resistance to acceleration so that the contraction rate would be dampened a bit, reversing the acceleration wouldn't reverse the contraction.
Notice that reversing the direction of the traveling twin does not reverse the way his clock records time either.
On the other hand, my simulation with four accelerated particles shows that they can stay synchronized if they all move with regard to light to do so. This way, the arms of the MM interferometer would both contract, and we would still get a null result.
It wouldn't change the recorded time of a roundtrip either if the horizontal contraction rate would be the same as the relativistic one. So what would it change exactly except permitting us to study how the limited speed of the information may affect motion at the particles' scale, a step that the relativists are not ready to make since it seems to contradict the reference frame principle?
This just isn't true. Two clocks (or twins if you like) can go from events A to B simply via different routes/schedules, with no round trip involved, and the time discrepancy between them when they meet up again should be computable in any frame of reference. The answer is frame independent.
Unless you can record the relative speed at which they're encountering the light, all they ever get is a perceived frequency, so it's only a reference for the gods.
You effectively have a lightclock, so if you have the wrong amount of contraction, the time that it records will be wrong. You have to get this to match up to the real universe before it has any relevance to any useful theory.
Acceleration clearly has no role in these events - the differences in timing are entirely caused by the speed of travel through space making some clocks run slower than others.
Excuse me while I look for a swatter.