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Post by Progenitor A on Dec 13, 2010 9:13:27 GMT 1
Time Dilation With Velocity -Hoary Old Chestnut
We all know that time dilates (expands, slows down) as a function of velocity.
Thus if we have 2 spaceships travelling at different velocities, we will have different time dilation won’t we? The faster of the two space ships will have time running the slowest, so the spacemen on the fast ship will be ageing less than those on the slower.
But the velocities are relative aren’t they? So when one spaceman looks out of his window at the light/mirror clock on the other spaceship he will see that time on the neighbour space ship is running slower than on his own light mirror clock
(Digression: A light/mirror clock consists of two mirrors vertically separated by 1 metre. Light is bouncing back and forth between these two mirrors and as the speed of light is absolute, the time taken to travel between mirrors is 1 x (3 108 )-1 seconds. But when spaceman A looks at his neighbours clock there is a relative velocity of the mirror/ light clock in the neighbour ship. Therefore the light has to travel further than 1m between the mirrors [the extra distance travelled is {(vt)2 + 12}0.5, where v is the relative velocity and t is the observation time (>1x (3 x 108)-1s).])
Lets imagine that our spaceman looking out of ship A sees ships B clock running at half the speed of his own.
Of course the spacemen looking out of ship B at ship A will also see ship A clock running at half the speed of his own
So both spacemen will think the other clock is going at half the rate of their own and that they themselves are ageing at twice the rate of their neighbour.
Now these two spacemen happen to be twins and they have no history of the velocity of their respective spaceships. They were placed in them as babies and off they have gone.
They have been travelling for 18 years now and it is time for their majority party. Each one knows he is 18 years old and calculates that his twin brother is only 9 years old because of time dilation.
To meet at the party, they both decelerate their spaceships at exactly the same rate until there is zero relative velocity, then, side by side, the doors are opened and one brother steps through the other’s door.
What will he find?
Will his brother be half his age, or will his brother be twice his age or will they both be the same age?
Neither can possibly know before they meet!
What will their relative ages tell them?
Moral of this puzzle?
Although there is no absolute velocity - all velocities are relative, (excluding light) there are real absolute increments of velocity.
Another way of putting this is that velocity is absolute, it is just that in general (in fact in all cases) we dont know what the initial 'rest' velocity was.
Odd that , don't you think? There are absolute velocities but we can never know what they are, so all we can deal with is relativities, just as Newton told us so!
Is time dilation a function of acceleration, of velocity or both?
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Post by speakertoanimals on Dec 13, 2010 13:25:15 GMT 1
The moral is quite simple -- velocity is relative, but acceleration (leaving gravity our of the picture) is absolute.
Saying that acceleration is absolute DOESN'T make velcoity absolute, just shows that once again, you don't know how to integrate or differentiate.
So, if both agree that what a = dv/dt is, doesn't mean they agree on v (or even changes in v) because the relation involves t as well (which in general they also disagree on!). You seem to have forgotten about time in making your silly claim......................
So, velocity and time are both relative (or rather, depend on your state of motion), but in such a way that we both agree on accelerations.
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Post by Progenitor A on Dec 13, 2010 14:14:24 GMT 1
The moral is quite simple -- velocity is relative, but acceleration (leaving gravity our of the picture) is absolute. You do not undestand what I have written. Velocity is absolute but because we have no idea of the initial conditions we can only deal in relativities. But then you hardly understand anything at all Saying that acceleration is absolute DOESN'T make velcoity absolute, just shows that once again, you don't know how to integrate or differentiate. Total miscomprehension on your part. So, if both agree that what a = dv/dt is, doesn't mean they agree on v (or even changes in v) because the relation involves t as well (which in general they also disagree on!). I see v does not involve t but a=dv/dt does involve t[/i].Hahaha! And what do you consider a change in v amounts to? Absolute innumerate gibberish displaying starkly that you are not a physicist. In fact the whole sentence is a fine example of your terrible English and muddled thinking leading to you putting forward your usual gobbldeygook You seem to have forgotten about time in making your silly claim..................... You really do not understand waht is written So, velocity and time are both relative (or rather, depend on your state of motion), but in such a way that we both agree on accelerations. More gobbldeygook You should be congratulated. You have read widely in popular science books. Unfortunately you do not have a clue as to what they are saying, are innumerate, and seek to hide your ignorance by attempting to shut other up by piling insult upon insult Yoy really do need help |
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Post by speakertoanimals on Dec 13, 2010 14:44:47 GMT 1
Utter bollocks.
Acceleration is an increment in velocity over time (I NEVER said didn't involve time, you forgot that it did!). Hence since time depends on your point of view, so in general will the increment of velocity.
I should have put this more clearly (NOT that it wil help you, you're still stuck on trying to understand a first derivative!) -- velocity in the sense of moving or not moving, depends on your point of view. Hence by choosing the right frame of reference, we can remove velocity. We can't do the same with accelerations -- whatever speed I chooose, even if I bring the object momentarily to rest, I will not be able to 'cancel out' its acceleration. Hence whether or not a thing is accelerating is (within special relativity) absolute, and doesn't depend on your frame of reference, even though the SIZE of the acceleration will depend on your state of motion.
And now all sense goes right out of the window! Perhaps a misguided attempt to use relativities on the same sense as absolutes, but an incorrect one, I don't need to add.
A simple example to show that 'absolute increments of velocity' stuff is wrong.
Suppose I am at rest with respect to A, who then increases his speed by 10 meters a second.
Suppose now B is moving past me at the speed of light less 5 meters a second. If this 'absolute increments' stuff meant anything, I would then predict that B should see A moving at the speed of light PLUS 5 meters a second. Which is wrong!
Hence 0 to 10ms^-1 relative to B will mean speed of light less 5ms^-1 to speed of light less something smaller than 5 ms^-1, but still less than the speed of light -- and always remains less than the speed of light, even if I say that A moves from rest to 0.9999999 of lightspeed.
Hence increments of velocity (like velocity) is relative. QED
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Post by Progenitor A on Dec 13, 2010 14:53:55 GMT 1
Utter bollocks. Acceleration is an increment in velocity over time (I NEVER said didn't involve time, you forgot that it did!). Hence since time depends on your point of view, so in general will the increment of velocity. I should have put this more clearly (NOT that it wil help you, you're still stuck on trying to understand a first derivative!) -- velocity in the sense of moving or not moving, depends on your point of view. Hence by choosing the right frame of reference, we can remove velocity. We can't do the same with accelerations -- whatever speed I chooose, even if I bring the object momentarily to rest, I will not be able to 'cancel out' its acceleration. Hence whether or not a thing is accelerating is (within special relativity) absolute, and doesn't depend on your frame of reference, even though the SIZE of the acceleration will depend on your state of motion. And now all sense goes right out of the window! Perhaps a misguided attempt to use relativities on the same sense as absolutes, but an incorrect one, I don't need to add. A simple example to show that 'absolute increments of velocity' stuff is wrong. Suppose I am at rest with respect to A, who then increases his speed by 10 meters a second. Suppose now B is moving past me at the speed of light less 5 meters a second. If this 'absolute increments' stuff meant anything, I would then predict that B should see A moving at the speed of light PLUS 5 meters a second. Which is wrong! Hence 0 to 10ms^-1 relative to B will mean speed of light less 5ms^-1 to speed of light less something smaller than 5 ms^-1, but still less than the speed of light -- and always remains less than the speed of light, even if I say that A moves from rest to 0.9999999 of lightspeed. Hence increments of velocity (like velocity) is relative. QED You are quite mad! An increment in velocity is acceleration. I wil not communicate with you further You should really seek help |
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Post by speakertoanimals on Dec 13, 2010 15:03:50 GMT 1
Maths versus common parlance again -- acceleration equals an increment in velocity divided by the elapsed time. Hence measured acceleration depends on BOTH, change in velocity AND time elapsed.
First derivatives again, stop being silly.
You seem to think all observers agree on the SIZE of the increment of velocity, which as I showed is nonsense. Yes, they all agree that A has changed their state of motion, but that is hardly news............
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Post by robinpike on Dec 13, 2010 19:23:42 GMT 1
We all know that time dilates (expands, slows down) as a function of velocity. The problem with Relativity, is that it is a theory constructed from measurement rather understanding. It is an assumption that the rate of time changes with velocity / acceleration. Until we understand how relativity works, there is the possibility that something different might be going on. For example, it might be that movement changes the ability to measure the rate of time, rather than the rate of time itself. A subtle difference, but nonetheless different. |
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Post by speakertoanimals on Dec 13, 2010 19:38:33 GMT 1
This is not quite the case. How Einstein came to it was the simple picture of what a lightbeam would look like if you were travelling with it -- then you would have oscillating but stationary em fields, a situattion that Einstein took as unphysical. Hence ideas that such a thing would not be possible.
SO, for some things in relativity, the concept of what an observer could measure matters. But when it comes to time, the point is not just that what we could measure as time matters, but that what we could measure using ANY and EVERY conceivable physical process comes into play.
No we could imagine a situation where there was some 'physical' effect of motion that altered the electron orbits in atoms and molecules to produce measured effects, but we would also have to postulate a process a that somehow mananged to change other timing devices (such as pendulum clocks, or clocks constructed using nuclear decay, or clocks constructed using chemical processes) and ALL in exactly the same way.
That's the kicker -- all possible timing devices, using physics/chemistry/electricity/biology, on various scales, ALL would have to change their rate in precisely the same way.
If it really is that general (all physical/chemical/biological processes), then isn't it simpler and more logical to say that what changes is time itself, if all processes change in the same way? In fact, isn't that what we eman by time, the thing that is common across all those different processes? What meaning can we assigbn, after all, to the concept of time independent of ANY process that marks that time? And if ALL processes mark the same thing, isn't that THE time?
IF we found a process that didn't vary in the same way, then we would need a rethink. But given that ALL processes we have tried give the SAME results.................
The point being that various commonsense alternatives have been tried, and rejected, it's not as if no one ever thought of them, or that the obvious was rejected in favour of the weird and abstract because Einstein said so.
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Post by Progenitor A on Dec 13, 2010 20:47:39 GMT 1
We all know that time dilates (expands, slows down) as a function of velocity. The problem with Relativity, is that it is a theory constructed from measurement rather understanding. It is an assumption that the rate of time changes with velocity / acceleration. Until we understand how relativity works, there is the possibility that something different might be going on. For example, it might be that movement changes the ability to measure the rate of time, rather than the rate of time itself. A subtle difference, but nonetheless different. physical theories are pragmatic. If they fit what is observed and allow predictions to be made, then they become that accepted norms -the paradigms. Unitil inevitably some new discovery highlights a flaw in the model/paradigm, and then the old model/paradigm is rejected then a new theory replaces the old theory. Nothing in physics should be regarded as 'true'. And indeed with QM there are various underlying (and opposing) theories that all correctly predict the same outcomes, whilst prominent physicists all agree that there is no real 'understanding' of QM. Mathematical models are fatally flawed as they invariably (in QM and Cosmology) come up with embarrassing 'infinities' that apparently have no place in the real world. Nearly all cosmological mathematical models involve 'normalisation' to get rid of these embarrassing infinities So our knowledge is limited and shaky. Despite all the tecnological advances that science has enabled, we still have no real understanding of what is going on around us That is what makes physics so interesting of course, and why, in my opinion, accepted mantras and dogmas should always be challenged if they are not understood - for there is a very good chance that they are not understood at all |
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Post by Progenitor A on Dec 14, 2010 9:49:24 GMT 1
Well what a shame!
Earlier I was here and Nick had two elegantly constructed problems on time dilation
I went away to consider them, cracked the problem, returned in triuumph to find them gone!
Is this spooky action at a distance once more? Did my thoughts somehow convey themselves faster than light to Nick, so he collapsed his wave function and warn't ther no more?
Very mysterious
Well the key was this
There is no way we can know that the two asteroids were travelling at the same velocity. They might have appeared to be travelling at the same velocity for example, but because one was , for example, moving faster than the other but we were moving with some velocity toward the second asteroid making it appear to have that same velocity as the first asteroid.
Even if they are travelling at the same velocity the relative time dilations will depend upon their history of acceleration.
However there is still the possibility that they are travelling at the same speed with the same past history of acceleration and then the paradox does seem unresolvable
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Post by speakertoanimals on Dec 14, 2010 13:13:59 GMT 1
Wrong. In QM, we understand why the infinities pop up in calculations, and how to perform the calculations in a way such that they don't occur.
As regards cosmology, in effect we know what some infinities turn up -- we don't know how to do quantum gravity, which we expect to modify things at the smallest length scales and largest densities.
Saying that infinity has no place in the real world (except the universe MAY be infinite) is just a statement of faith, NOT a statement of physics. A fatal flaw in a theory is the inabiolity to predict, or the ability to predict two different things from same data -- not just, we get an infinite answer at that point, we probably can't push the prediction that far then.
I'd like to know what you propose rather than mathmatical models.....................
Not quite -- we have different interpretations of quantum theory, that all produce the same predictions. Hence one way to look at it is IF they all produce the same predictions, then can any one be said to be any more true than any other? Why ASSUME that one must be?
I don't know why you keep banging on about physics being only provisional -- yes that is true, somewhat, but doesn't alter the fact that Newtonian gravity gave good predictions of the orbit of the moon at the time of Newton, AND STILL DOES! Rather than being replaced, we see it now as an approximation to the truth, we don't throw it out totally. So the fact that the motion of the moon and the planets approximately follow newtonian predictions is as true as it ever was.
Rather than throw it out, start again, a better picture would be ditch the word paradigm, and view it as a building, where more encompassing explanations and more precise explanations are built on to decayed remnants of earlier explanations, but we don't throw away the old foundations as we lay new blocks on top.
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Post by robinpike on Dec 14, 2010 14:53:05 GMT 1
So, for some things in relativity, the concept of what an observer could measure matters. But when it comes to time, the point is not just that what we could measure as time matters, but that what we could measure using ANY and EVERY conceivable physical process comes into play. Now we could imagine a situation where there was some 'physical' effect of motion that altered the electron orbits in atoms and molecules to produce measured effects, but we would also have to postulate a process a that somehow mananged to change other timing devices (such as pendulum clocks, or clocks constructed using nuclear decay, or clocks constructed using chemical processes) and ALL in exactly the same way. That's the kicker -- all possible timing devices, using physics/chemistry/electricity/biology, on various scales, ALL would have to change their rate in precisely the same way. If it really is that general (all physical/chemical/biological processes), then isn't it simpler and more logical to say that what changes is time itself, if all processes change in the same way? In fact, isn't that what we mean by time, the thing that is common across all those different processes? What meaning can we assign, after all, to the concept of time independent of ANY process that marks that time? And if ALL processes mark the same thing, isn't that THE time? Although our theories do not describe energy (and mass) as existing in "blocks", nonetheless in all physical phenomina, we find that the amount of energy / mass going in and coming out can be counted, and the going in and coming out amounts always agree - just as if the energy / mass were to be made of "blocks". So on that basis, I would say that a simpler explanation than time itself changing with motion, is: that the energy changes with motion, such as its shape, its ability to further increase its speed, etc. |
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Post by speakertoanimals on Dec 14, 2010 14:57:07 GMT 1
The block picture is 100% wrong.
We KNOW why energy and momentum and angular momentum are conserved. Its just MATHS and a french mathematician called Emmy Noether.
The answer is - if the laws of physics don't depend on when you happen to be at, or where you happen to be, than there is inevitably a quantity that is conserved. In the case of when, that quantity is energy, and in the case of where, that quantity is momentum.
None of which has anything much to do with why time changing with motion is or isn't a good explanation......................
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