Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Madidus_Scientia on 07/01/2009 18:35:26
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Say you had some sort of device that accurately measured the energy released from the collision of a very small amount of anti-matter and matter, and the amount of energy was x. What if you measured two different samples of the same masses but one of them has significantly more kinetic energy, will the one with extra kinetic energy measure x + KE? Or will they both measure just x.
If it did measure as x + KE, wouldn't the extra kinetic energy have to be relative to something? eg. I wouldn't say my desk has any KE right now but relative to the sun it's moving quite fast. So by finding what the KE was relative to could we find some kind of static point of the universe?
If that's not how it works, and the energy released was just x regardless of KE, then where does the kinetic energy go?
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I don't know the answer to this off-hand, but the first thing that comes to mind is that the KE would be evenly distributed and accounted for in the frequency of the EMR produced.
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Say you had some sort of device that accurately measured the energy released from the collision of a very small amount of anti-matter and matter, and the amount of energy was x. What if you measured two different samples of the same masses but one of them has significantly more kinetic energy, will the one with extra kinetic energy measure x + KE? Or will they both measure just x.
If your device measures the total energy, it will of course measure kinetic energy too, so it'll measure x + KE.
If it did measure as x + KE, wouldn't the extra kinetic energy have to be relative to something?
Yes, it's relative to the detector frame of reference. If you put that detector in a moving train, it will detect a different energy. Energy, as velocity or momentum, it's not frame-invariant; for this reason you can't ascribe it to an intrinsic property of the object.
(Yes, I know, it's simple, but not so immediately intuitive! [;)])
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Yes, it's relative to the detector frame of reference. If you put that detector in a moving train, it will detect a different energy.
But what i'm trying to get my head around is, the detector could be in any frame of reference, why would the amount of KE released be relative to the train station rather than to the centre of earth, or the sun, or another galaxy?
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Yes, it's relative to the detector frame of reference. If you put that detector in a moving train, it will detect a different energy.
But what i'm trying to get my head around is, the detector could be in any frame of reference, why would the amount of KE released be relative to the train station rather than to the centre of earth, or the sun, or another galaxy?
When you say "KE released" you are making a meaningless phrase if you don't specify the frame of reference. Think about velocity, instead of energy, it's the same.
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Yeah ok because KE depends on velocity and velocity is relative to something, I get that but i'm still confused about reference frames.
If you have 2 spaceships travelling in different directions, spaceship A and spaceship B, you could consider them both to be moving apart or you could consider one to be stationary and the other to be moving. If each spaceship has one of these detectors, and you considered spaceship A to be stationary, they would just get the energy E from converting the mass to energy. But relative to them spaceship B's matter/anti-matter has alot more energy because of their velocity, so they would predict spaceship B to get more energy out of their matter than they did from theirs, because they would get E + KE, wouldn't they?
But if you think of the spaceships as both moving they would yeild equal amounts of energy. Or if you considered spaceship B to be stationary and spaceship A moving, spaceship A should yeild more energy.
Or to think of the train example again If you put that detector in a moving train, it will detect a different energy.
If you considered the train still and the train station moving, wouldn't it be the train station that detects more energy from their device than the one on the train?
But obviously the detectors can only detect one result, so what will it be?
<head explodes>
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I think you need to clarify your points of reference. If the two spacecraft have only each other for reference then either could conclude that they were stationary and the other was moving (this assumes that there is no acceleration, as your question seems to do). If however, they both use a common reference, such as the planet from which they left in different directions, they would both conclude that they were moving. Finally though, and going back to both spacecraft only using each other for reference, while they would both conclude that they were stationary and only other was moving, the hypothetical distant observer could see whether one or both were moving.
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Yeah ok because KE depends on velocity and velocity is relative to something, I get that but i'm still confused about reference frames.
If you have 2 spaceships travelling in different directions, spaceship A and spaceship B, you could consider them both to be moving apart or you could consider one to be stationary and the other to be moving. If each spaceship has one of these detectors, and you considered spaceship A to be stationary, they would just get the energy E from converting the mass to energy. But relative to them spaceship B's matter/anti-matter has alot more energy because of their velocity, so they would predict spaceship B to get more energy out of their matter than they did from theirs, because they would get E + KE, wouldn't they?
"they would predict spaceship B to get more energy out of their matter". This phrase is meaningless if you don't know which is the relative velocity between A and B. A detects energy coming from B, but A cannot establish how much mass (invariant mass) was associated to that energy, if you don't know the relative velocity. Example: if A and B are stationary to each other and B converts 1 kg of uranium into photons sent to A, then A will receive 1*c^2 = 9*1016 Joules of energy in the form of photons, and A will be able to deduce that 9*1016/(3*108)2 = 1 kg of matter has been converted into energy from B. But if A and B are moving relative to each-other and you don't know which is the relative velocity, then, from the photon's energy that A receives from B (let's say it's still 9*1016 Joules), it's impossible to establish how much it was the invariant mass converted into photons inside the spaceship B.
If, instead, A knows which is the relative velocity between them, he can compute how much mass B has converted into energy: let's say A receives E joules and the relative velocity is V, approaching; then the energy E0 which B has converted into photons is:
E0 = E(1-V/c)/(1+V/c)
and the mass he has converted is E0/c2
This without knowing if A is stationary and B moving or the other way around or both moving, it doesn't matter, you only need to know the relative velocity V. Clearly, if the velocity is of receding, you only have to change the sign of V.
Example: A receives 9*1016 Joules of energy as photons, and he knows that the relative velocity of approaching between them is 0.5c. So:
E0 = E(1-0.5)/(1+0.5) = E/3 = 3*1016 Joules
So B has converted 3*1016/(3*108)2 = 3*1016/9*1016 = 0.333... kg into energy.
I don't know if this helps you, in case ask again.
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I'll familiarise myself with those equations when I get more time and hopefully make sense of it :P
In the meantime, thank you for your patient explanation :)
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Yeah ok because KE depends on velocity and velocity is relative to something, I get that but i'm still confused about reference frames.
If you have 2 spaceships travelling in different directions, spaceship A and spaceship B, you could consider them both to be moving apart or you could consider one to be stationary and the other to be moving. If each spaceship has one of these detectors, and you considered spaceship A to be stationary, they would just get the energy E from converting the mass to energy. But relative to them spaceship B's matter/anti-matter has alot more energy because of their velocity, so they would predict spaceship B to get more energy out of their matter than they did from theirs, because they would get E + KE, wouldn't they?
But if you think of the spaceships as both moving they would yeild equal amounts of energy. Or if you considered spaceship B to be stationary and spaceship A moving, spaceship A should yeild more energy.
Or to think of the train example again If you put that detector in a moving train, it will detect a different energy.
If you considered the train still and the train station moving, wouldn't it be the train station that detects more energy from their device than the one on the train?
But obviously the detectors can only detect one result, so what will it be?
<head explodes>
Madidus
:)
I think Lightarrow made sense.
Would you agree that the possible 'interaction' between two frames would result in the same amount of 'energy' released no matter from which frame you measured it?
As the only way you would be able to measure something would be in a 'interaction/comparison' between reference frames.
And that not knowing who is moving won't change the interaction/result, or energy, when you discuss 'the station versus the train.
When I think of it I see it as a 'relation'.
In that 'relation' we can say that it doesn't matter who is moving, that as the observation from either frame will yield the same result, no matter if 'observational fore knowledge' of the observers own situation/frame (if being moving/still etc)
What seems special to me is acceleration.
But then again, it can if 'uniformly accelerating'(one G) easily be seen as a 'gravity'.
When not accelerating, 'moving uniformly' (coasting:) it is an open definition who is doing what when comparing frames.
But the thing about reference frames that confuses me is.
How do we define one?
Take our Earth, the 'gravity' is uneven even if on the same height.
Where do one 'reference frame' start and another stop.
Is it all arbitrary defined?
Do we define it as having the same 'time' or 'gravity' or?
And I hope this makes sense:)
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Say you had some sort of device that accurately measured the energy released from the collision of a very small amount of anti-matter and matter, and the amount of energy was x. What if you measured two different samples of the same masses but one of them has significantly more kinetic energy, will the one with extra kinetic energy measure x + KE? Or will they both measure just x.
If it did measure as x + KE, wouldn't the extra kinetic energy have to be relative to something? eg. I wouldn't say my desk has any KE right now but relative to the sun it's moving quite fast. So by finding what the KE was relative to could we find some kind of static point of the universe?
If that's not how it works, and the energy released was just x regardless of KE, then where does the kinetic energy go?
I would say yes. I have speculated that the term ''rest mass'' could just be something experiencing a uniform or static field with the earths field, because in the end of the day, everything is always moving.
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Here is a very nice 'recollection' of inertial frames through our times:)
http://plato.stanford.edu/entries/spacetime-iframes/
And the wiki http://en.wikipedia.org/wiki/Inertial