Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: barneyboy on 03/11/2013 21:33:48
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If matter shrinks as it reaches the speed of light, does it expand again when slowed down? and does time slows down the closer to the speed of light matter gets? and finally I have read an article regarding photons having mass, if they have mass, do they not act in the same way as matter at the speed of light.
please keep answers simplistic as im no prof :D
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No takers? :o
I was hoping someone might spare a mo to put me straight :o)
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If matter shrinks as it reaches the speed of light, does it expand again when slowed down?
Yes. That you asked this question implies that you believe that it’s the fact that the object changed its speed that it contracted. It’s not that the object changed its speed that caused its length to change but that it has that speed and at that speed the length is shorter. That might seem to be an irrelevant nit pick but you should consider where that comes from. I wrote a derivation of that fact here http://home.comcast.net/~peter.m.brown/sr/lorentz_contraction.htm
Notice that the length of a moving object is
L = L_o/sqrt(1 – v^2/c^2) = gamma L_o
Where gamma =1/sqrt(1 – v^2/c^2)
When its speed non-zero then gamma < 1 and therefore L < L_o => Length contraction.
Now let’s say that the speed is smaller so that at the new speed gamma is less than the original one. Then L is larger than the previous one and it’s no longer that slow.
Notice that the increase or decrease in the speed of the object doesn’t necessarily mean that the speed of the object changed. It might very well be that the speed of the observer changed
…and does time slows down the closer to the speed of light matter gets?
Yes.
…and finally I have read an article regarding photons having mass, if they have mass, do they not act in the same way as matter at the speed of light.
I’m not sure what you mean by act in the same way. Do you mean that they both have momentum? They both interact with matter, etc.?
And no matter can go the speed of light if they have non-zero proper mass (aka rest mass)
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It's about frames of reference Barney. The observer defines the observation, but always in a relation to the observed. Then there also exist what is called a 'rest frame', valid for anything moving at less than the speed of light in a vacuum. The rest frame is where you are at rest with what you observe, as you resting on Earth :) Well, more or less. In a uniform motion you can be said to be at absolute rest with what you measure on. I'm not entirely sure that the same can be said of a acceleration though, 'being at rest with something else accelerating equivalently'? I used to think it should be possible though, to define it in a acceleration.
Anyway, if you are at absolute rest with something moving fast, as defined relative incoming light becoming blue shifted, then it, to you, will share a (macroscopically defined) same 'frame of reference', without measurable time dilations and Lorentz contractions (loosely defined).
If you now decelerate, to then move uniformly at a slower pace, the object you measured on before, as having a exact same time etc, now will present you with both time dilations, as well as a Lorentz contraction.
Google for 'light clocks' for a more extensive non technical description.
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thanks for the posts.
what I meant by "and finally I have read an article regarding photons having mass, if they have mass, do they not act in the same way as matter at the speed of light." is that if matter shrinks the closer to the speed of light it gets and it grows again when it slows again, then if does a photon get larger when it is slowed or even stopped? or does its wave alter when slowed?
I will look up the Lorentz contractions and light clocks asap when work duties allow. (I need the non tech description yor_on as im very non tech myself :o) )
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what I meant by "and finally I have read an article regarding photons having mass, if they have mass, do they not act in the same way as matter at the speed of light." is that if matter shrinks the closer to the speed of light it gets and it grows again when it slows again, then if does a photon get larger when it is slowed or even stopped?
A photon only has inertial mass = m = p/v. It doesn't have rest mass. A particle with zero rest mass always moves at the speed of light and therefore can never be stopped.
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Didn't Lene Hau in 1999 succeed in slowing light down? and then was there not and experiment that succeeded in stopping light -- accomplished by two teams. One was led by Ron Walsworth, a physicist at the Harvard-Smithsonian Center for Astrophysics, and the other by Lene Hau of Harvard University's Department of Physics.
If so my question still stands, if Lene Hau succeeded in slowing and then stopping light did the photons size increase or its wave expand as matter does when slowed from near the speed of light.
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Didn't Lene Hau in 1999 succeed in slowing light down? and then was there not and experiment that succeeded in stopping light -- accomplished by two teams.
Those are very misleading claims. They can neither slow down nor stop light in a vacuum. Those experiments didn't use a vacuum. I.e.
http://www.sciencedaily.com/releases/2013/08/130806111151.htm
To stop the light, the physicists used a glass-like crystal that contains a low concentration of ions -- electrically charged atoms -- of the element praseodymium. The experimental setup also includes two laser beams. One is part of the deceleration unit, while the other is to be stopped. The first light beam, called the "control beam," changes the optical properties of the crystal: the ions then change the speed of light to a high degree. The second beam, the one to be stopped, now comes into contact with this new medium of crystal and laser light and is slowed down within it. When the physicists switch off the control beam at the same moment that the other beam is within the crystal, the decelerated beam comes to a stop.
Relativity only states that the speed of light is constant and invariant in a vacuum.
In anycase Lorentz contraction does not apply to such things.
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Please explain why ,in simple terms, Lorentz contraction does not apply. And are you saying that the speed of light is not constant in a medium, only in a vacuum? ?
Im sorry to ask for the explanations in the simplest terms but I have no science degree, maths degree or even an o'level grade :I. I am one of "those" that now would be classed as dyslexic but way back when I was at school, I was just put to the back of the class and told to be quite >:(. This means that the only way I can hope to gain answers to the questions I have is not to stand on the shoulders of giants but to politely tug their sleeves and hope that they will indulge me. All I hope is that even if proved wrong (and im under no illusion that I will be) it will stir the little grey cells of those who can understand, as much as anyone, the physics of the universe and maybe give them a fleeting thought to check that all other possibility's have been explored.
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I think you safely can ignore Lorentz contraction for this Barney. Google doesn't seem as good as it used to be? Tried to find me writing about Lene Hue but failed. Anyway "Working at the Rowland Institute for Science, overlooking the Charles River and the gold dome of the state Capitol in Boston, she and her colleagues slowed light 20 million-fold in 1999, to an incredible 38 miles an hour. They did it by passing a beam of light through a small cloud of atoms cooled to temperatures a billion times colder than those in the spaces between stars. The atom cloud was suspended magnetically in a chamber pumped down to a vacuum 100 trillion times lower than the pressure of air in the room where you are reading this."
And check out this too. http://phys.org/news/2013-07-seconds.html
A Lorentz contraction involves you measuring something not belonging to a same 'frame of reference' as yourself. You can argue that microscopic time dilations and so, complementary Lorentz contractions, can exist in all observations of another 'frame of reference', but if so, they are not specific to just this experiment, but to everything you will measure on. Translate 'light' into 'energy', and then read the link above. They transfered a added 'energy' to a normally non transparent crystal, making it transparent for this purpose, immediately after changing it back to non transparent, 'storing' that 'energy/light' inside the crystal for up to 60 seconds.
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Ah, Pete :)
Same link, although different site.
Hmm reading I get a weird impression that they do not store 'energy' per se, but rather a 'gestalt' of a wave? And that the energy added releasing this wave would be the control laser beam. But that can't be right.
They add a energy, a light pulse creating a pattern. And that energy can't stay there for ever, so they have 60 seconds on them to release it, using the control laser to change the crystals properties, before it dissipates (kinetic energy into heat), right?
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I think I got you, are you saying that instead of slowing light they caught a photon in a crystal, and kept it bouncing around :P
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Well, there are such things as 'standing waves (https://en.wikipedia.org/wiki/Standing_wave).' although this one seems weirder than that :) A photon is also a wave, it's the 'duality' of it that counts, and depending on your experiment you will find one or the other. And if you describe it as a 'spring' it's the 'springs' oscillation in one place, as I gather. Actually I suspect this experiment is close to JP:s field of work, he should be the one for optical phenomena at a quantum level.
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Barney, the idea behind this 'capture' is tricky. As far as I get it it's about resonating aka oscillations creating a pattern inside some substance, be it atoms in some gas or inside some other material as a crystal. And also about something called a super position. And it's the super position that makes it truly weird. A standing wave is a form of a superposition, as described here.. (http://www.acs.psu.edu/drussell/demos/superposition/superposition.html)
But in Quantum physics a super position not only refer to lights duality, the ability to be both a wave and a 'photon' simultaneously, your final result depending on what experiment you've done ('forcing' it into a outcome), but also on HUP (Heisenberg's uncertainty principle). That states that you can't know really know the state of a 'particle/wave', until measured. What you can 'know' though is the probability of what state it, most probably, have using statistics. It seems as if this is a 'ground rule' for the very small. So instead of saying that we have a particle, or a wave, it seems better to define it as we have the probability of a wave, or a particle, as I get it :)
And statistically it seems as if you can get something in between. So when you send in a 'probe photon' into some substance to get 'stored', you set that substance/material, be it a gas or a crystal, into a superposition. The superposition is the 'storage' here. Using the 'control probe', sending in a new photon, for example, to make the crystal transparent you actually add further energy to that 'super position', forcing the super positions wave function to collapse (as I think then:) and release the original 'probe photon' that was stored there. And you can know it is the original coming out by measuring its energy, and frequency (wave).
That's as far as I get it, so far.
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Now, another way to see it could be that you add a energy to a configuration of particles. Those particles are now in a unstable configuration in where that energy won't stay, as I think. That's why we find a time limit for it existing. Adding more energy to this unstable configuration (making the crystal transparent) forces a outcome in where the stability of the original configuration is reinstated, and a photon released. What is remarkable in this is that it 'remembers' the photon stored. That way we get back to your idea of 'c' being 'c', as it will be true in any material, 'storing' a photon. The reason it is able to 'store' is also the about way we chill those atoms down, they can't 'oscillate' as it is so cold.
A 'photon' propagating in a vacuum is in reality a very weird proposition.
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It's weird though. One could assume that you by adding the control probes energy, to the 'probe photons' stored, would get another photon outcome, not the exact same. Definitely JP:s table this one :). There is one more way to see it and that's about this oscillation. If we assume that the crystals atoms stops oscillating after the energy from the probe photon is gained by those particles. Then there is no way they can release that energy, possibly? Although you have HUP there too. Because a temperature is motion/oscillations interacting. As in a gas, or a material.
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thanks yor_on, I was able to follow your explanation and reasoning through and now I believe I understand, as best as I ever will, why a photon does not act like matter even if it can be said to have mass :-)
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Barney, remember the first time I read that one, 'stopping light', getting very confused. Swanson was good enough, and Vern, to try to explain it.
Check this out. Can light stopped in a Bose Einstein Condensate "jump" between condensates? (http://www.thenakedscientists.com/forum/index.php?topic=20457.0)
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nice thread, thanks. ill give it all some more thought :-)