I heard only a couple of months ago of an experiment in which a pulse of light was stopped on its way through a chamber through the use of a laser, then it was allowed to keep going again. Does that count as photons being stopped?
In a way. But the focus of that experiment was on capturing the photon's "information" and then recreating another identical photon from that information. They didn't exactly stop a photon, but they did succeed in storing the information it contained and then replicating an exact copy of it. Interesting applications to computers I would think Please Register or Log in to view the hidden image!
Sagnac effect c'est moi: "Despite the ease and clarity with which special relativity accounts for the Sagnac effect, people who lack a sound grasp of basic physics and mathematics sometimes imagine that this effect entails a conflict with the principles of special relativity." That is an extract from the following site: <a href="http://www.mathpages.com/rr/s2-07/2-07.htm" target=_blank">http://www.mathpages.com/rr/s2-07/2-07.htm</a> If you do a search you will find many such pages. The Sagnac effect is entirely compatible with and explained by special relativity.
I have no problem, I can't deny anything about SR since I know very little about it. But I have seen no reason in any of this for people to even consider the possible existence of time travel or time dilation. The Sagnac effect is a plain old physical mechanical property. You go round in one way, it takes you longer to reach the starting point because that point is moving. There's no time trickiness going on there at all. It's a very ordinary mechanical effect. Here is another question though. If a light pulse going around one way takes long enough for it to become 360 behind the phase of the other pulse, and they meet up again precisely in phase (except one is one cycle behind the other), how is the difference detected? Can it be detected at all, or do we just assume from the distance covered that it is one cycle behind?
god, it's been just a few days a posted this thread and it's already on page 6... uf! (haven't had the time to read it all yet) hey crisp, for "rest" do you mean inercia? if you do, wouldn't a photon be in a constant velocity (moviment) in the vaccum of the universe, therefore in some kind of inercia (so you can use the e=mc2 formula)? i'm sorry, i'm just starting in this, so if i said some bs please correct me. wouldn't "movement" be only when the particle is in a accelerated moviment? also, i was thinking, as mass becomes more energy as much as it gets near to the velocity of light (according, again, to einstein), wouldn't the reason for the lack of a photon mass be exactly that: all its mass is in all its energy state since it travels in light's velocity in the vaccum? that doesn't mean it doesn't have mass, but that the mass presents itself in the energy "mode". i don't knoe if i could explain it right... english is not my first language anyway...
Hi Lua, "for "rest" do you mean inertia? if you do, wouldn't a photon be in a constant velocity (movement) in the vaccum of the universe, therefore in some kind of inertia (so you can use the e=mc2 formula)? i'm sorry, i'm just starting in this, so if i said some bs please correct me. wouldn't "movement" be only when the particle is in a accelerated movement?" With "rest" I mean that for you, as an observer, the particle has a velocity of v = 0. It does not move at all. "Also, i was thinking, as mass becomes more energy as much as it gets near to the velocity of light (according, again, to einstein), wouldn't the reason for the lack of a photon mass be exactly that: all its mass is in all its energy state since it travels in light's velocity in the vaccum? that doesn't mean it doesn't have mass, but that the mass presents itself in the energy "mode"." Hrmmmm... your first sentence isn't entirely correct. Mass does not become more energy as speed increases. there's no "conversion" going on. You have to be careful about the word mass (that's one reason why I have always argued to use the words "rest mass" and "relativistic mass" instead of just mass Please Register or Log in to view the hidden image!). When looking at the energy of a particle, there are two contributing terms, as you can see in the formula I stated above: - There's a term that is related to the rest mass (m<sub>0</sub>c<sup>2</sup>)<sup>2</sup> - There's a term related to the momentum (pc<sup>2</sup>). For a particle with rest mass, this means that the minimum energy that particle can have is E = m<sub>0</sub>c<sup>2</sup> (that is when the particle is at rest, v = 0, which for particles with mass comes down to p = 0). As the velocity (and hence p) increases, the particle just gets more energy, but the rest mass is unaltered. You can also look at a thing called "relativistic mass" (which is briefly called "mass" when talking about the theory of relativity). This quantity is only definied for particles that have a rest mass. The (relativistic) mass can be deduced from the energy as follows: E = mc<sup>2</sup> (only for particles with a rest-mass!!!) Where m = m<sub>0</sub> / sqrt( 1 - (v/c)<sup>2</sup>). You can easily deduce this formula yourself by inserting p = m<sub>0</sub>v / sqrt( 1 - (v/c)<sup>2</sup> (which is also only valid for particles with restmass) in the E<sup>2</sup> formula you quoted above. So in a way you are right: the "mass" m deduced from the energy-mass equivalence relation above increases as the energy increases - and the only way for the energy to increase for a particle with restmass is for it to move faster (so at higher speeds, relativistic mass consists more of energy due to motion than of energy due to restmass). This however does not apply to photons which only source of energy is its momentum (cfr. quantummechanics, special relativity). Hope this explains it somehow Please Register or Log in to view the hidden image! Bye! Crisp
Re: Sagnac effect "The Sagnac effect is entirely compatible with and explained by special relativity." I have read the link. The argument he cites (and disproofs) is not exactly the one I have given. Please just say where I go wrong. According to what I know, trying to detect ABSOLUTE velocity is needless viewed from the principle of relativity, hence the very name "relativity". But because of the Sagnac effect, which I agree, anyone can understand, we do detect the absolute velocity of rotation of the earth. Just tell me why this is compatible with relativity.
c'est moi, The Sagnac effect is not detecting an absolute velocity. It arises because the apparatus is rotating. The principle of relativity says the laws of physics are the same in all inertial frames of reference. A rotating frame is non-inertial, so the Sagnac effect does not conflict with relativity.
thanks crisp, i did understand that. Please Register or Log in to view the hidden image! i just always read books using the e=mc2 formula for all objects including those on movement. i guess when you try to simplify mathematics something comes wrong. i'm soon starting my phisycs college, so i probably get those concepts right again later, i was just being curious. take care.
"The Sagnac effect is not detecting an absolute velocity. " because of the Sagnac effect, we can caculate the velocity of the moving observer ( = velocity of the earth) which is the Absolute velocity why would this be a relative velocity???
might be helpful in regard to the sagnac effect: http://www.ldolphin.org/sagnac.html lots of stuff concerning special relativity (but also papers on general r.) http://www3.sympatico.ca/wbabin/paper/index.htm the look of a philosopher on relativity and quantum physics: http://www.geocities.com/Athens/Atrium/8041/ and let it be said: Most scientists are terrible philosophers, a MAJOR problem as discussing stuff like relativity is BASED on philosophical appraoches etc. and not firstly on Experiments as most think
Groovy links C'est Moi. Now, I've only started reading on special relativity and all that, but so far it seems a lot simpler than I had expected. As yet, I don't see any big deal about it. All very straightforward. Velocity of EM waves/emissions is a constant divided by the material it is passing through. That's no problem. My fastest running rate is slower if I'm in water, for an analogy. So at the very start it is a very basic mechanical idea. Now if I'm running along in water for example, my speed will be the same regardless of whether any observers are flying by in an aircraft or sititng there have a drink on the beach. Motion of observers makes no difference. Again, nothing more than a very obvious mechanical idea. Same for light in both cases. If I'm shot out of a torch which is in that aircraft, or if the torch is on the beach having a drink, I can still only run at that certain speed in the water. Maybe if I'm shot into the water eXtremely fast, I may enter the water at very great speed, but that will almost instantly be transferred into force against me and splashing and such. I'll still be left running through the water at the same speed as usual. These three analogies seem to me to represent the core of relativity as I understand it so far. I see no disparity with basic mechanics. There is nothing there which Newton would have a fit over. What exactly are the supposed problems with relativity? Could someone explain to me in simple terms any problems they see with it all?
Adam, No, this subject is unlike you say VERY complicated. Many times it has been said that nobody really understands relativity, and these scientists didn't say it just like that. <<What exactly are the supposed problems with relativity? Could someone explain to me in simple terms any problems they see with it all? << the problem starts: it is quite impossible in simple terms, it gets complicated very quickly! when i have the time, i'm gonna do some new research and write a piece on that i'll give you the link then @ James r: Explain why you think "a rotating frame is non-inertial".
Explain why you think "a rotating frame is non-inertial". Imagine a small cart moving at constant velocity and spraying paint dots passes in front of a wheel rotating at constant angular velocity. The paint dots on the white rectangle placed to the left of the wheel record the motion of the cart as seen by an observer; the dots on the wheel describe the motion of the cart as seen by an observer at the center of the wheel and rotating with it. Standing on the ground in our reference frame, we see that the cart is subject only to forces which we can ascribe to specific objects in the environment, i.e. the drive, friction, etc. We call this frame an inertial reference frame. To an observer on the wheel, however, the cart must be subject to some additional force since both the direction and the magnitude of its velocity change. We call the rotating frame a non-inertial reference frame because we must postulate pseudo forces (sometimes called inertial forces) to account for the motion of the cart in this system.
Very nifty analogy. I suspect it is all going to end up being quite easy. Most things are in the end. I suspect many people simply get caught up in the maths and fail to see basic principles. But then, I have yet to learn it all, so feel free to deliver words of impending doom for my future studies.
Using the Sagnac effect you can only calculate the angulr rotational velocity of the apparatus relative to a particular local inertial frame. You cannot calculate any absolute velocity, and in fact, if you use the Lorentz transformations to look at a different inertial frame (with a different velocity), the Sagnac effect remains the same.
let's take a look at that again: the velocity of the observer on earth can be calculated with the Sagnac effect with following equation: v = c - V where 'v' is the MEASURED velocity of light and 'V' the velocity of the observer (and this also the velocity of the rotation of the earth) --> the observer can simply be considered as a marked point on earth this agrees with the results expected from an absolute frame of reference, in which light is traveling at velocity c with respect to a fixed frame at rest. for a circular trajectory, the measured velocity of light is not "c" but "v", in the frame of reference used by the observer moving with the disk. The velocity of light v is equal to (c-V) or (c+V) depending on the relative direction of V with respect to c if you say we have calculated a Relative velocity "V", then what is the local inertial frame in this specific example? is it the universe that is turning and not the earth?
c'est moi: Bear in mind that the speed of light measured by an inertial observer outside the Sagnac apparatus <i>or</i> in an inertial frame co-moving with a particular point on the rotating apparatus is the same. Light travels at a constant speed in all inertial frames. There are no absolute frames of reference. Adam, Relativity is somewhat more complex than you currently think. You said: <i>Now if I'm running along in water for example, my speed will be the same regardless of whether any observers are flying by in an aircraft or sititng there have a drink on the beach. Motion of observers makes no difference.</i> That's wrong. If you are running along at 1 metre per second as seen by somebody sitting on the beach, and a plane flies past you (in the opposite direction) at 100 metres per second, your speed as seen by the pilot of the plane will be 101 metres per second. The motion of the observer clearly makes a difference in this case. However, if you shine a torch light in front of you, the person on the beach will see it travel at speed c metres per second. You might expect that the pilot of the plane will see the light travelling at c+100 metres per second, but that's not what happens. Actually, the pilot <i>also</i> sees the light going at c. That is a very counter-intuitive idea once you understand it. <i>There is nothing there which Newton would have a fit over.</i> There is much in relativity that shows that Newton had things right for low speeds, but completely wrong for speeds approaching the speed of light. <i>What exactly are the supposed problems with relativity?</i> There aren't any, as far as we can tell. Countless experiments have shown relativity to be correct and Newton to be wrong. There are some niggling problems with producing a quantum theory of gravity, but quantum theories already incorporate special relativity.
c'est moi: I looked at your links. With the first one, alarm bells started ringing as soon as I read the qualification of the author (from the "Association for Biblical Astronomy"). It then goes on to quote a 1938 paper for the supposed incompatibility of the effect with relativity. (Nothing more recent?) While I haven't read that paper, from the quoted sections of it it does not seem to support the author's conclusions. Besides, the problem pointed out is addressed in the link I gave earlier. I haven't read Babin's paper in full, but reading the conclusion is sufficient to show that he does not understand special relativity, because he refers to a "universal reference frame" or similar defined by the speed of light. Lindner (MD) seems to be more concerned with philosophy more than physics. (Does he have any physics training?) While I agree with your point that relativity is based on some axioms which could be considered a kind of philosophy, the veracity of those axioms is supported by an enormous body of experimental and observational evidence. Thus, relativity cannot be considered to be a philosophical construct. It is solid science.
JamesR: "C is a constant regardless of frame of reference." I believe that is what people have been telling me. The water thing is an analogy. Looks like what that means is that regardless of your own motion *relative* to the light/runner, the light/runner is still only moving at a certain absolute velocity. The *relative* velocity depends on the motion of the observer. But the light/runner is still moving at the same velocity as always. Or so it seems to me.
