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Domino effect and light

  1. May 17, 2014 #1
    I am not a physicist but a few years ago I was thinking about this, and thought to ask in a physics forum to satisfy my curiosity.

    Supposing we set up a number of domino tiles. When we topple one it creates a chain reaction and it knocks over the other tiles until the last one falls. Assuming that we have set it up so that the tiles cannot fall sideways and cannot fall completely off the table and thus interrupt the chain reaction and tha they always fall the same way being affected only by gravity.

    That means that regardless who, or what, and how starts the chain reaction, the time taken for the last tile to fall will always be the same. Regardless if there was a gust of wind or a bullet fired from a moving train in the opposite direction; the last tile will always fall down at the same time.

    Would that not explain the perception that speed of light is constant because we are only measuring the tiles in between, not the conditions under which the first tile fell ?

    Supposing light is photons which get absorbed and emitted by electrons, which means that the a photon hitting our retina is a photon probably emitted by the fluid inside the eye, and behind that by a photon emitted by the lens, and by the atoms in the air and what not, but it is not the original photon that left a distant star 100 light years ago ?

    Many thanks
  2. jcsd
  3. May 17, 2014 #2


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    Measuring what quantity "in between"?

    You are forgetting that c is a constant in ALL reference frame. The time taken in between one tile hitting the next may be a constant for all the tiles, but it isn't a constant for all reference frame. Someone travelling at v=0.9c, for example, will measure a different time for one tile to hit the other. Yet, everyone measures the came value for c.

    How would you reconcile that with your domino model?

    This isn't as easy as you think. If you read the FAQ in the General Physics forum, there is a discussion on the speed of light/photon in a medium, and it has a naive picture of what goes on.

    Any measurement of light will require an interaction. In fact, any measurement of anything requires an interaction. It is the only means to trigger a form of a signal. I don't have to always view light with my eyes. I can use a photodetector. So in that case, the final signal is no longer light, but electrons and a current.

    Whether it is the "same" original photon or not, I am not sure if it makes a difference within the context of what is being detected. Your eyes can't see radio waves and others outside of the visible spectrum, so already your eyes and optical system already lost a lot of other information coming from many sources.

    The thing you have to ask yourself is, when we observe the spectrum of light coming from these faraway sources, and when we analyze such spectrum to detect the various elements, to detect the red-shift amount, etc, what doubt is there that these are not correct simply because our eyes are doing what you are claiming they are doing?

  4. May 17, 2014 #3
    Sorry I am a layman.

    It is/was my understanding and assumption that a wandering photon will hit an atom's electron and get absorbed by it. And another electron somewhere else will (re)transmit a photon. Since we are surrounded by matter, and since we are matter ourselves, it follows that the photons that trigger our retina or some other sensor, have not travelled very far, they have just been emitted by another atom very, very near by.

    If the above is true then we have a relay of photons, like a domino. And the speed of the emitted photon is not related to the speed (or direction) of a previously swallowed photon.

    That in turn means that light is a sequence of those relays, absorptions and emissions, and therefore photons that have originated in a distant star or a near by street lamp are not the same photons that I actually detect using a sensor like my eyes or a photo transistor.

    That also means that all the photons that I perceive, using any of my sensors, my eyes, or a photo transistor, are MY photons, they have been emitted by some silicon substrate on top of the photo transistor or by my lens/iris/fluid - therefore these photons have been emitted by an atom that is always on the same reference plane as I am, because they are all my photons. Therefore I will always measure their speed to be the same regardless what it is I am actually measuring.

    I am not claiming anything, I am just thinking of a sequence of events.
  5. May 17, 2014 #4
    This assumption is only partially correct at best. Photons will not be absorbed when traveling through a transparent medium though the medium may affect the speed of light.

    But that's all besides the point since relativity doesn't claim that the speed of light is constant when light is traveling through a medium. It claims that the speed of light is constant when traveling through vacuum.
  6. May 17, 2014 #5


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    Your initial assumption is just not correct, andy_g.

    When you look at a source of light, you see the very photons that got emitted from it and had travelled continuously until hitting your retina. There is no in-between relaying involved.
    When photons get absorbed and reemitted, the reemission is in random direction, unlikely to be the same as the incident beam. This is perceived as increased opacity of the medium.

    An example is Rayleigh scattering of the sunlight. When you look at the setting Sun, you see only those photons from the initially white spectrum that did not interact with molecules in the air. Those which did interact got scattered in random directions. The more atmosphere is in the way(the lower the Sun over the horizon), the more photons fail to reach your retina. This results in dimmer and redder(red spectrum scatters less) Sun the lower it is. What is left there to see, however, comes at you directly from the photosphere of the Sun.
    These photons have travelled quite far indeed for you to see them.
  7. May 17, 2014 #6
    OK I will think more about that.

    One more question, again very in layman's terms, and I mean really laymans :)

    Can you have two objects moving away from each other? What happens when they reach a speed of c/2 each? Actually can we not say that for every particle in the universe there will be some other particle that may be moving at an opposite direction such that the relative speed between the two particles is greater than the speed of light? Would that then not imply that for the rule "nothing can move faster than light" that all particles in the universe are all linked together so nothing can ever move in some direction and speed that would be in danger of breaking the rule with respect to something else no matter how far away?

    Please excuse the primitive thinking.
  8. May 17, 2014 #7


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    Yes you can have two objects moving away from one another. You can have them each moving away from a central object in opposite directions at c/2 relative to that object.

    But that does not mean that they are travelling at c relative to each other. If you have the velocity of a with respect to b and the velocity of b with respect to c, the two velocities do not add in the straightforward way that you expect to produce the velocity of a with respect to c.

  9. May 18, 2014 #8
    why do we need a central object? can we not have a universe with just two particles in it? and can't those two objects move faster than light even if they do not know it?
  10. May 18, 2014 #9
    The central object isn't needed. The point remains that the faster than light speed doesn't exist. If the objects are moving at c/2 in opposite directions their relative speed is [tex]v_{rel} = \frac {v_1+v_2}{1+\frac{v_1v_2}{c^2}} = \frac {c/2+c/2}{1+\frac{(c/2)(c/2)}{c^2}} = \frac {c}{1+\frac{c^2/4}{c^2}} = \frac {c}{1+ 1/4} = \frac {c}{5/4} = (4/5)c = 0.8c < c[/tex]
  11. Sep 12, 2014 #10
    I think you ar right about eh domino effect. It can also explain why a photon acts like a wave.
  12. Sep 12, 2014 #11
    String theory also like domino effect


    The starting point for string theory is the idea that the point-like particles of elementary particle physics can also be modeled as one-dimensional objects called strings. According to string theory, strings can oscillate in many ways. On distance scales larger than the string radius, each oscillation mode gives rise to a different species of particle, with its mass, charge, and other properties determined by the string's dynamics. Splitting and recombination of strings correspond to particle emission and absorption, giving rise to the interactions between particles. (domino effect) An analogy for strings' modes of vibration is a guitar string's production of multiple distinct musical notes.[clarification needed] In this analogy, different notes correspond to different particles.

    In string theory, one of the modes of oscillation of the string corresponds to a massless, spin-2 particle. Such a particle is called a graviton since it mediates a force which has the properties of gravity. Since string theory is believed to be a mathematically consistent quantum mechanical theory, the existence of this graviton state implies that string theory is a theory of quantum gravity.

    String theory includes both open strings, which have two distinct endpoints, and closed strings, which form a complete loop. The two types of string behave in slightly different ways, yielding different particle types. For example, all string theories have closed string graviton modes, but only open strings can correspond to the particles known as photons. Because the two ends of an open string can always meet and connect, forming a closed string, all string theories contain closed strings.

    The earliest string model, the bosonic string, incorporated only the class of particles known as bosons. This model describes, at low enough energies, a quantum gravity theory, which also includes (if open strings are incorporated as well) gauge bosons such as the photon. However, this model has problems. What is most significant is that the theory has a fundamental instability, believed to result in the decay (at least partially) of spacetime itself. In addition, as the name implies, the spectrum of particles contains only bosons, particles which, like the photon, obey particular rules of behavior. Roughly speaking, bosons are the constituents of radiation, but not of matter, which is made of fermions. Investigating how a string theory may include fermions led to the invention of supersymmetry, a mathematical relation between bosons and fermions. String theories that include fermionic vibrations are now known as superstring theories; several kinds have been described, but all are now thought to be different limits of a theory called M-theory.
  13. Sep 12, 2014 #12


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    It is not correct to say that the photon is absorbed and re-emitted by each atom in the transparent material, because then the photon would be scattered in some different direction. For this picture to be correct, there must be an intervening state between photon absorption and photon emission. This means there has to be an excited atomic or molecular state that contains all of the photon's energy, and this state has some short lifetime before the photon is re-emitted, usually in some random direction. Because of the rules of quantum mechanics, this can only occur at very specific wavelengths which make up the absorption spectrum of the material.

    If the photon never ceased existence, it makes sense to call it the "same" photon, even if some properties have changed somewhat by a material.
  14. Sep 16, 2014 #13
    How sound waves are transmitted in the water? I think in the same way by sub-atomic level materials can transmit the light.
  15. Sep 19, 2014 #14
    Is there a simple experiment to show that the speed of light is measured constant regardless of the relative motion of the observers ? To clarify, not an experiment to show that there is no medium on top of which light travels. And not an experiment that depends on light's wave properties, eg phase, but rather an experiment that treats light as a stream of particles.

    Edit: clarified "simple experiment"
    Last edited: Sep 19, 2014
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