World Science Scholars

3.1 The Speed of Light

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    • With a NATO standard grenade, once the pin is pulled, 3 seconds after the handle is released it will explode. Putting the pin back in will not stop the explosion! Don’t try it.

    • perfect

    • If I were riding on a photon at the speed of light, how would I experience space and time?

    • If I were riding on a photon at the speed of light, how would I experience space and time?

      • if you were riding on photon , it means that you are travelling at the speed of light. As you are travelling with constant speed , meaning you are in constant motion. From your prospective , entire universe is travelling at speed of light in opposite direction. As speed =Space/time (in broader sense), to make the speed of light to be constant , space must need to be shrink in the direction of motion and time need to be slowed down for you , but you are unaware of the effect, and has not way to find it out.

        • thanks for that fascinating insight sukhveer !

        • The basic assumption In special relativity is that light always moves away/towards you at the speed of light. Assuming (even theoretically) that you are “riding on the photon” invalidates that assumption. If you invalidate the assumption you also invalidate any further calculations or conclusions or more accurately any conclusion you draw does not draw into doubt any special relativity conclusion since you have invalidated one of SR’s basic assumptions.

    • Its is interesting and quiet non-intuitive as well, the fact that speed of light is constant irrespective to all perspectives.

    • very nice vid 🙂 i LOVE the speed of light!

    • very nice vid 🙂 i LOVE the speed of light!

    • very nice vid 🙂 i LOVE the speed of light!

    • very nice vid 🙂 i LOVE the speed of light!

    • very nice vid 🙂 i LOVE the speed of light!

    • very nice vid 🙂 i LOVE the speed of light!

    • Einstein simply had the audactiy at the time, and perhaps a little of his rebellious youth, to take what was considered to be Michelson’s and Morley’s experimental “failure” at face value.

    • Einstein simply had the audactiy at the time, and perhaps a little of his rebellious youth, to take what was considered to be Michelson’s and Morley’s experimental “failure” at face value.

    • Einstein simply had the audactiy at the time, and perhaps a little of his rebellious youth, to take what was considered to be Michelson’s and Morley’s experimental “failure” at face value.

    • I just have one question:
      If we travel with light i.e. we ride along a beam of light,according to einstien then, we must measure it at 300000 km/s,but that would indicate that another observer who is at rest wrt the the sun, stars, earth etc. should calculate the speed of light as 600000 km/s. But then that goes against what einstien had stated. So I just want to get this cleared, what prevents this from happening

    • I just have one question:
      If we travel with light i.e. we ride along a beam of light,according to einstien then, we must measure it at 300000 km/s,but that would indicate that another observer who is at rest wrt the the sun, stars, earth etc. should calculate the speed of light as 600000 km/s. But then that goes against what einstien had stated. So I just want to get this cleared, what prevents this from happening

      • The basic assumption In special relativity is that light always moves away/towards you at the speed of light. Assuming (even theoretically) that you are “riding along [with] the beam of light” invalidates that assumption. If you invalidate the assumption you also invalidate any further calculations and conclusions.

    • I just have one question:
      If we travel with light i.e. we ride along a beam of light,according to einstien then, we must measure it at 300000 km/s,but that would indicate that another observer who is at rest wrt the the sun, stars, earth etc. should calculate the speed of light as 600000 km/s. But then that goes against what einstien had stated. So I just want to get this cleared, what prevents this from happening

    • What if Gracie was running at a speed faster than that of light away from George and holding a meter that measures the speed of light and George fires a beam of light? What would the meter measure?

      • Nothing because the light beam would never reach the meter.

    • What if Gracie was running at a speed faster than that of light away from George and holding a meter that measures the speed of light and George fires a beam of light? What would the meter measure?

    • I used to imagine, like a thought experiment, what if I had a rod that was one light year of lenght, and I was to poke an object that was floating in space one light year out from mye earthly station… would that poke take me a whole year to execute?

    • Yes, at least a year. This is because you are transferring a force along the rod which is mediated by force fields between atoms. Ripples in those force fields cannot move faster than the speed of light.

    • That the speed of light is the same for all observers is actually, in a real sense, the simplest theory about nature that we can come up with:

      Imagine you are floating in space and there is absolutely nothing else. Are you moving or not? You have no idea – in fact the concept of moving really has no meaning in this context. The question is ill defined. And so is the concept of direction. Am I rotating or not? If I felt a centrifugal force then I would know but there is a real discussion, starting with Newton and carried on by Leibniz and Mach among others, as to whether I would feel a centrifugal force in an otherwise empty universe (https://mathshistory.st-andrews.ac.uk/HistTopics/Newton_bucket/). If I don’t feel any force then the concept of rotating loses its meaning here too.

      Lets shine a light and measure its speed (by, for example, holding a clock at arms length and starting the clock at the same moment I shine the light). The clock stops when the light beam hits it. The very simplest assumption is that I always measure the same value. Also, if a beam of light is coming to me the simplest assumption is to assume that the speed is also the same.

      In short, the theory in which we make the least amount of assumptions about the nature of space or the speed of light with no concept of movement or direction except movement of light with respect to me and no differences in the speed of light inbound or outbound – implies a constant speed of light outbound or inbound – with respect to me. Of course, the same argument applies to you. My outbound light travels with a certain speed and your inbound light travels with the same speed in this simplest theory. The fact that my outbound light beam may be your inbound light beam really has no bearing on the theory. I measure a speed c on the outbound beam, you happen to measure the same speed c on the same beam inbound.

      What is important to note is that the concepts of an object having speed or direction are constructs having nothing to do with space itself but only define interrelationships between objects in space. Except for the speed of light, space doesn’t “care” how or even whether you define speed or direction for objects. From its point of view these are merely artificial constructs that have nothing to do with “real” space. All space (or nature) cares about is to keep nature as simple as possible with as few assumptions as possible and that is by giving light the same speed coming or going. If that creates weird effects when making use of your definitions of object movement or direction then that is your problem not nature’s. Those weird effects are due to your artificial constructs (definitions) and have nothing to do with the nature of space.

    • I do have one point I would like to make towards this part and the following Office Hours (section 3.2).

      In both Brian Greene quickly brush over the concept of the Ether (or Aether) and how Einstein showed it wasn’t required, this is a mistake most people make when discussing this topic. Below is a list of quotes on the topic from Einstein’s May 5th 1920 address at the University of Leidon entitled “Ether and Relativity”, you can find the full transcript here https://mathshistory.st-andrews.ac.uk/Extras/Einstein_ether/ and appear in the order in which he states them. One quote found below which I would ask those to ponder is Einstein’s belief that all of Space is made up of the Gravitational Ether.

      “More careful reflection teaches us however, that the special theory of relativity does not compel us to deny ether. We may assume the existence of an ether; only we must give up ascribing a definite state of motion to it”

      “The special theory of relativity forbids us to assume the ether to consist of particles observable through time, but the hypothesis of ether in itself is not in conflict with the special theory of relativity. Only we must be on our guard against ascribing a state of motion to the ether.”

      “To deny the ether is ultimately to assume that empty space has no physical qualities whatever. The fundamental facts of mechanics do not harmonize with this view. For the mechanical behaviour of a corporeal system hovering freely in empty space depends not only on relative positions (distances) and relative velocities, but also on its state of rotation, which physically may be taken as a characteristic not appertaining to the system in itself. In order to be able to look upon the rotation of the system, at least formally, as something real, Newton objectivises space.”

      “Mach’s idea finds its full development in the ether of the general theory of relativity. According to this theory the metrical qualities of the continuum of space-time differ in the environment of different points of space-time, and are partly conditioned by the matter existing outside of the territory under consideration.”

      “The ether of the general theory of relativity is a medium which is itself devoid of all mechanical and kinematical qualities, but helps to determine mechanical (and electromagnetic) events.”

      “The ether of the general theory of relativity is transmuted conceptually into the ether of Lorentz if we substitute constants for the functions of space which describe the former, disregarding the causes which condition its state. Thus we may also say, I think, that the ether of the general theory of relativity is the outcome of the Lorentzian ether, through relativation.”

      “If we consider the gravitational field and the electromagnetic field from the standpoint of the ether hypothesis, we find a remarkable difference between the two. There can be no space nor any part of space without gravitational potentials; for these confer upon space its metrical qualities, without which it cannot be imagined at all. The existence of the gravitational field is inseparably bound up with the existence of space. On the other hand a part of space may very well be imagined without an electromagnetic field; thus in contrast with the gravitational field, the electromagnetic field seems to be only secondarily linked to the ether, the formal nature of the electromagnetic field being as yet in no way determined by that of gravitational ether. From the present state of theory it looks as if the electromagnetic field, as opposed to the gravitational field, rests upon an entirely new formal motif, as though nature might just as well have endowed the gravitational ether with fields of quite another type, for example, with fields of a scalar potential, instead of fields of the electromagnetic type.”

      “Since according to our present conceptions the elementary particles of matter are also, in their essence, nothing else than condensations of the electromagnetic field, our present view of the universe presents two realities which are completely separated from each other conceptually, although connected causally, namely, gravitational ether and electromagnetic field, or – as they might also be called – space and matter.”

      “Recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this ether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it.”

      • Thanks for posting this link. Indeed, Einstein very much states that an ethereal concept is required however we cannot ascribe any concept of motion to it. The concept of motion has no meaning in space – or in an ether concept – as I already conjectured in my previous reply. Motion is an object based construct.

    • Its not the ether Einstein is objecting against. Its the “stationariness” of the ether.We cannot ascribe either motion or stationariness to the ether.

    • Is the speed of light a constant or is it that we always measure it as a constant since any velocity we have as we measure it would affect the clock we are holding to measure it and thus change the V=d/t equation so that the answer is always the same?

    • Dark matter and dark energy may allow us to see light travel in a medium. This slows the light.

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