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  1. #1 speed of light 
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    We say that 300 000km/h, the speed of light, is the fastest speed possible. And i have never gotten a good answer on why this is the case. Can anyone here help me out and tell me how we know nothing can be fa ster than light? The only thing I can think of is because of length contraction, but I know there must be a better reason. Can anyone please explain this?


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    I think you mean 300,000 km/s.. not km/h. Typo.

    I don't think you would like my answer to your question....

    But here it is anyway.

    I believe Einstein, in as far as his postulates in regards to special relativity, simply took that the speed of light was the fastest possible speed... sort of... a priori.

    Photons are massless. That might have something to do with it. :wink:

    But who knows... maybe there are particles that can travel faster. We just have yet to discover them....

    Cheers,
    william


    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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    oops didnt notice the km/h... well I don't mind your answer but there is something ive always assummed... are photons massless because of length contraction or is there some other reason?
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    Off hand, I don't think it has to do with length contraction (unless there is some deeper phenomenon going on...).

    From what I gather, you are asking "What makes a photon massless?"

    Hmmm.... :?

    Well...
    I really think you are not going to like this answer....

    I think they are massless because they just are....

    This is different than asking "Why is the moon round?" Here, "round" is not a fundamental property. There are indeed reasons why the moon has its current shape - and under different circumstances, it could take on a different shape. But ask the question "Why does the moon have mass?" and it becomes extremely difficult to answer. Mass is a fundamental property.

    In short, a particle either has certain fundamental properties or it doesn't... but I don't know why and I'm not sure that why has an answer....

    Cheers,
    william
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  6. #5  
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    You may like to read up on the "Higgs boson" though.... :wink:
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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    well i agree there does not have to be a reason why photons are massless but there might be because energy and matter are interchangable and I figured length contraction would make sense. And if it did then that would explain the speed of light because if through length contraction photons become wieghtless then you can't move faster and are also coincidently (dont think i spelled that right) at the fastest possible speed. Also I thought length contraction could explain light because you can see light head on but not from the side. I may not have spoken clear so if you want me to clear anything i said up let me know.
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    Then look up tachyons and that'll really confuse you....

    :wink:
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    lol... those tachyons are really confusing... all i got from them is that they go faster than light and can't slow down to below the speed of light... but anyone have anything to say about what I said before?
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  10. #9  
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    Quote Originally Posted by shawngoldw
    lol... those tachyons are really confusing... all i got from them is that they go faster than light and can't slow down to below the speed of light... but anyone have anything to say about what I said before?
    What you said before:
    well i agree there does not have to be a reason why photons are massless but there might be because energy and matter are interchangable and I figured length contraction would make sense.
    Only a minor adjustment to your statement... but energy and mass have a relation via E=mc^2, not matter.

    And if it did then that would explain the speed of light because if through length contraction photons become wieghtless then you can't move faster and are also coincidently (dont think i spelled that right) at the fastest possible speed.
    Again, a minor adjustment to your statement, massless - not weightless. There is a distinction. I'm not picking on you, but there are subtle differences.

    Also I thought length contraction could explain light because you can see light head on but not from the side. I may not have spoken clear so if you want me to clear anything i said up let me know.
    Hmmm...
    I believe that photons are point-like particles. If so, then there is no length to contract. Also, for any given massive particle, as it approaches the speed of light, its mass increases. If you have an extended object that is approaching the speed of light, while its length contracts, its mass also increases. Length contraction does not simultaneously decrease the mass. There is no correlation in that regard. But keep thinking. If nothing else, it's fun to think about these things.

    Regards,
    william
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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    I actually appreciate people shooting down my ideas! :-D ... Thanks for those minor corrections! but umm photons are packets of energy right? and they get formed when electrons get "bumped" off of atoms right? So they somehow become point-like particles and i guess i figured it was because of length contraction but you say mass increases when length contraction occurs, which I didn't know. And that kind of destroys my idea. So how do they become point-like?
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  12. #11  
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    Quote Originally Posted by shawngoldw
    I actually appreciate people shooting down my ideas! :-D ... Thanks for those minor corrections! but umm photons are packets of energy right? and they get formed when electrons get "bumped" off of atoms right? So they somehow become point-like particles and i guess i figured it was because of length contraction but you say mass increases when length contraction occurs, which I didn't know. And that kind of destroys my idea. So how do they become point-like?
    This is a tricky subject. Okay... photons are waves 'sometimes' and particles 'othertimes.' That is the "wave-particle duality." They are formed when electrons change energy states in atoms. And... curiously enough, each atom has its set of energy states that are distinct to that species of atom. By observing a photon and measuring its energy (i.e., wavelength), we can know which kind of atom it came from (e.g., hydrogen, helium, etc.). That's how we can tell what stars are made of. A photon hitting a plate and ejecting an electron (i.e., the photoelectric effect) can be thought of as a particle as it hits the plate. And yes... they are little packets of bundled up energy. Kind of like babies. In short, they are confusing little critters....

    Length contraction/mass increase:
    This falls in the regime of special relativity. Good ol' 'stein figured out that, as one approaches the speed of light, his mass will increase, his length will decrease (in the direction of travel), and time will slow down for him (though he will not notice it until he returns home to find his wife is now an old hag).

    Now, one minor point;
    you say
    ...but you say mass increases when length contraction occurs....
    Only if the object has both a length to contract and a mass to increase. In other words, if we imagine a massless stick approaching the speed of light, its length will contract but it has no mass to increase. The point being that they are not necessarily 'tied' to each other.

    Cheers,
    william
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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    okay! thanks
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    shawngoldw: light travels at 300,000,000 metres per second, but when you look at what a metre is, it's defined by the distance travelled by light in a given fraction of a second. And a second is defined as the number of "radiational" vibrations in a Caesium atom, which is again related to light. Relativity tells us that if you're travelling fast, you still measure light at 300,000 kilometres per second, because what you call a second and what you call a metre is different to what I call a second and a metre. Your velocity determines your metres and seconds, so quoting your velocity in metres and seconds is the wrong approach. It's the same with c. It isn't really a speed. This subtlety is at the heart of your problem.
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    im not arguiing the speed of light... im asking how we know it is the fastest possible.
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    Because it's infinite.
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  17. #16  
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    Hi shawngoldw

    There is nothing particularly special about the speed of light in particular except that it is invariant in all inertial frames (so if you are a "normal observer" you always measure the same speed for a light beam no matter how fast you are going). Its the fact that it is invariant that is special, and anything with this property would do (so you can imagine a universe without light but with river_rat rays that travel at a nice invariant pi m/s for all intertial observers) and you would still arrive at special relativity

    So lets do a little thought experiment with light- I'm flying towards you at some velocity v and you shoot a laser beam my way which i use to measure the velocity of light. Now no matter what my velocity is i always measure that light beam to be traveling at around 300 000km/s (even if i am going at 299 000km/s). Now that makes sense up until i am traveling faster then 300 000km/s at which something really weird has to happen for me to still measure the correct velocity for the light beam and this forces me to put an upper limit on my possible velocities i.e. i cant go faster then a light beam.

    Hope that made sense
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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    Quote Originally Posted by Farsight
    Because it's infinite.
    it is?
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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    as to the speed of light,

    the mass of an object increases exponentialy as the speed increases, up to the speed of light, where it is infinite (i beleive that is one of einsteins laws...). Therefore, when you get closer and closer to the speed of light, acceleration becomes closer and closer to being imposible, due to the increased mass, untill, when you are nearly at the speed of light, you require something of infinite mass in order to go further. in order to go faster still, you would need something of greater mass still, which is impossible.

    Photons have no mass, and so can reach the speed of light, explaining how they go so quickly.

    When accelerating a body to the speed of light, i beleive that time also slows down in relation to your suroundings, therefore once you have reached the speed of light, time for you in relation to other things has also stopped, and so you apear not to be moving.

    all this is based on theories that i have picked up from places, some may have been unrelaiable (GCSE Physics text books/a very intresting book called "the physics behind star treck") and so i may be entirly incorect in my asumptions, but i like to think enough of it is correct to offer some sort of explanation.
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    thanks, very well said... made a lot of sense and answered a lot of questions but it leaves me with 1 question... mass increases exponentially as something goes faster and at the speed of light it has reached infinite mass so nothing can go faster. But since a photon has NO mass when it reaches the speed of light, what stops it from going faster?
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  21. #20  
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    Just for the record - your mass does not increase exponentially as you approach the speed of light, thats a little too quick. It goes about as fast as v<sup>-1/2</sup> diverges at the origin.

    You cant talk about time and distance for someone traveling at the speed of light as these are not valid observers, a common mistake but a mistake never the less.
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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    because time stops? is that why they are invalid observations? but my problem is with understanding why nothing can travel faster than light. I don't really care how it would look or be observed.
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    Go back to my first post and read the thought experiment there - it should help.
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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    about the light coming at you and measuring it? that tells me what would happen but im confused about why it happens
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    How are you getting a "how" statement from my post? The how requires maths.
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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    im not getting a "how" statement... i said im getting a "what" statement. :P
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    Ditto for a what statement - which requires a different thought experiment.
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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    wait... what? what requires another thought experiment? now im confused!
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    ok...photons can not travel faster than the speed of light because of the relationship between time and the speed of light.

    I beleive that time also icreases exponentialy as speed increases (proved by planes flying round the equator...), until they reach the speed of light, where time stops. At this point, theoreticly if you go any faster, then you will travel backwards in time. It is not theoretcly possible for something to be in 2 places at once, and so it can not travel any faster. This would mean that it arrived before it set off, and so it was actually traveling backwards, so theoreticly, (just me guessing) the reason for a black hole being black could be that it is giving out particles faster than the speed of light, giving the apearance that it is "sucking stuff in" - unfortunatly, this wouldnt relay work, as the mater would have to come from somewhere, but its a nice idea...

    incedentally, the mass of a photon can not possibly be 0 according to Einsteins equation e=mc^2, and based on the fact that photons MUST have energy, in order to carry light, photons must have mass. Also, the simple fact that theyare affected by gravity, which only affects mass (i think) is proof enough that they should have mass. Having said that, assuming that Photons do have a mass would completly destoy our current understanding of physics, which could get a little confusing.
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    You were talking about special relativity there and theoretically if you travelled faster than light, yes you would go back in time but you wouldn't be in 2 places at the same time. you would just be back in time. So this doesn't explain my problem. Also I am pretty sure gravity does not only effect mass. And photons are massless.
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    1) if you travel back in time, you are in 2 places at once.

    ok, an example:

    I am in america.

    1 week lator, i go to europe

    then, i travel back in time, 3 days.
    i am then in america and europe at the same time.

    2) Photns have mass:

    Photons carry energy

    Mass=energy/speed of light^2

    Photons must have a mass - albeit very small.
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    you wont be in america and europe at the same time because your whole body goes with you back in time. You experience time differently than everyone else if you were to go faster than the speed of light (which you can't) but if you did you would experience time backwards and end up in the past, not be in america and europe at the same time. About photons ARE massless.
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    Aw come on guys. Time travel is bunk. It's as crazy as negative motion, or uneventing an event. Read this TIME EXPLAINED for more information.

    http://www.thescienceforum.com/viewt...t=4471&start=0

    The special thing about the "speed of light" is that it really isn't a speed. That's why you see it the same regardless of your velocity. A photon experiences total time dilation and so doesn't "experience" any time, but it does "experience" events, including the oscillation events that give it a wavelength. These wavelengths are happening in a no-time situation as far as the photon is concerned, but they have distance by our definition, so with zero time experience the photon's speed is infinite. It's arguably more proper to say the photon's speed is undefined because the Lorentz contraction factor reduces the wavelength to zero, but that tells you nothing. Apart from reminding you that c isn't a speed. And that's the thing you have to grasp.

    Speed is distance over time, and if your distances and times have gone out of the window, your speed has too. What you're left with is a c which is a fundamental property of the universe. Call it the elastic limit of space. Or how fast things happen. Or the speed of time. Anything but the speed of light.
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    oooo that's what you guys have been saying... Now it all makes sense! I already knew everything you said there but had never put it together like that... Thanks a lot guys!
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  35. #34  
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    Quote Originally Posted by Farsight
    Aw come on guys. Time travel is bunk. It's as crazy as negative motion, or uneventing an event. Read this TIME EXPLAINED for more information.

    http://www.thescienceforum.com/viewt...t=4471&start=0

    The special thing about the "speed of light" is that it really isn't a speed. That's why you see it the same regardless of your velocity. A photon experiences total time dilation and so doesn't "experience" any time, but it does "experience" events, including the oscillation events that give it a wavelength. These wavelengths are happening in a no-time situation as far as the photon is concerned, but they have distance by our definition, so with zero time experience the photon's speed is infinite. It's arguably more proper to say the photon's speed is undefined because the Lorentz contraction factor reduces the wavelength to zero, but that tells you nothing. Apart from reminding you that c isn't a speed. And that's the thing you have to grasp.

    Speed is distance over time, and if your distances and times have gone out of the window, your speed has too. What you're left with is a c which is a fundamental property of the universe. Call it the elastic limit of space. Or how fast things happen. Or the speed of time. Anything but the speed of light.
    Um farsight- you do realise that non of that made any sense what so ever. A photon experiences no time dilation as time dilation is what inertial observes observe between their different frames of reference and any reference frame strapped onto a photon is not inertial (it is not even a frame of reference as any observer in a reference frame measures his velocity to be zero, but in a photon reference frame he would still measure his velocity to be c). So you cant talk about the lorentz contraction and time dilation of a photon for starters. Secondly your velocity is something other observers calculate, in your eyes you are stationary and they are flying all over the place. So why exactly cant you call c the speed of light?
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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    river_rat:

    Um farsight- you do realise that non of that made any sense what so ever.
    That's a bit sweeping and dismissive. Try reading it again.

    A photon experiences no time dilation as time dilation is what inertial observes observe between their different frames of reference and any reference frame strapped onto a photon is not inertial (it is not even a frame of reference as any observer in a reference frame measures his velocity to be zero, but in a photon reference frame he would still measure his velocity to be c). So you cant talk about the lorentz contraction and time dilation of a photon for starters.
    You're nitpicking and using "reference frame" as a fig leaf here. You really can talk about these things, just imagine yourself going faster and faster.

    Secondly your velocity is something other observers calculate, in your eyes you are stationary and they are flying all over the place. So why exactly can't you call c the speed of light?
    Because c is a velocity, and velocity defines your experience of distance and time, so you can't define it using speed = distance / time. Read TIME EXPLAINED for more.
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  37. #36  
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    Quote Originally Posted by Farsight
    That's a bit sweeping and dismissive. Try reading it again.
    Have reread it and your response and it still does not make sense.

    Quote Originally Posted by Farsight
    You're nitpicking and using "reference frame" as a fig leaf here. You really can talk about these things, just imagine yourself going faster and faster.
    Special relativity is all about reference frames - so if you are using something that does not count as a reference frame then you are in serious trouble. Going faster and faster is not the same as going at the speed of light - in fact starting at any speed with as much acceleration as you could wish for you would not make it to c in finite time, its the bane of having mass. Can you see the problem with your work around, even if you let v -> c on the left you still have that v < c for all time, which is a very different case to the one you are suggesting.

    [quote="Farsight"Because c is a velocity, and velocity defines your experience of distance and time, so you can't define it using speed = distance / time. Read TIME EXPLAINED for more.[/quote]

    No, velocities are in the equations for how other intertial observers translate your observations into their own frames to make sense of them. There is nothing sinister about that fact, as it has always been the case, since Galileo and newton. In any one frame you have the same definition of a second (the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.) and thus the same definition of a meter and none of that depends on how fast you are going (which is a meaningless concept of itself - we are going at 0.95c relative to some places in the universe and we don't notice anything funny, you need to state who is measuring your velocity). So your whole thesis seems to be a bit shaky.
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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    Quote Originally Posted by river_rat
    So your whole thesis seems to be a bit shaky.
    That doesn't seem to be sweeping and dismissive enough. :wink:
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    Quote Originally Posted by Ophiolite
    Quote Originally Posted by river_rat
    So your whole thesis seems to be a bit shaky.
    That doesn't seem to be sweeping and dismissive enough. :wink:
    Lol - should i use your choice phrase in the swearing thread
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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    Sorry about this shawngoldw. Here's a link to the wiki article on special relativity:

    http://en.wikipedia.org/wiki/Annus_M...ial_relativity

    Here's an excerpt from TIME EXPLAINED:

    What is Time? Let’s start by looking up the definition of a second:

    "Under the International System of Units, the second is currently defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. This definition refers to a caesium atom at rest at a temperature of 0 K…”

    OK, a second is nine billion periods of radiation, of light. Now, what’s a period? We mentioned light, so let’s have a look at frequency:

    Frequency = 1 / T and

    Frequency = v / λ

    Flipping things around, I see that period T is wavelength λ divided by velocity v.

    A wavelength is a distance, a thing like a metre:

    “The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second...”
    Both a second and a metre are defined in terms of the "speed" of light, but a speed is distance divided by time. This is a circular definition, and lies at the root of your problem.

    It's why c is not a speed.

    Imagine you're in a spaceship going very fast towards a planet that's 6 light years away. OK you have a "reference frame" and you know you're undergoing time dilation and length contraction compared to back on Earth, but this doesn't affect you locally, and you measure c at the same old value.

    You don't consider your velocity to be zero, but you're not sure what it is so you look at the known stars passing by and calculate that your "speed" is .75 light years per year. Then you put your foot on the gas and see the stars go by at a "speed" of 1.5 light years per year. But you're also confused, because when you measure forward distances via radar they appear to have shrunk. Then when you arrive at your destination you find that more than 6 light years have passed according to their clocks. Your realise that your "speed" didn't really exceed 1 light year per year, but according to your clocks it took you 18 months to travel 6 light years.

    You know that the distances in the Universe didn't really shrink in one direction just because you were travelling fast, so you look at the time. A light year is the distance travelled by light in one of your years, and your years are all over the place. They vary according to your motion as a fraction of c. OK you have to compare it against the motion of those stars and planets and the people on them, but nevertheless this dictates how you experience time relative to them. Time is not objective, it isn't something fundamental that's actually out there. It's subjective, to you and your motion. Relative motion if you like, but the Universe and those people are out there.

    You can't rely on the time because it's subjective, you know your seconds are different to their seconds. So you can't talk sensibly to those guys about your velocity of .75c in terms of distance and time. And since speed is distance over time, you can't talk about c as a speed either.
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    We say that 300 000km/h, the speed of light, is the fastest speed possible. And i have never gotten a good answer on why this is the case. Can anyone here help me out and tell me how we know nothing can be fa ster than light? The only thing I can think of is because of length contraction, but I know there must be a better reason. Can anyone please explain this?
    When you have mass, more energy is required to move. If you take for example a clock made of aluminum parts that ticks out perfect 1sec intervals, and you changed all the parts in that clock with a material 10x more massive than gold, the seconds on that clock would reduce.

    Thus when you observe time to speed up in a reference frame, its mass must be reduced relative to you, in order for actions to be performed at the velocities you see them performed at. Such as, a scientist in his labratory moving around at what looks like hundreds of mph, he would have to be very low in mass.

    likewise, when a fast traveling frame is observed, and its time slows down, this would suggest its mass would be increased relative to that observer at rest. That is, when each and every particle is heavier time as it were may only go so fast, because energy to make motion is limited.



    So if you see time dilate. You can pretty well conclude the rest of observable physics.
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    One more addition.

    If you are talking about being in first person view of going near light speed. Its works a tad different.

    As you speed up faster and faster, the time of events around you speed up. That is a clock runs faster at a planet way our infront of you as you speed up.

    So a catch 22 of sorts comes along. When speed up, and so does the time around you. So even though space contracts - http://en.wikipedia.org/wiki/Special_relativity - the time speeds up, and the mass is dependant on time so mass decreases outside your frame.

    So eventually, there will come a point where you would convince the universe to run at infinite time and zero mass if you traveled the speed of light. Obviously this would violate a few laws. But even if you travelled at 99.999999% the speed of light, light would still be measured to travel at C or (aprox 300,000,000m/s).

    The thing that complicates this the most is our conception of time. Your mind remains constant. Or that is a frame remains constant. So while everything you observe in your -spaceship as it were- appears to remain the same, at this high velocity, your space ship is dilated with time, mass, and length to all kinds of different observation points.

    There is a fundamental thing at work. This is a frame of reference. You can not leave one, and they can not change. That is where constants come from. Thus a frame is all there is, and a frame has zero dimensions. It is a present moment, and a spaceless experience (at the basic operation). Everything observed is observed in that place, a frame, and a observation POINT, is a frame that has zero dimension.

    So everything else you see in the universe can fluctuate to meet your frames needs, in relation to physics. That is nothing is constant outside your frame. Only in your frame.

    Mathamatics shows this relationship, I just meant to give you the simple concpet for starters.

    [/url]
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    I figure time is spatial for a photon, why it can oscillate without moving a distance, since any distance moved along time would not be observeable to us. Hence it can vibrate without really vibrating. Why c is told to be max.

    Since IMHO, time will be spatial, if it can.

    So is time spatial for a photon?
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    space-time one thing. Energy is space and mass is time.

    E=MC^2

    E / M = C^2

    C = D / T

    E / M = D^2 / T^2 ---> Energy = space and mass = time.

    In everyday life there is space then there is time.

    But in the trueness of physics, it is one thing. space-time.



    --------------------

    Testing latex

    [latex]\frac {E}{M} = \frac {D^2}{T^2}[/latex]
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    What is Time? Let’s start by looking up the definition of a second:

    "Under the International System of Units, the second is currently defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. This definition refers to a caesium atom at rest at a temperature of 0 K…”

    OK, a second is nine billion periods of radiation, of light. Now, what’s a period? We mentioned light, so let’s have a look at frequency:

    Frequency = 1 / T and

    Frequency = v / λ

    Flipping things around, I see that period T is wavelength λ divided by velocity v.

    A wavelength is a distance, a thing like a metre:

    “The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second...”
    Okay here is our first problem - this whole argument is a bit loopy! You are taking little bits of physics from all over the place and misapplying them to a single problem. A second is a counting problem, count the pulses of caesium-133 and when you have counted around 9 billion you will find that one second has passed. What the equation magic is trying to show is anyones guess, if you have to think of the resulting photons hitting a photon detector.

    Both a second and a metre are defined in terms of the "speed" of light, but a speed is distance divided by time. This is a circular definition, and lies at the root of your problem.
    Once again, the speed of light does not come into the definition of time - all you need is something regular, hell any harmonic oscillator will do!

    It's why c is not a speed.
    Huh?

    Imagine you're in a spaceship going very fast towards a planet that's 6 light years away. OK you have a "reference frame" and you know you're undergoing time dilation and length contraction compared to back on Earth, but this doesn't affect you locally, and you measure c at the same old value.
    You are not undergoing anything, other observers notice length contraction and time dilation in your frame (nothing local here) but you notice the same thing in their frames. It seems as if you have some universal preferred frame in mind here and we should compare ourselves to that one.

    You don't consider your velocity to be zero, but you're not sure what it is so you look at the known stars passing by and calculate that your "speed" is .75 light years per year. Then you put your foot on the gas and see the stars go by at a "speed" of 1.5 light years per year. But you're also confused, because when you measure forward distances via radar they appear to have shrunk. Then when you arrive at your destination you find that more than 6 light years have passed according to their clocks. Your realise that your "speed" didn't really exceed 1 light year per year, but according to your clocks it took you 18 months to travel 6 light years.
    Actually would consider your velocity to be zero, put yourself in a sealed space capsule and you would not believe you are moving. Velocity is dependent on who is taking the measurement, there is no such thing as your actual velocity. This whole explanation seems wrong to me as well. You would only measure 18 months in your example, it would not be something found after the fact - every clock would have only counted out 18 months.

    You know that the distances in the Universe didn't really shrink in one direction just because you were travelling fast, so you look at the time. A light year is the distance travelled by light in one of your years, and your years are all over the place. They vary according to your motion as a fraction of c. OK you have to compare it against the motion of those stars and planets and the people on them, but nevertheless this dictates how you experience time relative to them. Time is not objective, it isn't something fundamental that's actually out there. It's subjective, to you and your motion. Relative motion if you like, but the Universe and those people are out there.
    Yep, there we go - preferred inertial reference frames and so we are not talking relativity here. The distance really does shrink, accept that and move on. You seem to be giving distance some mythical existence beyond the idea that it is a measurement.

    You can't rely on the time because it's subjective, you know your seconds are different to their seconds. So you can't talk sensibly to those guys about your velocity of .75c in terms of distance and time. And since speed is distance over time, you can't talk about c as a speed either.
    Huh again?
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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    Actually you would consider your velocity to be zero..
    This is where your talk of reference frames gives shawngoldw no answer at all. He's just seen a lightbeam go past him at 300,000,000 m/s, so he chases it, but he can never catch it. And you think you're explaining it by basically telling him: you can never go faster than the speed of light because your speed is always zero and the universe shrinks.

    If you'd like to answer shawngoldw's question to his complete satisfaction, go ahead. Otherwise listen: The answer is related to the nature of time, and it is subjective rather than objective and fundamental.
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    Quote Originally Posted by river_rat
    Hi shawngoldw

    There is nothing particularly special about the speed of light in particular except that it is invariant in all inertial frames (so if you are a "normal observer" you always measure the same speed for a light beam no matter how fast you are going). Its the fact that it is invariant that is special, and anything with this property would do (so you can imagine a universe without light but with river_rat rays that travel at a nice invariant pi m/s for all intertial observers) and you would still arrive at special relativity

    So lets do a little thought experiment with light- I'm flying towards you at some velocity v and you shoot a laser beam my way which i use to measure the velocity of light. Now no matter what my velocity is i always measure that light beam to be traveling at around 300 000km/s (even if i am going at 299 000km/s). Now that makes sense up until i am traveling faster then 300 000km/s at which something really weird has to happen for me to still measure the correct velocity for the light beam and this forces me to put an upper limit on my possible velocities i.e. i cant go faster then a light beam.

    Hope that made sense
    LOL. I rest my case.

    Sorry about this shawngoldw. I'll give river-rat time to give a better answer to your original question.
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    so... does it have anything to do with what farsight said about photons experience total dime dialation which means that they would go as fast as possible and since they go 300 000km/s we know that is the fastest possible?
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    Quote Originally Posted by shawngoldw
    so... does it have anything to do with what farsight said about photons experience total dime dialation which means that they would go as fast as possible and since they go 300 000km/s we know that is the fastest possible?
    Ignore Farsight - he is talking crap. Photons do not experience time dilation, as you cannot strap a inertial reference frame to a photon. Light travels at the speed of light because that is what it does, this velocity is special though because no matter what your velocity is you always get the same velocity for a photon. What i am trying to explain is why an invariant velocity forces you to have a universal speed limit, it is more fundamental then time dilation etc which only says you cant accelerate past the speed of light not that nothing can travel faster then that speed.
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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    I'll chip in and say: Yes, but I'd suggest a better way of getting a handle on it is to say photons move at c in no time at all.
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    Quote Originally Posted by Farsight
    I'll chip in and say: Yes, but I'd suggest a better way of getting a handle on it is to say photons move at c in no time at all.
    What does that even mean? How do you measure a photons velocity? Take two points in your reference frame and place a T.A. with a clock at each of those points. Then shoot a photon past them and get each one to note the time the photon passes, work out the time difference and then you arrive at the velocity by the normal equation v = s/t.

    Where does this no time idea come from?
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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    Quote Originally Posted by river_rat
    What does that even mean?
    Put some thought into it. Measure the velocity of an object in your reference frame, then accelerate to a new reference frame to match it. Measure the velocity of another object, then accelerate to a new reference frame to match it. Repeat. And explain to shawngoldw why he can't catch that photon.

    this velocity is special though because no matter what your velocity is you always get the same velocity for a photon..

    Actually would consider your velocity to be zero..
    Come on, do you have any velocity or not, and why can't you catch that photon? Something rather more substantial than it's special.

    How do you measure a photons velocity? Take two points in your reference frame and place a T.A. with a clock at each of those points. Then shoot a photon past them and get each one to note the time the photon passes, work out the time difference and then you arrive at the velocity by the normal equation v = s/t. Where does this no time idea come from?
    Godel and Einstein, 1949. And before that, Presentism, 1908.
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    Wouldnt the fact that some subatomic particles can affect each other instantaneously even if they are on the other side of the universe (see quantum entanglement) mean that they can travel faster than light ?

    relevant article :

    HERE

    ----edit by jeremyhfht, to reduce the fact it eats the page coding when you post insanely long links. Use the URL bind feature to prevent this. -----
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    I don't think so Gus. I don't think "affect each other instantaneously" is the right way of thinking about it. I think it's more like slicing a coin down the middle and each of us keeping half. You take a peek and find you've got heads, which means I've instantaneously got tails. Aw, I'm skeptical about quantum computing and David Deutche's "multiverse", which I think hinges upon the wrong concept of what a photon actually is. But I can't justify this without writing a whole essay about it, so all I can offer is this bit of negative opinion for now.
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    Quote Originally Posted by river_rat
    Where does this no time idea come from?
    From the point of view of the photon. (Remember there only is one photon, travelling eternally and recursively from the start of the Universe to its end, all in a timeless instant. )
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    Quote Originally Posted by Farsight
    I don't think so Gus. I don't think "affect each other instantaneously" is the right way of thinking about it. I think it's more like slicing a coin down the middle and each of us keeping half. You take a peek and find you've got heads, which means I've instantaneously got tails. Aw, I'm skeptical about quantum computing and David Deutche's "multiverse", which I think hinges upon the wrong concept of what a photon actually is. But I can't justify this without writing a whole essay about it, so all I can offer is this bit of negative opinion for now.
    Thanks for the explanation - however dont quantum computers work ?
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    Quote Originally Posted by Gus
    Thanks for the explanation - however dont quantum computers work ?
    do you see them being used?
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    Sorry, of course they do Gus. I tell you what, nip on down to PC World over the weekend and ask one of the sales guys to give you a demo of one.

    8)
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    Hello!

    I think this is not fully correct. The speed of light is a measure expressing some velocity
    which for today was pretty fast. Interesting I think was Einstein himself said nothing can
    be faster than the speed of light and he himself squared c which was said to be the
    broadening of light in the vacuum. There are other interpretations what c was but this was
    one of them. Right? Please, don't hesitate in correcting me.

    It's quite interesting how the same man does say nothing can be faster than light and
    almost at the same moment alters this statement to the opposite judgement.

    Please, same here, as you think I'm wrong please let me know.

    Now, what I could think of is that the statement nothing was faster than light was
    deriving from not knowing how to measure speeds that are, or might be faster than the
    speed of light. Light itself as a measure (laser today ), for example, had to be omitted.

    In physics as a fact the speed of devices used today were not to increase significantly as
    inertia would take effect on any kind of bodies in motion.

    That means, for the reason of inertia taking effect even the speed of light was a velocity
    not to reach and to survive if you hadn't a couple of years left to accelerate and having
    a couple of years left to negatively accelerate and to get to a full stop again.

    The region but you had as a test site was got to today. Space. So, while I was thinking
    about gravity and interesting different statements of different scientists by what it was
    originated I noticed the supposed relation of acceleration and speed or (and ) velocity
    was not really mentally to grasp for me. I noticed as well the term of a tempo which was
    share of rhythm but was an option to kind of doge to and was still left uninterpreted in
    physics. As I thought the speed of light or a velocity well beyond would eventually just be
    reached in space anyway I was confronted with other difficulties more of simpler nature
    like how to measure the motion of an alleged space ship in terms of a spot in space
    from which to measure and how. Like hanging in space freely (hi guys and girls ) and
    having a stop watch in your hand. And this was quite funny to think of as you can
    imagine.

    I'm left here just to say, the speed of light is number which is of such a large scale we
    don't measure these days. This was one point making waves. Second was the speed of
    light as with todays approaches we will not get in the reach of surely, but more as for a
    philosophical status than in real culture.

    In fact one as I think will move space ships in kind of a energized hull, separating the
    ship informatively from space completely and so taking away the chance to set a starting
    point to measure the ships speed from there. While the hull will move with the ship, the
    ship will actually neither being accelerated not reaching a speed but be changing position
    with a today unknown art. Sorry I don't have an other word at hand.

    So don't let yourself be fooled by the number of about 300 000 m/s. This is just a number
    and will not really effect the material or the humans or physical bodies. It will be possible
    but be done technologically and differently than todays approaches are.

    Steve
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    k... guys this was what seemed to be my answer... since photons don't experiuence time they move at the fastest speed possible, and since everyone views light at 300 000km/s we know that is the fastes possible speed. is this finally the correct answer?
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    Quote Originally Posted by shawngoldw
    k... guys this was what seemed to be my answer... since photons don't experiuence time they move at the fastest speed possible, and since everyone views light at 300 000km/s we know that is the fastes possible speed. is this finally the correct answer?
    Thats a bit convoluted, but anyway. Light travels at light speed because that is what it does - light speed is special because that is the universe we find ourselves in. Thats about as deep as it gets until you have a TOE that actually specifies these constants.
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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    Quote Originally Posted by river_rat
    Quote Originally Posted by shawngoldw
    k... guys this was what seemed to be my answer... since photons don't experiuence time they move at the fastest speed possible, and since everyone views light at 300 000km/s we know that is the fastes possible speed. is this finally the correct answer?
    Thats a bit convoluted, but anyway. Light travels at light speed because that is what it does - light speed is special because that is the universe we find ourselves in. Thats about as deep as it gets until you have a TOE that actually specifies these constants.
    May I say that you learnt from the best of the best?

    I see you agree with my example. In the other thread.

    But I guess you allready knew all that.
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    Hello!

    Have you ever seen the attempt with the kind of two plates positioned on opposite sides on kind of a
    slide?

    Than, there are light particles or others particles bouncing back and forth between the two plates. The
    slide starts moving forward.

    I'm not, at the moment, really know what it is being good for but feel some of you got confused by this
    experiment, the set up of it. It was said to be measuring or illustrate some of Einstein's 'theory' I think.

    In fact, what was not mentioned several times I saw the experiment was if the slide will move forward
    the particles of light between the plates will not follow the then moving plates fixed to the slide but
    succumb inertia and will be drawn out of the back side of the slide by it.

    I'm not really sure why I have been bringing this up but think here was some wrong explanation confusing a few of you
    and me as well, and, it was related to some kind of a light issues I would say. But, obviously. as we could today reach
    the speed of light not cause of it was light speed only about the number 300 000 m/s which is for today's standards
    'mad fast' and reaching the velocity seen in strict sense even not, one has to prove the velocity some way or have some
    gauges showing the actual position after full stop.

    Its surly a topic for mathematical calculation but having a measure in some way we are used to these days as well. So,
    do not muddle up the number 300 000 m/s and the actual speed of light which was said of having these same number as
    a physical characteristic (with sight of a more complex thinking related to science previously, and craftsmanship but as a
    need to let the issues thought about come true ). Sorry for the first even to me confusing version. Its hard to think here
    where I'm still living, oftentimes.

    Cheers,
    Steve
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