# Speed of light is a universal constant

1. Jun 18, 2005

### aura

I wonder that speed of light is a universal constant but why its so that it doesnt change wrt observers in different ref frames. an object moving with some velocity V1 wrt another object will be at rest wrt a third object (that is also moving with a velocity V1). but speed of light will be same be we on earth or on moom irrespective of their motions.Does anybody ever thought of the same thing or has anything to say about this?

ps-i tried to post it in general forum but couldn't access that hence posting here.

2. Jun 18, 2005

### HallsofIvy

I don't know that anyone can say "why" the speed of light is independent of frame of reference- the fact that it is constant is confirmed by experiment.

One way to look at it is this: Force equals mass time acceleration and we "feel" forces. If we were in a closed box moving at a constant speed no (mechanical) experiment could tell us the speed since there is no acceleration to give a force.
BUT the force due to a magnetic field on a charge DOES depend on speed. We could do some kind of electro-magnetic experiment (i.e. light) to determine speed! That's exactly what the Michaelson-Morley experiment was designed to do- and it gave a null result. The only way to reconcile those is to postulate that the speed of light is independent of the reference frame.

3. Jun 18, 2005

### OlderDan

Certainly people have thought about it. For a long time most people assumed there had to be some medium that light traveled through (the ether) at constant speed, and that if you were moving through that medium you would see light traveling at a different speed wrt to yourself. Experiments were performed to measure the speed of the earth through this ether. The novel thought that the speed could be the same to everyone regardless of their state of motion is a relatively recent idea, and the foundation of special relativity.

Imagine a universe where light could be caught by an observer, or even passed. Suppose photons carried with them little snapsots of some event, and you could catch up to and collect photons that left that event and travelled off into space. As you collected and viewed them, you would be seeing the event in reverse time order, like running a movie film backwards. What's more, as far as you know the event took place at some point in front of you instead of behind you. Rather preposterous isn't it? At least as preposterous as the idea that everyone sees light travelling at the same speed. Of course photons do not carry snapshots, and from a wave perspective light behaves very differently from what I described, but the idea of traveling faster than, or catching up to light always leads to a rather preposterous view of the universe.

4. Jun 18, 2005

### StatusX

Newtonian physics is actually inconsistent with Maxwell's electrodynamics. Imagine you have a uniform line charge of positive charge per legth $\lambda$, and there is a positive test charge nearby. The charge feels some repulsive force F. Now switch to a frame where the wire and charge are moving along the direction of the wire. The line charge is the same (according to Newton), so the E field is the same. But there is also a magnetic field which pulls the charge in. The force is different, and thus the movement of the particle depends on the frame you're in, which is clearly wrong. I'm suprised this wasn't a bigger deal back in the 19th century, but they probably had all kinds of ad hoc explanations to patch up the holes.

Relativity made electrodynamics consistent again, and in fact showed how electricity and magnetism are two aspects of the same thing. For this particular situation, the resolution is that length contraction compresses the wire, making the charge per unit length increase. This increases the repulsive electric force just enough to counter the attractive magnetic force and yield the same particle trajectory in all frames.

Last edited: Jun 18, 2005
5. Jun 20, 2005

### aura

what you are saying is there is no means to measure the speed when we are in the system thats true but every system in motion has its own velocity and the velocity of any other object will depend on the vel of the ref frame of observer but the same is not true incase of light though this holds good for sound then why light is only ruled out?In Michaelson-Morley experiment, change in speed was not detected as the shortenning of one arm of the interferometer just cancelled the fringe shift...can there be SOME basic problem that gives this const theory of light[i am sounding wierd, i know]

your 1st para is michaelson morley experiment...just seriously think how light can violate the principles that every other object follows..what is there in light that makes it so diff. i already gave the example of sound wave

statusX i will reply to u...

6. Jun 20, 2005

### aura

well this is the description of what the principle of relativity states...but again as EM wave holds good is nothing of much significance as light is a form of EM wave...so well i reiterate that why its so that the properties of this particular wave does not change while they do for all other. is it because we perform experiments with eyes and it cant catch the change in the medium (light here) on which the observation is based on..i can see only this difference in light and other waves that sound wave's perception is by our other sensory organ i,e ears and not light!!(eyes) and hence we can measure the change by observing objectively..r u getting what i am trying to say? then we will never be able to find the change in its speed as the thing we are using for doing so itself depends on light....i am wierd ..don't take it seriously

Last edited: Jun 20, 2005
7. Jun 20, 2005

### StatusX

c is the universal speed limit. Anything travelling at c in one reference frame will travel at c in all others. This is just the way the universe works, and it would be so even if there was no such thing as electromagnetism or light or eyes. The reason light happens to travel at c is because photons are massless, and relativity predicts that all massless particles will travel at c.

8. Jun 20, 2005

### matt grime

why is this in homework?

anyway, no theories give a causal explanation of the phenomenon; it just is (as far as we can tell).

sound waves are not the same as light waves. light exhibits wave particle duality for a start and the wave portion is latitudinal, whereas sound waves, which require a medium in which to propogate (unlike light) are longitudinal, though i find it hard to understand what you're saying here.

9. Jun 20, 2005

### aura

well i just meant that is it not possible that our observation is limiting ourselves somewhere? anyway it has been in a lot in disussion so i am not quite enthusiastic abt discussing it all over again.only wanted to know other's opinions here. i will have this curiosity if not others that why its so...i know its the principle on what universe works that is of no concern...i am not discussing its implications....i don't want to say anything and i don't think that i should at least not at this stage ...may be i am wrong and also may be its 99.99999999.....9% correct that i am wrong but i want to explore the .000000.................1% genuinity of what i think[not in this particular matter but everything]....i couldn't get any proper view and its fine as its not expected also....

can you link your explanation to the Q asked..please stick to the topic...its known to every XII th std student what light and sound waves are...please don't undermine the question (don't digress from topic) well forget it ...

ps-where it should be? i don't know..i am new here...

Last edited: Jun 20, 2005
10. Jun 20, 2005

### matt grime

"can you link your explanation to the Q asked..please stick to the topic...its known to every XII th std student what light and sound waves are...please don't undermine the question (don't digress from topic) well forget it ..."

i replied to your own question. not that it hjas a question mark to indicate where it ends. as i said, it is difficuilt to decipher what you mean; try writing more clearly before accusing others of wandering off topic.

wrong? right? who knows, so far i've not been able to ascertain what it is you're saying. nor do i care now.

11. Jun 20, 2005

### rbj

i think i have a feel for why the speed of E&M propagation should be the same for all inertial reference frames. it really just comes from Maxwell's Eqs. and the knowledge that there is no ether medium that E&M is propagated in (that's what the Michaelson-Morley experiment null result really meant). i mean, how do we tell the difference between a moving vacuum and a stationary vacuum? if we can't, if there really is no difference between a moving vacuum and a stationary vacuum, that such a concept is really meaningless, then whether the light that you are measuring originated from a flashlight mounted on a rocket moving past you at $c/2$ or from a stationary flashlight, how does that change the fact that a changing E field is causing a changing B field which is causing a changing E field, etc.? that propagation of an E field and B field disturbance, which has velocity $1/ \sqrt{ \epsilon_0 \mu_0 }$, how is this propagation different whether you are moving with whatever device initiated it or moving relative to that device (after this E&M disturbance has moved away from that device)? whether you are holding the flashlight or moving past it at high velocity, Maxwell's Equations say the same thing regarding the nature of E&M in the vacuum.

the force due to a magnetic field on a charge is a psuedoforce that is really a manifestation of electrostatic forces but with the effects of special relativity tossed in.

the null result of the Michaelson-Morley experiment really meant that there was no ether that was a medium of absolute reference for light or any other E&M wave to propagate in. sound has air, but light has no counterpart to air for it to propagate in. the null result was a surprise for them. they tried the experiment at different times of the year expecting the earth to be moving through the ether at some time as it revolved around the sun. and the speed of the earth moving through space was a sufficient fraction of the speed of light that the interference fringe shift in the experiment should have been noticeable (and it was not noticeable).

not meaning to dispute anything, Halls, just wanted to add another POV.

r b-j

12. Jun 21, 2005

### aura

My question is in my first post if you care to read that...i am not supposed to repeat that again and again in every post. my posts are well linked to my original Q but your ans doesn't have any correlation with the original Q..thats why i said so...try finding your fault rather than just blaming others...u find lack of clarity in my language but i find lack of connectivity in your suggestions and what is asked for..lets forget it...you must have obtained more than me in AWA but not more than point 1 i am sure well i didn't get what i expected as the topic i chose was not known to me that much ...i know least abt politics but i chose that as it was having morality in it which i didn't want to leave

cyao

ps- well everyone here is just repeating the michaelson moreley experiment but not trying to find the why behind the ans...as i told ether couldn't be detected as the fringe shift was nullified by the contraction of one arm of interferometer and that happenned as we were in the system...anyway thanks for all answers.....

Last edited: Jun 21, 2005
13. Jun 21, 2005

### aura

wow i just noticed that someone called madness also had posted this sometimes before...i have read relativity and i have solved problems considering C a const as that is the fundamental law of relativity but still i am not convinced ahy its a const as only einstein said was that (i am para phrasing)if the vel of light were not a universal const then the principle of relativity would fail and a special inertial frame would be singled out(the one at rest in the ether) but the form of maxwell's eqns as well as failure of any experiment to detect motion through ether, suggests that the speed of light is constant independent of the motion of the source. absence of ether preserved the simplicity of the principle of relativity.

BUT this is no explanation of the reason behind the phenomenon.his argument was based on the fact that vel of light predicted by elctromagnetic theory involves no reference to a medium and hence c MUST be a constant...thats all..NO reasonings

14. Jun 21, 2005

### matt grime

you posted more than one question and i was answering what seemed like another question, a question that i quoted back when you asked what i was respondng to (is it... then something about light and sound and sensory organs... , is it generally indicates a question) which apparently asked about the diffrence between light and sound. and now we've wasted yet more posts on it.

I have no idea what AWA is, and no interest in comparing "scores" with you.

as hallsofivy (or was it hurkly) said in the very first post we have no "why" explanation (it is a postulate), as for a great many things in science.

Last edited: Jun 21, 2005
15. Jun 21, 2005

### aura

i posted ONE question and what you told was just the explanation of something that i asked to illustrate my point in original question.so your every answer should relate to the first question even if u are answering the 2nd one...sound waves are longitudinal and requires medium in no way associates as to why its change in speed can be detected while change in speed of light cannot be? my illustration does include the fact that light is the medium that we are using for detecting the change in speed of light or in other words we are using a medium to detect the change in the medium only...if you couldn't see the question in my post which was

yes i forgot to put Q mark and if u couldn't get because of that then may be i am wrong, don't know!...

and yes i don't have any interest to argue on this matter with you.answer it if you know else don't...else try to find the reasons and then answer.i also don't want to compare my scores with you as you are no way my compettitor[stick to the topic]...don't want any further argument here..if u want u can pm me...lets stick to the topic here ..better say that "einstein didn't mention any why to it!!" not that there is "no why" to this as neither you nor i can say that...have you read about the quarks in quantum mechanics...my reasoning for this Q lies near to some logic similar to one of its properties....but i wont discuss it anymore...reason-you can and should understand...all the best and thanks a lot....

Regards

Last edited: Jun 21, 2005
16. Jun 21, 2005

### Staff: Mentor

Well, I can answer one question for you: we do not perform experiments with our eyes. Our eyes are only used to read the results, not to make the actual measurements.

As for the "why" - "why" is not a question adequately dealt with by science. For that, you may need religion because eventually the long string of "why"s will have to end with either 'God made it that way' or 'it just is'.

17. Jun 21, 2005

### Staff: Mentor

Actually all objects and light follow the same rules, at least as far as their measured speeds go. What's different about light is that its speed happens to equal the universal speed limit.

18. Jun 21, 2005

### Danger

All laws of nature appear to be random consequences of the specific conditions of the Big Bang. In another universe (parallel to ours, or if ours had formed differently, or preceding ours), c could be 50,000,000 kph, Planck's Constant could be 6, Avagadro's Number 14.8, etc. This just happens to be the one that we live in. To know why any of these things are as they are, we would have to know every single detail of the initial conditions. It's extremely unlikely, if not impossible, that we ever will.

19. Jun 21, 2005

### Danger

You have every right not to, but I'm sticking with it until something better comes along. So far 'Steady State' has been disproven, and the alternative is something supernatural. If evidence leads to a different scientifically derived theory, I'll give it equal credence.

20. Jun 21, 2005

### panthera

do you know some stars have shown a blue shift instead of red shift? and scientists always refute this fact..i had once attended a lecture of jayant narlikar in IUCAA and he was much convincing and it did strengthen my perspective.