Speed of light is a universal constant

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.

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HallsofIvy

Homework Helper
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.

OlderDan

Homework Helper
aura said:
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.
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.

StatusX

Homework Helper
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.

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aura

HallsofIvy said:
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.
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]

OlderDan said:
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.
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...

aura

StatusX said:
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.
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

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StatusX

Homework Helper
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.

matt grime

Homework Helper
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.

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....

matt grime said:
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.
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...

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matt grime

Homework Helper
aura said:
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..:
"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.

rbj

HallsofIvy said:
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.
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 DOES depend on speed. We could do some kind of electro-magnetic experiment (i.e. light) to determine speed!
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.

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.
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

aura

matt grime said:
"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.
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.....

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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

matt grime

Homework Helper
aura said:
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

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.

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aura

matt grime said:
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.
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

aura said:
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
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

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russ_watters

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'.

Doc Al

Mentor
aura said:
...just seriously think how light can violate the principles that every other object follows..what is there in light that makes it so diff.
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.

Danger

Gold Member
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.

Danger

Gold Member
panthera said:
i had just posted 1 sentence...don't know how that vanished...i had posted that i do not believe in big bang!!...
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.

panthera

Danger said:
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.
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.

Doc Al

Mentor
Let's not turn this into a debate over the Big Bang. Please stick to the topic.

rbj

russ_watters said:
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'.
actually, Russ, i don't totally agree. we don't have all the answers (and i'll admit we never really will, because the more we learn, the more new questions that crop up) to the "why" questions, but if we ask the right questions, maybe someday we'll get answers to them.

we do perform crude experiments with out eyes and ears and skin and other senses. this fancy equipment of ours is really an extension of those senses. we judge distances relative to approximately the size of our bodies and we judge time relative to approximately the rate that our brains can perform simple compare-like operations (i heard, when we're about 16, we can do about 40 per second). it is no accident that a meter is roughly how tall we are and a second is roughly how long a heartbeat is. this is a "natural" anthropocentric result.

now the speed of light is simply what it is, but its numerical value depends solely on the anthropocentric units we have decided to use to measure it (the meter and the second). the gravitational constant and Planck's constant and the Coulomb force constant and Boltzmann constant are also simply what they are, but again, the numerical values they take on have to do with the anthropocentric units we have sorta arbitrarily decided to use.

if you want to get away from those man-made units, then you need to use Planck units (or something very similar) and then the speed of light is just naturally equal to 1. same for Planck's constant and G and $\epsilon_0$ and k.

the dimensionful numerical values they take on is just because of the size we happen to be (that is close to the meter) relative to the Planck length $$l_P$$ and the way we experience time (which is in the ballpark of a second) relative to the Planck time $$t_P$$.

now, i don't know why an atom's size is approximately $$10^{25} l_P$$, but it is, or why biological cells are about $$10^{5}$$ bigger than an atom, but they are, or why we are about $$10^{5}$$ bigger than the cells, but we are and if any of those dimensionless ratios changed, life would be different. but if none of those ratios changed, nor any other ratio of like dimensioned physical quantity, we would still be about as big as $$10^{35} l_P$$, our clocks would tick about once every $$10^{44} t_P$$, and, by definition, we would always perceive the speed of light to be $$c = \frac{1 l_P}{1 t_P}$$ which is the same as how we do now, no matter how some "god-like" manipulator might change it.

so the questions to ask are not "why is the speed of light what it is?", but is why we are as big as we are relative to the Planck length, why we think as slow as we do relative to the Planck time, and why things we deal with weigh as much as they do relative to the Planck mass. if we answer those questions, then we can answer why light travels 299792458 of these lengths we call "meters" in the time elapsed in one of these periods we call "seconds". the speed of light, Planck's constant, the graviational constant, and Coulomb's constant merely define, in our already established unit system, where the scaling or tick marks of nature are. once we look at these quantities from the perspective of nature's scaling, they're all naturally just 1 unit large. then it's like asking "why is it that a Newton of force is just the right amount that will accelerate a kilogram of mass at 1 meter/sec^2? why don't we measure it to be some other amount of force?"

best,

r b-j

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aura

i don't understand how people can correlate size of atoms with speed of light...size of anything is not a variable whereas speed can be variable....i am getting agitated sorry..its purely underestimating the question...how can 1 compare size and planks const etc etc with speed of light? the consts terms were found after the expression came into being but light exists in itself!!!!!! i see no point in the argument...see i am not a fool and have discussed this issue with many famous physicists so i cant tolerate this..it seems people have 0 knowledge or simply trying to stiffle the discussion.......

aura

What is light?

Its having both wave and particle nature. Hard core facts---- it consists of certain particles which we named as photons and the quantum physics has proved its existence.Then optical phenomenon like diffraction, interference, refraction,polarization etc have proved its wave nature…they are TRUE properties of light but cannot be quantified. Speed of light is NOT its internal property like the size or mass of any particle…it is something which is a variable quantity that is different from the constituents of light..it’s a PHENOMENON…a phenomenon is changeable but the internal properties are not…I don’t consider the speed of light as its intrinsic property as no object’s speed is considered so.well I better term it as velocity. For a moment forget about relativity.velocity of any particle in an atom need not be const always.it depends on the energy the particle posseses.we can see from the tunneling effect itself that more the number of attempts a particle makes, more is the probability of its emission[any particle] and the K.E in every case is diff which in turn implies that velocities are different…so while the SIZE(mass here) of the emitted particle remains same every time its emitted[ we have given fixed numerical values to every particle even to the degenerated ones, so mass is fixed], it can travel with diff velocities depending on the KE in diff cases. So velocity is something which VARIES…so its not comparable with quantities as size or color etc….Now considering relativity every objects vel is relative and there is NO SPACE for “absolute” in relativity.then why the vel of light is NOT relative…I hope atleast I could make the diff between size and velocity[meaningless!]

i dont know better ans than this for the above post.....

russ_watters

Mentor
rbj said:
we do perform crude experiments with out eyes and ears and skin and other senses.
Well, maybe I should have said scientific experiments. There is very little, if anything, left up to the senses in a laboratory experiment.

And yes, units are arbitrary, but that has nothing to do with my post.
aura said:
i don't understand how people can correlate size of atoms with speed of light...size of anything is not a variable whereas speed can be variable
Size is variable. The size of an object (distances) depends on the frame of reference of the observer.

There are a great many other misconceptions in your other post as well. I know its strange when you hear about it the first time but the universe really does work the way people are describing it. The evidence is incontrovertible.

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