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# Light Speed, Are We Talking Relativity?

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If an observer is moving (at a high enough speed to be considered)  between two light sources (at fixed distance), does the light from both sources reach the observer at the same speed? I am led to believe the answer is yes.

Could someone either correct me or (even better if I'm right) explain the phenomenon?

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Without a concept of time, I don't see how you can have any concept of cause and effect and thus no way of making sense of experience.

Yes it does you are correct in thinking that refer to your previous post(http://www.scienceforums.com/topic/35879-light-speed-photons-and-redshift/).

The effects are not linear (red) but exponential, insignificant at common speeds and significant at near light speed. The H-K experiment used the earth center as the origin.

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If an observer is moving (at a high enough speed to be considered)  between two light sources (at fixed distance), does the light from both sources reach the observer at the same speed? I am led to believe the answer is yes.

Could someone either correct me or (even better if I'm right) explain the phenomenon?

Yes it does you are correct in thinking that refer to your previous post(http://www.scienceforums.com/topic/35879-light-speed-photons-and-redshift/).

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If an observer is moving (at a high enough speed to be considered)  between two light sources (at fixed distance), does the light from both sources reach the observer at the same speed? I am led to believe the answer is yes.

Could someone either correct me or (even better if I'm right) explain the phenomenon?

Yes you are right it does. This is what is observed.

Special Relativity accounts for this by deducing, from this observation, that measures of distance and time are different for objects in relative motion, so as to allow the speed of light to remain the same irrespective of the relative motion. (See any account of special relativity for details.)

As for "explain", it all fits if you accept that measures of distance and time are motion-dependent in this way.  That is far as "explanation" goes, counter-intuitive thought it may be.

Edited by exchemist
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So by speed you mean velocity=distance over time and by between you mean one light source behind you and one in front of you. The light source will travel a further distance and take more time to reach you from behind than from the front. Normally velocity is defined from an initial starting point so that if someone is tossing a football at you, your relative velocity to the football will increase if you move toward the football because you've decreased the time the football takes to reach you compared to the time it would have taken if you stood still. This doesn't happen if the football was light because c is the fastest relative velocity allowed so your velocity towards the light can't be added to it. So you're moving, your initial separation remains unchanged and  the light hits you sooner so using the formula for relative velocity = initial separation/time, the relative velocity should increase but it can't. So what gives? Relativity states that your movement warps time and space such that your initial separation magically shrinks and your time moves slower so the light has more time to cover less initial distance so your relative velocity to it can remain constant. My threads show another way this paradox in the definition of relative velocity can be resolved without using fairy dust and magic.

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If an observer is moving (at a high enough speed to be considered)  between two light sources (at fixed distance), does the light from both sources reach the observer at the same speed? I am led to believe the answer is yes.

Could someone either correct me or (even better if I'm right) explain the phenomenon?

The correct answer depends on his direction and initial position.

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Thanks to all and especially thanks to ralfcis for the big effort on
my behalf. Thanks to your explanation I suppose that the only possible
conclusion lies with time. What is really baffling is that we have to
accept that even relative velocity can't exceed the speed of light! We
are not expecting light to change its velocity, so it seems intuitive
to add and subtract the relative velocity. I recognise that you have
explained this ralfcis but its still very hard to grasp - especially
when I wonder what causes this constraint - and I suppose that leads
back to time and nothing else.
Even more baffling is the question of how photon wavelength changes
when its speed does not! I will start looking at all your recommended
Mattzy

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Thanks to all and especially thanks to ralfcis for the big effort on

my behalf. Thanks to your explanation I suppose that the only possible

conclusion lies with time. What is really baffling is that we have to

accept that even relative velocity can't exceed the speed of light! We

are not expecting light to change its velocity, so it seems intuitive

to add and subtract the relative velocity. I recognise that you have

explained this ralfcis but its still very hard to grasp - especially

when I wonder what causes this constraint - and I suppose that leads

back to time and nothing else.

Even more baffling is the question of how photon wavelength changes

when its speed does not! I will start looking at all your recommended

Mattzy

On the last point, you might perhaps consider this in the context of light having momentum and energy while being massless. Newtonian rules clearly do not apply!

For a photon, E=pc =hν.

The energy and momentum of a photon are proportional to its frequency. These do change with relative motion, due to the relativistic Doppler effect. Light from a receding source is red-shifted, which means its momentum and energy per photon are indeed reduced, as intuitively expected. The thing that - counterintuitively - does not change, though, is the speed at which it arrives at the receiver.

Edited by exchemist
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" I wonder what causes this constraint "

Mattzy I will answer your question in my relativity and algebra thread. Be it known I came on this forum in January a fully indoctrinated believer in relativity with just a couple of nagging questions. When I got the answers to those questions on the physics stack exchange, I turned on the cult of Einstein so anything you buy from me will be outside the mainstream.

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If an observer is moving (at a high enough speed to be considered)  between two light sources (at fixed distance), does the light from both sources reach the observer at the same speed? I am led to believe the answer is yes.

Could someone either correct me or (even better if I'm right) explain the phenomenon?

How do you measure the speed of a light pulse if you don't know when it was emitted?

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When it's turned on makes no difference. His question is not whether the light from the front and back will hit him simultaneously, his question is about whether his movement away from or towards a light will change his relative velocity to the light.

Edited by ralfcis
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When it's turned on makes no difference. His question is not whether the light from the front and back will hit him simultaneously, his question is about whether his movement away from or towards a light will change his relative velocity to the light.

It's not clear what his velocity is relative to the sources.

To measure a speed, requires an interval of time and space.

Where is that?

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His speed is irrelevant relative to the light it will always be c according to the velocity combo law. (v+c)/(1+vc.) always equals c no matter what value of v you plug in.

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Yes ralfcis, that is what I'm asking. Light does not behave like bullets fired from guns in front and behind a moving car (in the same context). The impacting bullets would hit at muzzle velocity plus and minus vehicle velocity respectively. But photons from the muzzle flash will all arrive at the same speed. (I'll copy this to sluggo).

I want to have a look at your threads as you may be the man to help me get to grips with this crazy monster!

I have been trying to resolve the question of redshift and blueshift while light speed remains constant and clearly have to start understanding relativity. You may be interested to see how my other post on that has been going along. Ref. two photons on a parallel course. But its all the same thing with no explanation on this site yet.

Thanks ralfcis.

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It's not clear what his velocity is relative to the sources.

To measure a speed, requires an interval of time and space.

Where is that?

Hello sluggo, ralfcis has perceived what I am asking - my phrasing may have misled you.

Light does not behave like bullets fired from guns in front and behind a moving car (in the same context). The impacting bullets would hit at muzzle velocity plus and minus vehicle velocity respectively. But photons from the muzzle flash will all arrive at the same velocity.

This is what I'm trying to grasp. I suspect it's the starting point in understanding special relativity - which is where I am right now!

What is even more baffling is that photon wavelength of the light changes - but only if the detector is moving towards or away from the source. Not for other photons moving in parallel that are not measured!  Maybe you know about this and can explain it?

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Hello sluggo, ralfcis has perceived what I am asking - my phrasing may have misled you.

Light does not behave like bullets fired from guns in front and behind a moving car (in the same context). The impacting bullets would hit at muzzle velocity plus and minus vehicle velocity respectively. But photons from the muzzle flash will all arrive at the same velocity.

This is what I'm trying to grasp. I suspect it's the starting point in understanding special relativity - which is where I am right now!

What is even more baffling is that photon wavelength of the light changes - but only if the detector is moving towards or away from the source. Not for other photons moving in parallel that are not measured!  Maybe you know about this and can explain it?

Mattzy I don't claim to be an expert on relativity (I'm a chemist), but I rather think it goes back to what I said in post 3 about the consequence of the speed of light being invariant is that measures of length and time MUST be observer-dependent.

The wavelength of light obviously depends on the observer measuring with his length scale. The frequency (cycles per second), involves a measure of time.  So if these scales differ in different frames of reference, measurements of wavelength and frequency will be affected.

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You're in a sealed train. You can't tell if you're moving or not. At the other end of the train, a light goes on and reaches you at a velocity c = length of train/time it takes to cross the train. Since light's propagation velocity is independent of the train's velocity (light's propagation velocity is relative to its medium), the light will always have that train's length to cross in the same time. But let's say you have two light detectors at your end, one outside the train and the other inside. You have two sources of light, outside and inside,  that go on together when they're side by side. If the train is moving toward the light you'd expect the outside light to hit the outside detector first because the train's motion has shortened the distance the light has to travel compared to the inside light which always has to traverse the full length of the train. Remember, the train's motion does not affect the speed of light inside the train or the distance that light has to traverse. The problem is the MMX proved that the light outside the moving train hits the end simultaneously on the outside as the inside light  does. This makes no sense from a standard definition of relative velocity. The train's velocity has no effect on the light speed and the light is traversing two different distances in the same time. So Einstein copied Lorentz and said the train must contractinside to match its length outside. Of course this is nonsense. What is happening is the time inside the train is slowed relative to the time outside the train allowing the light more time to cover the greater distance so it can arrive simultaneously with the light covering the shorter distance  outside the train. I hope you can see how wrong Enistein was in his crazy assumption about how the length of the train inside contracted while the outside remained the same.

Now relativists will lie to you and say light travels as a photon and not a wave. The photon as bullet would be affected by the movement of the train. It's not because a wave propagates at a velocity relative to its medium. But since light's medium is electromagnetic and not material (it's a vacuum) The vacuum inside the sealed train can't be pushed relatively to the vacuum outside the train. If the medium can't be moved, and the relative velocity of the light to the medium can't be changed, then the relative velocity of the light to the train can't be changed by the train's movement. The MMX verified the earth registers no velocity relative to light's medium whatever its nature.

Edited by ralfcis
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Mattzy I don't claim to be an expert on relativity (I'm a chemist), but I rather think it goes back to what I said in post 3 about the consequence of the speed of light being invariant is that measures of length and time MUST be observer-dependent.

The wavelength of light obviously depends on the observer measuring with his length scale. The frequency (cycles per second), involves a measure of time.  So if these scales differ in different frames of reference, measurements of wavelength and frequency will be affected.

exchemist, yes I'm sure you're right on the money there. I noted your post 3 at the time. Getting a grasp of it is going to be fun. falfcis has something to say too!

Victor

Mattzy I don't claim to be an expert on relativity (I'm a chemist), but I rather think it goes back to what I said in post 3 about the consequence of the speed of light being invariant is that measures of length and time MUST be observer-dependent.

The wavelength of light obviously depends on the observer measuring with his length scale. The frequency (cycles per second), involves a measure of time.  So if these scales differ in different frames of reference, measurements of wavelength and frequency will be affected.

exchemist, yes I think you are right on the money there, and I have noted your post 3 (I sometimes forget who said what while I'm replying though). Clearly that is what I'm going to have to struggle with.

Victormedvil talked of the redshift in measuring universal expansion and the theory of dark energy, that makes me wonder if redshift is only a phenomena in universal expansion. Maybe redshift doesn't occur in local movements or experiments (or across a solar system at right angles to universal expansion) if so, I suppose that would support the dark energy theory. I must check that out!

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