problem with michelson morley experiment?

Well for a start the Earth is not an inertial frame of reference because the earth is rotating and thus accelerating, which breaks the "inertialness" of the earths rest frame.

Reply 2

OP

Why isn't it possible for light to travel faster than c, from another inertial frame, ie from outside Earths magnetic field, light would whizz by at c + the speed of earth through the solar system.

Reply 3

div curl F = 0

PHKnows

Why isn't it possible for light to travel faster than c, from another inertial frame, ie from outside Earths magnetic field, light would whizz by at c + the speed of earth through the solar system.

Light is never observed to do this in nature and so this is a fundamental postulate of special relativity, which has big ramifications to the whole of physics. To actually pin down why this is so is like saying "why does mass produce gravity?" We don't know, all's we know is that it does and the consequences of this.

Reply 4

PHKnows

ie from outside Earths magnetic field, light would whizz by at c + the speed of earth through the solar system.

No it wouldn't. One of Einstein's postulates is that the speed of light is the same in all inertial frames of reference and is independent of the motion of the source.

Reply 5

OP

But einstein developed special relatively using the michelson-morley experiment which was carried out on Earth. There's no way that proves that light cannot move faster with respect to space unless experimants have been done up there as well?

Reply 6

teachercol

Actually they have. But the space up there is the same as the space down here.

Reply 7

From Einstein's postulates we can derive the Lorentz velocity transformation,

v_x' = \frac{v_x - u}{1 - uv_x /c^2}

, with

v_x

the speed in reference frame S,

v_x'

the measured speed in reference frame S', and u the speed of one frame relative to the other. When

v_x=c

, so the speed of light is c in the reference frame of the light, then the observed speed when measured in the frame

S'

is

v_x' = \frac{c-u}{1 - uc/c^2} = \frac{c(1-u/c)}{1 - u/c} = c

. Hence the speed of light is independent of the velocity,

u

of the reference frame from which the measurement is being made. It is an invariant.

Reply 8

OP

Lusus Naturae

From Einstein's postulates we can derive the Lorentz velocity transformation,

v_x' = \frac{v_x - u}{1 - uv_x /c^2}

, with

v_x

the speed in reference frame S,

v_x'

the measured speed in reference frame S', and u the speed of one frame relative to the other. When

v_x=c

, so the speed of light is c in the reference frame of the light, then the observed speed when measured in the frame

S'

is

v_x' = \frac{c-u}{1 - uc/c^2} = \frac{c(1-u/c)}{1 - u/c} = c

. Hence the speed of light is dependent of the velocity,

u

of the reference frame from which the measurement is being made. It is an invariant.

I think you contradicted yourself in the last two sentences. Speed of light is the same from every inertial frame? And has this been proven? And how?

Reply 9

Experimental evidence for Special Relativity

PHKnows

I think you contradicted yourself in the last two sentences.

It was a typo (now corrected).

PHKnows

Speed of light is the same from every inertial frame? And has this been proven? And how?

Reply 10

F1 fanatic

16

Lusus Naturae

From Einstein's postulates we can derive the Lorentz velocity transformation,

v_x' = \frac{v_x - u}{1 - uv_x /c^2}

, with

v_x

the speed in reference frame S,

v_x'

the measured speed in reference frame S', and u the speed of one frame relative to the other. When

v_x=c

, so the speed of light is c in the reference frame of the light, then the observed speed when measured in the frame

S'

is

v_x' = \frac{c-u}{1 - uc/c^2} = \frac{c(1-u/c)}{1 - u/c} = c

. Hence the speed of light is independent of the velocity,

u

of the reference frame from which the measurement is being made. It is an invariant.

Im not really sure you can use that as justification, since you've just proved the postulate on which the theory is based. Provided that the theory is self-consistent which any good theory must be it will obviously result. The constant speed of light was hypothesized and forms the fundamental assumption. Based on this, special relativity makes further predictions such as time dilation etc which can be experimentally verified.

Reply 11

F1 fanatic

Im not really sure you can use that as justification, since you've just proved the postulate on which the theory is based. Provided that the theory is self-consistent which any good theory must be it will obviously result. The constant speed of light was hypothesized and forms the fundamental assumption. Based on this, special relativity makes further predictions such as time dilation etc which can be experimentally verified.

I'm sorry as it seems I didn't make my point clear: self-consistency was the point of that post. I see the postulate as an axiom for a self-consistent Mathematical theory. The formulae of SR are derived from the two postulates and the ideas of Galilean / Newtonian Mechanics. The results predicted by the theory have been tested to an incredibly high accuracy over the past few decades, therefore we feel justified in using this theory as a Physical model. Does that make it clearer? :smile:

Reply 12

estel

18

Oh my; if I hear those two postulates again I might just shoot someone.

And has this been proven? And how?

I think you're approaching the problem from the wrong direction: Science is obviously based on theorems which cannot be proven, only disproven. To formulate them, you need to take some facts as unprovable but which must be accepted to be true: in this case, that the speed of light is invariant in all inertial frames and independent of the speed of its source. There's no mathematical proof for this: only countless experimental verifications that it is true. Without proving it, Einstein had to assume that it was true when deriving SR.

Reply 13

OP

You're missing my point. Has the speed of light, relative to space, been measured. Can physicists be certain that the speed of light is c, from the frame of reference of somewhere that does not move with the Earth, ie in space. And if they have measured it, how on earth, or off it, did they do it.

Reply 14

Wangers

16

PHKnows

You're missing my point. Has the speed of light, relative to space, been measured. Can physicists be certain that the speed of light is c, from the frame of reference of somewhere that does not move with the Earth, ie in space. And if they have measured it, how on earth, or off it, did they do it.

The fundamental flaw with your argument is in the highlighed text. Space is not an inertial frame of reference. There is no stationary frame of reference. Only degrees of relative motion. Hence what you are saying does not make sense.

What M&M did was prove that the speed of light was invarient both travelling te same distance paralelle and perpendicular to the motion of the Earth. Hence they did away with the theory of the aether. It does not exist, otherwise there would have been a frame shift detected.

Now on a more general scale, when Einstein proposed special relativity, raised to assumptions to the status of postulates (postuli?)

1) - Physical laws hold true in all inertial frames of reference, and therefore it follows that: there are no preveilaged frames.
2) The speed of light

C

in vaccumn is a fundamental constant regardless of the motion of either the source or observer and also independent of their relative positions.

I can't be bothered to post all the equations, but basiclly the consequence of the invarience of speed is either time dialation, or distance contraction. These are goverened by another constant - the lorentz factor.

from these, once you've worked out which frame is which ie

t

t'

or as the case may be

m

and

m'

or even

l

and

l'

you should be able to persue all necessary calculations at A level.

A word for the exam - I learnt the derivations for changing the subjects of those equations - because that kind of algebra easily buggers up, and spotting a mistake is time consuming. I'm not saying you should just learn to regurgitate those forms, understanding is vital in such questions, but its no good if you cant gimme the money as it were.

Reply 15

OP

but how do they know that the speed of light is c all the time, If they've only measured it from earth, that is my question.

Imagine you travelled to space and you could hold onto it, remaining stationary. The earthb would pass you by at some speed, and maybe the speed of light would be c + this speed. the michelson morley experiment can only be valid if its done in space, whilst earth passes by, ie by some massless technology.

Reply 16

teachercol