# Special Theory Of Relativity

ajclarke
Hello PF Folks.

So Einsteins Special Thoery of Relativity is based upon two principles (im shortening them for speed and effort):

1) Speed of light in a vacuum is independant of the motion of the source
2) Same laws of Physics apply in all inertial frames of reference.

My question is this, which of the two do you think is the most profound, meriting the name principle of relativity?

Gold Member
Hello PF Folks.

So Einsteins Special Thoery of Relativity is based upon two principles (im shortening them for speed and effort):

1) Speed of light in a vacuum is independant of the motion of the source
2) Same laws of Physics apply in all inertial frames of reference.

My question is this, which of the two do you think is the most profound, meriting the name principle of relativity?
Einstein postulated that the two halves of the round-trip measurement of the speed of light by any inertial observer are equal is what makes his theory stand out from all the others.

ajclarke
So you consider postulate 1 to be the most profound then? I thought this also personally since postulate one effectively removed the theoretical ether which then agreed with the MM experiment

Gold Member
So you consider postulate 1 to be the most profound then? I thought this also personally since postulate one effectively removed the theoretical ether which then agreed with the MM experiment
No, it does not effectively remove the theoretical ether because it is a postulate. It, in effect, affirms the possibility of an absolute ether rest frame, and, in fact, declares that any inertial frame of reference is indistinguishable from an absolute ether rest frame of the type that Lorentz proposed in his ether theory that predated Einstein's SR theory.

ajclarke
So i'm wrong in saying that Einstein's postulate was reinforced by the MM experiment because no ether drift was found to effect the value of c?

Gold Member
So i'm wrong in saying that Einstein's postulate was reinforced by the MM experiment because no ether drift was found to effect the value of c?
MMX was about the round-trip speed of light, Einstein's postulate arbitrarily defines both halves to be equal.

Staff Emeritus
My question is this, which of the two do you think is the most profound, meriting the name principle of relativity?
Neither one. This is like asking which of Newton's three laws of motion is most profound. In both cases the person doing the asking is missing the forest for the trees.

In the case of special relativity, neither one of the two postulates taken by itself is all that new. The roots of the first postulate, the principle of relativity, predates Newton. Galileo established the underpinnings of the principle of relativity in his Dialogue Concerning the Two Chief World Systems. The roots of the second postulate, the invariance of c, comes straight out of Maxwell's equations. While Einstein may or may not have been aware of the Michelson-Morley experiment, he most certainly was aware of Maxwell's equations. The second half of his 1905 paper on special relativity was devoted to Maxwell's equations.

What makes special relativity profound is the packaging of these two relatively simple but apparently conflicting principles as the basis for a new theory.

ajclarke
I did wonder that when It came up on the past paper for last year tbh. It seems a question that is personal and somewhat vague to me.

Either way, It had to be answered. I imagine the marks were given for either one argued, as long as the reasoning and the physics behind it were sound and clearly thought about.

I just thought I would see your inputs. =]

Gold Member
In the case of special relativity, neither one of the two postulates taken by itself is all that new. The roots of the first postulate, the principle of relativity, predates Newton. Galileo established the underpinnings of the principle of relativity in his Dialogue Concerning the Two Chief World Systems. The roots of the second postulate, the invariance of c, comes straight out of Maxwell's equations. While Einstein may or may not have been aware of the Michelson-Morley experiment, he most certainly was aware of Maxwell's equations. The second half of his 1905 paper on special relativity was devoted to Maxwell's equations.
What is the apparent conflict between Einstein's two postulates?

I ask because Maxwell, who was undoubtedly familiar with the two roots that you state, proposed a method to measure the speed of the solar system through the absolute ether rest frame. Don't you think someone as brilliant as Maxwell would have seen whatever conflict Einstein thought was apparent?

Staff Emeritus
What is the apparent conflict between Einstein's two postulates?

I ask because Maxwell, who was undoubtedly familiar with the two roots that you state, proposed a method to measure the speed of the solar system through the absolute ether rest frame. Don't you think someone as brilliant as Maxwell would have seen whatever conflict Einstein thought was apparent?
The conflict in the latter part of the 19th century between electrodynamics and Newtonian physics was in a sense much deeper than today's issue of reconciling general relativity and quantum mechanics. Electrodynamics and Newtonian mechanics were in direct conflict. There is no room in Maxwell's equations for a Galilean boost.

Maxwell believed in a luminiferous aether. He couldn't quite take his equations at face value. Maxwell died at a young age, eight years before the Michelson-Morley experiment. Poincare believed in an absolute but undetectable ether. In his philosophical writings he wondered whether the concept of an ether should be discarded, but in his physics writings he continued to use the concept of an absolute but undetectable ether well into the 20th century. Poincare could well have been taken to be the founder of special relativity had be taken his own philosophical musings to heart.

That's the key: The greatest minds of the time couldn't quite take their own works at face value. They were too attached to older concepts. Einstein's brilliance was in selectively keeping/discarding key parts of those older concepts.

Gold Member
The conflict in the latter part of the 19th century between electrodynamics and Newtonian physics was in a sense much deeper than today's issue of reconciling general relativity and quantum mechanics. Electrodynamics and Newtonian mechanics were in direct conflict. There is no room in Maxwell's equations for a Galilean boost.
Thanks for the historical background. I always love learning more about what these early pioneers went through.

I realize that there were a lot of conflicts going on during that time but I don't see how your statement that the real conflict between Electrodynamics and Newtonian mechanics could have been the apparent conflict that Einstein mentioned in his 1905 paper when he introduced his two postulates.

Staff Emeritus
Galilean relativity comes in two parts: That the laws of physics are the same in all inertial frames of reference and that all inertial frames are related by a Galilean transformation. The second part is so obvious that it is barely worth mentioning; that is just how the universe works. Except of course it isn't how the universe works.

Einstein kept the first part of Galileo's principal of relativity but threw out that second, barely worth mentioning part. The conflict between Newtonian mechanics and electrodynamics does not result from the principle of relativity itself. It results from the assumption of a Galilean boost that went along with the principle of relativity.

Gold Member
I don't think you are addressing my question. I'm not asking about a real conflict. I'm asking about an apparent conflict that is not real. I'm asking what was the apparent conflict between his two postulates that turns out not to be a conflict. (Einstein used the phrase "only apparently irreconcilable" when he introduced his two postulates.)

Staff Emeritus
The apparent conflict comes from the second part of Galilean relativity. I don't know how else to say it.

Homework Helper
I wouldn't say the first is "more profound" but the second is true in any "relativity" theory, including "Galilean relativity" that was the basis for classical physics. It is, therefore, the first statement that "defines" special relativity.

The "apparent conflict" in Galilean relativity is with the laws of motion due to eletromagnetic force which, of course, neither Galileo nor Newton knew anything about.

Gold Member
The apparent conflict comes from the second part of Galilean relativity. I don't know how else to say it.
But didn't you say the second part of Galilean relativity was:
that all inertial frames are related by a Galilean transformation.
And isn't that an actual conflict not just "only apparently" in conflict?

Staff Emeritus
Einstein threw that part out. He kept the first part, that the laws of physics are the same in all inertial frames.

lugita15
But didn't you say the second part of Galilean relativity was:

And isn't that an actual conflict not just "only apparently" in conflict?
Before Einstein, people thought that the principle of relativity MEANT that the laws of physics were invariant under Galilean transformations. So when it was discovered that Maxwell's Equations were NOT invariant under Galilean transformations, everyone including Maxwell himself immediately reached the conclusion that it contradicted the principle of relativity, and thus that it could only be true in one preferred frame. But then Einstein was able to show that the principle of relativity did not require the Galilean transformations. So he showed that there was no real contradiction between the assumption that Maxwell's equations held in all frames and the principle of Relativity, and that the only thing you need to throw out are the absolute notions of space and time which give rise to the Galilean transformations.

I don't think you are addressing my question. I'm not asking about a real conflict. I'm asking about an apparent conflict that is not real. I'm asking what was the apparent conflict between his two postulates that turns out not to be a conflict. (Einstein used the phrase "only apparently irreconcilable" when he introduced his two postulates.)
Galilean relativity has a prescription for the addition of velocities. According to Galilean relativity, the speed of light should depend on the speed of the observer. The constancy of the speed of light for all inertial observers (the 2nd postulate) therefore conflicts with Galilean relativity. In order to 'splice' these postulates together, the inertial transformations needed to be changed -- Lorentz symmetry supplanted Galilean symmetry.

roineust
Hey lugita15,

So at this point it seems - '...there is no contradiction between the assumption that Maxwell's equations held in all frames and the principle of Relativity...' - But this creates an apparent new contradiction, between time dilation and the invariance of C (that if I understand correctly, is what you call - 'Maxwell's equations held in all frames'), the contradiction is that for C to be invariant in all frames, it (C - light) can't itself undergo time dilation. So even under the replacement of the Galilean transformation by the Lorentz transformation, you have along and besides the new (Lorentzian) possibility of Maxwell's equations holding in all frames, other properties of the laws of physics, that have nothing to do with electro magnetism (but actually with matter), which must change between different inertial frames (in order for the new contradiction not to exist), and this apparently must restructure the principal of relativity against the notions of both Galileo and Einstein - Which affirms the possibility (when an experiment will take place) of an absolute notion of space and time.

Roi.

Last edited:
Yes, Roi. You've just proven Einstein wrong.

Gold Member
Hey lugita15,

So at this point it seems - '...there is no contradiction between the assumption that Maxwell's equations held in all frames and the principle of Relativity...' - But this creates an apparent new contradiction, between time dilation and the invariance of C (that if I understand correctly, is what you call - 'Maxwell's equations held in all frames'), the contradiction is that for C to be invariant in all frames, it (C - light) can't itself undergo time dilation. So even under the replacement of the Galilean transformation by the Lorentz transformation, you have along and besides the new (Lorentzian) possibility of Maxwell's equations holding in all frames, other properties of the laws of physics, that have nothing to do with electro magnetism (but actually with matter), which must change between different inertial frames (in order for the new contradiction not to exist), and this apparently must restructure the principal of relativity against the notions of both Galileo and Einstein - Which affirms the possibility (when an experiment will take place) of an absolute notion of space and time.

Roi.
Roi, have you given up on learning SR? I haven't given up on teaching you. You need to go back to your previous thread:
and finish our learning exercise. You can do it. I know you can.

Gold Member
So he showed that there was no real contradiction between the assumption that Maxwell's equations held in all frames and the principle of Relativity, and that the only thing you need to throw out are the absolute notions of space and time which give rise to the Galilean transformations.
I like the way you worded that. I think that is the answer I was looking for. Thanks.

roineust
Hey ghwellsjr,

Did not forget for a moment the exercise.
Will be there soon.

Roi.