# Question on General Relativity from a layperson

## Main Question or Discussion Point

I'll begin by admitting that I did not scroll through the entire list of threads to see if there was already a post similar to the one I am beginning.
I do apologize if I've broken any rules but I gave up searching after 6 pages and noticed there were at least a hundred more.

My question is,
How general is general relativity?

As a layperson best described as having an inquisitive mind and an abiding respect for learning I wonder if the term, "general" is appropriately applied when it seems applied to the specific field of physics.

I have observed that all things in nature are related on at least some level through which "general relativity" should be truly generally applied.

I concede that I may be naive or that my basic understanding of nature may be faulty or misinformed.
My posting here is an honest attempt to seek validation from those who devote their time/life to study in areas of which I only have a fundamental understanding.
I mean no disrespect nor offense when I say that to my understanding of nature and the word relative, relativity is manifest in all matter and all matter is related.

I applied why to the question of life by asking, "why is life?", "what function does it serve?" as a philosophical exercise.
After eliminating all known possibilities, I concluded that life's purpose is to exist and in that, all life forms are universally related to one another.

Related Special and General Relativity News on Phys.org
Hi,
as far as I know, general is Vs special, or restricted. The latter, is actually applying to inertial frames only, i.e. stating the laws of physics are actually the same in all inertial frames, while the general one is somehow an extended ( i.e. general ) version of the special one, leading to same laws of physics for all frame of reference, even for the accelerated ones.

The principle of relativity was already known from Galileo, however, but the Einstein version is actually encompassing a new law, i.e. the speed of light is constant in vacuum. So, since the laws of physics are the same in all
inertial frames, and the constantness of speed of light IS a law of physics, you get the speed of light is constant in all inertial frames.

It's always difficult to be precise using the proper words, and I'm not a professional physicist, so eventually sorry if it was not fully clear.

Regards

Ricky

atyy
General relativity is generally considered badly named. General relativity is a theory of gravity.

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A.T.
How general is general relativity?
The generality of general relativity is relative. It is more general than special relativity.

Riccardo, and others, I appreciate the responses.
While the last is a fun play on words, it offers no explanation and the second only acknowledges that the term is inappropriate while not offering an answer as to why the inappropriate is acceptable.
Riccardo's response explains what the term means in the world of physics but I think misses the point of my question.
If relativity is accepted in one area of nature, is it not applicable to all areas of nature and if so, does that not make it truly general as well as a fact rather than a theory?

PeterDonis
Mentor
2019 Award
If relativity is accepted in one area of nature, is it not applicable to all areas of nature and if so, does that not make it truly general as well as a fact rather than a theory?
The theory of relativity (general and special--but special relativity is really just a sub-theory of general relativity, so we can consider them one theory for this discussion) serves two main functions in physics:

(1) It covers all non-quantum phenomena involving gravity. The "non-quantum" qualifier is necessary because GR is not a quantum theory. It is believed, however, that whatever the correct quantum theory of gravity turns out to be, it will have GR as its low-energy, classical limit, so the fact that GR is non-quantum does not make it wrong; it just limits its domain of validity.

(2) It provides a framework in which theories covering all types of phenomena, whether they involve gravity or not, can be developed and tested. For example, when scientists analyze the results of particle physics experiments (such as those done in the LHC), relativity plays a crucial role in the analysis; without it, we would not be able to make sense of the experiments. These experiments don't involve gravity at all (technically, gravity is present since the LHC is on Earth and not floating in deep space, but the effects of gravity on the results are far too small to matter, or even to detect), but we still need relativity as a framework to analyze them.

Note that quantum effects do play a crucial role in these experiments, so it's not that we don't know how to combine quantum physics with *special* relativity; quantum field theory does that. But we don't know how to add gravity to the picture and still retain all the quantum stuff. (Actually, that's not quite correct: we can do quantum field theory in curved spacetime, where gravity is present, as well as flat spacetime; but we can only do that by treating the spacetime, and hence the behavior of gravity, classically, not quantum mechanically, so it's still not a real combination of gravity with quantum physics.)

Bandersnatch
It looks to me like a simple misunderstanding between the meaning of relativity as in "things are related", and relativity as in "motion is relative".

Not at all Bandersnatch.
That motion is relative enhances my sense that all things in nature must obey the laws of nature equally in proportion to their time and space, regardless of velocity or influences from other natural forces.
In this way, all things, including relativity are related in that they are in existence within nature and therefor must obey the laws existent within nature.
My curiosity is in trying to learn why would anything not be considered relative or applied to the full spectrum of life both as beings in existence and in our attempts to understand specific areas within the spectrum.
I am a wordsmith of sorts and I find integrity in the root meaning of words unblemished by the lack of imagination required to create new words when redefining old words is easier.
I suspect that the physicist finds as much integrity in equations as I do the root meaning of words though I wonder if it is possible that specialists are looking too closely to fully realize how their special area relates to all other areas of life both personally and collectively.

For instance, my neighbor is a math teacher in our local school district and yet I found him trying to force a 4 inch screw into a 3 inch void without trying to solve for why it wasn't working.
Being handy, I saw the problem immediately but showed him how to use his taught math skills to work out the solution.
Upon explaining it to him as an algebra problem, he readily resolved the problem with a shorter screw.
In doing so, he found that he can apply his teaching to all areas of his life and hasn't been found trying to drive a screw with a hammer since.

Nugatory
Mentor
If relativity is accepted in one area of nature, is it not applicable to all areas of nature and if so, does that not make it truly general as well as a fact rather than a theory?
I am somewhat concerned by the way that you're using the words "fact" and "theory" here. In lay usage (where people are sloppy about these things) a "theory" is something uncertain, provisional, or speculative. However, within the scientific community, a theory is something much more solid: a way of organizing and understanding facts, and which has been demonstrated to produce good predictions and new insights.

For example, it is a fact that every single object that I have ever dropped or seen dropped in my entire life has fallen to the floor. This fact leads me to accept the theory of gravitation; and this theory in turn leads to insights about the behavior of the planets orbiting the sun, and much more besides. And even though gravitation is "just a theory", it makes a prediction about what will happen the next time I drop a waterglass - and I doubt that you would bet against that prediction. Nonetheless, we must not forget that the observed falling of dropped objects is a fact; gravitation is a theory that explains this fact.

Thus, when we say that relativity is a theory, we aren't limiting its scope and applicability. Special relativity works everywhere that spacetime is flat; general relativity agrees with special in the case of flat spacetime, and also works in non-flat spacetimes. You may reasonably object to these names, but nomenclature once established is hard to correct - in the US European settlers have been referring to the descendants of the indigenous population as if they were inhabitants of the Indian subcontinent for centuries.

Thus, when we say that relativity is a theory, we aren't limiting its scope and applicability. Special relativity works everywhere that spacetime is flat; general relativity agrees with special in the case of flat spacetime, and also works in non-flat spacetimes. You may reasonably object to these names, but nomenclature once established is hard to correct - in the US European settlers have been referring to the descendants of the indigenous population as if they were inhabitants of the Indian subcontinent for centuries.
This is exactly what I meant in my first reply, even if my version was a bit more fuzzy. "General" must be intended as a word about the "extension" of the domain in which the principle of relativity
can be applied. So the special one, which of course a subset of it, applies to inertial frames only, while the general applies to accelerated frames as well ( which is a different way of stating this is a theory of gravity, of course ).
So it's just a matter exactly of accepting the nomenclature. If you are looking to "general" as a sort of "Great Unified Theory" this is not true, of course. As stated in a wise previous post, the quantum effect are currently not taken into account by general relativity, so this is not "general" in that sense.

UltrafastPED
Gold Member
"Special Relativity" provides the rules for all of the laws of physics when they are observed in "inertial reference frames". Maxwell's equations for electrodynamics were already in this form, and it was quickly shown that it was possible to reformulate Newton's Laws of Motion to follow these rules - it was not possible to express Newtonian gravity so that it would follow these rules. Our most accurate rules for quantum mechanics also obey these rules.

"General Relativity" is an extension of these rules for the laws of physics in any reference frame, including accelerated, rotating, etc. With these extensions it was possible to create relativistic equations for gravitation. However, we do not yet have a version of quantum mechanics which obeys these rules - this is a great problem.

Nugatory
Mentor
So the special one, which of course a subset of it, applies to inertial frames only, while the general applies to accelerated frames as well ( which is a different way of stating this is a theory of gravity, of course ).
You and I seem to be in agreement about the big concept here, so I feel a bit diffident about disagreeing on a tangential question... but I have to say it...

Special relativity applies to and works just fine for accelerated frames as well as inertial frames, as long as the spacetime is flat. The math is appreciably messier and often contributes no new insights, so you don't see examples of SR applied to non-inertial accelerating frames in most introductory courses - but it works.

The step from SR to GR is the step from flat to non-flat spacetime, regardless of acceleration.

Nugatory
Mentor
"Special Relativity" provides the rules for all of the laws of physics when they are observed in "inertial reference frames".
.... and non-inertial frames as well, as long as the spacetime is flat. Try googling for "Rindler coordinates" to see one example of how SR successfully handles acceleration.

"The theory was originally termed "special" because it applied the principle of relativity only to the special case of inertial reference frames, i.e. frames of reference in uniform relative motion with respect to each other.[7] Einstein developed general relativity to apply the principle in the more general case, that is, to any frame so as to handle general coordinate transformations, and that theory includes the effects of gravity.

The term is currently used more generally to refer to any case in which gravitation is not significant. General relativity is the generalization of special relativity to include gravitation. In general relativity, gravity is described using noneuclidean geometry, so that gravitational effects are represented by curvature of spacetime; special relativity is restricted to flat spacetime. Just as the curvature of the earth's surface is not noticeable in everyday life, the curvature of spacetime can be neglected on small scales, so that locally, special relativity is a valid approximation to general relativity.[8] The presence of gravity becomes undetectable in a sufficiently small, free-falling laboratory."

Or from here:

http://en.wikipedia.org/wiki/Special_relativity_(alternative_formulations)

you can read better what I meant:

"As formulated by Albert Einstein in 1905, the theory of special relativity was based on two main postulates:

The principle of relativity — The form of a physical law is the same in any inertial frame.
The speed of light is constant — In all inertial frames, the speed of light c is the same whether the light is emitted from a source at rest or in motion. (Note this does not apply in non-inertial frames, indeed between accelerating frames the speed of light cannot be constant.[1] Although it can be applied in non-inertial frames if an observer is confined to making local measurements.)

There have been various alternative formulations of special relativity over the years. Some of these formulations are equivalent to the original formulation whereas others result in modifications"

When I said the definition of special relativity I really meant the REAL definition gave to this theory at the beginning when it was born. As saw above, currently you can think of it as a special case in which the space is flat for sure. But the usual way of defining is different, and it's more closed to Einstein's initial definition.
Anyhow, this is just a matter of terminology as always. That's why I consider post like these ones pretty unuseful, just because we are talking a bit the naming of things, instead of their contents.

As Feynman told us "it doesn't matter if you call a bird "robin", or if you call it in many different languages. All in all when you see it, you can recognized it".

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PAllen
2019 Award

"The theory was originally termed "special" because it applied the principle of relativity only to the special case of inertial reference frames, i.e. frames of reference in uniform relative motion with respect to each other.[7] Einstein developed general relativity to apply the principle in the more general case, that is, to any frame so as to handle general coordinate transformations, and that theory includes the effects of gravity.

The term is currently used more generally to refer to any case in which gravitation is not significant. General relativity is the generalization of special relativity to include gravitation. In general relativity, gravity is described using noneuclidean geometry, so that gravitational effects are represented by curvature of spacetime; special relativity is restricted to flat spacetime. Just as the curvature of the earth's surface is not noticeable in everyday life, the curvature of spacetime can be neglected on small scales, so that locally, special relativity is a valid approximation to general relativity.[8] The presence of gravity becomes undetectable in a sufficiently small, free-falling laboratory."

When I said the definition of special relativity I really meant the REAL definition gave to this theory at the beginning when it was born. As saw above, currently you can think of it as a special case in which the space is flat for sure. But the usual way of defining is different, and it's more closed to Einstein's initial definition.
Anyhow, this is just a matter of terminology as always. That's why I consider post like these ones pretty unuseful, just because we are talking a bit the naming of things, instead of their contents.

As Feynman told us "it doesn't matter if you call a bird "robin", or if you call it in many different languages. All in all when you see it, you can recognized it".
By the definition of a theory as restricted to the founder's first paper, Maxwell's equations as you learned them are not actually Maxwell's equations.

Anyway, there is no difference between the covered phenomenology of the two different definitions of SR. However, defining the theory by inertial reference frames is equivalent to saying that doing Newtonian physics in polar coordinates is beyond Newtonian physics.

OK, I see your point, which is correct to me of course. I was just trying to refine a bit my post from my initial definition, in order to specify it better.

You and I seem to be in agreement about the big concept here, so I feel a bit diffident about disagreeing on a tangential question... but I have to say it...
All in all, it seem we were not completely on agreement about it, I mean we were using a slightly different
definition. Not a big problem, however.
Sorry for having quoted you probably improperly, eventually.

Regards

Ricky