ok, hi. I'm a senior in high school, taking an astronomy class. I have a question about the four forces. (Strong, weak, electromagnetic, gravity, you know...) I understand that three of the four can be combined in a 'grand unified theory', and that gravity is the one that doesn't fit. My question is, why are we even calling gravity a force, when according to Einstein gravity is nothing except a byproduct of the dent in spacetime caused by massive objects? I know this is a criminal oversimplification, but please be gentle: high school. Thanks
~Savanna~

WARNING! Amateur about to attempt an answer!

Why is gravity a force?

Well, my Macquarie Dictionary (budget edition) includes the following definition of a force: "an influence which produces or tends to produce motion or change of motion."

I think you could apply that to gravity.

But I'm happy to be corrected by people who know more in this field.

For an alternative view of what gravity is look for the book "Pushing Gravity, New Perspectives on Lesage's Theory" coming out soon. It explains how gravity is an aspect of electromagnetic forces. It's just the push from the very long wavelength cosmic background radiation from a static universe, pushing everything together.
This idea has a long history, from Lesage to Charles Brush to more modern views.

Kierein has omitted an important disclaimer: he is a co-author of the book in question, and it is far from a mainstream view of gravity.

Why is gravity considered a force? You're correct in pointing out that Einstein changed our view of gravity considerably. However, one often see the four forces described as the "four traditional forces"; this is a nod to the fact that e/m and gravity were long known as forces before Einstein redefined the latter. And now, of course, we have a possible fifth force (or a new aspect of an old force): there seems to be something causing the universe's expansion to accelerate.

I suppose you can consider gravity to be a force in that it is "that which causes spacetime to warp".


<font size=-1>[ This Message was edited by: Donnie B. on 2002-03-20 08:33 ]</font>

Kinda ironic, isn't it?

I guess the short (and long) answer of why we consider gravity a force is that it behaves as a force and can be treated as one. It offers a plausible and testable explanation for why you can walk on the floor of a house in Nova Scotia and on the floor of a house in Brisbane, but you can't walk on the ceiling of either one. We even have a quantum mechanics view of gravity that fits fairly well with the other three (known) basic forces.

Does this mean we have all the answers? Of course not. But we do have most of the questions (so far). There are competing explanations for the effects we attribute to gravity (as John alludes in his post), but so far they have not proven as effective as gravity in answering those questions.

Just another amateur's opinion.

__________________
Never attribute to malice what can be adequately explained by ignorance or stupidity.
Isaac Asimov

Simple answer: On Earth, you don't notice the difference. You can list on one hand the practical applications of general relativity (Einstein's theory of gravity), and GPS is the only one which most people will ever use. Newton's theory of gravity works just fine for everything from building bridges to launching the shuttle. It's simpler and more intuitive; so why not use it?

Philosophical answer: Both Netwon's and Einstein's theory are just mathematical representations of reality. Einstein's theory is more correct in that it is more accurate quantitatively; however qualitatively, one can not judge. Gravity is neither a force nor a warping of space-time, but treating it as either a force or a warp allows us to make quantitative predictions. Given this, why not go with the simple answer?

P.S. Welcome to the BABB, Savanna.

Don't forget that there are physicists also who are trying to develop a quantum theory of gravity, where particles exchange "gravitons".

I don't really know the current state of their work however.

__________________
...And that, my liege, is how we know the Earth to be banana-shaped. --Sir Bedevere

I disagree with that characterization. Any practical application of Newton's theory is also a practical application of Einstein's, as Newton's is included as a limiting case of Einstein's.

As far as simpler--that depends upon how you look at it, doesn't it? And the argument for "more intuitive" just boils down to what you've been used to.

Savanna's question is basic, and not yet really answered, I think.

I agree that any problem that can be solved using Newton's theory can be solved using Einstein's theory. And I think you'll agree that the vast majority of our everyday experience can be accurately predicted by Newton's theory. My point is that when scientist and engineers are trying to solve a problem, they reach for their old beat-up (Newtonian) dynamics text not Misner, Thorne & Wheeler.

To fully appreciate Newton's theory, one only needs calculus -well, differential equations doesn't hurt. To appreciate general relativity, one also needs tensor calculus, differential geometry, and a little helping of non-associative algebras. Mathematically, Newton's theory is simpler. Even when using Einstein's theory to determine the acceleration due to gravity, once it's found, you use Newtonian mechanics (assuming the speeds are not relativistic). I do agree that "more intuitive" corresponds to "what you've been used to". But consider, since our everyday experience is not relativistic, isn't "what we're used to" Newtonian? Everyone feels the pull of gravity; we've all have had to apply force to lift an object and counteract gravity. My personal opinion, and feel free to disagree, is that everyday experience translates better or more intuitively into Newton's theory.

Yep, it's a simple question but a good question. Aren't those questions the hardest to answer? [img]/phpBB/images/smiles/icon_smile.gif[/img]

We've been batting similar questions around on a physics forum where I go to spread bad advice, bad temper, and bad jokes, so here's my two cents.

Savanna, the word 'force' has a pretty specific meaning in physics--from a subatomic perspective it describes the possible interactions between particles. Different particles respond to the different forces, and all matter particles (protons, neutrons, electrons) respond to gravity, just as all charged particles respond to electromagnetism. In that sense gravity belongs right with the other three forces.

However, a lot of physics can be understood using more than one conceptual framework, and gravity is no exception. Einstein showed that we can think of gravitation as spacetime curvature, and relativity provides beautiful equations to model this 4-dimensional curvature. But we can also think of spacetime as a field, one with the peculiar property of distorting spatial and time measurements. And there are ways to model gravity as a quantum interaction (I think somewhat unsatisfactory, but under development). These models use gravitons as the subatomic particle which mediates the force, just as gluons mediate the strong force.

No one way is right--as relativist Kip Thorne wrote, sometimes thinking about gravity as curvature leads a physicist to the right formulation for a problem, and sometimes thinking about spacetime as flat and gravity as a field is more rewarding. Hopefully the theorists will get quantum gravity worked out in all its details and that will add more mathematical tools to the physicists toolbox.

OK, guys, is the above accurate? [img]/phpBB/images/smiles/icon_wink.gif[/img]

--Don Stahl

Somehow, these seem fitting:

Those who are not shocked when they first come across quantum theory cannot possibly have understood it.
Niels Bohr

...During the Planck (Grand Unification) era, the Universe can be best described as a quantum foam of 10 dimensions containing Planck length sized black holes continuously being created and annihilated with no cause or effect. In other words, try not to think about this era.
U of Oregon

Thus, there is a difference between something and nothing, but it is purely geometrical and there is nothing behind the geometry.
U of Oregon

~Savanna~

I'm always happy [img]/phpBB/images/smiles/icon_biggrin.gif[/img] to answer 2 days late since it gives me the opportunity to review the answers of others first. Maybe I'll even learn something; I don't get my hopes up, but there's always that possibility.

In this case, the first answer you got from Peter was very appropriate, (even if it did come from a dictionary I never heard of).[img]/phpBB/images/smiles/icon_biggrin.gif[/img] Sometimes the amateurs get right to the heart of it. And of course, Grapes and Wiley's discussion is enlightening.
So let me take it one step further:

Even though Newton defined force, (F = ma; if mass accelerates there must be a force), Einstein's space-time continuum, (contrary to popular belief), is not just an extention of Newtonian physics. Rather, it requires one to dispose of Newton's 3rd law ( 'for every action there is an equal & opposite reaction'.) [img]/phpBB/images/smiles/icon_eek.gif[/img]
Such was the radical departure from classical Newtonianism required by relativity; and most physicists, so desirous of a solution for the misfit gravity, willingly departed without even the blink of a eye.[img]/phpBB/images/smiles/icon_wink.gif[/img]

Furthermore, it requires one to believe (or at least act as though) relativistic space-time is a real physical entity, with real physically independent properties which exert real physical effect -something conceptually incomprehensible classically.

Comments?

G^2

Hear that Grapes, we're enlightening! [img]/phpBB/images/smiles/icon_smile.gif[/img]

You know, everytime I get to feeling like I might be getting a better handle on this relativity thing, somebody tries to explain it and says something like "dispose of Newton's 3rd law" and I'm just totally lost again. Arrrgh!

Well, don't feel so bad; you're not SUPPOSE to realize that's what you are really doing. I mean nobody tells you directly, "You've got to give up Newton's 3rd law";(except for some blunt goofball like me who wants you to know all the repercussions BEFORE you get married..) It's much more subtle, usually to the point that most physicists don't even realize they've been scouped.

Tell you what; I'll just give you an excerpt below from a fellow from Germany (who seems to understand this stuff better than me):

"The principle of inertia, in particular, seems to compel us to ascribe physically objective properties to the space-time continuum. Just as it was consistent from the Newtonian standpoint to make both the statements, 'tempest est absolutum, spatium est absolutum', so from the standpoint of the special theory of relativity we must say, 'continuum spatium et temporis est absolutum'. In this later statement
'absolutum' means not only "physically real," but also 'independent in its physical properties, having a physical effect, but not itself influenced by physical conditions'
As long as the principle of inertia is regarded as the keystone of physics, this standpoint is certainly the only one which is justified. But there are two serious criticisms of the ordinary conception. In the first place, it is contrary to the mode of thinking in science to conceive of a thing (the space-time continuum) which itself acts, but which cannot be acted upon"....
A. Einstein
The Meaning of Relativity
Princton Univ Press, 1953

Note the last sentence. Yea, not only is it contrary to 'the mode of thinking in science', it is contrary to Newton's 3rd law; which is exactly what you are being asked to relenquish!

Have a wonderful honeymoon.[img]/phpBB/images/smiles/icon_biggrin.gif[/img]
G^2



<font size=-1>[ This Message was edited by: Gsquare on 2002-03-23 23:30 ]</font>

I dunno, G^2, if that is your justification for saying that Einstein's space-time continuum requires one to dispose of Newton's third law, it seems that that is backwards. Isn't Einstein in that quote saying that the ordinary conception (i.e., Newtonian conception) of inertia is contrary to the sense of the third law? That it is Newton who would say that spacetime acts, but is not acted upon?

Believe me, Grapes, I am not trying to justify the disposal of Newton's 3rd law; what I am presenting is how Einstein (presumably) justified it. You are free to draw your own conclusions.[img]/phpBB/images/smiles/icon_biggrin.gif[/img]

No, I don't believe so. If that were the case then why (in the 1st paragraph) would Einstein make such a point of saying that in relativity "absolutum means...having a physical effect, but not itself influenced by physical conditions" ? (Newton's concept of inertia, on the other hand, is in everyway consistent with the 3rd law).

Quite the contrary; If Newton believed space-time was physical, he would say space-time acts and IS (re)acted upon, consistent with the 3rd law. Einstein, anticipates the objection of Newtonians, and addresses it .
It may help if you change the word “of’ to “from”: so it reads, “ But there are 2 serious criticisms (against relativity) FROM the ordinary (Newtonian) way of thinking. In the first place, it is contrary to the mode of thinking in science to conceive of a thing (like space-time) which itself ACTS, but which cannot be acted upon”. In other words, this is what relativity requires and he is anticipating the objections to ignoring Newton’s 3rd law and preparing to overcome it.
In other words, he is saying, I know relativity is contrary to the normal newtonian 3rd law of action / reaction, but your just going to have to get over it.
It is a subtle way of invalidating the 3rd law in favor of implementing the physical S-T continuum which does not obey action/ reaction principles.

Thanks for the interesting discussion.

G^2



<font size=-1>[ This Message was edited by: Gsquare on 2002-03-24 20:42 ]</font>

By putting "in relativity" in bold, you make it seem as if Einstein were emphasizing that point in relation to relativity in general. He certainly didn't. The passage is on page 55 of my copy, the first page of the intro to the general theory of relativity. The comments about absolute space and/or time are in reference to Newtonian space and time and Special Relativity spacetime. He uses that as an introduction to Mach's objections to Newtonian inertia. Apparently, Newton felt the same unease about the situation.

No, I think that is what Mach was objecting to, as pointed out by Einstein.

Hold up. The "(against relativity)" does not appear in the passage, nor the "(Newtonian)" although I agree that the "(Newtonian)" is inferred. So, the passage should read "But there are two serious criticisms of the ordinary (Newtonian) way of thinking." That's quite the opposite of your interpretation. Einstein is introducing one of the concepts that he used to build the theory of general relativity, what he called Mach's principle.

I disagree. I see it just the opposite.

G² and GoW,

Interesting discussion, but I want an example where GR violates the Newton's 3rd law. If you don't treat gravity as force, some interesting 1st law violations occur, but I can't think of any 3rd law problems.

Mach said Newton's laws really reduce to one law:
This seems to jive with GR. Note: no mention of force whatsoever in Mach's formulation.

The passage in The Meaning of Relativity seems to be about the interaction of compact objects and spacetime, not two compact objects. In other words, Einstein is pointing out that Newton and Special Relativity both have the problem that spacetime somehow acts on that compact object without that compact object acting on spacetime.

I believe he thought he remedied that with the principles of general relativity.

Danka, GoW.

I'm beginning to understand.

Ya know, I just reread the title of this thread and realized it completely wrong. No quanta need to be involved ... yet.

'Wow' is about all I can say. thanks for the great answers. I'm sorry the thread title was a little less than accurate. I was trying to be cute, silly me. 0:-) anyway, my astronomy teacher is going to be very happy when I show him all of your answers, so thanks again. You may have gotten me some extra points! [img]/phpBB/images/smiles/icon_biggrin.gif[/img]
~Savanna513~

I'm glad to see you're still following the thread; I was afraid we may have scared you off. And thread title is just fine. The alliteration is nice. In that tradition, let me suggest "a classical conundrum". [img]/phpBB/images/smiles/icon_smile.gif[/img]

Good luck on your report/project/whatever.


<font size=-1>[ This Message was edited by: Wiley on 2002-03-25 18:08 ]</font>

When you express General Relativity mathematically you get the famous Einstein equations. And they basically say that “energy=geometry”. They’re the same. Energy, we all know is equal to matter times c^2, and geometry is the shape of things.

David> It depends on whether or not you believe in string-theory I’d say. In modern M-theory there is supergravity along with eleven dimensions. The problem with string-theory is that it’s not currently a falsifiable theory. That is, we can’t test through experiments whether or not it works.
A quantization process for gravity as has been done for the strong and electroweak forces (canonical quantization) can’t be carried through for gravity. Or rather, no one has succeeded yet.

Dstahl> Spacetime is not the same as the metric it’s endowed with. And the metric is so to speak the gravitational field if you like. While the Riemann tensor is the curvature. They’re related, but space-time is not a field. It’s a four-dimensional space with some very specific mathematical properties.

__________________
-All it takes to fly is to hurl yourself at the ground... and miss.
Douglas Adams

Congratulations for the good question, savanna.

Omega--

Well, I probably wasn't clear or accurate in my post on alternate ways to view gravity. I based my comments on some of Kip Thorne's discussion in his 1994 book Black Holes and Time Warps (Savannah, Dr. Thorne is a physicist specializing in relativity, and co-author (with Charles Misner and John Wheeler) of a landmark text, Gravitation--and that's why I trust his writing.)

In the chapter titled "What is Reality" Thorne discusses the curved spacetime model versus a flat spacetime model, and says:

"In the flat space of the right half of Figure 11.1 there resides a gravitational field that controls the sizes of fundamental particles, atomic nuclei, atoms, molecules, everything, and forces them all to shrink when laid out radially....The shrinkage-controlling gravitational field has other effects. When a photon or any other particle flies past [a black hole], this field pulls on it and deflects its trajectory; it is curved, as measured in the hole's true, flat spacetime geometry..."

"What is the real, genuine truth? Is spacetime really flat, as the above paragraphs suggest, or is it really curved? To a physicist like me this is an uninteresting question because it has no physical consequences. Both viewpoints, curved spacetime and flat, five precisely the same predicitons for any measurements performed...Which viewpoint tells the "real truth" is irrelevant for experiments; it is a matter for philosophers to debate, not physicists. Moreover, phyisicists can and do use the two viewpoints interchangeably when trying to deduce the predictions of general relativity." [Emphasis in the original.]

Now, your point about the Riemann geometry is perfectly valid, however, what I think Thorne is saying is that the mathematics can also be interpreted as spatial and temporal distortion by a field in flat spacetime, and this yields insights not as readily available when thinking of curved spacetime. He continues:

"The curved spacetime paradigm is based on three sets of mathematically formulated laws: Einstein's field equation, which describes how matter generates the curvature of spacetime; the laws which tell us that perfect rulers and perfect clocks measure the lengths and the times of Einstein's curved spacetime; and the laws which tell us how matter and fields move through curved spacetime...The flat spacetime paradigm is also based on three sets of laws: a law describing how matter, in flat spacetime, generates the gravitational field; laws describing how that field controls the shrinkage of perfect rulers and the dilation of the ticking rates of perfect clocks; and laws describing how the gravitational field also controls the motions of particls and fields through flat spacetime."

Thorne further notes that he might use the curved spacetime picture when thinking about black holes, but the flat spacetime picture when thinking about gravitational waves. Each has its uses.

As a complete doofus amateur, I continually struggle with the idea that there can be several perfectly valid ways to picture fundamental physics, and so my outlook tends to be overly rigid. I'm a plumber who only has one tool in his toolbox, and so no matter what the task at hand may be I attack it with a pipe wrench! Thanks for your post--I hope Thorne has made clear what I did not.

(The quotations are from Black Holes and Time Warps, pages 402 and 403.)

--Don Stahl