Maybe there is no grand unified theory?

[Warp]

This is a complete layman question because the topic goes well beyond my head, but I was thinking... There has been a quest of a "grand unified theory", a single theory of physics that would explain spacetime, gravity, quantum mechanics, and everything in between.

But what makes scientists so sure that there is such a unified theory?

I mean, couldn't it be that gravity (ie. spacetime geometry) and quantum effects are two completely separate and disconnected features of our universe? They are not related, they are independent of each other, they exist as completely separate phenomena, and there is no underlying common ground between them, and trying to search for a unified theory is a completely futile endeavor because there isn't one?

It's a bit like the shape and color of an object. Both are independent features of the object, and neither one defines the other, nor is there any third feature that affects both and creates a connection between them. Trying to develop a "unified theory of color and shape" is futile because they simply are not connected. An object can be of any color and any shape, completely independently of each other.

 

[mathman]

The problem that physics has to address is what happens when both quantum theory and general relativity apply, such as what happens inside a black hole or how the big bang started. The current theories are incompatable.

 

[bapowell]

Yes. For example, how does a Planck mass particle behave? Both general relativity and quantum theory are intertwined at this scale.

 

[PAllen]

Finding a sound framework for dealing with both quantum field theory and gravity when both are significant is required (else there is no theory at all to address conditions presumed to exist in the universe).

However, that is separate from a grand unified theory. It is conceivable that such framework still has a separate representation for QCD, electroweak, and gravity. A grand unification is, IMO, motivated only by aesthetics and historically informed physical intuition. Part of that intuition is that known problems in SM seem to go away in partially successful GUTs, and there is promise of M-theory. But none of that amounts to an argument that nature must be so unified.

[Edit: I see the OP wasn't necessarily asking about what is normally called grand unification. For what they were asking, I agree with the prior answers.]

 

[detective]

...warp has brought up a simple question...my simple thoughts are that gravity, being intimately related to mass, has the problem of allowing for the component of time...

...anything massive has a ''life''...and all things no matter how massive ''die'' or reduce...this exactly is the proper definition of entropy, and entropy taken to it's finality and most extreme means...nothing ...

we invoke the ''time taken'' for that to occur...cheers

 

[bapowell]

But a single electron floating in space will remain a single electron floating in space for eternity.

 

[Warp]

The problem that physics has to address is what happens when both quantum theory and general relativity apply, such as what happens inside a black hole or how the big bang started. The current theories are incompatable.

Of course, but my question was about the quest for finding the one single unifying theory that explains both at the same time. Maybe there isn't one. Instead, we have to simply find out how exactly GR and QM interact with each other.

I understand, however, the motivation behind the idea of there being one single theory that explains everything at once. After all, the history of science has consisted in big part of us discovering that phenomena that we initially thought were separate and completely independent of each other were, in fact, the one and same, and that the same laws of physics actually apply to the entirety of the Universe.

 

[friend]

Of course, but my question was about the quest for finding the one single unifying theory that explains both at the same time. Maybe there isn't one. Instead, we have to simply find out how exactly GR and QM interact with each other.

We have to keep looking for a common cause for events in the universe (for QM and GR). For if there is no common cause, then we would have to explain how two totally unrelated phenomena can fit together everywhere at all times. If there is a logical explanation for everything, then there is a common reason why the universe is both quantum mechanical and generally relativistic. The fact that the universe came from a single point in the big bang means that all reality shared everything in common at some point so that there must have been a common cause for everything in the universe.

 

[Warp]

We have to keep looking for a common cause for events in the universe (for QM and GR). For if there is no common cause, then we would have to explain how two totally unrelated phenomena can fit together everywhere at all times.

If one phenomenon applies to the whole universe, then why not two?

 

[friend]

If one phenomenon applies to the whole universe, then why not two?

What are the odds that that two arbitrary theories should be completely consistent with each other. That's probably a contradiction of terms. Can a contradiction actually exist in the universe? I think not. At some level all of physics laws are consistent with each other, which means by definition that there is a common cause. It's just a matter of finding it.

 

[marcus]

Two comments: we laypeople and non-experts should make an honest effort to use words more or less consistently with the pros, else more confusion results. I have NEVER heard someone use "GUT" the way you do. Normally a GUT does NOT include gravity. See WikiP for example:
http://en.wikipedia.org/wiki/Grand_Unified_Theory
So please to stop saying GUT when you mean unification of GR with QM.

The other thing is
1) physics is a mathematical science
2) math is a man-made human language
3) theories do not exist in nature, they are predictive testable manmade DESCRIPTIONS and they are always provisional. We use the best available description until we discover it's limitations and are forced to improve or replace.
4) we NEVER pretend that an manmade theory is 100% eternally RIGHT, and all theories are manmade.
5) human languages evolve, mathematics evolves, if our language is not good enough to describe GR and QM in the same equation-model TODAY, that does not prove anything. The language may evolve, the concepts and equations become more powerful, we might have a unified description tomorrow or in 10 years.
6) Nothing is gained by asking a question like you do which is essentially a speculation about human impotence: "What if our species is incapable of evolving an adequate conceptual quantitative language?" "What if we are so lame and weak we cannot do this?" What if what if what if.
That is the logical content of the question you are asking. It is about the future evolution of the human mind and it is uselessly defeatist, or so it seems to me.
==================

My personal view is that gravity=geometry and QG means both quantum gravity and quantum geometry. Spacetime geometry is necessarily dynamic and uncertain especially in certain regimes like the presumed BIG BOUNCE that presumably occurred at start of expansion. At least some equation models find a bounce as you go back in time to before inflation and the start of expansion---the models find a bounce due to quantum geometric effects. And other models do not, so we have to compare what they predict with observations of the most ancient light, to see which models are in closer agreement with nature. And I see that in the past 2 to 3 years certain QG research programs have made remarkable progress and have been attracting many more young researchers. The QG enterprise is certainly not STUCK but on the contrary is making rapid progress. Plus I see that interest in String program has been declining, citations are down, string jobs are down, publication has slowed, fewer gifted young people going into string. Fashions change and it's all to the good AFAICS.

 

[friend]

The other thing is
1) physics is a mathematical science
2) math is a man-made human language
3) theories do not exist in nature, they are predictive testable manmade DESCRIPTIONS and they are always provisional. We use the best available description until we discover it's limitations and are forced to improve or replace.
4) we NEVER pretend that an manmade theory is 100% eternally RIGHT, and all theories are manmade.

This kind of argues that there can never be a completion of physics, because all theories are provisional and evolving. I would take issue with this. But the only way for a theory to be absolutely true and complete is if you start your development from the concepts of true and false. If you derived a theory of everything from "pure logic alone", then it could not be argued with, and you'd know it was complete. Yet we don't know if a development from logic is even possible or not at this time.

 

[marcus]

This kind of argues that there can never be a completion of physics, because all theories are provisional and evolving. I would take issue with this. But the only way for a theory to be absolutely true and complete is if you start your development from the concepts of true and false. If you derived a theory of everything from "pure logic alone", then it could not be argued with, and you'd know it was complete. Yet we don't know if a development from logic is even possible or not at this time.

come on, friend
True and False are human words which people use in various different ways in different contexts.
What you call "pure logic alone" is a subset of human language with its own excellent rules of syntax (special high-performance rules of grammar). It is a credit to us as a species. But it is not eternal and static---what we think of as logic continues to evolve--- it was not handed to us once-and-for-all by some absolute Authority. We made it up.

AFAIK physicists are not looking for a final "completion of physics", they show every sign of being quite human and it is human to want to always go beyond, to keep on exploring. when nature does not show them something new for 20 or 30 years they get fretful and despondent, they want their (provisional evolving) theories to be defied, challenged by nature. It is naive to imagine that they want to "complete" the job of finding out how she works and what gives rise to her and why she is the terrifically beautiful way that she is. Forget completion.

 

[friend]

come on, friend
True and False are human words which people use in various different ways in different contexts.
What you call "pure logic alone" is a subset of human language with its own excellent rules of syntax (special high-performance rules of grammar). It is a credit to us as a species. But it is not eternal and static---what we think of as logic continues to evolve--- it was not handed to us once-and-for-all by some absolute Authority. We made it up.

Yes, well, I find myself asking whether your statement is "true". I don't think you can escape the relevance of true and false in debate. All theories in a sense are statements about which we ask if they are true or false representations of reality. If you can "falsify" a theory, then there must be some truth-value content to theories.

To say that logic evolves only underminds the process of reasoning. You can't argue that the principles of reason are changeable to something we don't know yet. That just negates any ability to debate or argue or investigate the truth of theories. If logic is not authoritative, then we could claim our theories are right so we must alter reason to accommodate our theories.

I think it is obvious that true and false must be relevant to theories of the universe. For we distinguish statements that represent what actually exists by saying they are "true". We even say that the existence of the universe as a whole is "true".

AFAIK physicists are not looking for a final "completion of physics", they show every sign of being quite human and it is human to want to always go beyond, to keep on exploring. when nature does not show them something new for 20 or 30 years they get fretful and despondent, they want their (provisional evolving) theories to be defied, challenged by nature. It is naive to imagine that they want to "complete" the job of finding out how she works and what gives rise to her and why she is the terrifically beautiful way that she is. Forget completion.

I think it's a bit misguided to suggest there are no all encompassing principles to guide us into what is actually the case about reality. Experiment is one way to determine if a theory is a correct statement about reality. Logical consistency of a theory is another, assuming that the logic of true and false is even relevant to the investigation of physical theories.friend

 

[marcus]

...I think it's a bit misguided to suggest there are no all encompassing principles to guide us into what is actually the case about reality. Experiment is one way to determine...

Who says there are no all encompassing principles? We have all encompassing principles which guide us, and they change. What underlies/selects/redefines them is the COMMUNITY and the TRADITION that it maintains. People.

The empirical Baconian tradition goes back 400 years to the time of Sir Francis Bacon, contemporary of Shakespeare.

Until not so long ago EUCLIDEAN GEOMETRY was an "all encompassing principle" just like the principles of Aristotelian LOGIC.

The community of scholars--nature-philosophers, etc.--redefines its tradition and by a kind of restless argumentative CONSENSUS chooses what are to be the "all encompassing guiding principles" for that generation.

There will always be persons so anxious to have certainty, or so mistrustful of the community consensus, that they cling to the floating debris from that previous shipwreck.

The rest of us just use what we collectively think are the best guiding principles we have so far.

 

[Chronos]

Perhaps science is inherently limited to compiling ever more accurate, but, never quite perfect approximations of nature. An interesting discussion is here http://arxiv.org/abs/physics/0510188.

 

[friend]

Who says there are no all encompassing principles? We have all encompassing principles which guide us, and they change. What underlies/selects/redefines them is the COMMUNITY and the TRADITION that it maintains. People.

Is that true? Yes, of course I think people can refine their theories and beliefs. To think otherwise would be another argument stopper. I'm not a religious fanatic.

 

[VantagePoint72]

If one phenomenon applies to the whole universe, then why not two?

As others have already said, the issue is that the two models are incompatible when applied at the same time (in certain circumstances). There's no reason you can't have two separate models needed to explain physical reality, but they must be consistent with each other. There are physical questions, answerable—in principle, at least—by an experiment which quantum theory gives one answer to while GR gives a different answer to. Thus, it's just a logical fact that one or both of them cannot be completely correct in its current form. So, it may not be the case that there is a theory of everything, but it is certainly the case that our current understanding through GR and quantum theory is not the whole story.

 

[julian]

Einstein's equation

R_{\mu \nu} - {1 \over 2} g_{\mu \nu} R = T_{\mu \nu}

tells us how geometry is coupled to matter. Since matter is in a quantum superposition, isn't geometry as well?

 

[VantagePoint72]

Whom is your question directed to, julian? That is in fact one the points of tension between QM and GR. The curvature tensor is purely classical and we don't have a well-defined notion for a 'superposition of curvature'. However, QM says it should be coupled to something that is in a superposition. Hence, one of the incompatibilities between QM and GR is the question of what the gravitational field looks like for a particle small enough to have quantum effects dominate.

 

[bapowell]

Einstein's equation

R_{\mu \nu} - {1 \over 2} g_{\mu \nu} R = T_{\mu \nu}

tells us how geometry is coupled to matter. Since matter is in a quantum superposition, isn't geometry as well?

This is the motivation behind so-called semi-classical treatments of gravitational phenomena, in which the stress-energy tensor is replaced by its quantum expectation value, \langle T_{\mu \nu} \rangle, but the geometric side of the Einstein Equations remain classical. This approach, while only approximating the full quantum theory of gravitation, is quite powerful. Hawking used it to discover that black holes radiate, and it is central to the derivation of the temperature anisotropies in the CMB.

 

[julian]

The origin question was can GR and QM be independent theories...and I'm saying they are incompatible.

 

[julian]

This is the motivation behind so-called semi-classical treatments of gravitational phenomena, in which the stress-energy tensor is replaced by its quantum expectation value, \langle T_{\mu \nu} \rangle, but the geometric side of the Einstein Equations remain classical. This approach, while only approximating the full quantum theory of gravitation, is quite powerful. Hawking used it to discover that black holes radiate, and it is central to the derivation of the temperature anisotropies in the CMB.

Really the right hand side is an operator. Attempts are made to replace the right hand side by an expectation value but an iterative procedure must be implemented to define the expectation values. It has been shown that the iteration does not converge in general, whence we must quantise the gravitational field...see p.g. 5-6 of Thiemann's book.

 

[bapowell]

Attempts are made to replace the right hand side by an expectation value but an iterative procedure must be implemented to calculate expectation values. It has been shown that the iteration does not coverge, whence we must quantise the gravitational field...see p.g. 5-6 of Thiemann's book.

Of course. Hence how I said that this approach approximates the full quantum theory of gravititation (but I'm unclear on the non-convergence issue. I'm not an expert on stress-tensor renormalization by any means, but I was under the impression that there were finite representations of \langle T_{\mu \nu}\rangle. In other words, aren't those dangerous UV modes Thiemann mentions integrated out?). Saying that GR and QM are incompatible isn't new -- this was recognized long ago, and forms the basis of the OP's question. It seems you already know the answer to the question you asked.

 

[julian]

People talk about singularities and how GR and QM must merge in certain circumstances. But if you take QM to apply at all levels (which I do!) then they are always incompatible.

 

[julian]

Hello Bapowell

The iteration is unstable in general. It was Robert Wald and Flanagan [gr-qc/960252] who showed this.

"In other words, aren't those dangerous UV modes Thiemann mentions integrated out?"

I'm not sure how to answer it, I'm not an expert either.

But what I do know is that Thiemann shows in his book that a canonical quantization of GR (with full backreaction of matter on the quantum gravitational field) is finite.

 

[bapowell]

Thanks for the reference julian. I'll have a look.

 

[julian]

Hi bapowell

I think people take they view that quantum mechanics stops at some length scale, beyond which matter is classical, and then put it into Einstein's equations. I'm of the opinion that there is no artificial boundary between classical and quantum...

 

[PAllen]

Hi bapowell

I think people take they view that quantum mechanics stops at some length scale, beyond which matter is classical, and then put it into Einstein's equations. I'm of the opinion that there is no artificial boundary between classical and quantum...

That may be (and I agree), but as a practical matter, there is enormous range of validity to the limits of measurement for Newtonian mechanics, Maxwell's equations, and GR. As a practical matter, one must try to understand the boundaries of when classical theories are good enough, unless you want to waste endless effort calculating trivial results.

 

[atyy]

The origin question was can GR and QM be independent theories...and I'm saying they are incompatible.

Here is a discussion of GR as a quantum theory.

http://arxiv.org/abs/1209.3511