Why Do We Need a Singe Theory?

[Lestat]

To a scientist or physicist the idea of unification is extremely seductive. My question is, why?

Why are scientists and physicists enamored with the idea that the laws of the universe should boil down to only a few theories or equations? I understand why they might *prefer* an elegant comact set of rules that govern all things, but why do they think this MUST be so?

As I understand it, many physical laws break down at the quantum level and physicists are desperately seeking ways to reconcile this. They come up with the theory (philosophy?) of strings to try and tie it all together. My question is:

Why can't there be different sets of laws for the sub-atomic world and that which we live in? What's so wrong with that? To a layman like myself, if you need two pages of laws (as borodog pointed out), what's the difference if you need a whole tomb of laws? What's the difference if you need 2 pages of equations to explain everything or if you need 1000 pages? In other words, who says (or why do physicists think), that it all has to boil down to just a few laws? Thanks.

[Matt R.]

I think the simple answer is that science wants to find a fundamental truth. It seems unreasonable to most physicists and scientists that two separate sets of laws should govern large, massive objects and tiny, sub-atomic particles. The main reason is when you mix the two and look at tiny, massive objects the theories are completely incompatible. Do tiny, massive objects behave completely illogically so that we can't understand them? Most likely not -- they are governed by the logic of our universe as well. So, physicists attempt to rectify the current incompatibility by coming up with a theory that incorporates both.

[Borodog]

It's not that it must, it's that it seems to. Quantum electrodynamics (QED) can already be written down very briefly (if inscrutably, if you don't understand the symbols involved), as can relativity. Maxwell's equations (all of classical electromagnetism), for example, can be written as a single equation that occupies half a line.

The problem is that relativistic theory and QED (everything else) each contain a fundamental description of spacetime (i.e the Universe), and they are incompatible. So if spacetime behaves in the way that QED describes, relitivity does not work, and vice versa. Hence, one or both must be incomplete. My hunch is that relativity is a stupefyingly elegent mathematical trick, like Maxwell's equations, but it doesn't accurately reflect the underlying nature of, uh, nature. /images/graemlins/wink.gif In classical electrodynamics, Maxwell's equations describe the oscillations of electric and magnetic "fields" and reduce to a wave equation, which is why light behaves like a wave. But in QED, we see there are no such things as "fields" or "waves"; they're all mathematical tricks. It's all particles, just particles that have strange properties that are more difficult for us to understand than waves. And since wave equations happen to describe the observed behavior, we think of them as waves, but they really aren't. It's all particles, all the way down, like turtles. /images/graemlins/wink.gif

[Lestat]

Do you think it ever will be possible to prove the existence of strings? It is my understanding that strings are so unimaginably small it is very unlikely they will ever be observed or tested for existence. If so, why do physicists put any more weight into string theory than a religious theory? At this point they are only philosophizing, no?

Uh, and that leads me to another layman question (which might be too complicated for you to explain to someone with a limited knowledge (and brain), like me). Just how is they can say that string theory has yet to make a bad prediction when they don't even know for sure strings exist?!?!? How are they doing this testing?

If Einstein were alive today, do you think he'd believe in strings? It's my understanding that he didn't concern himself much with the chaotic world of quantum physics. Was this because he simply didn't like chaos in the universe? Or because he didn't agree that the sub-atomic world should be any different from the world of the large? Thanks Borodog.

[Borodog]

Quite honestly, I don't know enough about string theory to understand it, much less explain it.

[chezlaw]

Its all beyond me but I hear that string theory does make predictions that can be tested. I think the theory predicts the existence of a new range of particles and if these were found that would be impressive. There's also the idea that some very large strings may have survived from the big bang and can be discovered.

I think Einstein would have loved it. A mathematical theory that described everything based on symmetries was just his sort of thing, and he did get heavily involved in quantun physics doing a lot of the ground work and winning the nobel prize in that field.

chez

[DcifrThs]

i was under the impression (elegent universe by brian greene) that the issue of incompatibility boils down to scale. on large scales, relativity rules. on atomic scales, QED rules. the issue is finding an underlying theory that relates both and can be used to explain the transmission of gravity via gravitrons as well as the unpredictable subatomic motions and actions of particles like electrons...

also the scale of most interest is the plank (sp?) length, which is like a 1 with 44 zeros in front of it.

and this is where scientists now think string theory comes in.

Barron

[Borodog]

My understanding is that while two theories do operate at different scales, but that isn't the source of the problem. The problem is that the two theories provide fundamentally different and incompatible descriptions of the nature of spacetime.

But it's not my field, so you should take anything I say with a grain of salt. My specialty was fluid dynamics.

[DcifrThs]

[ QUOTE ]
My understanding is that while two theories do operate at different scales, but that isn't the source of the problem. The problem is that the two theories provide fundamentally different and incompatible descriptions of the nature of spacetime.

But it's not my field, so you should take anything I say with a grain of salt. My specialty was fluid dynamics.

[/ QUOTE ]

this is CLEARLY not my field either (mine was stats/economics now finance) but i really do enjoy reading about it.

that being said, the roiling nature of spacetime given by QED appears to be in conflict with the smooth nature of spacetime given by relativity. is that the main case in your posts? if so then it makes more sense than what i thought was the case which was a result of the analytical scale.

interesting stuff

Barron

[Borodog]

Yes, that's exactly what I meant.

[atrifix]

The idea is that the theory of relativity and quantum theory are incompatible, i.e., they make competing predictions. If you're going to accept that there are multiple physical theories, then you're going to have to reject that the world is closed under natural laws. Most people believe that it is.

[atrifix]

[ QUOTE ]
Do you think it ever will be possible to prove the existence of strings? It is my understanding that strings are so unimaginably small it is very unlikely they will ever be observed or tested for existence. If so, why do physicists put any more weight into string theory than a religious theory? At this point they are only philosophizing, no?

[/ QUOTE ]
String theory does make some observable predictions--but those are quite difficult to test, based on current technology. I don't know enough about string theory to expand on that further.

[ QUOTE ]
If Einstein were alive today, do you think he'd believe in strings? It's my understanding that he didn't concern himself much with the chaotic world of quantum physics. Was this because he simply didn't like chaos in the universe? Or because he didn't agree that the sub-atomic world should be any different from the world of the large? Thanks Borodog.

[/ QUOTE ]

This is not really correct. Einstein was arguably the most influential person in the development of quantum physics. He ended up rejecting some of what he helped to develop, as evidenced by his quote "God does not play dice with the universe."

[Metric]

[ QUOTE ]
In classical electrodynamics, Maxwell's equations describe the oscillations of electric and magnetic "fields" and reduce to a wave equation, which is why light behaves like a wave. But in QED, we see there are no such things as "fields" or "waves"; they're all mathematical tricks. It's all particles, just particles that have strange properties that are more difficult for us to understand than waves. And since wave equations happen to describe the observed behavior, we think of them as waves, but they really aren't. It's all particles, all the way down, like turtles. /images/graemlins/wink.gif

[/ QUOTE ]
Well, I have to interject here -- this isn't the way it's seen by people working on the subject. QED, the standard model, etc. are very much field theories, dispite being fully quantum theories (the dynamical variables being quantized are fields). The particle interpretation shows up because of a certain specific fact -- in flat spacetime the energy spectrum of these fields come in integer lumps, which we can interpret as particles. In the more general setting of quantum field theory in curved spacetime, however, this is not necessesarily the case -- one can still have a consistent quantum field theory, but the particle interpretation breaks down. In this sense, then, the concept of "fields" is much more general and fundamental than the concept of "particles" which only make sense in certain specific settings.

[Borodog]

Yeah, I regretted [the whole description after further thought] , but it was too late to edit. /images/graemlins/frown.gif

I can only plead that again, it wasn't my "field," if you will.

[Metric]

Unification always means understanding at a deeper level. For example -- the physics of heat was originally seperate from the physics of particle mechanics, but their unification (statistical mechanics) now shows how to think of one in terms of the other. Another example: Electric and magnetic fields were described by two seperate laws -- one for each. But such a view was incomplete: They were eventually understood as different aspects of the same thing, and one found that once unified, one could also understand something seemingly unrelated, light, on a far deeper level as another aspect of electro-magnetism. Still another example was the "particle zoo" situation in the 1950's and 60's -- people had discovered dozens of new particles but did not understand the relationship between them. Eventually the quark model and eventually the "standard model" was able to describe them all in terms of a few "elementary" particles.

So, as you can see -- unification expresses deeper understanding. Eventually, understanding gravity as a quantum theory instead of a classical theory would express much better understanding of the fabric of space and time -- as it is now, one can ask questions which cannot at present be answered confidently due to the lack of such a "unified" theory.

[Rduke55]

I'm definitely ignorant about this stuff but one thing I'm wondering is how much of this is just based on them making the equations work? Like the empirical stuff they get says there should be more matter that we find so, bam, must be dark matter out there. So then the predictions based on that equation hold up and everyone says "See predicitons made on dark matter hold up." when it actually is that equation that holds up.
Are strings similar to this?

After reading this post (and my other ones) I'm going to refrain from posting anymore today because my brain is obviously barely functional. You're welcome.

[Scotch78]

It might have something to do with living in a single universe.

Scott

[Lestat]

Yes, this is kinda what I was wondering. That is, how do they know they aren't just "forcing" a particular equation to work?

[tyrus72]

[ QUOTE ]
Just how is they can say that string theory has yet to make a bad prediction when they don't even know for sure strings exist?!?!? How are they doing this testing?


[/ QUOTE ]

You are missing the point. String theory is a mathematical construct. It makes "predictions" based on application of the concept to observable phenomena. This is analagous to "wave theory" being applied to electromagnetic radiation. What is a wave? It is a mathematical construct, that's it! There is no electromagnetic "wave" you can see, touch, hold -- just observable phenomena that we can describe using a mathematical model we call a "wave". So, the string theory is a higher-dimensional mathematical construct which, to varying degrees, explains some very complex phenomena. There are still possible results of this theory that are not yet observed, but such observations may one day occur, just as Einstein's theory of relativity predicted certain phenomena which were not observable at the time but later confirmed. Don't get hung up on if we can "see" the strings -- what matters is if such a construct can accurately describe observable phenomena. If it can, then it really doesn't matter if these strings exist or not, so long as the model is useful in predicting experimental results. Just like it doesn't matter that an electromagnetic wave may not really have any physical meaning so long as it is useful as a model.

[Lestat]

Very instructive post. Thanks tyrus!