when relativity is factored in?

9/28/2005 4:16:32 PM

FDT

9/28/2005 4:18:10 PM

Not with special relativity.

The "a" fucks it up.

[Edited on September 28, 2005 at 4:21 PM. Reason : ...]

9/28/2005 4:20:52 PM

NEGATIVE NINE POINT EIGHT METERS PER DEEZ

9/28/2005 4:29:02 PM

i didn't think so...

we were having a debate last night about the meaning of "law" vs "theory"

then the newtonian "law of gravity" and all that shit came up, and I said they were not true, but only a very good approximation of reality.

9/28/2005 4:36:31 PM

Quote :

"we were having a debate last night about the meaning of "law" vs "theory""

I wrote my research paper in 11th grade about exactly that and how it applies to relativity.

Unfortunately, I was on the wrong side of that argument: I thought it should be a law.

Quote :

"then the newtonian "law of gravity" and all that shit came up, and I said they were not true, but only a very good approximation of reality."

Technically, they're only a very good approximation of how humans perceive reality, but yeah, you're basically right.

9/28/2005 4:41:45 PM

Newton's law of gravity was true enough until 1) einstein came along and described relativity, and 2) quantum mechanics was born. It's still true enough today for most everything in between. That is, until we discover the http://en.wikipedia.org/wiki/Theory_of_everything

9/28/2005 4:42:19 PM

Quote :

"they're only a very good approximation of how humans perceive reality"

HOLOGRAPHIC UNIVERSE!

9/28/2005 4:44:16 PM

Quote :

"The primary problem in producing a TOE is that quantum mechanics and general relativity have radically different descriptions of the universe, and the obvious ways of combining the two lead quickly to the renormalization problem in which the theory does not give finite results for experimentally testable quantities."

Are there any physics majors who can translate this bolded bit into English?

9/28/2005 4:49:21 PM

i'm no physics major, but i'll take a stab at it.

we know that relativity deals with objects that are extremely huge - the universe, stars, planets, black holes, etc. (it also "normal sized" objects like houses, people, rocks, but not in any measurable way that I'm aware of).
Quantum physics applies to extremely tiny objects, like electrons, quarks, sub-atomic particles.

The theories are completely incompatible with each other. That is, you cannot take a quantum mechanics formula and use it to determine how 2 planets will interact; likewise, relativity formulas will not tell you how two electrons will interact. That problem is the justification of searching for the TOE, as physicists have to make the assumption that all objects in the universe are ultimately subject to a single set of rules, or a single theory, not multiple sets depending on their size. The normal ways of combining two theories (for the sake of simplicity, 2 equations) do not work - that is, in math, for example, if you have 2 equations and 2 unknowns, you can simply solve one equation for X in terms of Y, then use the answer in the 2nd equation to solve for Y, then use that answer to finally solve for X. I guess that's what they're referring to as "re-normalization".

So I think what that sentence is saying is that we've yet to find a way to combine the theories and be able to test the results on measurable figures, which I'm guessing means that the figures are too small or too large to be tested accurately (e.g. we could only test the theory by weighing a black hole to the nearest milligram, or something like that, which is impossible).


edit - now that I actually read the sentence in Wikipedia where you copied from, here's an (equally convoluted?) explanation of re-normalization - http://en.wikipedia.org/wiki/Renormalization But without fully reading that article, renormalization appears to be a way to reduce the complexity of, or abstract, a theory/equation/formula into it's simplest terms. The first example shows an interaction between a photon and electron. Technically, there's all sorts of shit that goes on between the two objects, including outside interaction, but by renormalizing the interaction, we can come up with a simple solution that ignores all the small shit and just tells us what happens in the end. Kind of like in EE when you have a circuit with a lot of sources and loads, you can simplify the circuit into its Thevenin's Equivalent and view a much simpler, equivalent circuit with one source and one load.


[Edited on September 28, 2005 at 5:15 PM. Reason : .]

9/28/2005 5:01:38 PM

omg, F=ma looks like FEMA.... think about it

9/28/2005 5:42:59 PM

i'm not reading all this

but physics equations have a lot more to do with engineering than with philosophy

you know what i'm saying?

that is we go with what works

9/28/2005 5:52:35 PM

eh... talk to a string theorist if you think that

9/28/2005 6:00:01 PM

I'll give you an easy way to think of renormalization.
In quantum field theory the things you are interested in, the physical observables, are often expressed as a power series where the coefficients of the expansion parameter typically involve an integral over the particle momenta where the integrand is a polynomial times an exponential: Int{ P(x)*exp(-x^2)} with the integration limits generally from 0->infinity. Say you're interested in electron-electron scattering and want to measure the differential cross section, let's call it D.
Then to repeat what I said you calculate:
D = c_0+c_1*alpha+c_2*alpha^2+....where c_i are your coefficients given above and alpha is the expansion parameter (in electrodynamics it would be the fine structure constant which tells us how strong the interaction is between the field and the particles and is equal to ~1/137).
The problem arises that these coefficients tend to be divergent integrals. This is bad because the experiment gives us a finite answer. To reconcile the two facts one tries to renormalize the integrals and the easiest way to do that is to pick a range of integration where the integral will be finite. That is, integrate from 0->L instead of 0->infinity.
This still isn't satisfactory, however, because now your final answer depends upon the L you picked. Now some smart guys came along with the names of Schwinger, Feynman, and Tomonoga and solved this problem by noticing that if you're clever for every L you pick you can pick a corresponding alpha(L) such that at the end of the calculation you set L->infinity and your answer becomes independent of L. You have renormalized your theory.
Of course, not every theory you play with can this be done. Gravity is one of these theories.

9/28/2005 6:00:46 PM

look if a horse is a sphere because it makes the numbers easier

then F = m*a

for v << c

9/28/2005 6:09:43 PM

^^^

i don't know how much "physics" string theory is all about

that particular area sounds alot more like math to me than anything else

9/28/2005 7:01:07 PM

Quote :

"rudeboy: omg, F=ma looks like FEMA.... think about it"

I thought the same thing when I first saw the thread (it's in TSB after all )

Good books on the TOE...





[Edited on September 28, 2005 at 7:28 PM. Reason : ---]

9/28/2005 7:28:14 PM

those two should be read in reverse order that they are listed in. Fabric of the Cosmos is a better introductory book

9/28/2005 10:06:06 PM