Why is String theory not taken seriously?

lpetrich · **#21** by **lpetrich** » May 03, 2010 12:43 pm

Zubin, do you have any direct evidence of that? I ask that because most physics fields are rather heavily mathematical.

Zubin · **#22** by **Zubin** » May 04, 2010 1:15 am

lpetrich wrote:Zubin, do you have any direct evidence of that? I ask that because most physics fields are rather heavily mathematical.

Not sure what qualifies evidence for you, but:
I'm a physics student and I've talked with many physicists and mathematicians of all ages.
I've also visited quite a few physics/math forums and the results have been no different.

Just because you use mathematics does not mean you're good at math, nor does it mean you're a mathematician. Physics is more intuition based whereas pure mathematics is more rigorous. This is why a physics student may struggle the first time they come across a course being taught on "Mathematical Physics" because those courses are often taught by math professors. Sure, in more analytical physics courses you will need to know more math and have a better understanding of math, but even then that does not mean you're good at math.

Look at the textbook Mathematical Methods In The Physical Sciences by Mary L. Boas and you'll get an idea of what I'm talking about.

amorrow · **#23** by **amorrow** » May 04, 2010 3:11 am

From
http://en.wikipedia.org/wiki/The_Elegant_Universe
Part.II Strings the Thing
http://video.google.com/videoplay?docid ... 914167356#

At minute 12:00 and and again at minute 35:00
Nobel Prize winner Sheldon Lee Glashow comments to the effect that:

Physics is an experimental science and yet we can make no experiments, observations or fasifiable predictions of String Theory.

hermionesotter · **#24** by **hermionesotter** » May 04, 2010 12:06 pm

amorrow wrote:From
http://en.wikipedia.org/wiki/The_Elegant_Universe

I've read this book and, as a layperson, would recommend it for anyone who wants an accessible introduction to string theory.

The reason claimed by Brian Greene for ST's predictions not yet being testable is due to the current technological limitations not allowing us to see on the microscopic scales explored by the theorists. Nevertheless, as far as I'm aware it is still the only theory that successfully unites quantum physics and gravity. Also, I believe the LHC's tests will include searching for extra dimensions, so perhaps when it's properly up and running, we'll finally get some answers.

Nautilidae · **#25** by **Nautilidae** » May 04, 2010 1:38 pm

amorrow wrote:From
http://en.wikipedia.org/wiki/The_Elegant_Universe
Part.II Strings the Thing
http://video.google.com/videoplay?docid ... 914167356#

At minute 12:00 and and again at minute 35:00
Nobel Prize winner Sheldon Lee Glashow comments to the effect that:

Physics is an experimental science and yet we can make no experiments, observations or fasifiable predictions of String Theory.

Many have been made.

twistor59 · **#26** by **twistor59** » May 04, 2010 6:07 pm

Would anyone care to summarise what, in their opinion, are the very best arguments for string theory...

Just to be clear, I'm asking because I'm interested, not because I'm going to come up with a load of counter arguments to demolish them. I don't know enough about the subject to do that.

hackenslash · **#27** by **hackenslash** » May 04, 2010 6:17 pm

I suspect the main reason that M-Theory is not taken seriously is simply the length of time it's been around with no hard empirical evidence. It's been anything but a quick fix. Still, I think it has some merit, though I am neither an advocate nor a detractor at the moment. At bottom it's an extremely simple idea, and one that has some degree of parsimony, simply because it's actually an attempt to reduce the number of entities required to make the universe work.

iamthereforeithink · **#28** by **iamthereforeithink** » May 04, 2010 6:18 pm

twistor59 wrote:Would anyone care to summarise what, in their opinion, are the very best arguments for string theory...

1. Some of the math can be used to create financial models
2. It helps MIT and Princeton PhD grads to get jobs on wall street

Nautilidae · **#29** by **Nautilidae** » May 04, 2010 6:45 pm

twistor59 wrote:Would anyone care to summarise what, in their opinion, are the very best arguments for string theory...

Just to be clear, I'm asking because I'm interested, not because I'm going to come up with a load of counter arguments to demolish them. I don't know enough about the subject to do that.

One of superstring theory's strengths is it's potential as a theory of everything. By describing phenomena in terms of strings and branes, the theory can reproduce the standard model of particle physics and produce quantum gravity. Essentially, rather than having many different kinds of particles and forces, all of those particles and forces can be described by the different vibrations of fundamental object: the fundamental string. While there are other theories of quantum gravity, no other theory incorporates both quantum gravity and the standard model. It can also be used to solve the black hole information paradox. Another very pleasing result of superstring theory is supersymmetry. Supersymmetry is a cornerstone of particle physics, and string theory was one of the first models (if not the very first) to predict it.

There are several tests for string theory, but almost all of them involve very high energy experiments, which haven't been created until recently (the LHC). For instance, string theories predict extra dimensions. If these dimensions exist, the lower limit of black hole mass, the Planck mass, is reduced to the TeV energy range. This means that if particle collisions at the LHC occur at high enough energies, CERN should detect both micro black holes and massive string states called string balls. Depending on the specific conditions, however, the string balls may simply collapse to form more black holes

hackenslash · **#30** by **hackenslash** » May 04, 2010 7:16 pm

It should be pointed out also that, if stings are Planck scale entities, then while they are testable in principle, they are untestable in any practical sense, since probing to those scales would require an accelerator on the scale of the solar system. As Hawking put it, it's unlikely that such a device would be built in the current financial climate. :lol:

twistor59 · **#31** by **twistor59** » May 04, 2010 8:04 pm

hackenslash wrote:It should be pointed out also that, if stings are Planck scale entities, then while they are testable in principle, they are untestable in any practical sense, since probing to those scales would require an accelerator on the scale of the solar system. As Hawking put it, it's unlikely that such a device would be built in the current financial climate.

Why would that be though ? Why can't you just take your LHC and leave it running for longer while the hadrons speed up to reach an energy appropriate for probing the small distances ? Is it because of engineering issues, or is there a physics reason why you can't do it ?

hackenslash · **#32** by **hackenslash** » May 04, 2010 8:32 pm

Physics reason. It has to do with keeping the ions confined in the circumference of the accelerator as the velocity increases. Think about how a car leans when you go around a corner. That's why accelerators keep getting larger and larger to probe smaller scales at higher energies.

Sityl · **#33** by **Sityl** » May 04, 2010 8:40 pm

I'm not gonna lie, I think the solar system sized collider is a FABULOUS idea.

iamthereforeithink · **#34** by **iamthereforeithink** » May 04, 2010 8:50 pm

num1cubfn wrote:I'm not gonna lie, I think the solar system sized collider is a FABULOUS idea.

How about we save the money and build a new planet instead? We could go there when this one runs out of oil.

twistor59 · **#35** by **twistor59** » May 05, 2010 6:31 am

hackenslash wrote:Physics reason. It has to do with keeping the ions confined in the circumference of the accelerator as the velocity increases. Think about how a car leans when you go around a corner. That's why accelerators keep getting larger and larger to probe smaller scales at higher energies.

You mean they can't build magnets strong enough to do the business at those energies and radius of curvature of the beam ? They need to try those magnets that Wile.E.Coyote uses in the Roadrunner cartoons.

klazmon · **#36** by **klazmon** » May 05, 2010 1:43 pm

twistor59 wrote:
hackenslash wrote:Physics reason. It has to do with keeping the ions confined in the circumference of the accelerator as the velocity increases. Think about how a car leans when you go around a corner. That's why accelerators keep getting larger and larger to probe smaller scales at higher energies.

You mean they can't build magnets strong enough to do the business at those energies and radius of curvature of the beam ? They need to try those magnets that Wile.E.Coyote uses in the Roadrunner cartoons.

Not only that but the more powerful the bending magnets, the greater the loss of beam energy to synchrotron radiation. This is why a 7 TeV electron synchrotron would have to be a lot larger in circumference than the LHC proton synchrotron. Some electron synchrotrons are deliberately designed and built to obtain the intense synchrotron radiation. See synchrotron light source.

lpetrich · **#37** by **lpetrich** » May 06, 2010 10:47 am

Let us now compare linear and circular accelerators.

Linear

From Larmor formula - Wikipedia, the free encyclopedia

a linear accelerator's radiated power is (2/3) * q2 * (d(g*v)/dt)2

q = electric charge, g = (1 - v2)-1/2, v = velocity, and

t is the external (coordinate) time, not the accelerated charge's proper time (that Wikipedia article isn't as clear on that as one might want).

The Stanford Linear Accelerator Center is about 3.2 km long, and it can accelerate electrons and positrons to 50 GeV. That's nearly 16 GeV/km. With that performance, one would need a 5-kiloparsec accelerator to get to GUT energies (1016 GeV) and a 5-megaparsec one to get up to Planck energies (1019 GeV). The center of our Galaxy is about 8 kpc away, and the Andromeda Galaxy 0.8 Mpc away.

Circular

From Synchrotron Radiation

radiated power = (2/3) * q2 g4 v4 / r2 (c = 1, cgs-like units)

r = radius

The orbit angular frequency w = q*B/(g*m) for magnetic field B and mass m.

The radius is thus r = (g*v*m)/(q*B)

The Large Hadron Collider has a radius of 4.3 km, and can keep 3.5-TeV protons inside of it. Now consider an accelerator that can accelerate protons up to 1016 or 1019 GeV with the same amount of magnetic field. One gets 0.4 parsecs in the former case, a little less than a third of the distance to Alpha Century, and 400 parsecs in the latter case, about the distance to the Orion Nebula.

Increasing the magnetic field? That has the problem that magnetic fields much greater than the LHC's fields will create internal pressures large enough to destroy the magnets. Pressure ~ B2, with 1 tesla producing 4 bar on the coils. The LHC goes up to about 8.36 tesla, with a resulting pressure of 280 bar, about 3 km deep in the oceans (1 bar = 100 kilopascal).

That is closest to the practical maximum for niobium-titanium superconducting magnets, about 10 tesla (The LHC supermagnets and cooling system).

New Iron based superconductors might resist magnetic fields over 100 Tesla - the strongest continuous-field magnet has a field of 45 tesla, giving a pressure of 8100 bar, while the strongest pulsed one has 90 tesla, giving 32 kilobar.

From, Tensile strength (Wikipedia), Kevlar has a yield strength of about 36 kilobar, and carbon nanotubes an ultimate strengh of about 110 - 630 kilobar, giving maximum magnetic fields of 170 to 400 tesla.

So it's going to be VERY hard to build magnets much stronger than the LHC's.

GreyICE · **#38** by **GreyICE** » May 10, 2010 9:05 pm

String Theory is more philosophy than science because science, ultimately, cares about testable predictions and String Theory, ultimately, doesn't make them.

That's really all it comes down to. Until they get over that 10^500th solutions nonsense and give us something we can straight up test, yes or no, it's more metaphysics than physics.

I agree a lot of good physics is theoretical, but a LOT of bad physics is too.

Nautilidae · **#39** by **Nautilidae** » May 11, 2010 1:20 pm

GreyICE wrote:String Theory is more philosophy than science because science, ultimately, cares about testable predictions and String Theory, ultimately, doesn't make them.

That's really all it comes down to. Until they get over that 10^500th solutions nonsense and give us something we can straight up test, yes or no, it's more metaphysics than physics.

I agree a lot of good physics is theoretical, but a LOT of bad physics is too.

What on Earth are you saying? Of course it makes testable predictions. I've already listed one.

GreyICE · **#40** by **GreyICE** » May 11, 2010 2:07 pm

Nautilidae wrote:
GreyICE wrote:String Theory is more philosophy than science because science, ultimately, cares about testable predictions and String Theory, ultimately, doesn't make them.

That's really all it comes down to. Until they get over that 10^500th solutions nonsense and give us something we can straight up test, yes or no, it's more metaphysics than physics.

I agree a lot of good physics is theoretical, but a LOT of bad physics is too.

What on Earth are you saying? Of course it makes testable predictions. I've already listed one.

Alright, what sort of energies will create string-balls? (Micro black holes can already be created and destroyed through simple relativistic acceleration).

Let me guess. The solution lies in a field given by the solution sets to certain mathematical functions related to D-Branes. Most solutions put it at above LHC energy, some below.

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