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Yep, the idea is that they cannot slow down to c.

Mehh

Also they have imaginary mass...

Another thing. You know how QED says something that what we observe in our everyday experience is the net result of infinite process occuring at an atomic scale? For example, on the Feynman Lectures on QED, Feynman talks about how photons of monochromatic light collide with the mirror at a load of different angles, but if you sum up all their paths as vectors, their net path would be enter an angle to the plane mirror and reflect off the mirror at that same angle. Well, I read somewhere that QED also states that the speed of light is also a mean figure of lots of photons travelling at speeds just under and just above the speed of light? Is this true?

If you cant measure it, it isnt Physics. "True" doesnt come into it. Every theory is true until it's false.

Tachyons are an inherent problem in any theory. The first versions of string theory had tachyons and they were crippled by it.

The problem with tacyhons is they make the vacuum unstable. Fluctuations in the vacuum cause tachyons to appear, which then accelerate to possibly infinite speeds (given there's no speed limit if you start faster than light from relativity's point of view), throwing out more energy as they do so, making more tacyhons. In a tiny fraction of a second the entire universe would explode in an exponentially growing quantity of tacyhons.

Bugger.

As such, any theory with tachyons is considered wrong. They are logically and physically inconsistent. Work in string theory found a way around them by adding fermion fields which cancelled out the tacyhons from the theory perfectly

The problem with tacyhons is they make the vacuum unstable. Fluctuations in the vacuum cause tachyons to appear, which then accelerate to possibly infinite speeds (given there's no speed limit if you start faster than light from relativity's point of view), throwing out more energy as they do so, making more tacyhons. In a tiny fraction of a second the entire universe would explode in an exponentially growing quantity of tacyhons.

Bugger.

As such, any theory with tachyons is considered wrong. They are logically and physically inconsistent. Work in string theory found a way around them by adding fermion fields which cancelled out the tacyhons from the theory perfectly

teachercol

If you cant measure it, it isnt Physics. "True" doesnt come into it. Every theory is true until it's false.

I wasn't asking about whether QED is 'correct' in the physical sense as in an accurate description of Nature. I was asking whether it is true that this idea was proposed by the theory of QED, to verify my memory. So, my question is 'historical' in wanting to know the characteristics of the theory and does not concern whether Nature operates in such a fashion.

AlphaNumeric

The problem with tacyhons is they make the vacuum unstable.

Alphanumeric

As such, any theory with tachyons is considered wrong. They are logically and physically inconsistent.

Dharma

What is the significance of a vacuum?

Dharma

So they were originally just a patch, a way of dodging some problems in string theory?

With the development of string theory and the inclusion of fermions (ie matter particles) new machinary naturally developed within string theory which disallowed the tachyon mass state. In a similar way to QFT naturally prevents 2 fermions being in the same place at the same time, string theory developed a mechanism which blocked a load of physical states, the tachyon being one of them. There was no 'fiddling' or 'patching', it was something which string theory's mechanics naturally just prevented.

Wangers

Unfortuneatly for the rest of us string theory needs a rigorous degree in maths to begin to understand. btw - is it true that most of the actual theory cannot be tested?

No...the existance of strings can not be tested. This is because they are by definition smaller than any Plank measurement.

However string theory MAY come up with some predictions in future that could be tested.

Unfortunately we are still struggling with the unification of the String THEORIES.

You need to be specific when saying things like that and it's often a point of confusion or contention.

- Does string theory give physical predictions of any kind?

Yes, it predicts a slew of physical processes and phenomena.

- Are any of them actually measurable now by our technology?

Yes, the main example being general relativity. String theory develops this within a quantum field theory framework. However, given this isn't a new set of phenonema, it is not seen as 'evidence for string theory' since you can get the same model without all the 'baggage' string theory comes with like extra dimensions etc.

- But why do people say string theory has no tested predictions?

Because they poorly word their description. String theory has tested predictions, what they are referring to is a prediction which no other theory does and which is found to be true by experiment. For instance, relativity superplanted Newtonian theory when we observed light bending around the sun in a way which nothing but relativity could explain. String theory has no such experiment for itself.

Part of the issue is that while string theory covers so much of physics, it covers theories we already have. It models gravity using general relativity, but if you want to model a gravitational system, why bother with string theory when the formalism of GR is simpler? It covers supersymmetry, but why not just use stand alone supersymmetry rather than all the complex string theory methods? It covers cosmological models, but why not just use a version of general relativity for that?

If you want to do a specific model, we already have so many well developed theories for various things, apart from quantum gravity it's pretty hard to find an area of physics that isn't already at least partly modelled by something. But the plus side of string theory is that it's ALL under the same heading, all part of the same formalism. Things like GR or susy or quantum field theories seem to be effective models of string theory (at least that's the general hope), you take a certain limit of the theory and you get quantum mechanics, you take another and you get GR. At present the problem lies in nailing down the details of this. GR is pretty much done, it's getting the QFT sector to play ball.

- But what about other physical predictions?

Unfortunately that's part of the problem with string theory and relates to the 'baggage' I mentioned. You want a model of GR and supersymmetry put together (otherwise known as 'super gravity') then you're going to need to consider 10 or 11 dimensions, reduce 6 or 7 of them to a minute size and then do your calculations. But what shape do the 6 or 7 dimensions take? That's the kind of thing my work is on and it's been the major problem to string theories for about a decade now.... Not to mention, as Mehh says, such things can't be measured directly and at present it's not known if a round-about way of measuring such predictions would ever be found.

- Does string theory give physical predictions of any kind?

Yes, it predicts a slew of physical processes and phenomena.

- Are any of them actually measurable now by our technology?

Yes, the main example being general relativity. String theory develops this within a quantum field theory framework. However, given this isn't a new set of phenonema, it is not seen as 'evidence for string theory' since you can get the same model without all the 'baggage' string theory comes with like extra dimensions etc.

- But why do people say string theory has no tested predictions?

Because they poorly word their description. String theory has tested predictions, what they are referring to is a prediction which no other theory does and which is found to be true by experiment. For instance, relativity superplanted Newtonian theory when we observed light bending around the sun in a way which nothing but relativity could explain. String theory has no such experiment for itself.

Part of the issue is that while string theory covers so much of physics, it covers theories we already have. It models gravity using general relativity, but if you want to model a gravitational system, why bother with string theory when the formalism of GR is simpler? It covers supersymmetry, but why not just use stand alone supersymmetry rather than all the complex string theory methods? It covers cosmological models, but why not just use a version of general relativity for that?

If you want to do a specific model, we already have so many well developed theories for various things, apart from quantum gravity it's pretty hard to find an area of physics that isn't already at least partly modelled by something. But the plus side of string theory is that it's ALL under the same heading, all part of the same formalism. Things like GR or susy or quantum field theories seem to be effective models of string theory (at least that's the general hope), you take a certain limit of the theory and you get quantum mechanics, you take another and you get GR. At present the problem lies in nailing down the details of this. GR is pretty much done, it's getting the QFT sector to play ball.

- But what about other physical predictions?

Unfortunately that's part of the problem with string theory and relates to the 'baggage' I mentioned. You want a model of GR and supersymmetry put together (otherwise known as 'super gravity') then you're going to need to consider 10 or 11 dimensions, reduce 6 or 7 of them to a minute size and then do your calculations. But what shape do the 6 or 7 dimensions take? That's the kind of thing my work is on and it's been the major problem to string theories for about a decade now.... Not to mention, as Mehh says, such things can't be measured directly and at present it's not known if a round-about way of measuring such predictions would ever be found.

Alpha,

any chance of a brief 'in a nutshell' type explanation as to why Ed Witten is considered such a hero?

any chance of a brief 'in a nutshell' type explanation as to why Ed Witten is considered such a hero?

AlphaNumeric

Part of the issue is that while string theory covers so much of physics, it covers theories we already have. It models gravity using general relativity, but if you want to model a gravitational system, why bother with string theory when the formalism of GR is simpler? It covers supersymmetry, but why not just use stand alone supersymmetry rather than all the complex string theory methods? It covers cosmological models, but why not just use a version of general relativity for that?

This is because Strings is being formalated as a ToE. But as opposed to having lots of theories that are incompatible at the moment, we have lots of string theories, which don't link together nicely...

If we can link the frame work up together nicely we might find some unique predictions. But that is some way to go...

Dirac Delta Function

any chance of a brief 'in a nutshell' type explanation as to why Ed Witten is considered such a hero?

Witten is one of those people who is quiet and unassuming but when he's got something to say, it's best you listen. Numerous times in the last 25 years or so he's turned his attention to a problem which has been plaguing both mathematicians and physicists and demolished it seemingly with little effort.

He proved the positive energy theorem in a very simple way (though wasn't first by about 2 years but the first proof was horrifically complex) in general relativity. He demonstrated that the 5 string theories of the 1980s were actually different sides of the same underlying theory. He provided mathematical methods which allowed otherwise uncomputable (due to complexity) particle interactions to be possible and he's done enormous work in topology.

He's got the highest 'h index' rating of any physicist, as well as having the highest total score ever. The h index is a way of measuring the importance of papers publised by number of citations.

I wouldn't be half suprised if he also fights crime during the night.

Mehh

This is because Strings is being formalated as a ToE. But as opposed to having lots of theories that are incompatible at the moment, we have lots of string theories, which don't link together nicely...

If we can link the frame work up together nicely we might find some unique predictions. But that is some way to go...

If we can link the frame work up together nicely we might find some unique predictions. But that is some way to go...

In fully uncompactified space-time they are all different but once you start crunching down the dimensions their differences rapidly shrink away. Compactifing onto a circle turns 11d supergravity into 10d IIA string theory. Compactifying that onto a cirlcle allows the duality with IIB string theory. Beyond that they are identical. $E_{8} \times E_{8}$ and SO(32) need some compactification I don't know, but it is there. The problem then comes on what the 6 dimensions which must be compactified ultimately become, 6d torus? 6d Calabi-Yau? $S^{6}$ hypersphere? K3? Some kind of combination.

Unfortunately while the dualities show the 5 different theories are just different limits of the same theory, the final choice of space-time shapes to compactify down on is mind bogglingly huge. A google is $10^{100}$. A googleplex is $10^{google} = 10^{10^{100}}$. That's the kind of number we're talking about. Same string theory, inequivalent vacuum states and since you build up your excited states from the vacuum (ala usual Fock spaces) you've got a googleplex of inequivalent systems.

Having spent the last month investigating one of them, I kinda see the enormous problem string theory currently has.... Someone needs to come up with a way of excluding or testing HUGE quantities of vacuua at the same time, not one by one.

Krystal

are u all uni students here...?

ah no wonder....i am merely an A level student..

must be exciting work eh? id love to do something on those lines...i just don't think i've got the potential*sigh*...so im goin for medicine instead.

could u elaborate on the M-theory? i dont fully understand it. doesn't it say that perhaps the beginning of the universe was merely 2 colliding 'membranes'. Also is there also something about gravitational force leaking through the membrane, as a reason for why it is such weak force...?

must be exciting work eh? id love to do something on those lines...i just don't think i've got the potential*sigh*...so im goin for medicine instead.

could u elaborate on the M-theory? i dont fully understand it. doesn't it say that perhaps the beginning of the universe was merely 2 colliding 'membranes'. Also is there also something about gravitational force leaking through the membrane, as a reason for why it is such weak force...?

Krystal

must be exciting work eh?

Krystal

could u elaborate on the M-theory? i dont fully understand it. doesn't it say that perhaps the beginning of the universe was merely 2 colliding 'membranes'. Also is there also something about gravitational force leaking through the membrane, as a reason for why it is such weak force...?

M theory itself was the solution Witten came up with to explain the fact 4 different string theories (along with 11d supergravity) existed in the early 80s with no way of connecting them. Witten showed that if you assume the existence of an 11 dimensional high energy theory, you can reproduce the 4 string theories and supergravity (sugra for short) as limits of M theory. As I explained in a previous post, if you crush one of M theory's dimensions down onto a tiny circle and then take the limit as that circle shrinks to a point, you get IIA string theory. If you instead shrink down one of the dimensions into an interval, you'd get the heterotic string theories.

In the same way that Newtonian dynamics is a low energy limit of relativity, the various string theories and sugra are low energy or lower dimensional limits of M theory. While this provides a framework for the theories to be linked, it doesn't solve all the problems. M theory is still a 'weak field limit', the high energy/coupling interactions within it are still not understood, hence why quantum gravity is still not worked out.

Within string theories, gravity is modelled by the graviton particle (or by space-time curvature, it's a little up in the air). The graviton is an excitation of a closed string. Typically strings are open, they don't join themselves at their ends and via some complicated work, the ends are confined to the branes previously mentioned. For instance a D3 brane takes up 3 spacial dimensions, so the ends of a string confined to such a brane cannot move through the other 6 or 7 spacial dimensions, in the same way we're sort of stuck on the surface of the Earth. Closed strings don't have ends, so they aren't confined to a brane. Hence they can wander through all 10 dimensions without restriction.

So if gravity can move between branes, it can 'leak' from region to region. Normal particles might be trapped on a D3 brane (since our universe or what we can see is 3 dimensional), gravity isn't. Hence, it can just leave the brane normal particles are on, greatly reducing it's influence. Alternatively, if the mass of the particles is held on other branes, not connected to our D3 brane, then gravity has to leak across to our brane and you have a reduction in strength. So while gravity seems to be able to affect any and all branes, it pays the price of diluting itself a lot in the process.

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