String theory / Plank length question

In summary: Planck length is. The idea of a Planck length as the smallest distance that one could conceivably detect quantum fluctuations is a misleading approximation, and has been abandoned in favor of a more general notion of a "Planck scale" as the smallest distance that is still accessible to classical physics.In summary, according to the summarizer, at the Planck scale, quantum mechanics and gravity are both fully in effect. Anything smaller than the Planck scale is affected by both, and anything larger is not. However, there is a scale smaller than the Planck scale where quantum mechanics and gravity are both in effect, and at that scale QG is needed to take into account gravity.
  • #1
Chas3down
60
0
From whst I have read, anything smaller than Plancks length, you need to account for quantum gravity.. also, it is assumed strings in string theory are Plancks length.. so how could anything be smaller than Plancks length if strings are Plancks length(or greater than)? I thought strings made up everything?

Sorry if my question is stupid or doesn't make sense, I am just a HS student who likes to look up and attempt to understand some ineteresting physics.
 
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  • #2
- String theory is just a hypothesis, it can be wrong.
- String theory does not have particles which are smaller than (the order of the) Planck length, but the strings itself can need descriptions on scales smaller than their own size.
 
  • #3
Okay, that was what I assumed...
anything smaller then Planck length is under the laws of quantum mechanics, correct? and anything larger is not..
 
  • #4
Anything follows the laws of quantum mechanics*. But for anything smaller than the Planck scale you need an extension of the (current) quantum mechanics, which takes gravity into account.

*well, depends a bit on your favorite interpretation of QM, but at least there is no fundamental size limit
 
  • #5
sorry, QG is what I meant. But if Planck length = length of a string, and nothing can be smaller than that, nothing would be affected by QG. I know this isn't true, I am missing a concept somewhere, but not sure where...

QG is needed when less than Lp(planck length)
Strings of string theory are assumed to be Planck length or higher...
Thus. nothing can be smaller than Planck length and QG wouldn't affect anything.
 
  • #6
Chas3down said:
and nothing can be smaller than that
That is not true.

Take apples as an example: An apple has a size of some centimeters. But there are apples with different diameter, and different geometry, and you can compare apples on a scale of millimeters or even micrometers.

In addition, we are talking about a specific theory here.
 
  • #7
Chas3down said:
sorry, QG is what I meant. But if Planck length = length of a string, and nothing can be smaller than that, nothing would be affected by QG. I know this isn't true, I am missing a concept somewhere, but not sure where...

QG is needed when less than Lp(planck length)
Strings of string theory are assumed to be Planck length or higher...
Thus. nothing can be smaller than Planck length and QG wouldn't affect anything.

It seems important to explain something about how approximations work in physics. It is not the case that quantum mechanics does not affect anything on long scales. It is the case that quantum effects are very small compared to classical ones when we probe a system on length scales which are long compared to the characteristic size of the object. Quantum effects do not suddenly turn on as we probe to shorter and shorter scales. They are always there, but at long scales they are small enough that we may ignore them for most purposes.

Similarly, QG effects are not zero at 1000 Planck lengths, but they are around 0.1% smaller than the classical description. Depending on the precision of a hypothetical experiment, they might be completely negligible. However, a sufficiently precise measurement could not neglect QG effects.

The Planck length is the length scale at which the quantum effect is as large as the classical prediction, so there is no confusion at all about whether or not quantum gravity effects can be neglected. It is not some magic scale where QG effects get turned on.
 
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  • #8
mfb said:
That is not true.

Take apples as an example: An apple has a size of some centimeters. But there are apples with different diameter, and different geometry, and you can compare apples on a scale of millimeters or even micrometers.

In addition, we are talking about a specific theory here.

This is really a serious misunderstanding, on two related counts. First, in string theory as a complete theory, there are no objects smaller than strings to probe a sub-stringy geometry. Thus "smaller" distances (roughly speaking, see below) are unobservable. If one tried to pump in more and more energy in a scattering process in an attempt to resolve smaller distances, what one actually would find is that the effective size of the string grows again and becomes classical at some point (there is circumstantial evidence for the latter claim, which has not been proven AFAIK). So in a sense the string scale is the smallest scale that can be accessed, and if the latter point turns out as expected, then quantum effects are maximal there.

Which brings me to the second point, which fzero has already touched upon. At the Planck scale, quantum fluctuations of the space-time geometry become of order one, so there is no good notion any more of what a metric is, and thus distance, etc. Thus it does not make sense to use the word "smaller". In the picture alluded to above, Apples can_not_ be compared with each other at a millimeter scale, because there is no way the specify what a millimeter is. This applies more or less to all theories of quantum gravity besides strings.
 
  • #9
Unobservable to direct measurements is not the same as "not there". As long as you don't consider a discrete spacetime, smaller distances are there - even if there is no direct (!) way to probe them, similar to the problem of measuring millimeters if "hit it with an apple" is your only option.
 
  • #10
mfb said:
Unobservable to direct measurements is not the same as "not there". As long as you don't consider a discrete spacetime, smaller distances are there - even if there is no direct (!) way to probe them, similar to the problem of measuring millimeters if "hit it with an apple" is your only option.

What is a "smaller distance" if there is no metric, no classical geometry left? How would you compare two "distances" ?
 
  • #11
If I would know that, I would publish that theory of everything and wait for the Nobel Prize :wink:.
 

Related to String theory / Plank length question

1. What is string theory?

String theory is a theoretical framework that attempts to reconcile quantum mechanics and general relativity by describing the fundamental building blocks of the universe as tiny strings rather than point particles.

2. What is the significance of the Plank length in string theory?

The Plank length, which is approximately 1.6 x 10^-35 meters, is the smallest length scale that has any physical meaning in string theory. This is where the effects of quantum gravity become relevant, and it is believed that at this scale, the concept of space and time as we know it breaks down.

3. How does string theory explain the dimensions of the universe?

String theory proposes that the universe has 10 dimensions - 3 spatial dimensions and 1 time dimension that we experience, and 6 additional compactified dimensions that are curled up and too small for us to detect. These extra dimensions are necessary for the math of string theory to work, and they provide a possible explanation for the different forces and particle types in the universe.

4. Is there any experimental evidence for string theory?

Currently, there is no direct experimental evidence for string theory. However, some predictions of string theory, such as the existence of supersymmetric particles, may be testable in future experiments. Additionally, string theory has provided important insights and mathematical tools for other areas of physics.

5. What is the current status of string theory in the scientific community?

String theory remains a highly debated and controversial topic in the scientific community. While some researchers believe it has the potential to unify all of physics, others criticize its lack of testable predictions and its reliance on unproven concepts such as extra dimensions. Further research and experimentation are needed to fully understand the validity and implications of string theory.

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