# String Theory: Vibrations & Plank Lengths

• pforeman
In summary, the conversation revolved around the concept of a string in string theory and its size in relation to the Plank length. The question of whether a string could be divided into smaller parts and if it could vibrate was also discussed. It was also mentioned that the Plank length is not necessarily the smallest possible length and the limitations of current technology in measuring length were brought up. The idea of a string being infinitely rigid and its ability to vibrate was also addressed.

#### pforeman

In string theory, a string is at or near the plank length. If a string is vibrating, or if it is curved into something like a circle, aren't you having something smaller than a plank length?

With a vibrating string part of the string is inverted to the other part, this would mean that you could (and should?) divide a plank length string into parts Which I thought should be impossible.

Is there some reason that the vibration isn't thought to be "lengthwise" with the vibration being from one plank length to two plank lengths or some integer thereof?
Thanks,
Paul

What would a longitudinal vibration on a string actually mean?

stretching from one length to another?

What is stretching from one length to another? You mean you are imagining the strings would change length?
What would it mean for the physics described by the string?

Basically string theory is set up to encode a lot of ideas at once as different dimensions.
The vibrations are transverse because it is difficult to draw a graph of longitudinal waves logitudinally.
So - how would it help to propose a string-model for fundamental interactions (etc etc) in which the strings carry longitudinal vibrations?

I'm taking this tack because I suspect you have been attempting to understand string theory based on your understanding of classical waves on a string. That approach will get you tied up i knots (could't resist).

pforeman said:
... aren't you having something smaller than a plank length?
l

You seem to have a belief that the Plank length is the smallest possible length. This is not true --- it's just the shortest length that we could ever even theoretically measure according to current theory. The fact that we could not measure shorter lengths doesn't mean they are physically impossible.

EDIT: and by they way, the Plank length using today's technology is MANY orders of magnitude smaller than anything that can be measured. But again, this does not rule out smaller lengths.

Assuming that a string is the size of 1 plank and thus the smallest possible entity. Also that it is not comprised of smaller particles then would it not be infinitely rigid? If so then how can it vibrate?

tanzanos said:
Assuming that a string is the size of 1 plank and thus the smallest possible entity.
... this question has been answered by phinds in post #5 when he said:
You seem to have a belief that the Plank length is the smallest possible length. This is not true...

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