# Question about concepts of distance

• Positralino
In summary, the conversation discusses the concept of distance in relation to quantum geometry and string theory, specifically mentioning two different notions of distance based on measurement methods using probes and strings wrapped around circular dimensions. The discussion also touches on the idea of T Duality and how the concept of distance changes at different scales. There is a mention of difficulty understanding this concept and a suggestion to explore string theory further for clarification.
Positralino
While reading "The Elegant Universe" by Brian Greene, I got stuck on the chapter about quantum geometry where it suggested that there are apparently 2 concepts of distance. I recall something about measuring distance with a probe traveling across the distance at a fixed and known speed and measuring how long it takes for it to cross the length. With the time and the speed known, the distance can be calculated. However, the book said something about using strings wrapped around a circular dimension and strings that are not as a probe, which gives two different notions of "distance" in our universe.

I do not understand how does wrapped and unwrapped strings give two different notions of distance. Can someone explain it to me?

Oh no - string theory.
http://en.wikipedia.org/wiki/String_theory

I suspect he's talking about T Duality.

When you head into more than three dimensions, the concept of distance is pretty vague anyway.
He's talking about how this changes with very large vs very small scales.

## 1. What is the concept of distance?

The concept of distance refers to the measure of physical space between two points. It is often measured in units such as meters, kilometers, or miles.

## 2. What are some common units used to measure distance?

Some common units used to measure distance include meters, kilometers, miles, and feet. Each unit is used for different contexts, such as meters for smaller distances and kilometers for longer distances.

## 3. How is distance different from displacement?

Distance is a measure of the total length traveled, while displacement is a measure of the shortest distance between the starting point and the ending point. Displacement takes into account direction, while distance does not.

## 4. Can distance be negative?

No, distance cannot be negative. It is always a positive value as it represents the length between two points.

## 5. How is distance related to speed and time?

Distance is directly related to speed and time through the equation d = st, where d is the distance, s is the speed, and t is the time. This equation shows that the greater the speed or the longer the time, the greater the distance traveled.

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