## Planck length and dimension

Hello all .
We know Planck length is and universe was in that density at big bang .

Is that mean there was dimension at that time ?
I mean , can we move in Planck length ? like up , down, right, left, forward, backward ؟

Mentor
 Is that mean there was dimension at that time ? I mean , can we move in Planck length ?
I don't understand these questions.

 like up , down, right, left, forward, backward
I am quite sure that we can move up, down, right, left, forward, backward. Note that those 6 directions are arbitrary definitions based on your current orientation in space.

 Quote by mfb I don't understand these questions.
what's initial size of the universe at big bang ? there was any direction at that moment ?

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Gold Member

## Planck length and dimension

 Quote by big_bounce what's initial size of the universe at big bang ? there was any direction at that moment ?
The general theory is that "size" is not a meaningful question about the singularity as we currently understand it. It was everything there is, so either (1) finite but unbounded or (2) infinite.

In any case it had no center, no edge, and no direction.

 Quote by phinds The general theory is that "size" is not a meaningful question about the singularity as we currently understand it. It was everything there is, so either (1) finite but unbounded or (2) infinite. In any case it had no center, no edge, and no direction.
How about in loop quantum gravity ?
The new theory and other new theory such as m-theory ( Brane ) said there was any singularity at that moment
So can we consider any direction at that moment ?

 Mentor I don't think you use "direction" and "dimension" the same way physics usually does. As a result, your questions look strange (at least to me) and I don't know how to interpret them.

 Quote by mfb I don't think you use "direction" and "dimension" the same way physics usually does. As a result, your questions look strange (at least to me) and I don't know how to interpret them.
Al right !
You consider direction and dimension is same ! and replace "dimension" to "direction" in my question .

So if we don't consider any singularity at big bang , can we say we have dimension or direction at that moment ?

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Gold Member
 Quote by big_bounce How about in loop quantum gravity ? The new theory and other new theory such as m-theory ( Brane ) said there was any singularity at that moment So can we consider any direction at that moment ?
This seems to be an exceptionally poor representation of anything realistic. Two problems jump right out at you

First, "space-time is classical" is nonsensical. Space and time are classical things, space-time is not.

Second, the time scale is just weird. I mean, "today" is 10E17 seconds and the first galaxies are 10E16 seconds. So galaxies just started forming 10 seconds ago? Doesn't seem likely.

Mentor
If you consider "classical" as "not quantum-mechanical" (but allow special relativity), it is fine.

10E17s=1017s (~3 billion years)
10E16=1016 (~300 million years)
10E17-10E16=9*1016 (~2700 million years)

The universe is older than 3 billion years, but the order of magnitude is still correct.

 Quote by big_bounce So if we don't consider any singularity at big bang , can we say we have dimension or direction at that moment ?
3 spatial dimensions plus one time dimension, as usual. Plus some (undiscovered) extradimensions, maybe.

 Quote by phinds This seems to be an exceptionally poor representation of anything realistic. Two problems jump right out at you First, "space-time is classical" is nonsensical. Space and time are classical things, space-time is not. Second, the time scale is just weird. I mean, "today" is 10E17 seconds and the first galaxies are 10E16 seconds. So galaxies just started forming 10 seconds ago? Doesn't seem likely.
10^17-10^16= 9e16s or 90'000'000'000'000'000s seconds.
Not ten seconds. This a logarithmic scale.

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 Quote by jetwaterluffy 10^17-10^16= 9e16s or 90'000'000'000'000'000s seconds. Not ten seconds. This a logarithmic scale.
doh !