# 2 time dimensions

Jack
Is it theoretically posible to have two time dimensions? Physicists often try to describe aspects of a 4-dimensional(spacially) universe but I have never heard of anyone try to describe a universe in which there are two time dimensions. If it is theoretically possible then can we make any predictions about what it may be like, however vague they may be?

## Answers and Replies

Science Advisor
There arre some forms of string theory that include another time dimenion, but this is curled up too small to be beasured by us, just like the extra spatial dimensions. However, one projected result of having freedom of movement through a additional time dimension would be that we could "circle around", come back, and pass through a moment we have already been through. I must admit I have some difficulty imagining what that would mean in practice.

VBPhysics
LURCH -- That is more of moving back in our own time than using another time dimension, but your right there are string theorist that try to incorporate other time dimensions into their work.

Jack, on a related note. There is no exclusion principle that I am aware of which says that objects couldn't float backwards in time. There is an argument to be made, and indeed feynman made it, that electrons use photons as a kind of universal road sign. where in every time that they get hit by a photon they reverse their direction in time. This is how he explained the existence of positrons. The theory also has a rather unique feature in that since electrons are just passing back and forth through time until they are hit by a photon, they can be in multiple, even an unlimited number of, places at once meaning that there could be only one electron in the universe.

What if time was more of a plane than a strait line? The second dimension would allow time to bend, which could explain time dilation because it could bend and allow time to go faster for one person and slower for another, relative to velocity.

eljose79
I think there is only a time dimension..

an example was given to us in a conference at university he argued that if there were two time dimensions then we could perform transform that changed one into the other so it would be possible to go back in time...what is of course unobserved.

hmm

I think the thing we need to remember, in my opionion, when we start talking about subjects like this is that we may be able to observe and analyze these types of particles and situations, but is beyond our human ability to manipulate and use to our advantage.

Science Advisor
Gold Member
Yes Hawking and Hartle's concept of imaginary time uses an extra time axis which elimnates singularites and can be used in conjunction with the wavefunction of the universe to help us understand why the universe is in the stae that we observe it to be in.

Ambitwistor
Originally posted by jcsd
Yes Hawking and Hartle's concept of imaginary time uses an extra time axis

In this context of this thread, that statement may be misleading. In Euclidean field theory (or quantum gravity), the imaginary time axis does not exist in addition to the real time axis, making two time dimensions; rather, it replaces the real time axis (making it into a space axis).

Hence, if anything, Hartle and Hawking's quantum cosmology should be considered a theory with no time dimensions.

Science Advisor
Gold Member
Originally posted by Ambitwistor
In this context of this thread, that statement may be misleading. In Euclidean field theory (or quantum gravity), the imaginary time axis does not exist in addition to the real time axis, making two time dimensions; rather, it replaces the real time axis (making it into a space axis).

Hence, if anything, Hartle and Hawking's quantum cosmology should be considered a theory with no time dimensions.

Interesting, I can't claim any technical knowledge of any of the different approaches to quantum gravity, but I thought though that it formed a closed surface with the real-time axis or are you saying that in this approach you cannot make any real distinction between the spatial and temporal dimensions?

Ambitwistor
In the "no-boundary" initial state in the Hartle-Hawking proposal, one considers path integrals over 4D Euclidean (or really, Riemannian) geometries (which locally have a Euclidean metric ds2 = x2+y2+z2+&tau;2),
instead of 4D Lorentzian geometries (which locally have a Minkowskian metric ds2 = x2+y2+z2-t2). So we can say that in Euclidean quantum gravity, the "imaginary time" dimension is literally a spatial dimension.

Science Advisor
Gold Member
So the imaginary time axis replaces the time? So in effect rather than considering 4 dimensional space-time, in imaginary time we consider 4-dimensional space?

Ambitwistor
Yes, that's right; that's why the approach is called "Euclidean".