# Fermilab

## Inquiring Minds

Early Universe Question

Greetings

Well, you most likely receive a billion messages a day from nuts like me, but here's another one for you. Before the big bang, when there was just random endergy fluctuations in this empty void. At this point, before the particle that blew up, is it possible that there could have been one dimension raining supreme, a dimension which, when seperated, become the four distinct dimensions we now live in? What I'm trying to say is, when the universe was formed, couldn't the matter have acted as a prism, seperating this dimension of emptiness into the ten distinct dimensions of right now, including the four we perceive. In a way, the empty dimension would be like a beam of sunlight, the matter acting like a prism and seperating it into distinct dimensions. Please write back and tell me how wrong I am, just so I can go walk in front of a truck :).

Dear Questioner,

I didn't know much about dimensions until your letter prompted me to learn. Three spatial dimensions (like east-west, north-south, and up-down) and one dimension of time is all I can easily grasp. But many physicists who study the theory of 'strings' think we live in a world of 10 or 11 dimensions, 6 or 7 of which we can't see or experience directly. To get a handle on this idea, and to try to answer your question about the possibility of fewer dimensions at an early time in the universe, I sought help from Ola Tornkvist and from Joe Lykken. They are two physicists at Fermilab who specialise in theory (as opposed to experiment).

The first thing I learned is that physicists and mathematicians think of dimensions differently. To a mathematician, a dimension is just another axis on which to plot numbers, like the 'x' and 'y' axes we use to plot, say, the Dow Jones index as a function of time. A mathematician might think of dimensions as more columns of numbers in a table, and not worry about trying to plot them all at once on a piece of paper. In the example I used, daily values of the Dow Jones index and time would be two columns of numbers. One could also have columns of data on the share price of twenty different companies. One could plot the data for one of the companies on the y axis against time on the x axis, or plot the data for a company on the y axis and the Dow Jones index on the x axis, or just think of all the data on the twenty companies, plus the Dow Jones index, and time, as data that exists in 22 dimensions.

To a physicist, the number of dimensions is fixed by how many directions a particle or an object can move in. So far, all anybody knows for sure is that particles move in three distinct dimensions. No matter what path an object follows through space, the path will be made up of components in three directions that are at right angles to each other, like east-west, north-south, and up-down (or southeast-northwest, southwest-northeast, and up-down, three directions that would do equally well as reference directions).

Theorists have imagined that there could be six or seven more dimensions, clinging to every point in space, but all "curled up" so tightly that they don't usually affect particles we study in the laboratory. To travel along these dimensions, a particle would have to move in a circle of extremely small radius, and that's hard to do, as you know if you've ever tried to take a hairpin curve in your car at 50 miles an hour. The first step in getting particles to travel into these other dimensions is to accelerate them to very high energy.

It might sound like it would be very difficult to prove the existence of these other dimensions, but there are a few ways physicists might get experimental evidence for their existence. When particles collide, as in the Tevatron, their behavior depends on how many dimensions are available for them to move in. If there are more directions than just the three we usually think of, there is less chance the particles will undergo a particular type of interaction; there are more alternatives available. One way physicists could get a hint of other dimensions is to determine that the rate of interactions doesn't jibe with their expectations based on the premise of three spatial dimensions.

Another clue to the existence of the other dimensions would be if physicists accelerated particles in collisions at higher energies than the Tevatron can presently achieve, and saw particles they already know about, but produced in new ways.

Some physicists speculate not only that these extra dimensions exist, but also that the number of dimensions could change in time. Perhaps all 10 or 11 dimensions of string theory existed on an equal footing at the time of the big bang; then three spatial dimensions expanded, while 6 shrank. To answer your question, it seems more likely, to physicists, that there were more dimensions than we know of now, at the time of the Big Bang, rather than fewer dimensions.

Leila Belkora
(Ph.D., Astrophysics)
Editor, Office of Public Affairs

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