This is Part 1 of a series on large extra dimensions.
I always say that one of the things that separates real science from pseudoscience is that while in both you’re allowed to say whatever crazy idea pops into your mind, in real science you’re obligated to take that idea seriously.
You can imagine a late evening. You’re hanging out with friends. Spit balling some wild ideas. Maybe there are some illicit substances involved. Maybe all you need is a good wine and cheese pairing. And then you get a wild idea, and you feel bold enough to share it with the group: “hey, everybody, listen. What if the universe had more dimensions?”
Powerful idea. Interesting idea. Probably wrong idea, but that’s never stopped us before. Now in pseudoscience this is where the line of thinking largely stops, or you just start spitting out loosely connected ideas that branch off of it.
But if you said this to a room full of physicists, you would see something magical happen. You would start to see these very serious, very smart people TAKING IT SERIOUSLY. And what does it mean to take a wild idea seriously? It means we’re going to explore if the idea ACTUALLY WORKS and if there’s a way we can test it.
So does the universe have extra dimensions? Well, let’s take the idea seriously and see where it takes us.
To see if a wild and crazy and potentially wonderful idea actually works in the real world that we happen to find ourselves in, we have to do two things. Actually, make that three. One, we have to see if this new idea fixes any problems that we might have. Does it resolve a longstanding tension? Does it offer a new insight? Does it explain an enduring mystery? For this new idea to work is has to, you know, DO WORK, and so it has to help us in our journey to a greater understanding of the universe.
Two, for this idea to work it has to fit into known physics. Now that doesn’t mean it can’t break anything. If there’s anything physicists like to do it’s break all of their toys. But we have to figure out WHAT breaks and WHERE and WHY so that we understand the implications of the new idea.
Two, we have to use this new idea to make predictions. We’re gonna test this sucker out, so we need something to point to. Something to probe. Something to peel apart and see what goes on inside.
And lastly, we need to design a way to make those tests happen. Do we need a new observatory or collider? Can we use old, archived data? Can we take an existing setup and twist a knob and look at things through a new angle? Nature is the ultimate arbiter of our ideas, and we have to put it to the test.
Now when it comes to potentially more dimensions in the universe, we’ve done all three! We still don’t know if there are large extra dimensions (sorry, spoiler alert I guess) but that’s because this is a really sticky, tough problem that requires a lot of nuance and care, and rarely in science do you get clear-cut black-and-white answers. You instead get a series of “oh, that’s interesting” and then “mayyyyybe it works” and then “nah, turns out that was a wrong approach” and then “well what if we tried this instead”
Like I said, we’re going to take our ideas, even the wild ones, seriously.
And the idea of extra dimensions has been floating around the popular imagination since well into the 1800’s. Just look at the monsters of Lovecraftian lore for example. But physicists didn’t really glom onto the idea until 1919 when a guy named Theodor Kaluza said “hey, albert, you’ve got a really great general theory of relativity that operates in four dimensions. Well, what if we added an extra dimension to it? I think it might solve some longstanding problems in science.”
See? Taking the wild idea seriously!
Kaluza’s theory solved a problem of unification. He found that if you added an extra spatial dimension, then you could write down a single set of equations that included both gravity and electromagnetism. That’s neat. That tells us that there might be deeper connections between those two wildly different forces, which would open up so many cool revelations and insights and technological marvels.
One problem: where exactly is that extra dimension? If I go out for a walk, I can choose any direction I want: left or right, forward or backward, even up or down. There’s no extra freedom of movement that I’ve somehow been ignoring this whole time. There’s no readily apparent DIRECTION that is perpendicular to all the other directions.
We’re taking this idea seriously, and this idea has to connect to everything else we know about the universe. So if an extra dimension brings us a new insight in physics, then where does this extra dimension exist?
Seven years later, in 1926, Oskar Klein proposed an idea of where the extra dimension could be. Right here. Right in front of us, but all curled up on itself so tightly that we never notice. You know those rolling tables that they use at the X-ray scanners at the airport? Think of those like compact dimensions. You roll your bag down, and it only moves in one direction. But in the process it circumnavigates all those little rollers which are curled up in a second dimension. You don’t notice that extra movement, just the bag rolling on down.
To make this work the extra dimension (or dimensionS if you’re feeling saucy) have to be way, way small, like all the way down to around the Planck scale, so we’re talking 10^-33 centimeters or so. To put that scale in perspective, you, as in your body sitting right there, is closer to the size of the entire observable universe than the Planck scale is to you.
That’s small, too small for us to notice in our day-to-day life, or even day-to-day high-energy physics.
For the aficionados, you already know that Kaluza-Klein theory (as this idea came to be called) would go underground for a couple decades and the reemerge as the backbone of String Theory, which would go on to require not one, not two, but 10 (or maybe 11, depending on what kind of mood Ed Witten is in today) compact extra dimensions. Once again, all those dimensions are curled up on themselves at nearly the Planck scale, so we have no hope of directly observing them.
But I’m not talking about string theory today. I’m not interested in super-tiny super-curled up all cutesy wutesy extra dimensions. I’m interested in LARGE extra dimensions. Obviously, these dimensions can’t be TOO big. Otherwise we would’ve noticed them by now. But maybe that can be large compared to the Planck scale (which to be fair, is literally everything) and maybe these large extra dimensions don’t have all that big of an interaction with everything else.
So let’s take this idea seriously. If there are extra dimensions that are NOT curled up at the Planck scale, can they do anything for us? Can they get some work done?
To be continued…
