In another universe you might have become the president of Micronesia. Or a pauper, subsisting on ketchup. Perhaps a different version of you already read this—in which case, read it again, for the first time.
All crazy ideas, but all completely plausible given the idea that there may be, in fact, multiple universes. Infinite, even.
I recently sat down with physicist and best-selling author Brian Greene at the 2015 Curiosity Retreat, a weeklong conference featuring scientists and other speakers in southwest Colorado, to talk about string theory, infinite worlds and cosmic bread loaves.
Let’s cut to the chase. Are there multiple universes?
I don't know. But I will say that to me it's provocative at the very least that so many pathways in science naturally bump up against the notion of other universes. Cosmology—the science of trying to understanding how our universe began—suggests our universe may not be unique, or the only one.
String theory also suggests the possibility of other universes. Quantum physics does too. That doesn’t mean it’s right, but means it’s worthy of attention.
You study string theory. What exactly is it?
The basic idea is that the most basic element of a matter is a little vibrating filament, rather than a dot [as is the case in quantum physics or quantum mechanics, which studies the behavior of tiny, subatomic particles]. That move from the old idea of a dot to a new idea of a filament allows us to meld the laws of the large, which are described by the theory of general relativity, with the laws of the small, or quantum mechanics.
Can you elaborate on how these other areas suggest there are multiple universes?
Take the origin of our universe, the Big Bang. There's reason to believe that wasn't a onetime event, that there were many Big Bangs each giving rise to many universes.
On the other hand you’ve got quantum mechanics, which describes the universe being probabilistic, the electron being over here or over there. When you measure the electron, you find it in one location, but what happened to the other possibility? The natural suggestion from the math is that the other possibility happened too, but in another universe.
In popular conceptions, many people think of multiple universes with us in it. Are they infinite, and would they contain copies of ourselves, but living in different circumstances?
Yeah, in many incarnations of the idea there are ultimately infinite universes. This would also include other copies of ourselves, although that’s a little bit of a [anthropocentric] way of thinking about it.
That’s fascinating. But it’s just so hard to imagine that there is enough space—
Bear in mind that another copy of you in another universe probably just said that too.
Right, of course. How would there be enough space for that, and would everything be happening simultaneously?
Our mind fills all the space with our universe, which speaks to limitations of our visual imagination. In some versions of this multiverse proposal our universe can be pictured as a bubble or sphere in a larger environment that can accommodate other universes or spheres.
It's also the case that the real estate of our reality could be just a tiny portion of a grander whole. The analogy I like to use: Think of a loaf of bread, where our universe is just one slice in this cosmic loaf. And the other slices which are separated from us along a different plane are different universes. An inhabitant on our slice would say that this slice goes on forever [in one dimension]. But their imagination is tied to this experience and its perception—while right next door is another piece of bread, another universe.
This is a weird question, but I’m going to ask it anyway. Some people who have taken a drug called DMT or tribal medicine called ayahuasca have said they can see other dimensions of space and time. Do you think they are seeing other dimensions?
Whatever they're experiencing, I don't believe they are the extra dimensions I'm talking about.
Because they wouldn't be accessible through the five human senses that I know about even if those senses are operating differently because [of the drug].
You're really seeing the same stimuli, but the internal brain is changed. There's no reason to think that a distinct chemical path is going to give us any kind of direct link to modern theoretical physics
Yeah. It would be nice.
Do you think that a unified theory, like the kind that Albert Einstein sought, will ever be found, and are you the person to find it?
There's a chance that we have it today. I consider it a small and unlikely chance, but we do have a proposal that’s been on the table for several decades called string theory that has all the trappings of a unified theory that Einstein was looking for —with the one main deficit being that we don't know if it's right. We've not been able to understand the theory well enough to bridge the math to observations or experimental data.
So we may have the theory. If we don't, I do anticipate we will find it in maybe 50, 100 or 1,000 years.
It doesn’t sound like you’re confident we’ll crack it soon.
Well, making a prediction about a timescale of big scientific discoveries is very difficult. You don’t know where insights will come from [or] what mind will be born.
It could happen tomorrow, and it could happen 1,000 years from now, and it could have already happened.
What are the “strings” of string theory made of?
It could be that they're made of string stuff, nothing finer. We're used to finding what things are made of—smaller and smaller particles—but that may be a chain that doesn't go on forever.
How many dimensions are there?
Human perception reveals three dimensions of space and one dimension of time. The math of string theory requires more dimensions of space than the three that you and I know about. That math seems to require seven additional dimensions of space, for a total of 10 dimensions.
Can you describe those dimensions a little more? Are we certain they exist?
We can’t prove anything coming out of string theory. But it's a natural idea mathematically—roughly speaking we believe those extra dimensions are crumpled up into tiny shapes that are all around us, but too small for our eyes to see or our equipment to detect. The analogy I love to use is to think of a garden hose stretched out before you. It looks like a straight line, but you zoom in and realize there's a circular part that you didn't see. That’s a second dimension on the hose that you missed. In the same way space could have curled up dimensions and pieces too small for us to see.
What would you say is your proudest achievement, and is there something you’re striving for you haven’t gotten to?
Work we've done showing that the fabric of space can rip apart and change its basic shape in ways that Einstein wouldn't have thought possible, that was an exciting discovery to be a part of. And in terms of big questions that have eluded me and others, the big one is: What truly is the nature of time?
Is it a fundamental idea that is stitched into the fabric of reality, or is time something we impose on reality to organize our perceptions? I don’t know the answer to that. Time is so basic to all of us, but we don’t really know what it is.
What do you suspect?
I suspect it's not fundamental. I suspect that the way a table is made of atoms, that time itself may be composed of some kind of basic ingredient. But what those ingredients are, or how they behave, is unknown.
If time is just a matter of perspective, does that have any implication for daily life or time travel?
It might suggest time isn't an intrinsic quality of reality, and therefore in some sense not as important as our experiences lead us to think.
We all think of past, present and future. Already Einstein taught us that that’s a limited way of looking at time. According to Einstein, much as different locations are real, multiple moments in time are real.
Walking through a door feels like something that happened, and then passes. But that may not be true. But no one really thinks like that; it’s hard to embrace that intuitively.
The one lesson that physics teaches us is: Don't go by your experience alone if you're looking for deep truths.
I can understand that for physics, but it is kind of irritating to always hear the same message in science over and over: that your own experiences can’t be trusted.
Human experience is the most precious things that we have toward understanding our lives. But the fact is, our human senses evolved so we could survive. And to survive you do not need to know the quantum behavior of an electron, or how time behaves around the event horizon of a black hole.
So it's not surprising that if you go beyond the scales of human experience, radically new ideas that are completely foreign may dominate. And indeed that appears to be the case.
(Note: this interview has been condensed and edited for clarity.)