The Thought Experiments That Fray the Fabric of Space-Time
A constellation of puzzles suggests that the space-time continuum we seem to inhabit is not fundamental but an approximation of something deeper, and that the concept will eventually be replaced by more basic ingredients in physicists’ next recipe for reality. “People smell it,” said Vijay Balasubramanian, a physicist at the University of Pennsylvania. “And then you’re trying to see where the smell comes from.”
Three thought experiments contribute to the intoxicating aroma. One shows that there is a minimum length scale below which no experiment can produce a result. Another reveals that no physical property — length, mass, speed or any other — can be measured from nearby with perfect precision. History has taught physicists not to ignore blind spots like these, because situations that defy observation may not be fundamental.
“We can’t have heavy reliance on something that isn’t actually there, that you can’t operationally give a meaning to,” said Nima Arkani-Hamed, a physicist at the Institute for Advanced Study.
The third thought experiment gets at a mysterious property of any region of space: Its information capacity is very high, but much less than it should be, given its volume. “If it’s much less,” Balasubramanian said, “then why does it look like there are all of these different locations in space?”
Together, these intellectual explorations tug at the weave of space-time itself, stretching it until it seems to tear.
Zooming In on Space-Time
Probe the laws of physics at smaller and smaller distances.
Physicists learn the laws of physics by colliding particles together. Particles have a wavelike nature. The higher their energy, the shorter their wavelengths.
Physicists learn the laws of physics by colliding particles together. Particles have a wavelike nature. The higher their energy, the shorter their wavelengths.
And the shorter their wavelengths, the smaller the area in which the particles will interact.
And the shorter their wavelengths, the smaller the area in which the particles will interact.
So probing reality at ever-shorter distances requires higher-energy particles with shorter wavelengths.
So probing reality at ever-shorter distances requires higher-energy particles with shorter wavelengths.
If a collision concentrates enough energy in a small enough region, the particles form a black hole and never reach a detector.
If a collision concentrates enough energy in a small enough region, the particles form a black hole and never reach a detector.
So there’s a minimum distance — known as the Planck length — below which we can’t even dream of gathering data.
So there’s a minimum distance — known as the Planck length — below which we can’t even dream of gathering data.
If no measurements can be made below the Planck scale, perhaps space-time as we know it doesn’t exist there.
Making Local Measurements
Measure any physical property of any object in space-time.
All measurements have some unavoidable uncertainty due to the quantum fluctuations of particles.
All measurements have some unavoidable uncertainty due to the quantum fluctuations of particles.
This uncertainty decreases rapidly as the number of particles grows, so it’s unnoticeable in everyday objects — even to experimentalists.
This uncertainty decreases rapidly as the number of particles grows, so it’s unnoticeable in everyday objects — even to experimentalists.
But it’s still there. To reduce the uncertainty of a measurement, we need a measuring device that has less uncertainty of its own and is therefore packed with more particles.
But it’s still there. To reduce the uncertainty of a measurement, we need a measuring device that has less uncertainty of its own and is therefore packed with more particles.
The density of the device can only increase so much until it forms a black hole. So we can’t measure the desired property exactly.
The density of the device can only increase so much until it forms a black hole. So we can’t measure the desired property exactly.
It might be impossible to define physical properties of objects in space-time, so perhaps there’s some other level of organization that is exact and true.
Storing Information
Pack as much information as you can into a fixed region of space-time.
Start by filling a region — a room, for example — with books. How much information do the pages capture?
Start by filling a region — a room, for example — with books. How much information do the pages capture?
Now try a denser medium: digital information stored in a modern hard drive. How much information does the region hold now?
Now try a denser medium: digital information stored in a modern hard drive. How much information does the region hold now?
Can we do better? Imagine the best information storage device in the universe. Perhaps such a “super hard drive” could encode information in neutron star material, the densest known matter. Now the region can accommodate far more information.
Can we do better? Imagine the best information storage device in the universe. Perhaps such a “super hard drive” could encode information in neutron star material, the densest known matter. Now the region can accommodate far more information.
But try to pack in just one more byte, and something dramatic happens. The room will collapse into a black hole.
But try to pack in just one more byte, and something dramatic happens. The room will collapse into a black hole.
Black holes, it turns out, are the densest possible containers of information.
But how much information can they carry? Strangely, according to calculations in the 1970s by Jacob Bekenstein and Stephen Hawking, the amount depends on their surface area — not their volume.
Black holes, it turns out, are the densest possible containers of information.
But how much information can they carry? Strangely, according to calculations in the 1970s by Jacob Bekenstein and Stephen Hawking, the amount depends on their surface area — not their volume.
This may imply that any event happening “inside” a black hole is recorded on the black hole’s surface.
This may imply that any event happening “inside” a black hole is recorded on the black hole’s surface.
Perhaps black holes — and by extension all regions of space-time — are holograms of data living on a two-dimensional surface of an unknown nature.
These thought experiments suggest that the space-time fabric that seems to form the universe we inhabit may break down in extreme circumstances. And if it does, we may need a deeper description of reality that doesn’t fall apart, comprised of more fundamental building blocks.
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