###### By Don Lincoln, Fermilab

## Alden Mead is an American chemist and was a professor at the University of Minnesota. In 1959, he had an interesting insight about the role of gravity in very small scales. He spoke with other scientists who didn’t find his arguments persuasive. However, after five years and countless arguments with referees, he published a paper in 1964 in the journal *Physical Review*, entitled “Possible Connection Between Gravitation and Fundamental Length”. What was it about?

### Can Gravity and Fundamental Length Be Linked?

What Mead’s paper did was to consider the effect of gravity on a phenomenon called diffraction, which describes what happens to light when you send it through a small aperture. Because gravity is so incredibly weak compared to electromagnetism, which is the force that governs the behavior of light, its effect is completely ignored in diffraction calculations.

But Mead was curious about quantifying gravity’s negligible effect. When you scatter a particle of light off another particle, say an atom, the atom’s gravitational attraction to the light particle causes an intrinsic uncertainty in the atom’s location.

### Heisenberg’s Uncertainty Principle

Mead applied Heisenberg’s uncertainty principle to the problem. Heisenberg’s uncertainty principle states that you cannot simultaneously measure the location and motion of a subatomic particle. Mead investigated how Heisenberg’s principle would affect the gravitational behavior of a scattered photon.

What he showed is that it is impossible to determine the position of an object to a precision smaller than the Planck length. His logic can then be extended to short time durations like the Planck time.

I actually talked to Mead back in 2013. I had written an online article about his work. He was an emeritus professor by then and he had Googled his name and my article came up. We had a long and interesting conversation about his work on the subject, he was also quite gracious that I hadn’t read his paper when it was published, especially since I was only a few months old at the time!

This article comes directly from content in the video seriesThe Evidence for Modern Physics: How We Know What We Know. Watch it now, on Wondrium.

### What Mead’s Work Says

It’s important to be specific about what Mead’s work said. First, it did not say that the Planck length was the smallest length and the Planck time was the shortest possible time. What it did say was that at these tiny distances and times, the laws of physics as we currently understand them all break down. What his work said was that at the Planck scales, the effect of gravity can no longer be ignored in quantum calculations.

It was later that the scientific community began to think that the Planck scale was somehow a smallest scale. In fact, Mead pointed me to a 2001 exchange of letters in the pages of the journal *Physics Today* with Nobel Prize winning physicist Frank Wilczek, who I also hold in high regard.

Mead said, “I don’t know when or how the transition of the Planck proposal from heresy to conventional wisdom took place, but I can attest that it had not even begun in the mid-1960s. I suspect that it did not really begin to take hold until at least the mid-1970s.”

### The Possibility of Quantized Time and Space

So, what do scientists claim today? They claim such things as that prior to the Planck time, which is to say in round numbers prior to 10 to the minus 43 seconds after the big bang, time didn’t exist. In fact, there are some who claim that at these small scales, time is quantized, where time was zero, then one Planck time, then two Planck times, like an old-school stopwatch, where the hand clicked from second to second. In this scenario, there simply isn’t any 1.5 Planck times.

Similarly, some researchers claim that space is also quantized, coming in discrete chunks of Planck length. If you were super tiny, you could only move in one, two, or three Planck lengths, but never one and a half.

But this idea of discrete space and time is really just a theoretical prejudice and not one that we can prove. What we can prove is that at scales like the Planck time, length, and energy, that gravitational effects that we always ignore are no longer ignorable.

This has to be the size scale where quantum gravity absolutely must become important, although, of course, it could become important at larger scales, but it’s mandatory at these scales. Furthermore, while it might be possible to look at space at shorter distances, it will require a completely new physical paradigm, with new laws, equations, the whole enchilada.

It is clear that general relativity breaks in the quantum world. It predicts infinities. And even if we somehow come up with a better theory of gravity, at the Planck length and time, we have to take quantum effects into account. What they will be, we don’t know. But what we do know is that the future of quantum gravity will be truly fascinating.

### Common Questions about the Quantum Effect and the Role of Gravity in Very Small Scales

**Q: How did Alden Mead consider diffraction?**Since gravity is too weak compared to electromagnetism, its effect is completely ignored in diffraction calculations. Using Heisenberg’s uncertainty principle, Mead managed to show that it is impossible to determine the position of an object to a precision smaller than the Planck length. This logic can also be extended to short time durations like the Planck time.

**Q: What do researchers claim today when it comes to the theory of quantized time and space?**As they claim, prior to the Planck time, time didn’t exist, as they claim time is quantized at these small scales, where time was zero, then one Planck time, then two Planck times, clicking from one to two and there can’t be any 1.5 Planck times. Some researchers also claim similar things about a quantized space. Although all of these are simply theoretical prejudice that can not be proved, even if we consider gravitational effects.

**Q: In which scales does gravity’s negligible effect become important?**According to Mead’s works, in the Planck scales, the effect of gravity can not be ignored in quantum calculation, as in this size scale, quantum gravity becomes absolutely mandatory.