How nature is telling us something important about the connection between light and gravity
My formal education was extremely haphazard until I reached university. Prior to that, my family had bounced from one country to the next and consequently I bounced from one system of education to the next, quite often delivered in languages I had to acquire quickly in order to have any idea at all of what was being presented to me.
Not surprisingly, I reached the age of fifteen with a disparate grab-bag of knowledge and had to perform a rapid catch-up in order to pass the British O-level and A-level examinations that were the gateway to higher education at that time.
Most of what I learned about mathematics was self-taught, pulled from a variety of textbooks most of which seemed to have been written with the sole purpose of obfuscating their subject-matter or, conversely, making it so dull that the reader would rather commit suicide than plod to the end of yet another tedious chapter.
Yet there was an upside: I learned to conceptualize ideas rather than simply memorize formulae, and because of this I found I could sometimes derive new concepts before encountering them in the standard textbooks. And the more I learned, the more I learned to love Physics because Physics is at heart a never-ending symphony of concepts.
Although I wasn’t able to study Physics at university, I was increasingly fascinated by the subject, most especially cosmology. In my thirties I balanced work and my private life against gaining greater depth in theoretical physics, and necessarily struggled to improve my mathematical abilities upon which adequate understanding relies.
Now, in my sixth decade, I find myself returning once again to the problems that stumped me all those years ago.
To the general reader, the problems of theoretical physics may seem arcane concerns of no practical value to everyday life, but in fact they are fundamental and ultimately will have the same utility as Maxwell’s equations of electromagnetism (which powered the 20th century) and Einstein’s General Relativity (which, among many other things, underpin GPS signal corrections, enable us to calculate gravity-assisted trajectories for interplanetary missions, and allow us to understand the evolution of the entire universe).
And frankly, physics is astonishing. Have you ever thought about gravity?
We stand on the surface of a decent-sized planet. The diameter of Earth is 12.742 x 10³ kilometers and its mass is 5.972 x 10²⁴ kilograms. In other words, it’s a pretty big piece of space rock. As we know from General Relativity, mass distorts spacetime and that distortion is what we call gravity. Every atom in our bodies is constantly being pulled by gravity towards the center of the Earth. Gravity is why we don’t float off into space.
Yet the really strange thing is this: given how absolutely massive the Earth is in comparison to our tiny bodies, its effect is extremely weak. We don’t need enormous muscles in order to be able to get up from the sofa or raise an arm to reach out for a drink. All that enormous mass beneath us exerts an effect that we can defeat simply by lifting our arms.
Meanwhile all the other fundamental forces are far stronger. Using a simple bar magnet we can pull metal paper-clips upward. A tiny magnet weighing perhaps 100 grams can overcome the gravitational field of our entire Earth. And that’s astonishing when we stop to think about it.
One of the great problems of physics for more than a century has been this discrepancy between the fundamental forces. Gravity is weaker than the other three fundamental forces (strong nuclear, weak nuclear, and electromagnetic) by many orders of magnitude. Yet, despite its weakness, it’s the only fundamental force that has theoretically infinite range. And it is the only force for which an adequate quantum interpretation is still lacking.
Meanwhile we also have the problem of electromagnetic radiation, commonly referred to as “light” but which is more properly termed photons and includes everything from x-rays and ultra-violet to infra-red (which we perceive through our skins as heat).
With modern observation satellites such as COBE and Planck we can detect photons that were first emitted more than 13.7 billion years ago and which have been traveling through our expanding universe ever since. We use light in our lasers and our entertainment screens. We use it to see in the dark. We use radar (photons beyond our visual range) to manage air travel and image the surface of planets and moons. Photons are everywhere.
Yet we don’t really understand photons at all.
The problem is that a photon can act like a particle, yet it can also act like a wave. It all depends on the experiment we use. The problem is: being a particle and being a wave ought to be mutually exclusive states. How can light be both, and therefore not really either? Physicists have puzzled over this ever since the famous “double slit” experiment was shown to validate the wave/particle duality problem.
For those who may be unfamiliar with the double-slit problem, here’s a brief description. At one end of the experiment we have a device that emits discrete photons (the same experiment has also been done with electrons, and the results are the same). At the other end is a detector that records the location of each photon as it arrives. In the middle is a barrier with two small slits. If the photon is a wave, it can pass through both slits simultaneously and then interfere with itself as it propagates out to the detector, causing the famous “striped lines” interference pattern we see when waves overlap on the surface of a pond. If the photon is a particle, it will simply pass through one or other of the two slits and hit the detector in a single spot.
The problem is, if we measure which slit the photon passes through on its way to the detector, it always behaves like a particle. But if we stop measuring which slit it passes through, it always behaves like a wave. This is illustrated as follows:
The more I’ve mulled over these two problems (gravity and photons) that lurk at the heart of modern physics, the more they seem to be connected.
Both gravity and photons are effectively infinite in terms of reach, although obviously the effect of gravity falls off as the square of the distance between any two objects and photons lose energy as the expansion of spacetime causes them to redshift. But here’s the thing: because of E =mc2 we know that energy and mass are equivalent. That means that even massless particles such as the photon create a (tiny) gravitational field due to their momentum. Each photon is therefore accompanied by a tiny gravitational field.
But what happens to this gravitational field in the classic double-slit experiment when the photon acts like a wave and passes through the two slits simultaneously and then interferes with itself on the other side?
It’s this question above all others than convinces me of a deep connection between electromagnetic radiation and gravity.
Unfortunately I’m entirely the wrong person to make any progress with this possible avenue of exploration. I’m too old, my mathematics is insufficiently sophisticated, and I lack the deep grounding in certain specialisms such as QFT that would be necessary. But I do hope to live long enough to learn that someone, somewhere, ultimately elucidates this deep yet elusive connection.
Because when they do, we will have achieved an enormous leap forward in our fundamental understanding of nature.