Experiment Reveals How Light Shoves Matter Around

By Bill Andrews | August 22, 2018 1:12 pm
Kenneth Chau, an engineer at the University of British Columbia, helped shine a light on how electrons impart momentum. (Credit: UBC Okanagan)

Kenneth Chau, an engineer at the University of British Columbia, helped shine a light on how electrons impart momentum. (Credit: UBC Okanagan)

Quantum mechanics, the science of the smallest stuff, is famously kooky. Light is both a particle and a wave, electrons zip around and travel instantaneously, cats are both alive and dead — it’s hard for our human brains to comprehend. One phenomenon that sort of makes a little sense, if you think about it right, is that light alone can push things around.

Formally known as ‘imparting momentum,’ the idea can also seem quantumly crazy. I go out in the sunlight all the time without feeling any pushing!

But here’s how I’ve heard it explained: Momentum is defined as mass times velocity, so even though photons (particles of light) have zero rest mass, their velocity is enormous (literally the speed of light) and they’re never really at rest, so they have a tiny effective mass.

That means they have a positive momentum, which can give a little kick to anything light crashes into. Plus, if you conceptualize light as a bunch of particles, it’s not crazy to imagine those things moving around matter as they bump into it.

Clearly, it’s still weird to think about, and physicists had never actually observed exactly how the process of light imparting momentum occurs. Until now.

Lasers and Mirrors

Since the effect of light’s momentum is so small — there’s a reason I can’t feel sunlight pushing on me, after all — sensitive enough equipment to capture it is only just becoming available. A Nature Communications paper out yesterday describes the ingenious experiment an international team of researchers came up with.

The real trick is making sure any effects were really caused by imparted momentum from light, rather than just the side effects from heating the material as it absorbs light. To do that, the team constructed a mirror designed to absorb as little of the light as possible. Then they shot lasers at it.

The mirror, upon feeling the shove from the laser’s photons, should theoretically start to ripple as waves propagate outward from the light like a pond that had rocks thrown in it. The researchers had installed acoustic sensors designed to pick up on those ripples (since sound waves are basically just ripples in the air). These are tiny, tiny measurements, on the order of picometers, but the team picked them up.

The best part is, when they compared the experimental results with predictions from computer models, the two lined up almost perfectly. It proved not just that the experimental setup worked and they really were measuring imparted momentum (not heating effects), but also that the current understanding of light momentum, which led to the models, is indeed correct.

Measuring Light’s Momentum

Great, so we understand light, and the quantum realm, just that much better now. But, unlike many particle developments, the finding may lead to various known applications.

First off, this type of setup can help scientists better characterize different materials, since different stuff will have ripples that move in different ways. Study their waves, and you’ll have an insight into their material makeup. And it makes sense to imagine it’ll help make better solar sails, which had already shown that light can impart momentum, but not exactly how.

But even more futuristically, the paper’s authors also suggest their work can “further advance optical tweezer technology.” Get a good enough handle on how light waves interact with matter, and you have an incredibly powerful and delicate means to manipulate individual particles.

Quantum mechanics may be kooky, but it gets results.

CATEGORIZED UNDER: Space & Physics, Technology, top posts
  • http://www.mazepath.com/uncleal/EquivPrinFail.pdf Uncle Al

    Compare momentum transfers of solar emission versus solar wind – the reason comets have two tails.

  • jimgrot

    I do not follow this part “and they’re never really at rest, so they have a tiny effective mass.”. I believe it but I do not follow the logic or the science. Can someone explain?

    • OWilson

      Describing a quantum particle “at rest”, is like describing a wave “at rest”, it doesn’t really exist in our conception.

      There no absolute “at rest” location anywhere in our universe anyway.

      Still confused? So am I!

      Anyone? :)

  • Erik Bosma

    if our three dimensions are a holograph of another dimension then that other dimension (and our three) is the wave carrier of light or what we ‘see’ mainly as particles only waves as probabilities. if we could be one with the whole deal then we would see light as it actually is – a type of wave in another dimension that appears to us as particles or waves. matter of fact we could see everything – all condensed matter – as it really is – waves. made sense to me for a few minutes anyways…

    • Erik Bosma

      I’m suggesting that the observable U is a 3D representation of a 4D object, in other words, saying that there exists an extra spatial dimension which we (quantum, virtual and 3D objects and forces) all also inhabit.
      Time is to me not a part of this scenario because it seems that time may just be a property (consequence) of living in more than one or two spatial dimensions and is not really a dimension by itself. Sorry Time D fans.
      So objects like photons and electrons lacking, probably, mass can easily cross-inhabit all four spatial dimensions. We are not able to ‘see’ their full or ‘real’ effects or properties since they move in and out of a dimension (4D) which we can’t really observe. Kind of like a fish in water which is not able to stick his head out into the air to see what is ‘really’ going on. Instead, our fishy friend – and she could be a girl fish too so don’t y’all be getting judgmental on me – would only be able to see the effects or measurements or properties as this 4D photon or electron had on its limited world view.
      So a photon could look like a particle or a wave depending how he or she observed (measured) it because said fish (which is really you and me) has only a limited view and ability as well as measurement toolkit (you can’t make a tool for something that may or may not exist) in order to make precise observations. Reninds me of the hilarious toolkit the Ghostbusters used to track down their cross-dimensional subjects. Ain’t gonna work. So what we need to do is develop a means to be able to use, probably, highly advanced topological math and subsequently brilliant engineers to give us the tools we need.
      Please, please… does this at least make it as clear as 4D mud? I would really like some input because discussion – for me – is so much more stimulating than victory. Thanks guys and gals.

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