I’m not entirely sure the Mail has the physics right here

Your body will be bombarded by about sextillion photons of light per second.
Most of these photons, or small packets of energy, originate from the sun.
But a very small fraction have travelled across the universe for billions of years before ending their existence when they collide with your skin.

Photons destruct on contact with skin, do they?

31 thoughts on “I’m not entirely sure the Mail has the physics right here”

  1. “destruct” would be an odd choice of words, but “ending their existence” seems fair enough. Those that aren’t scattered back again hit atoms, are absorbed, and will no longer be photons. What did you think happened? That they bounce around as teensy little photons inside of you?

    Daily Mail 1, Timmy 0. Might be a first 🙂

  2. Bloke in Costa Rica

    Some will be reflected, some will be absorbed, some will be scattered. But photons don’t have identity like macroscopic objects. A photon goes in, makes some electrons wiggle about and a photon comes back out again, at the equal but opposite angle to the one that went in. Is it the same photon? That’s not even a meaningful question. So the Mail is basically correct, albeit probably inadvertently.

  3. As it is possible to see people as a result of photons bouncing off them I tend to think that “destruct” isn’t right.

  4. Your brain will be bombarded with sextillions of absurdons and who-the-hell-are-theyons every time you follow a link from the Mail‘s Sidebar of Shame.

    A guilty pleasure, not to be overindulged. Possibly injurious in the long term.

  5. You can see people as a result of photons heading your way from them. Some of those are bouncing off, and some of them are emitted by you as a result of the absorption of incoming photons.

    Simple example: in an otherwise dark room, UV light hits white cloth. You can see the cloth because the UV is absorbed and you are seeing photons in a range of wavelengths coming off – you are not seeing it by virtue of bouncing UV photons.

  6. I think BiCR is correct, except that I’m not sure that all will be emitted at the equal but opposite angle; that’s only the reflected ones. I think new photons will be in infra-red wavelengths at random angles. Assuming black-body radiation (am I allowed to say that?)

  7. I consider this debate should be no-platformed. Some of it is coming dangerously close to being factually correct.

  8. Photons ‘destruct’ in the same way they ‘create’.

    The electrons in matter are charged, which means they have a field of electrostatic attraction/repulsion around them. If an electron wiggles very fast, the field wiggles too, and creates a wave that travels across space. Absorption is just the time-reversed version of the same physics. A wiggle in the attraction/repulsion field pushes on an electron in matter (i.e. attracts/repels it) and the wave’s energy transfers to become the kinetic energy of the electron. The electron being wiggled in this way creates its own field, exactly in time with the incoming wave, which cancels the wave out. That’s just a version of Newton’s third law – two bodies acting on one another apply equal and opposite forces on one another, so the electron being speeded up pushes back and causes the wave to slow down, and energy is thereby conserved.

    That’s the classical physics version of the story. It’s because of the mysteries of quantum physics that the process is an all-or-none one, with the entire wave getting absorbed or ignored. But that’s a bit weird, and hard to explain, so I’ll skip that.

    What happens next can vary. Sometimes the wiggling electron immediately emits a photon again, with exactly the same energy, so it looks like it ‘bounced off’. Sometimes it collides with other particles and the energy becomes heat. Sometimes there’s a long delay before re-emission, so you can get that glow-in-the-dark stuff that absorbs light and then glows for an hour or two. Sometimes you get some frequencies absorbed and others re-emitted, so the colour of the light changes on contact. And sometimes you get more complicated stuff going on like fluorescence.

    By the way, the equal-and-opposite-angle thing is because of the combined effects of many atoms emitting together. As a wave comes in, all the atoms along the surface absorb and re-emit in synchrony, (each atom emitting its own spherical wave,) so the re-emitted waves line up and interfere with one another. If they’re lined up on a smooth plane, the result is to bounce the incoming wave back out at the equal-but-opposite angle. If the atoms are arranged more irregularly, the outgoing wave is disordered and random.

  9. Too many dangerously Newtonian terms like “bounce” and “reflect” being used here. Most things that we see are objects that absorb light and re-emit some of that energy in the physical spectrum. The re-emitted light is a different photon corresponding to a change (drop) in the energy levels of electrons in the absorbing material that were excited by the incoming photon. Light is reflected only in the case where its path is diverted as the wavefront crosses the boundary between media with different refractive indices.

  10. Since you can’t tell if it is the “same” photon that is either directly reflected or absorbed and immediately re-emitted, using the word “bounce” seems like a perfectly reasonable classical interpretation at the macro scale.

    It is mere quantum quibbling to insist that all such photons are absorbed because then they haven’t travelled all that distance as claimed as they have ‘probably” been absorbed and re-emitted countless times from interstellar matter anyway. That is, you can’t tell, so why bother.

  11. “Since you can’t tell if it is the “same” photon that is either directly reflected or absorbed and immediately re-emitted, using the word “bounce” seems like a perfectly reasonable classical interpretation at the macro scale.”

    Absolutely. If you look too closely at the quantum stuff, it becomes rather questionable whether photons are “the same” from one moment to the next, anyway. A photon doesn’t simply fly through space in a straight line from one point to another; it actually follows every possible path and history, turning into a zoo of other particles and back again along the way. The closer you look, the more of this there is going on, until down at the Planck length it all breaks up into a mad, random, space-time foam of particles.

    The idea of a single particle with a fixed identity going from one place to another is a macro-scale approximation, that only works because all the lower-level goings on obey certain conservation principles.

    If you want an economic analogy, think of money passing from one account to another. At the macro level, you can treat the money as a fixed “lump” that goes from one place to another. But look closer, and you will see it keeps changing form along the way. One moment it’s an electronic record, then it’s cash withdrawn from an ATM, then it’s a gift voucher bought from a shop, then it’s posted to a second person, who exchanges it for a valuable painting from an art shop, and puts it in his bank vault. All the time the money is changing form, combining and splitting with other bits of money, and there is no single physical entity that passes from bank account all the way to the bank vault at the other end. But we can treat matters as if there was because at each step the conversions are constrained by the rule that the total value of the items exchanged must stay the same. Monetary value is conserved. So it’s perfectly valid to think of the money your employer puts in your bank account to be the “same” bit of money you take out of the hole in the wall.

    Photons are the “same photon” in the same way the money can be thought of as “the same money”.

    But I think what the DM was thinking of was the cases where the photon is absorbed and *not* immediately re-emitted, which happens quite a lot. They don’t say *all* photons come to an end when they hit you, only some of them.

    I think it’s pretty clear the DM journalist didn’t write this stuff from their own knowledge – they’re just quoting some physicist they asked. What’s remarkable is that they managed not to muck it up when the sub-editors got hold of it. But “not changing the words you’ve been given” is not really much of an achievement, in the grand scheme of things, so you can happily retain your low opinions about journalists intact.

  12. Deconstruct.
    1. Unlikely it (they) collide with skin
    2. Why would it only be the old ones from out of space that destruct
    3. Bombarded seems inappropriate. I would go with flooded, but what do I know?

    That they might not appreciate the physics we can accept. But you would expect them to do better with language.

  13. Where’s the Feynman diagram? To pick up on BIS, Murph would have deleted all the responses here. And DBCR would write in to applaud his sagacity to ignore thicky neo Liberals.

  14. Bloke in Costa Rica

    If anyone wants the real scoop on this at a semi-classical optics level, read Vol. I of the Feynman Lectures on Physics. You will learn why the sky really is blue! With actual equations!* Actually, read the FLP anyway. They’re as significant a testament to the glories of the human intellect as the collected works of Shakespeare. Everyone I know (including me) who made a decent fist of his physics degree owed it in some degree to FLP. It doesn’t even have all that much tricky maths in it.

    * I.32.19

  15. “Photons are clicks in photon detectors.”

    I’d have said photons are eigenstates of the number-of-photons operator applied to the joint observer-‘electromagnetic field’ system. Same epistemology, but a different ontology. 🙂

    Can’t say I entirely agree with Anton. As Dave Mermin put it: Is Anton a click in an Anton counter? Or to put it another way, aren’t photon detectors just clicks in a photon detector detector? The whole world is quantum.

  16. Go on – you make this all up.
    It is a conspiracy – ‘photons’, ‘dark matter’ etc etc.
    And the scientists have the gall to mock religion.

  17. Bloke in Costa Rica

    Difference between a pilot and a sky pilot is that aerodynamics works and religious mumbo-jumbo don’t.

  18. “And the scientists have the gall to mock religion.”

    Religions sound a lot *more* plausible and comprehensible than this, which is oddly one of the better reasons for thinking religion is made up and quantum physics isn’t.

    Gods and creation myths generally are all built around human concerns, human motivations, human ways of thinking. God X had a big fight with God Y, they made a big mess, and that became the world. God X made the world to show how great he was, God Y came along and spoiled it. God X ran around smashing things with a bolt of lightning and having sex with anything that moved and shouting and making lots of rules up and punishing people for breaking them. We have a lot of Gods and Goddesses, but in a strange way, they all look the same: like humans, only bigger and louder.

    But if you look at the one book that we *know* was written by the creator – the universe itself – we see a different story. It’s written in the language of mathematics, which does not feature highly in any of the more conventional holy books, and is done in a way that no human would think of. It’s all terribly elegant and obvious once you’ve seen it, with 20:20 hindsight, but no human would ever have chosen quantum mechanics as the obvious way to do it. It’s clearly not written by a human, and no human could or would have ever made something so mindblowingly weird up.

    Dark matter, I agree, is a very human-style kludge to try to explain why our gravitational predictions don’t work. It’s early days for that idea yet – I’m sure that whatever is going on will turn out to be even stranger.

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