If you’re curious to know what my book is about and why it’s called “Waves in an Impossible Sea”, then watching this video is currently the quickest and most direct way to find out from me personally. It’s a public talk that I gave to a general audience at Harvard, part of the Harvard Bookstore science book series.
My intent in writing the book was to illuminate central aspects of the cosmos — and of how we humans fit into it — that are often glossed over by scientists and science writers, at least in the books and videos I’ve come across. So if you watch the lecture, I think there’s a good chance that you’ll learn something about the world that you didn’t know, perhaps about the empty space that forms the fabric of the universe, or perhaps about what “quantum” in “quantum physics” really means and why it matters so much to you and me.
The video contains 35 minutes of me presenting, plus some Q&A at the end. Feel free to ask questions of your own in the comments below, or on my book-questions page; I’ll do my best to answer them.
8 Responses
I was lucky enough to see your lecture in person this spring, in Seattle. I have had a question ever since, nagging at the back of my mind. I read your book cover-to-cover, and it is not addressed therein. It is not philosophical or profound, nor is it in any way practical. It is simply something I have been wondering about.
I was thinking about all of the light illuminating the space — how at every moment, depending on their frequency, some photons were reflecting back from the dark curtains that were the backdrop while others were absorbed into the material.
I couldn’t help wondering… what exactly happens when light transforms into energy? Is the transition gradual or abrupt? Can it be measured with a unit of time? Are the precursors to the moment when a photon loses its structure and becomes “merely energy?” And of course, what happens in the other direction… when a substance is heated to such a degree that it gives off electromagnetic radiation — either in the form of visible light or at some other wavelength?
Does this shift happen all at once, or are there precursors that can be detected at the subatomic or quantum level? Are the functions that describe this transition between light and energy best described as continuous or discrete?
To phrase it another way, when light is absorbed into matter, does it shatter or melt?
Looked around a bit on the Internet for an answer to this question but could not find one that was satisfactory. Apologies if this is a nonsensical or irrelevant question. It’s just been bugging me for several months, and I finally got a bit of free time.
It’s a very sensible question, and very tricky to answer. I’ll give you my best shot, but maybe if I think about it for a while I can do a better job.
Let me first clarify one point: when a photon is absorbed, it does not “become energy”. Energy is not a object; it is something that objects have. Photons have energy, and when they are absorbed, their energy must also be absorbed or transferred to some other object. So that’s part of what we have to track down.
Okay… Now, typically, a visible light photon is absorbed by a single atom (though more complex processes are possible) so let me focus on this simplest case. When the atom absorbs the photon, it recoils, just as any ball recoils when struck by another. The photon is gone, and the energy that it carried now is motion-energy of the atom. But the atom is not free-floating; it is integrated into the curtain material and can’t move freely. And so it bumps into other atoms, transferring some of its energy to them; those atoms are also not free-floating, so they transfer the energy they receive to even more atoms; and thus, in an instant, the photon’s energy is dissipated across the whole material. The effect is to slightly increase its temperature. (Indeed, if we shine a strong beam of light onto the curtain, it will soon become noticeably warmer… and that heat will then cause the atoms to radiate yet more photons back into the room, though these will have too low a frequency for our eyes to see.)
I have ignored a detail: an atom is a complex object, with electrons around an atomic nucleus, so the absorption of the photon may do interesting though temporary things to its internal structure.
Now: about the process of absorption (or emission, which is just the same thing in reverse). You asked, “what exactly happens when light transforms into energy? Is the transition gradual or abrupt? Can it be measured with a unit of time?”
You are asking questions for which quantum physics doesn’t allow precise answers, at least not of the sort you are looking for. There are precise answers to other questions: “how likely is a particular type of atom to absorb a photon of a specific frequency?”, or “what is the rate of emission of photons from a certain curtain with a particular temperature.” The process of absorption or emission is a straightforward calculation, but nowhere in that calculation does one see or say “what exactly happens.” No quantum process ever has an “exactly happens” story, even if it has an exact calculation of its rate and probability. And the question of gradual or abrupt, continuous versus discrete, are core conceptual challenges of quantum physics. On the one hand, I would tell you that “yes, these processes take time.” The reason is that there is a rough quantum relation between time t and energy E, in which E is related to Planck’s constant h divided by t. This relation implies that anything that takes an infinitesimal amount of time requires an infinite amount of energy. In general, the time for some process to occur is therefore no shorter than h/E, where E is the maximum amount of energy involved in that process. Sometimes one can be somewhat more precise than this. On the other hand, there are limits to what one can hope to say.
This is ever so prominent in famous discussions of radioactive decay, which can include an unstable atomic nucleus emitting a photon (typically an X-ray or gamma-ray rather than a visible photon, but the issues are the same.) This is a long story, and I’m not surprised it’s hard to find a good discussion online. I will keep looking for one, but here’s my own brief sketch.
Suppose I have a pile of such atoms in front of me. I can use quantum physics to calculate precisely how much energy each emitted photon will have, to very high precision, and check this experimentally. I can calculate the rate at which these atoms decay — depending on the atom, it might be quite long, perhaps minutes or more — and I can check this experimentally too. I can infer, partly from the fact that each decay leads to a single photon that I can detect at a particular moment, but also by more clever experiments, that each individual decay is much faster than minutes. I can also observe that’s what left after these atoms decay is other atoms, so I can conclude (via the time-energy relation I mentioned) that the decay must be slow enough that the energy of the decay leaves the atoms more or less intact. That’s an ultra-microscopic time scale, but not infinitely short. Finally, I cannot calculate when any one atom will decay; measurements show no pattern, as though the time of the decay is completely random, even though the probability that governs that randomness is precisely known.
The question of what I now know is debateable, and often debated.
I can try to do better. With more detailed but gentle experiments, I can confirm that the process of decay takes very little time. But to measure more precisely what happens, I have to do more and more intrusive experiments. When they are intrusive enough, they actually change the process of decay. That happens just around where I start to do experiments that would show me exactly how long the photon emission takes. And so I am blocked, by the very nature of experiment, from getting a clear answer.
The point is that measurement is not a passive process. It is an active process, by its very nature. Precise measurements are more active than imprecise ones, and they change the system to a degree that it is no longer what it was. The unmeasured decaying atom is not undergoing the same process as the precisely-measured decaying atom.
And so even though I am comfortable telling you that we do know there’s a time-scale involved in photon absorption and emission, using the time-energy relation, and that this time scale can sort of be observed, in the sense that we see it gives us correct intuition for what to expect in other processes involving atoms and light, it’s not a precise understanding built of watching atoms like a hawk to see exactly what they do. It’s born, in my case at least, of long experience with quantum physics in complex systems like atoms and nuclei, and the intuition that comes from seeing how things work in many different examples. I’m sure that’s not very satisfying for a non-expert to hear.
I think we can safely say that when light is absorbed into matter, it neither shatters (ultra-short time-scales) nor melts (rather long time-scales). I would probably suggest that the right intuition is that the time scales are such that neither happens; that the process is as short as it can be while remaining smooth and nondisruptive. But another physicist might debate me on that, perhaps arguing that the time scales are shorter, or that they are completely ill-defined. With no experiment that can settle the debate, I’d be hard-pressed to say my colleague is wrong.
Someday I may write a book on quantum physics. By that time, I need to have a much clearer answer to your questions, or at least, more confidence that no clearer answer exists. Maybe one of my readers is wiser than I am on this point. In any case, thank you for forcing me to start thinking again about this century-old issue.
Thanks for this answer. It left me with a sense of wonder and bemusement.The subsequent blog post and associated links only led further down the rabbit hole.
I think what you are saying is that we cannot directly measure the dissolution of a photon because unlike other elementary particles (with the exception of far more elusive bosons) it has no mass. Bombarding it with a beam of light or other instrumentation changes its position and momentum. But although a single photon can be detected and measured, it would appear they don’t have any “structure” comparable to the innards of an atom.
This is what is wild to me. The LHC and its experimental precursors reveal levels of structure and endless motion at the core of ordinary matter. Light is, just light. Until it isn’t.
Hi Doc. A big question: Reading you book, at Chapter 20, “Higgs Field in Action”, I had a doubt: if all massive particles acquired mass at the origin of the universe, in the Big Bang, who can guarantee me that at this moment at billionth of billionth second, the Higgs field gave Mass to all particles, or even, if there was in time the Higgs Field or the Higgs Boson, if the quarks, electrons, etc., was before created, that I ask you about whom was created before all these particles, at which hierarchy, etc. Or what will be my guarantee it, what formula, equation, what theory says that Higgs and its field came before all the particles, and even it had time to give mass to the infinite amounts of particles in this infinite time lapses. Theoretically the Higgs field can give mass, ok, but in the practice, this is an absurdly unthinkable, at this infinite lapse of time, do you agree? Or to summarize, if a one particle in this time was able to acquire mass on itself, or by Big Bang, without needs this field that it created all the particles, then there was usefulness the Higgs Field and its Boson, that is an the extreme Doubt its abilities, dont is? And then comes the question of the “egg and the chicken”, that ask, which came before about the Boson and its own mass, even about the first Boson in this whole story, asking, who gave its mass. There are many doubts and a lot of confusion to submit HB at the Big Bang. Regards
So, I’m afraid that no physicist would guarantee you any of those things you mentioned. That’s because none of them are believed to be true and/or meaningful.
For instance, you asked, “what theory says that Higgs and its field came before all the particles”? The answer: no theory would say that. No theory says that the Higgs and its field came before anything else.
Similarly, “which came before about the Boson and its own mass, even about the first Boson in this whole story, asking, who gave its mass…”; this suggestion, too, is not related in any way to what the Big Bang theory, or experiment and observation, has to say about the universe. The Higgs boson gives mass to nothing, and the origin of its mass has nothing to do with how any other particle in the universe got its mass. Nor does one thing have to have mass before another does.
1) The universe is believed to have had all of its fields since the Big Bang. The Big Bang may or may not be the ultimate origin of the universe, but certainly seems to have been a moment about 13.8 billion years ago following which the universe was very hot and dense. There is no sense in which some fields came first and others came later. For information about what we do and do not know about the Big Bang, see my articles on the subject: https://profmattstrassler.com/articles-and-posts/relativity-space-astronomy-and-cosmology/history-of-the-universe/hot-big-bang/
2) The reason these fields are believed to have been present at the origin of the universe is that they are integrated into spacetime, as far as we can tell. By contrast, particles are ripples in those fields; they can come and go. It’s somewhat like air pressure and sound; there’s always air pressure (a field) anywhere where there is air, but sound (ripples) can come and go. There was no first field, and there was no first particle with mass; it’s not even meaningful to ask about them.
3) The Higgs boson gives mass to nothing. Particles do not give masses to particles. Particles get masses from other sources, and in our universe, many of them get mass from the Higgs field. This happened because the Higgs field affected the resonance frequency of other fields. It did so when it switched on. Thus, all fields that got resonance frequencies from the Higgs field did so simultaneously, simply as a result of the Higgs field switching on. The fact that particles in those fields — again, just ripples, nothing more — have mass is a consequence of this more fundamental phenomenon, the stiffening of the various fields by the Higgs field.
4) The time it took for the Higgs field to switch on and settle down can be estimated, or in a detailed theory, calculated precisely; and any theory will tell you that it takes a tiny fraction of a second. And so the particles that happened to be present in the universe at that time got their masses in a tiny fraction of a second. (Remember that human time scales are very long compared to particle physics time-scales; a second is an eternity for an atom.) As for nowadays, particles do not have to go out and buy masses at the store; it takes no time for them to get them. The very act of creating a particle creates its mass simultaneously; that’s the lesson at the end of Chapter 20. Particles don’t exist without the mass that they have to have, by their very nature: mass is their energy-of-being, and could no more exist than a human could exist without breathing.
So I suggest you read chapter 20 again slowly and think about what it is really saying about fields and their particles. Your emphasis should be more on the fields than the particles if you want to understand what is fundamental in the universe and what the early universe was like.
Now, as for guarantees: I can pretty much guarantee what the universe was like when it had cooled just enough to form the first atomic nuclei (the time of “nucleosynthesis”). That’s because we have both detailed calculations and measurements, and they agree. https://en.wikipedia.org/wiki/Big_Bang_nucleosynthesis However, the further back from there we go (and the hotter the universe at that time), the less we know for sure. We do not know from observation how and when the Higgs field switched on, so there are assumptions built into any such statement.
What we know with confidence is that by the time of nucleosynthesis, the Higgs field was switched on and had settled into its present value (since otherwise the calculations for nucleosynthesis wouldn’t work at all.) What happened before that is educated guesswork, in which we take the current best equations of particle physics (the Standard Model) and calculate what would happen to them at very high temperatures. If, however, the equations for the Higgs field require significant modification (no sign of which has yet been seen at the Large Hadron Collider or any other experiment, but may still be present) then some of the details may require revision someday. So (a) there is a Higgs field and it is switched on and stiffening other fields — that’s guaranteed; (b) the universe was once hot enough to cause nucleosynthesis, and already by that time the Higgs field was switched on — pretty much guaranteed; and (c) at higher temperatures the Higgs field might have been switched off, though we can’t yet be certain of the details, and so even though I’m pretty confident of the general story, the details are not guaranteed.
Thank you very much. Your approach about the BB and Higgs Field is rare on publications, only you do it. I even ask you to any day launch a book about BB and Higgs Field, a fact that no publications, because the last publication that I read about BB, was the book “The First 3 minutes” from Weinberg, but all is outdated at new discoveries . Regards and thanks.
Very informative and enlightening on a subject dear to us. One of our own doctrinal excerpts align.
37 And there are many kingdoms; for there is no space in the which there is no kingdom; and there is no kingdom in which there is no space, either a greater or a lesser kingdom.
38 And unto every kingdom is given a law; and unto every law there are certain bounds also and conditions.
Worlds without end
Eager pattern search is not evidence though. (And if some part could not be loosely read to align with modern facts the myth text was not sufficiently long and irrelevant.)