• Benj96
    163
    What is space, change, time and interaction at the speed of light?

    I'm not going to lie I've spent the best part of a month absolutely blown away by the concept of the speed of light. The more you think about it the less sense it makes and the more wonderful and mysterious it is.

    At the speed of light, Time stops.
    At the speed of light distance contracts to nothing.

    So the "speed" of light doesnt appear to be a speed at all. At the speed of light the speed is zero because there is no distance to travel in no time at all. At not the speed of light the speed is 299 million and something meters/ second.

    At the speed of light no interactions can occur forever. Everything is completely instantaneous because nothing is happening. Yet that same interactionless non-acting state hits a "slower" mass like a leaf and bam it's used to create sugar which we eat. What the hell.

    At the speed of light there is no mass. No mass can travel at that rate. Then how did energy ever give rise to mass (e=mc2)? If it cannot do anything to itself in a state of pure timelessness then how did it just spontaneously slow down and get "heavy" with matter in the first place.
  • Pfhorrest
    2.5k
    Fundamentally all particles travel at the speed of light always: apparent slowness is just a particle being rapidly absorbed and re-emitted (by the Higgs field if nothing else), and that slowing-down also manifests as rest mass. From a lightspeed particle’s perspective, its entire existence just is the interaction between whatever emitted it and whatever absorbed it. Time and space and everything else are constructed out of a network of such interactions, in a way elaborated by philosophers like A N Whitehead.
  • EnPassant
    336
    Then how did energy ever give rise to mass (e=mc2)? If it cannot do anything to itself in a state of pure timelessness then how did it just spontaneously slow down and get "heavy" with matter in the first place.Benj96

    That's Higgs theory; mass is a slowing down of energy. Mass and matter are not substances, they are processes.
  • jgill
    667
    At the speed of light no interactions can occur forever. Everything is completely instantaneous because nothing is happening.Benj96

    Let's see, it takes a message from the Mars Rover about twenty minutes to reach Earth. So, nothing happens here while we are waiting. Sounds about right. :roll: :smile:
  • Benj96
    163
    Let's see, it takes a message from the Mars Rover about twenty minutes to reach Earth. So, nothing happens here while we are waiting. Sounds about right.jgill


    If you put a clock on that message (which you cant because of the mass of a clock) and it was sent at the speed of light from Mars to earth, and you had another clock here, the clock on earth would have elapsed 20 minutes while the clock returning at the speed of light would have barely ticked a second due to time dilation. From the perspective of the message the interval was instantaneous but to us it was 20 minutes. So yes nothing happened here while the message travelled (from the perspective of the message). No? Or am I getting relativity completely wrong
  • Pfhorrest
    2.5k
    You’ve got it right.
  • TheMadFool
    6.3k
    What is intriguing is that there is a speed limit for the universe. Granted that assuming there is such a speed limit makes for a beautiful theory - relativity - that not only explains well but also predicts well, we still lack an explanation for it. Yes, having the limit as a postulate appears to make my question meaningless; however, remember that this is a postulate of a different kind - it's drawn from observation i.e. it's a posteriori rather than a priori. In my humble opinion, it's not as stupid to ask for an explanation for an a posteriori postulate than it is to ask the same for an a priori postulate. After all, the former kind is a physical observation that needs explaining while the latter kind is simply an arbitrary starting point.
  • Mr Bee
    234
    Fundamentally all particles travel at the speed of light always: apparent slowness is just a particle being rapidly absorbed and re-emitted (by the Higgs field if nothing else), and that slowing-down also manifests as rest mass.Pfhorrest

    Hmm, I don't think I've heard of that description before, of the HIggs field absorbing things like quarks and stuff like that (and for that matter, I don't think that quarks are absorbed and produced either like force carriers are). I think you're mixing up two different explanations here, one for why light moves slower than c in a medium, which does involve photons being absorbed and re-emitted, and how some particles acquire mass through the Higgs field.
  • Outlander
    363
    At the speed of light, Time stops.
    At the speed of light distance contracts to nothing.
    Benj96

    Imagine if you were a snail. At the speed of an elderly woman walking down Main Street with a cane, the same is true. In a way. Yes?
  • Outlander
    363
    I suppose you could say I'm not quite (or perhaps even at all) hitting the nail on the head. However, the speed of light is not as mystical as you say. It is measurable. 287,000 miles (or something) per second. It's lack of being timeless is why we can stargaze and "see" distant stars and planets that no longer exist. It can be an emotional idea, whether positive or negative. I believe the idea you're looking for is what happens in a black hole. Which I may or may not should've mentioned.
  • tim wood
    4.7k
    At the speed of light, Time stops.Benj96
    Whether the rate of the passage of time ever actually reaches zero is more than I know. But in any case, any sci-fi notion of time stopping is an error. I'm at home and my clock seems to me to run at a certain rate. My twin on the spaceship describes exactly the same phenomenon. But on his return I'm sixty years older, and he twenty. But neither of us noticed or observed any difference in the rate of the passage of our own local time.

    There are good videos on Youtube that make all of this reasonably clear. Search Fermilab for their now-large series of excellent and interesting short lectures.
  • Kenosha Kid
    622

    And here's a scary thought: what if the photon's perspective (no time, no distance) is the correct one? :o

    At the speed of light there is no mass. No mass can travel at that rate. Then how did energy ever give rise to mass (e=mc2)? If it cannot do anything to itself in a state of pure timelessness then how did it just spontaneously slow down and get "heavy" with matter in the first place.Benj96

    Because we get energy from two things: our mass and our momentum. Light is pure momentum: E=pc. A body at rest is pure mass: . This leads to the interpretation that any restful body is not actually at rest but is moving through time at the speed of light. So in that sense everything moves through spacetime at velocity c, but photons can only move through space, hence no time passes for a photon.
  • Pfhorrest
    2.5k
    Hmm, I don't think I've heard of that description before, of the HIggs field absorbing things like quarks and stuff like that (and for that matter, I don't think that quarks are absorbed and produced either like force carriers are). I think you're mixing up two different explanations here, one for why light moves slower than c in a medium, which does involve photons being absorbed and re-emitted, and how some particles acquire mass through the Higgs field.Mr Bee

    Admittedly I am not a physicist, though I think @Kenosha Kid is, so maybe he can back me up. As I understand it, the Higgs mechanism involves massive "fundamental" particles like electrons actually being a kind of "blended" particle created when more fundamental particles rapidly interact with the Higgs field. In the case of the electron, IIRC, there is a particle that is mostly exactly the same as an electron, except it has no mass (and so moves at c), and a fixed spin, let's say left. Such a particle can't travel any notable distance at all though, without immediately smacking into the Higgs field, which converts it into a different particle with the opposite spin. That can't travel without immediately smacking into the Higgs field again, which converts it back to the first kind of particle. And so on, over and over, immeasurable rapidly, back and forth. The net result is what appears to be a single particle that's like both of those two, except it moves slower than light and has mass, and an indeterminate spin: the electron as we know it.

    Likewise, all the other "fundamental" particles with mass, except IIRC the neutrinos, which don't couple with the Higgs field and so whose tiny mass is still unexplained (because in the Standard Model, all particles should by default be massless, unless interaction with some field is slowing them down and converting some of their kinetic energy to rest-mass, which was my main point).
  • Mr Bee
    234


    I'm not a physicist either, so I am just going off of what I've heard on the topic as a layperson. The usual explanation describes the Higgs field as some kind of field of molasses which slows some particles down, and the fact that your description sounded similar to the explanation of why light travels more slowly in a medium is what gave me the idea that you're overlapping them. Perhaps you have a source that you can provide that can clear things up?
  • Pfhorrest
    2.5k
    PBS Space Time has a nice easy explanatory video:



    Another good (but more difficult) place to start would be Wikipedia's article on the Higgs mechanism, and possibly also their article on weak hypercharge (which is what e.g. the proto-electrons give and take from the Higgs field to convert between their two types).
  • Mr Bee
    234


    Thanks for the links. I am still not sure where the absorption and emission aspect of your description comes in. What was described in the video was a particle being constantly bombarded by the Higgs field and converting into two distinct states. That is not the same as the reason why light moves slower in a medium.
  • Pfhorrest
    2.5k
    In a very strict sense, every time a particle changes it is one particle being destroyed and another created, because particles moving at c experience no time and so cannot change.

    Even physicists don’t usually speak so strictly though.
  • Mr Bee
    234


    Even if I were to grant such a description, again I wouldn't call that the Higgs field "absorbing and re-emitting" fundamental particles like quarks, which was what I initially took issue with.
  • Mr Bee
    234
    Likewise, all the other "fundamental" particles with mass, except IIRC the neutrinos, which don't couple with the Higgs field and so whose tiny mass is still unexplained (because in the Standard Model, all particles should by default be massless, unless interaction with some field is slowing them down and converting some of their kinetic energy to rest-mass, which was my main point).Pfhorrest

    Oddly enough, apparently the Higgs Boson (which is also a fundamental particle) itself has mass that is not fully accounted for by the Higgs field, at least going off of this helpful source that I've been reading: https://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-known-apparently-elementary-particles/the-known-particles-if-the-higgs-field-were-zero/.
  • Pfhorrest
    2.5k
    Issues with the terminology don't bother me, so if you just dislike the words that's fine (unless someone like @Kenosha Kid confirms it's accurate), but as I meant them that is tantamount to the same thing. A proto-quark with no mass and left spin hits the Higgs field and ceases to exist; Higgs field then immediately spits out a proto-quark with no mass and right spin, which immediately hits the Higgs field and ceases to exist; etc.

    apparently the Higgs Boson itself has mass that is unexplained by the Higgs fieldMr Bee

    Yes that's a good point, I had forgotten that that was also an exception.

    And that article is one that I've seen before that helped me learn about this, too! I'm glad you found and shared it.
  • Kenosha Kid
    622



    Sorry, it was bedtime here. I had to actually check the Feynman diagrams to see what Pfhorrest meant. A couple of clarifications.

    The Higgs field is a weak-force (radioactivity and stuff), spin 0 (scalar) field: it does not couple to spin but weak isospin and to hypercharge. Weak isospin, annoyingly, is not a kind of spin but is so called because it behaves in a similar way to spin mathematically. It is the property that couples in electroweak (electomagnetic + weak) interactions along with hypercharge, which together comprise the conserved electric charge.

    Both isospin and hypercharge can be flipped under interaction with the Higgs field, conserving charge. All quantum field theories are something called perturbation theories. If you imagine adding an electron to the universe, that electron will perturb space around it, and that perturbation will affect the electron, which in turn changes the perturbation, etc. Perturbation theory allows you to calculate e.g. the electron energy by cutting off this infinite series of interactions when the higher-order terms become negligible.

    One of these terms correspond to what Pfhorrest said, which is the transformation of an electron from one isospin to another. As a Feynman diagram, it looks like becomes , so it's valid to say that the electron in this term is destroyed and a new one with opposite isospin is created. However, individual terms (Feynman diagrams) in the series don't necessarily have a physical meaning. It is only the sum over an infinite number of terms that is physical. That said, it is usual to associate particular diagrams with a sort of approximation to a part of the physical process.
  • Mr Bee
    234
    First off, thanks for responding to Pfhorrest's request for comment. It's nice to have an expert chime in just to clear things up.

    As a Feynman diagram, it looks like eL+heL+h becomes eR+heR+h, so it's valid to say that the electron in this term is destroyed and a new one with opposite isospin is created.Kenosha Kid

    Okay, I don't really take issue with that sort of description, but is it also valid to say that the Higgs field is absorbing and emitting things like electrons and quarks then? That was what I was hung up about earlier in our conversation.
  • Kenosha Kid
    622
    Okay, I don't really take issue with that sort of description, but is it also valid to say that the Higgs field is absorbing and emitting things like electrons and quarks then? That was what I was hung up about earlier in our conversation.Mr Bee

    It is valid within quantum field theory models, yes. Motion of particles themselves is formulated in terms of destroying a particle in one position and creating another in another. It depends on how slavishly you want to interpret the models. Plus, as I said, individual Feynman diagrams don't necessarily have physical meaning. These are really mathematical tools, not analogies to reality. When you work in QFT, it is helpful to think of these as physical processes, but that isn't guaranteed. Destroying a particle and creating an almost identical one is equivalent to the particle changing state. There are quite a few ontological degrees of freedom in quantum theory. That's where philosophers should come in :)
  • Mr Bee
    234
    It is valid within quantum field theory models, yes.Kenosha Kid

    Okay that settles that then.

    Plus, as I said, individual Feynman diagrams don't necessarily have physical meaning. These are really mathematical tools, not analogies to reality. When you work in QFT, it is helpful to think of these as physical processes, but that isn't guaranteed. Destroying a particle and creating an almost identical one is equivalent to the particle changing state. There are quite a few ontological degrees of freedom in quantum theory. That's where philosophers should come in :)Kenosha Kid

    Well, you can say that about any scientific theory really. Varying interpretations aren't just unique to quantum mechanics (though it is uniquely infamous for having alot of them). For instance relativity theory, contrary to popular opinion, is also open to interpretations which allow for an absolute time for instance (like the Lorentz ether theory that preceded special relativity), but of course those are less well known.

    That is to say that there is no such thing as a purely scientific ontology of the world. If you want to attach a particular world view to a model you're gonna have to dip your toes in philosophy to a certain extent and make arguments that go beyond the science. That or you could shut up and calculate.
  • Kenosha Kid
    622
    That or you could shut up and calculate.Mr Bee

    :up: That's me!
  • jgill
    667
    I appreciate your knowledgeable commentaries. Thanks. :cool:
  • fdrake
    3.9k
    :up: That's me!Kenosha Kid

    Do you actually though? I don't mean to say you have a standard philosophical position on the matter, but don't you have conceptual insight/imagination that you apply to the thing to interpret it? Like an imaginative background of the calculation.

    Maybe off topic.
  • Kenosha Kid
    622
    Do you actually though? I don't mean to say you have a standard philosophical position on the matter, but don't you have conceptual insight/imagination that you apply to the thing to interpret it? Like an imaginative background of the calculation.fdrake

    Shut-up-and-calculate is a sort of atheistic position on the interpretation of QM as a whole, which describes me well. There are some I find intriguing, some just plain wrong, but I think I have good reason to not adopt a premature position. The ball-rolling of interpreting QM comes from a time when there were only a few simple many-body states we could calculate, and yet intelligent people started generalising to cats rigged to particle detectors. My research group tended to be open to the idea that the maths would sort itself out and give us answers, which, to a limited extent, it has done. Experiment has also pointed in quite a different direction again. These have been far more insightful than adopting a position.

    Concerning specific methods used, it is sometimes helpful to picture things as meaning something physical when you're learning, but beyond that short-term utility, it's more misleading than anything. Besides, these things tend to get their own terminology that's abstracted from any interpretation beyond "The Feynman diagram looks like a ladder/bubble/whatever." My particular research tended to live in quite abstract domains.

    It's the results that are important. Those are what demand interpretation. I don't particularly question the underlying meaning of a hammer when putting pictures on my wall :rofl:
  • fdrake
    3.9k
    Concerning specific methods used, it is sometimes helpful to picture things as meaning something physical when you're learning, but beyond that short-term utility, it's more misleading than anything. Besides, these things tend to get their own terminology that's abstracted from any interpretation beyond "The Feynman diagram looks like a ladder/bubble/whatever." My particular research tended to live in quite abstract domains.Kenosha Kid

    :up:

    Thanks!
  • Benj96
    163
    Because we get energy from two things: our mass and our momentum. Light is pure momentum: E=pc. A body at rest is pure mass: E=mc2E=mc2. This leads to the interpretation that any restful body is not actually at rest but is moving through time at the speed of light. So in that sense everything moves through spacetime at velocity c, but photons can only move through space, hence no time passes for a photon.Kenosha Kid

    This is so cool
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