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I got this question in a university entrance exam; I'm not sure what could've been the answer. I've scoured the web and could hardly find a decent answer. The question and choices were:

What property of light allows it to propogate in space?

  • The Source
  • The Wavefront
  • The Waveguide
  • The Medium

I simply chose "The Source" since that is how most waves are produced. Do correct me if I'm wrong. An explanation with the answer is appreciated.

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    $\begingroup$ This is a really bad exam question. $\endgroup$
    – Ghoster
    Commented Dec 26, 2023 at 22:34
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    $\begingroup$ In retrospect, this question might not even be worth keeping in this site forever given how its going to confuse more people than it is going to help. This is not a reflection on you OP, but on the question creator. $\endgroup$
    – JohnA.
    Commented Dec 26, 2023 at 22:53
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    $\begingroup$ I understand; I was just startled by this question since it seemed completely irrational. $\endgroup$
    – Sohaib Mubashir
    Commented Dec 26, 2023 at 23:07
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    $\begingroup$ I would say the Wavefront, which is the part that propagates, i.e. creates the next set of E and M waves by induction. The latter two are certainly wrong, and the Source seems like it can't be correct because waves propagate irrespective of the source, which could be a billion light years distant. $\endgroup$
    – RC_23
    Commented Dec 26, 2023 at 23:13
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    $\begingroup$ this question is shockingly bad. I'd probably have gone for the wavefront because this is the thing that propagates, but it seems to me that waveguide is the only one that can't be correct from some perspective $\endgroup$
    – Tristan
    Commented Dec 27, 2023 at 9:42

3 Answers 3

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None of available answers suited me. There's no single property which would let alone electromagnetic waves travel in space. But I would need to name this property of EM field somehow, - it would be self-consistency or self-generation. There's a pair of Maxwell equations which best illustrates this point : $${ {\begin{array}\\\nabla \times \mathbf {E} &=-\partial _{t}\mathbf {B} \\\nabla \times \mathbf {B} &=\mu _{0}\left(\mathbf {J} +\varepsilon _{0}\partial _{t}\mathbf {E} \right)\end{array}}} $$

It reads like this. Varied electric field creates magnetic field and varied magnetic field creates electric field. So it happens that EM field is just "locked into movement" and like dancing of binary stars does not need any medium apart from space itself, - so does propagation of EM wave.

Hence, if I would be forced to choose an answer from the given set, - I would opt for "wavefront", because to some degree it summarizes this pair of Maxwell equations.

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  • $\begingroup$ This is the closest I can get to an answer with a rational explanation. $\endgroup$
    – Sohaib Mubashir
    Commented Dec 28, 2023 at 14:15
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Let me start by saying that as far as I know this is an odd, non-standard question. I will interpret the word “space” as outer space here; I will answer it in the context of classical E&M as that is what is tagged.

Also, I might get downvoted for this but the correct answer (out of the choices given) is the medium.

The way I am reading the question, its basically asking “why can light travel in space but other waves cant”, what’s so special about it?”. The answer to this is that light can travel through space by using the electromagnetic field as a medium, no other type of wave can do this. Relevant.

Also, the 3 other choices are basically nonesense and this question is terrible exam question, specially for a multiple choice without nuance.

Edit: I like the reasoning people are using to justify wavefront (don’t disagree) so I will say the only two nonsense choices are source and waveguide.

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  • $\begingroup$ +1. My initial impulse was "There is no medium. Nothing is waving." See In what medium are non-mechanical waves a disturbance? The aether? But your link is relevant. $\endgroup$
    – mmesser314
    Commented Dec 27, 2023 at 3:02
  • $\begingroup$ I concur with the consideration that contributor Neumaier offered in the answer you link to: in terms of quantum mechanics the quantum entity that is the mediator of electromagnetic interaction is omnipresent; in the absence of electromagnetism in a state of zero excitation. Depending on the context the answer to that odd, non-standard, ill-posed question is very different. Classical mechanics: there is a field if and only if some electromagnetism is happening. Quantum: EM field is omnipresent, but is in absence of electromagnetism taking place in a state of zero excitation. $\endgroup$
    – Cleonis
    Commented Dec 27, 2023 at 7:57
  • $\begingroup$ I see a contradiction here. While "light can travel in space but other waves can't", in the end, both EM and other waves use some medium, as you so clearly describe. So in the end, "the medium" is not a difference, but a common point. Which would nullify its usefulness as the answer. $\endgroup$
    – George Menoutis
    Commented Dec 27, 2023 at 8:48
  • $\begingroup$ George, I think the point is that light's choice of medium allows it to propagate through (empty) space. Whereas sound waves need a medium of air. light needs the medium of the photon gauge field, which like all quantum fields is omnipresent, so it can propagate through "empty" space. But I agree with all other responders that this is a stupid exam question, that should never have been asked. $\endgroup$
    – Nadav Har'El
    Commented Dec 27, 2023 at 10:01
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I see light as propagated in Space as being sustained in its wave form by the speed of which the photon can be pushed out of and dropped into the wave, sustaining the wave and carrying the particle. Source then might be self sustained but not origen, wave front and wave guide resultant, not causative and medium likely to be an restrictive value.

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