Asked by my 8yo today and left me scratching my head.


closed as off-topic by Daniel Griscom, Qmechanic Mar 31 '16 at 0:13

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  • 2
    $\begingroup$ an object can't travel at the speed of light... $\endgroup$ – user98038 Mar 30 '16 at 22:18
  • $\begingroup$ a hypothetical object that has always been travelling at the speed of light then? $\endgroup$ – Marc Scott Mar 30 '16 at 22:33
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    $\begingroup$ it has to be massless then... $\endgroup$ – Martin Mar 30 '16 at 22:38
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    $\begingroup$ It seems only Einstein is allowed to ask hypothetical questions like that. $\endgroup$ – Bill Alsept Mar 30 '16 at 22:52
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    $\begingroup$ @BillAlsept too bad he's dead. Otherwise he'd bail us all out of the question being closed. If an 8 yo truly asked - reflects a highly creative imagination. $\endgroup$ – docscience Mar 30 '16 at 22:56

Whenever you run into a "can't go there" question, the next best thing is to see what happens when you approach the limit.

So let's imagine an object traveling at 0.999...c . If the sun is overhead, and we look for a shadow below it, will there be a shadow?

Let's slow everything down. The photons of the sun are now falling with the speed of raindrops, and our rocket is traveling at the speed of the rain drops. The front of the rocket will get very wet, and a "cylinder" of dry air will follow behind it. If you are standing below the rocket, it will briefly stop raining as it passes overhead.

So yes, there will be a shadow.


Obviously if the object is traveling towards me "at the speed of light", it intercepts very few photons (at the limit of the speed of light it will never hit any) and will in essence have no shadow (although it will seem the lights go out when it reaches us...); and when it travels towards the light it will cast a "normal" shadow.

  • $\begingroup$ good answer and another way of looking at it is having the object pass across the face of the sun at 99% speed the light. There should be a brief shadow. You could continue the hypothesis with a stream of particles that would leave a shadow line. $\endgroup$ – Bill Alsept Mar 30 '16 at 23:15
  • $\begingroup$ But what about Lorentz contraction? From a photon's perspective, does the spaceship have a near 0 length? In which case, they wouldn't strike the spaceship. $\endgroup$ – Marc Scott Mar 30 '16 at 23:20
  • $\begingroup$ @MarcScott even if the spaceship shortens to a disk, it still has a finite cross sectional area ("it is just a windshield") and will still hit all the photons that were in its path (the "raindrops"). $\endgroup$ – Floris Mar 30 '16 at 23:22
  • $\begingroup$ What if the object was the size of the galaxy? $\endgroup$ – Bill Alsept Mar 30 '16 at 23:35
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    $\begingroup$ It doesn't I totally agree with your answers. I was just trying to help out the Lawrence contraction situation. Like you said it doesn't really change anything. $\endgroup$ – Bill Alsept Mar 31 '16 at 2:37

First of all, no object can travel the speed of light if it has any mass at all. If an object travels, say 99,999999% the speed of light, than yes, it will cast a shadow, since the speed of light is the same in all reference frames.

Edit: you should watch this video, it covers a lot of related questions: http://m.youtube.com/watch?v=JTvcpdfGUtQ


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