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A few questions about Relativity, Light, and Physics that dont seem to make any sense

  1. Oct 25, 2011 #1
    This isn't homework. I just read another person's post and it jogged my head into having questions, so this is just for my curiosity.

    Question 1:

    Aside from the practical problem of finding a fuel source that travels the speed of light or faster...

    Why is this a fundamental rule?

    I really do not get this.

    i get that:
    speed = distance/time

    Ok here is scenario #1:

    2 spaceships are travelling at 75% the speed of light toward each other in opposite directions, one from earth to Proxima Centauri and another from Proxima Centauri to earth. Each spaceship's speed is relative to earth. Are not technically BOTH spaceships travelling at 1.5x the speed of light relative to each other? Why would this be possible but one spaceship travelling from earth to Proxmia Centaruri at 1.5x the speed of light not be possible? (again excluding the problem of finding a propellant/fuel source to do so)

    I mean if gravity can affect light how do we know what the "Actual" speed of light is? How do we know that this is in face a constant, as opposed to everything else in the universe where if an object is shot/thrown/launched/etc off a moving object that it's speed is added to the speed of the object it was shot/thrown/etc from (newton's 2nd and 3rd laws)? Gravity is everywhere pulling in multiple directions with all different amounts of force.



    Question 2:

    I read https://www.physicsforums.com/showpost.php?p=66811&postcount=25" by chroot (the thread actually brought me to this site)
    "The term "wave" is meaningless unless you also specify what it is that is waving."


    and it got me to wondering.
    In ocean waves it is the water that is waving
    in sound waves it is the air (usually) that is waving

    So what is waving in light waves?
    It cant be matter because they propagate in a vacuum.
     
    Last edited by a moderator: Apr 26, 2017
  2. jcsd
  3. Oct 25, 2011 #2

    Pengwuino

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    Re: A few questions about Relativity, Light, and Physics that dont seem to make any s

    To answer your first question, that's not how velocities add in special relativity. The simple [itex]v_{total} = v_1 + v_2[/itex] is not valid. The two spaceships, as seen from Earth would appear to be closing in on each other at 1.5c, which is perfectly valid in special relativity. However, when you go into the reference frame of either spaceship, they will see each other speeding towards one another at 0.96c based off the way velocities add in special relativity (see: http://en.wikipedia.org/wiki/Special_relativity#Composition_of_velocities ). This is neglecting the expansion of the Universe, however, which absolutely would be important if you want to talk about traveling between entire galaxies.

    To answer the rest, the gravitational bending of light is so insignificant that it can pretty much be neglected in any measurement of the speed of light on Earth.

    To answer your 2nd question, there is nothing waving in the sense you are thinking. Light is a traveling electromagnetic wave. It was believed that what was "waving" was something called the ether. They thought this substance permeated the universe and light was the disturbance that propagated through the ether. However, it was determined that there was no experimental evidence that the ether existed. So, this electromagnetic wave is in a sense, waving, but it's not waving in anything. It's self-propagating.
     
  4. Oct 25, 2011 #3
    Re: A few questions about Relativity, Light, and Physics that dont seem to make any s


    so for question #1 does that mean newton was wrong?

    for question #2 "waves" require energy to oscillate ....
    so i guess my question is:
    what exactly is "Waving" in an electromagnetic wave?
    or what is causing the photon itself to oscillate, if it is the photon that that is doing the waving on its own
     
  5. Oct 25, 2011 #4

    ZapperZ

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    Re: A few questions about Relativity, Light, and Physics that dont seem to make any s

    Newton has been proven "wrong" twice, first by Relativity, and second by Quantum Mechanics. However, the word wrong here is stated with caveat, because what has been shown is that Newtonian mechanics applies only within certain limits (your bridges and your houses are still built using Newtonian mechanics). We now know when we can no longer apply Newtonian mechanics and when we need to use a more general description.

    The "energy" doesn't oscillate. It is the electric and magnetic field components of light that oscillate. This applies both within the photon picture, and the classical electromagnetism picture of light.

    Zz.
     
  6. Oct 25, 2011 #5
    Re: A few questions about Relativity, Light, and Physics that dont seem to make any s

    ohhhhhhh ok so that's what the left hand rule and this picture are about

    http://openlearn.open.ac.uk/file.php/3922/!via/oucontent/course/469/s381_1_010i.small.jpg

    (Electronic Engineering student)


    so what exactly causes these fields to oscillate, or is it not really oscillating but just appear to do so because the field is spinning while moving forward in a direction?
     
  7. Oct 25, 2011 #6

    ZapperZ

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    Re: A few questions about Relativity, Light, and Physics that dont seem to make any s

    We know when something is circularly polarized, and when they are plane polarized. Look up "waveguides".

    Zz.
     
  8. Oct 25, 2011 #7
    Re: A few questions about Relativity, Light, and Physics that dont seem to make any s

    Yeah I learned about waveguides last quarter. Those things make a lot more sense now.

    So if the field itself isnt spinning then what is causing the field to change?

    or is that something we havnt figured out yet?
     
  9. Oct 25, 2011 #8

    vela

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    Re: A few questions about Relativity, Light, and Physics that dont seem to make any s

    As ZapperZ noted, Newtonian mechanics is perfectly valid when speeds are small compared to that of light. To Newton's credit, he was aware he was assuming that everyone experiences space and time the same way, and at the time, there really was no evidence that wasn't true.
    That picture depicts linearly polarized light. The electric field oscillates in one plane, and the magnetic field oscillates in another plane. It turns out it's the oscillating magnetic field that produces the magnetic field and vice versa. It's a consequence of Maxwell's equations.

    It's also possible to have a wave where the electric and magnetic fields rotate. This would be circularly polarized light. You could actually look at it as two linearly polarized wave which happen to be 90 degrees out of phase with each other. Similarly, a linearly polarized wave could be thought of as a combination of two circularly polarized waves. So it's not really an either-or thing. You can use choose either description as they are equivalent.
     
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