1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What makes a radiowave / lightwave ect move

  1. Mar 1, 2013 #1
    At first it may seem obvious but then I started to think what actually propells these to the speed they move?

    If I radio through to my friend using a walkie talkie, what actually makes the radio wave travel at the speed at which it does?

    The same when I turn on a laser pen or flip a light switch... Do these things build up speed or are they just instantly at there max speed?
     
  2. jcsd
  3. Mar 1, 2013 #2

    Drakkith

    User Avatar

    Staff: Mentor

    Light and Radio waves are different parts of the Electromagnetic spectrum. That is, they are electromagnetic waves. These waves are oscillations of the electric and magnetic fields in space. Since the fields aren't physical objects they are not accelerated. As soon as the EM wave is created it propagates outward at a velocity of around 300,000 km per second, or 186,000 miles per second. This also happens to be the maximum speed that anything can travel locally through space, and is known as "c".

    If there is some underlying "why" that answers why EM waves travel at c, I do not know it. If c were a higher or lower value I would expect light to move at that velocity instead.
     
  4. Mar 1, 2013 #3

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Imagine connecting up an electromagnet. A N pole would form one end and an S pole at the other. Someone at a distance from you (say 1000m) would not know you had switched on until a time of about 3microseconds had elapsed. Their (ideal) compass, would not twitch until that short time had elapsed. It wouldn't /couldn't be instantaneous. In fact nothing is instantaneously communicated - not magnetic fields, not electric fields or even gravity. There is always a delay - even over a short distance between your hand and your eye. (In this case, only about 2nanoseconds).
    Now imagine using a changeover switch on your electromagnet - changing it every 3 microseconds (that's a frequency of about 2/3 MHz. Their compass would also buzz away at the same rate but it would always point one way when your electromagnet was exactly opposite - a delay in phase of 180 degrees.
    You would effectively be sending them an EM wave with frequency 2/3 Mhz and they would receive a delayed version. This EM wave would carry on beyond them, spreading out and getting progressively weaker 'for ever', like ripples on a pond. All EM waves behave the same way, whether they are Radio Waves, Light or X Rays.
     
  5. Mar 1, 2013 #4
    I understand that but what I didn't understand is what gives a photon the things it needs to travel.

    Basically take a grenade, the shrapnel inside the grenade travels at whatever speed, that speed is based on the power of the explosion. So I was trying to relate a photon in a similar way.

    If I turn on a very very weak light bulb, or I turn on a 10,000 watt bulb they both propell photons are the same speed, but what allows them to go from point a to point b? What gives them the power to move.
     
  6. Mar 1, 2013 #5

    Drakkith

    User Avatar

    Staff: Mentor

    EM waves require energy to be created. Once they are created they propagate outwards at c and carry that energy with them. Because EM waves are massless, they always travel at c, no matter how much energy went into creating them.

    What gives them the "power to move"? Whatever created them in the first place. It could be an accelerated charge in an antenna or the transition of an electron between atomic electronic states. It doesn't matter. Once they are created they simply move, just like how an object keeps moving once it is accelerated unless something works to stop it.
     
  7. Mar 1, 2013 #6

    Dale

    Staff: Mentor

    You can think of it this way. A piece of shrapnel's speed is determined by two things: its energy and its mass. If you keep the energy constant then the less mass the faster it goes. In the limit as the mass goes to 0 the speed goes to c. So light is essentially the limiting case of "shrapnel" with 0 mass.

    Don't take the analogy too far.
     
  8. Mar 1, 2013 #7
    That doesn't sound like an analogy at all to me. I'm pretty sure a grenade will emit light when it explodes. That light will move at c for exactly the reason you said. The heavier shrapnel will move at lower speeds. All this is powered by the chemical energy of the explosive. 1 important difference is that the heavier shrapnel is propelled by expanding vapors while the EM radiation results from accelerating charged particles.

    In other examples that have been mentioned...
    The energy that makes the light from a light bulb go is the energy used to light the bulb. The energy that makes a radio signal go is the energy used by the transmitter.

    As to why c is 186282 miles per second instead on 1 or 10100 miles per second, I think that's just a matter of units. If a mile or a second were longer or shorter the number would be different. The important thing is that there is a number. It is nothing more or less then the speed of causality.
     
  9. Mar 1, 2013 #8

    Dale

    Staff: Mentor

    Well, the big weakness of the analogy is that the shrapnel consists of identifiable bits of metal that are stationary before the explosion and are accelerated up to speed. The light neither exists before nor does it accelerate up to speed.
     
  10. Mar 1, 2013 #9
    The Impedance of the space you're transmitting in needs to be 376.73031... Ohms, or light doesn't want to travel without reflecting back.
     
  11. Mar 1, 2013 #10
    My thought is that you should consider the equations that light's wave equation is derived from. Faraday's law and Ampere's law. The idea behind them is that a changing electric field creates a magnetic field and a changing magnetic field creates an electric field. With the right conditions you can get a changing electric field which makes a changing magnetic field which makes a changing electric field.... This is what makes light "go", the changing fields. How fast it goes depends on the "response" the electric or magnetic field "feels" when changing, that is the permittivity and permeability already mentioned.
     
  12. Mar 1, 2013 #11
    If correct this is exactly what I was trying to ask about in my question. Thanks.
     
  13. Mar 1, 2013 #12

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    This is the same problem that many people have. You want an analogy that you hope will give you some more understanding. The best analogy for an EM wave is another wave. Perhaps a wave on the surface of water. You can forget about physical objects flying around because they are really nothing like a wave. A water wave involves regular disturbances on the surface (and below) which carry energy, spreading out as they go. Water waves travel at a certain speed and they have a frequency (rate of going up and down) and a wavelength (distance between crests or troughs). The can be reflected (at a harbour wall) and refracted (as they approach a shelving beach the direction changes).

    If you want to talk in terms of photons, there aren't really any decent analogies because photons are 'really special' entities that behave like nothing else, so don't expect to get very far with that. They have no mass and no identifiable 'size' so they are nothing like 'little bullets' which is how many people try to describe them. Despite the attractive mechanical image presented, I can guarantee that, if you try to come to conclusions about how light (and other EM waves) will behave, your cuddly photon model is certain to let you down. Stick to waves and you may get somewhere, until you know an awful lot more about the subject. I am not being patronising about this. Many, better informed people than me will agree with me, I am sure.
     
  14. Mar 2, 2013 #13
    You essentially asked what propels electromagnetic waves. Why they move.

    The statement [answer] to which you refer, while correct, does not really explain why either the electric field nor the magnetic field 'moves'.....It's a perspective, but if I said the wave particle duality is what makes light 'go' that would be about as fundamental an answer.

    You are essentially asking why this form of energy behaves as it does...If you study quantum electrodynamics you'll get a lot of insights, but not why such a form of even energy exists. We have math that describes what we observe: movement at light speed.

    Or, on the macro scale, the 'laws' quoted by Modus are codifications from observations.....not something based on first principles. We also have a lot of mathematical theories that do NOT apply....do not match observations.

    Physics deals almost exclusively with WHAT happens rather than WHY.
     
  15. Mar 2, 2013 #14

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Exactly. There are no answers to the 'Why" questions. You can only replace one why with another why.
    The best you can really do is to come up with a model that will predict, accurately, the outcome of an experiment. When it works, you can feel that sense of 'understanding' on which we thrive - but it's an illusion more than anything.
     
  16. Mar 2, 2013 #15

    Dale

    Staff: Mentor

    Yes. And, as any 4-year-old child knows, that game can go on as long as the grown-ups decide to keep speaking to you.
     
  17. Mar 2, 2013 #16

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    You have obviously talked to my grandson.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: What makes a radiowave / lightwave ect move
  1. Photons and lightwaves (Replies: 8)

Loading...