Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What's the longest wavelength possible?

  1. Feb 16, 2006 #1
    What's the longest wavelength possible???

    1) If radio waves have wavelengths around 100 meters long and a frequency of about 10*7 Hz, are there any wavelengths in the EM Spectrum that are longer?

    2) I've heard the Gravity Probe LISA is using interferometers spaced 5 million km apart to measure gravity, does this mean gravity waves are supposed to be about 5 million km long? If so why?

    3) Also, if Wavelength = 300,000km/s / frequency and frequency = 1, then that means the Wavelength is 300,000km for f=1. So then, how could a gravity wave be longer than light speed's frequency of 1? Did space's expansion stretch the 300,000km into 5 million km?

    Please help me, I'd really like to know, thanks.
  2. jcsd
  3. Feb 16, 2006 #2
    1) Theres no theoretical upper limit on the wavelenght of the EM-spectrum. On the horizon of our visible universe theres an barrier of "infinite redshift". Also, EM-radiation radiated by a body falling into a black hole will also redshift into "infinity" at the event horizon.

    2) No, the 5 million km is because gravity waves have such a small influence on the geometry of spacetime that the difference can't be noticed with small distances. LISA can observe lenght differences in space at the atomic scale.

    3) I don't follow... gravity waves are distortions of spacetime travelling at c, and the "frequency" (or the volume of distortion) of the gravity wave depends on what caused the gravity wave. A head-to-head collision by two blackholes would cause noticeable gravity waves.

    I'm just an undergrad so someone who has actually studied this stuff might answer more clearly.
  4. Feb 16, 2006 #3


    User Avatar
    Gold Member

    1) As drez ponits out, there's no upper limit on wavelength; it's a continuum. Thge label radio waves is purely arbitrary and human-centric. Everything below 10^7Hz is considered radio.

    2) 5 million km is merely a long enough baseline. It has nothing to do with the frequency of gravity waves.

    3) Setting gravity aside for a moment, lets just deal with the general topic of wavelengths and frequencies of EM radiation for the moment. There is nothing special about f=1; it is not a lower limit. An EM wave of f=0.5 has a wavelength of 600,000km.
  5. Feb 16, 2006 #4
    Then can EM waves have such long wavelength that they can never be refracted?
  6. Feb 18, 2006 #5
    Thanks guys, great answers. Most people reply with either a paragraph about hyper-dimensions and transcendence of space-time mumbojumbo, or with winded answers that show how smart they are but cleverly dance around the question because they are too pompous to admit they don't know the answer. Can I just say, amazing replies and so concise!!

    Back to Gravity waves....
    a) Are Gravity waves like EM waves then? Is there an inifinite spectrum of them possible?

    b) What wavelength of Gravity wave should we hope to find first if they exist (the one's created by cojoining blackholes), any idea on the expected value of this wavelength?

    c) If gravtiy waves moves at a velocity = C, then why can't they be considered EM waves? I know EM waves are created by electron excitations or energy level drops, but doesn't it seem like gravity waves are created by the same process or a sub-process thereof, considering both EM and Gravity waves both travel at C?

    Thanks again, anticipating another great round of replies!!

  7. Feb 18, 2006 #6
    c) "gravity waves" would be "created" by Higgs particles, not electron excitation or deexcitation. The wavelength issue depends on the Higgs particle, which we have yet to prove experimentally, so further speculations may prove futile.
  8. Feb 18, 2006 #7
    What is a Higgs particle? Why is it inferred from theory and how?
  9. Feb 18, 2006 #8
    My favourite page on Higgs is:

    http://hands-on-cern.physto.se/hoc_v21en/index.html [Broken]

    Click on the link "Higgs" in the left scroll menue and you will find what you are loooking for.

    In short, you could say that the Higgs particle is neither a force carrier nor a matter particle. It appeared in the equations of the Standard Model when these where changed to allow particles with mass.
    Last edited by a moderator: May 2, 2017
  10. Feb 18, 2006 #9


    User Avatar
    Gold Member

    In short, the Higgs field is the closest thing to the pop term "the fabric of space". The Higgs field supposedly causes a form of "drag" on matter, and thus is the source of the property of mass.
  11. Feb 18, 2006 #10


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The relationship between wavelength and frequency is:

    [tex] \lambda = \frac c f [/tex]

    Now if you were to take a perament magnet and wave it up and down once per second you would be creating an electromagnetic wave with a freqence of 1 Hz which would have a wavelength of 3X108m.

    The Extremely Low Frequency band (ELF) 3 - 30 Hz is used to communicate with submarines when they are submerged.
  12. Feb 21, 2006 #11

    Whoa... Hold on a second here.
    Are you suggesting that I can take my grade N38 neodymium magnet, vibrate it laterally at 10 cps, and generate an emmited 10 hz EM photon field?
  13. Feb 27, 2006 #12

    Physics Monkey

    User Avatar
    Science Advisor
    Homework Helper

    What are talking about, Order? If you tell me which of your questions didn't get answered to your satisfaction I will try to remedy the situation.
    Last edited: Feb 27, 2006
  14. Feb 27, 2006 #13


    User Avatar
    Gold Member

    Ah. You wish to insult mentors (I refer specifically to the ego comment) - but ask them not to take it as an insult.

    I guess that's kind of like getting questions answered and then claiming they weren't answered.

    [STRIKE]Perhaps you could be more specific.[/STRIKE]
    Actually, never mind.
  15. Feb 28, 2006 #14


    User Avatar

    Even if you think they don't answer the questions well, unarguably mentors teach you a lot, and give you a good background, that's why they were chosen to have the title under their name.

    mentor - to serve as a teacher or trusted counselor; a wise and trusted guide and advisor
  16. Feb 28, 2006 #15


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes, that is what I mean. But wait, what is a "photon field" I have never heard of that. I mean that if you wave your magnet at 10cps you will be generating photons of the given wavelength, I will trust your math.
  17. Feb 28, 2006 #16


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I answered your question about electromagnetic waves.

    Since Gravity waves remain undetected and so far are purely theoretical constructs I did not even try to touch that part of your question, because you are correct, I don't know. :rolleyes:
  18. Feb 28, 2006 #17
    Something offered freely upon request should be accepted or rejected graciously. The value a gift possesses when offered is retained only when its value is appreciated by the recipient.

    The price of knowledge is not fully paid by accumulation but by assimilation. We are indebted not only to those who convey knowledge but to those who first acquired it from whom they learned through respect and admiration. Those who discover knowledge have paid their debt in full by conformity to the reality that enabled them to explore it.

    Knowledge is the food the mind requires to obtain the food the body requires to sustain the mind.

    In other words, “Don’t bite the hand the feeds you”, if you prefer food to poison.

    What's the longest wavelength possible??? About that long.
    See why I am not a mentor? Can you appreciate the difference?

    Integral, Loved your broadcast! Received it load and clear.
  19. Mar 2, 2006 #18
    Ok, ok, sorry.

    I think you know why I was frustrated.

    Anyhow, anyone have a guess as to a process arising from within an atom that may cause a graviton to be emitted. I'm thinking along the lines of an exotic particle being deexcited at a level even smaller thean quarks, which would produce a graviton, analogous to how an electron deexcitation creates a photon. Can anyone rule out this theory with already established axioms? Or is it at least plausible, even though it is currently unprovable?

    Again, sorry for the earlier comment. Maybe moderators should strike it and the replies to it from the board, so it doesn't dilute the interesting topic at hand.
  20. Mar 3, 2006 #19
    The idea seems highly unlikely. Gravitions must be produced at an enormous rate. Quarks will try to pull back everytime you try to put something between them or try to pull them apart with a stronger and stronger force the longer separated they are. If your theory were true, the quarks pattern would be constantly "interrupted".

    Also, you are trying to compare excitation with something below quark level. Just that part of the theory is incorrect. Since the quarks act together to form a proton, it is the proton as a whole compared to the electron that gives, the excitation/deexcitation. What do you mean?

    According to your theory, an atom produces a gravition. However atoms interacts with an atom. How would this go together?

    It is hard producing theories when we don't even know how quarks interact or is made.

    NOTE: If you would like to make a theory to PF members, I suggest that you turn to Independant Research subforum in Physics.
  21. Mar 3, 2006 #20


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I'm closing this thread. The answers have been given to the original question, and now we're going into totally speculative no-mans land.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook