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Hello all! new member with question

  1. Sep 20, 2007 #1
    I am currently a college student who has always had a love for math and science. I was recently thinking and discussing with some of my friends, how mass increases with speed as it gets closer to the speed of light. Then I started thinking about the concept of speed of light and how it is the fastest speed (at least to my knowledge) due to the fact that it doesn't need to push things out of the way to continue moving...

    Anyways, I started to think about that law that states, for each reaction there is an equal reaction. I was wondering what effect this has on the rules of light.

    I want to state, I am neither majoring in, nor am I taking any courses in physics. I don't have too much knowledge about a lot of physics, so go easy on me please :). Thanks.
  2. jcsd
  3. Sep 20, 2007 #2


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    hello and welcome to PF !

    If you want to learn about physics, this is certainly the right place to come.

    As to your questions, I have to say I don't know how to answer. Physics (as many other fields of human intellectual activity) has a well-defined vocabulary and rather precise meanings for many words.

    As such, when you say:
    then this doesn't mean probably what you have in mind. Certainly, lightspeed is not the maximum speed because of any reason that could correspond to what you write ; in fact nobody knows *why* lightspeed is the maximum speed. It is simply that because of the structure of a theory that seems to describe quite well concepts of distances and times (and much more), namely Einstein's theory of relativity, that there is a universal speed "build in the structure of space and time". But nobody really knows why nature is *this* way, and not another way.

    action = reaction is a law of nature that was first discovered in Newton's time in the frame of mechanical forces. Later it was realised that this was a consequence of the conservation of momentum, something which had a broader application than simply mechanical forces. And in the end, it is again realized that this follows from a certain symmetry property of space and time (namely, the symmetry that if you move the whole universe "1 cm to the right", all laws of physics remain the same).
    As such, action = reaction, in its modern form, namely, conservation of momentum, also applies to light, because light carries momentum (although it is tricky to say that you can exert a mechanical force on light).

    The problem is that in order for all this to make any sense, one first needs to learn what all the words mean.
  4. Sep 21, 2007 #3


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    I disagree. If you want to learn physics then take a course at your university. This place is fair at best for learning physics... let's be honest.

    On the other hand, vanesch's answer to your question is perfectly fine--it's a good answer... but it doesn't really teach you physics...
  5. Sep 21, 2007 #4


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    There was the word "about" :wink:

    Also, one should distinguish "if one wants to become a physicist", then yes, you'll have to go to university for that - or if one wants to learn SOME physics, while one has other professional interests.

    The answer: "if you want to learn physics, go to university" implies that if you don't go to university, you won't be able to learn anything about physics. But that's suicide for a field ! Imagine movie makers say that if you want to see movies, you should go to an academy for that!
    Places like PF are there for people who are not physicists, or who do not intend to become a physicist, to get some answers to their questions about the field, where they can be guided to good books on the matter, and discuss the difficulties they may have in their understanding. In other words, to provide information for the interested lay man. I agree fully that that doesn't replace formal courses.
    Last edited: Sep 21, 2007
  6. Sep 23, 2007 #5
    well, I am at a college, and even though I have a lot of interests in physics, like vanesch has stated, I'm currently not looking to take a course. I have specific interests, and due to people having different preferred learning styles. I really like to take things at my own pace and at "sort of" a one on one experience.

    What I was mainly getting at with the law stating equal reactions, is that in my mind, I assume light has the fastest possible speed (even though it might technically not) because it doesn't depend on matter. For a bullet to move, even one more atom forward, it has to push atoms out of the way and create the vacuum that places the atoms back behind it. The reason why I think sound isn't as fast as speed, is due to it depends on the matter it is being transported through. But now that I'm thinking about it more, I do remember my Physics teacher saying that black holes are capable of warping the path of light, to add curvature. As to all of the vocabulary, I am sorry I might not thoroughly express things as clearly as I mean.

    I hope for some more posts in this thread to see what others are thinking/know about the topics I am talking about. I also thank you vanesch for going easy on me, a lot of forums aren't so welcoming. :)
  7. Sep 23, 2007 #6


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    I think the main key to this thread is the statement Vanesch pointed out; the idea that lightspeed is the fastest possible speed because light doesn't have to push anything out of the way (I confess that I don't really understand the meaning of that statement). Lightspeed's title as "The Fastest Speed Anything can Go" is more related to another statement you made in your original post; that things gain mass as they speed up. At lightspeed, the mass of any object becomes infinite, and so that object would require infinite force to accelerate it. In fact, just before lightspeed, the mass of any massive objcet becomes "close to" infinite, and so requires more power than exists in the Universe to accelerate any faster.

    Massless partices like photons travell exactly at lightspeed, and can't go any slower. This is not because of lack of drag or good aerodynamics, but because they have no inherent mass to be multiplied as they accelerate (that's way simplified, so don't take it too literally). There is a theory that some particles may travel faster than lightspeed, but it is impossible for them to slow down to the speed of light (which retains the prohibition against massive particle travelling at lightspeed), but this remains unconfirmed.
  8. Sep 24, 2007 #7
    wow, thank you very much, that helps a lot, and opens my mind up to a lot of other thoughts. :)
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