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Confusion about relativity

  1. Sep 21, 2009 #1
    When I first started reading up on physics I thought I understood it all, then someone told me that time is relative, and i went through a long process of disillusionment. After I thought I understood time being relative, I proceeded to develop a series of theories based on what I understood, and as far as logic was concerned, i was fine, but my foundations were or were not, depending on how you view the rest of this post. My physics teacher is great at teaching as far as my personal learning goes, but I'm not sure that he corrects me where I am mistaken and he has never told me that my theories were correct, rather he only states that my logic is solid. Then I had a similar conversation with my math teacher, and he said some things that shook my foundations a bit.

    -Until today I believed;"Nothing can travel faster than the speed of light" meant that if something moved at the speed of light, and then was slowed down by the speed of light, it would be absolutely stationary. (c-c=0 and 0m/s being relative to the universe)
    -This was where my confusion started, that the speed of light is completely and entirely relative (is it?); If an object is moving in a straight line at the speed of light, if you subtract the speed of light from it's velocity, is it absolutely stationary relative to the universe?(I realize that this question is fairly abstract because I didn't specify a point of reference for that object to be traveling by) I believed that there is a point at which you cannot accelerate any more (light speed), and if you stayed at that speed forever, you would never age, and you would never die. That was my understanding of the relativity of time and the relativity of the speed of light to an imaginary, stationary point. I'm very confused because of contradictory answers I have received to this question.
    -If light always travels at the same speed in a vacuum, regardless of how fast the light source is traveling, then wouldn't it be subject to the Doppler effect? Is this the case? I'm very confused, please set me straight. Please post any valid, credible, and tested resources that I can read on the subject of relativity, my understanding has been shaken, and I'd really like to understand it all again.

    Has it ever been proven that aging is relative? (twin paradox) That the effects and nature of time changes with speed? Is aging also relative to an observer?

    My math teacher, in essence, said that the light travels relative to the source of light, that a light source traveling away faster the speed of light relative to an observer would not be seen by the observer because the light coming off of that light source would be traveling ((velocity of light source)-c) away from the observer and therefore the light would never reach the observer. and that is why nothing can travel faster than the speed of light relative to the observer because the observer would not see it. Is it true that the only acceptable truths in physics are those that can be seen? That is to say that if I establish that a flashlight is flying away from me in space faster than the speed of light relative to me, that if i cannot see the light from that flashlight, does it exist? Is observation everything, or can things be observed logically rather than physically? I'm very very confused, please correct any and all things that I've said incorrectly, and post any good resources. I'm shaking as I write this because I'm so insanely curious, I doubt this stress is healthy, please please please give me something to read, or concepts to grasp, and correct my misconceptions. Thank you
  2. jcsd
  3. Sep 21, 2009 #2


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    You misunderstand: no object with mass can be accelerated to the speed of light. Ie, it can't go "at the speed of light".
    You're not getting what "relative" means. The speed of one object is measured relative to another which is assumed to be stationary: Speed is the rate of change of distance between any two objects. "The universe" is not an object. There is no absolute reference frame from which all velocities are measured (this is one of the critical implications/bases of Einstein's Special Relativity).
    You can, in fact, accelerate forever. But that doesn't mean you will ever get to the speed of light.
    It is.
    To be honest, your understanding is so thin, you need to start with the basics. Here's the wiki on SR as a starting point: http://en.wikipedia.org/wiki/Special_relativity
    You mean has it ever been demonstrated on human timescales? No. But there is no reason for this to be considered necessary when there are a number of physical processes, many related to what goes on inside your body, that have been demonstrated to be affected by SR.
    Unless you misunderstood, your math teacher doesn't understand SR. Though no such object exists, a hypothetical object traveling faster than the speed of light could be seen just like a real object traveling faster than the speed of sound can be heard.
    Observed, yes. Obviously, we cannot test a theory that has no implications we can observe.

    The rest of that is talking along the same flawed lines...
  4. Sep 21, 2009 #3
    Maybe we can help each other. I wrote a blog describing special relativity and a general relativity one (don't worry about general relativity for now), both in layman terms because I'm basically a layman. Luckily you don't need a degree or even a basic education in physics to understand relativity. It's fairly intuitive if you look at in the right way. You just need to get used to it. I'm interested in seeing what someone who initially has very little understanding of the concept makes of it, no offence. Take a look at this then tell me if it helped or made you more confused. https://www.physicsforums.com/blog.php?b=743 [Broken]
    Last edited by a moderator: May 4, 2017
  5. Sep 22, 2009 #4
    You're mostly right, except that the object would not be slowed by the light. Think of the equation E=mc^2. As an object approaches the speed of light, it gains relative mass. Therefore, the object will require more kinetic energy to maintain that speed. Photons are purely kinetic energy, so photons always appear to be travelling at the speed of light in any frame. If you were to instantaneously accelerate to the speed of light, you would still see light travelling away from you at about 300,000 km/s. Needless to say, you would be stationary relative to that light.

    That would depend on the frame that the observer is in. Is the observer in a uniformly moving frame which is stationary wrt the source?

    I don't know if there has been an experiment using radioactive materials to test the twin paradox. There have been cases in which an astronaut flies off to the moon and returns theoretically younger than his twin. It would take an enormous change in speed to produce any observable effects. But here's an explanation. Imagine two identical rods laid parallel to the direction of travel. Only one of the rods is sent off into outer space, and a virtual copy of it is displayed on a computer screen. The other remains on earth. The rod on earth and the simulation on the computer screen both have a particle inside that oscillates in the direction of motion, from one end of the rod to the other. Now as we observe the rod on the computer screen approaching the speed of light, we see that it's length is contracted so that the simulated particle, which is only a blip on the computer screen, takes less time to travel the length of it than the particle of the earth-stationary rod. This seems to mean that time is accelerated for the accelerating rod, but what it actually means is that time has less of an effect on the accelerating rod than on its twin because time is relative and accelerates with the observer, or the rod, in a particular frame. So when the accelerating rod finally returns to earth it is not as aged as the rod that never left.

    From the point of view of an ant on the light source (assuming that it survives) the light does reach you. But you would never see the light because of the motion of the source. That's not why the speed of light is considered the universal speed limit though. The real reason is that, to travel the speed of light, something would have to have infinite kinetic energy, meaning that it would have to be entirely made of kinetic energy. Photons are purely kinetic energy, but nothing else we know of is. So as far as anyone knows nothing can travel at the speed of light, let alone exceed it.
  6. Sep 22, 2009 #5


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    I assure you, if you ever really start learning physics, that feeling goes away.
    As long as it is there, you definitely know nothing, and shouldn't waste your time "developing a series of theories".
  7. Sep 23, 2009 #6
    I'm addicted to using my brain; using logic and reason to innovate and manipulate established ideas is my best way to learn, and I love to "waste my time" doing it, but yes, while there is soooo much that I am ignorant of, I shouldn't post and share them with anyone that is unwilling to correct me and further my understanding; thus I have not.

    Thank you all for clearing up my foggy idea of relativity.
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