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Speed is relative

  1. Feb 10, 2009 #1
    I'm sitting at my computer at home while I'm typing this thread; i am traveling 0 mph. Im sitting onthe earth typing; i am spinning on an axis at 1000 mph. I am also traveling around the sun, through the milky way. Considering my maximum speed, which would would be pretty a darn fast one around the center of our galaxy, can i only travel, say, half the speed of ligh faster until i reach the limit of speed that is the speed of light?


    consider this: since light travels the same speed always, i feel like turning on a light at my house. i then chug a rd bull and shoot off into space at the speed of light. Since my reference point is where i started, (my house) is my only limit is traveling slower than the light in my house is going from my house?


    since light is always traveling these speed of light faster than I am, can I continue to accelerate forever?

    which of these is true? Please try to take a crack at all of them
  2. jcsd
  3. Feb 10, 2009 #2
    It's no more true to say you are moving relative to the Sun, than it is to say the Sun is moving relative to you. Think of it like this, you should suffix all speeds with what they are relative to, as in 1000mph relative to the surface of the Earth. You can always give your speed relative to yourself as 0. Light will always move at c, measured by you or any other observer. If you were in a ship traveling at .99c relative to the Sun, and turned on a light you'd measure it at c, in every direction, so would someone measuring it from the Sun.

    As an example let's say the Sun is traveling at .50c relative to the Galactic Center. You launch a space ship and begin accelerating in the same direction. What speed you think you are traveling will depend what you measure your speed relative to. If you are traveling at .90c relative to the Sun (you'll tend to think of this as your true speed, although it's no more true than any other), you could then measure your speed relative to the Galactic Center, and it would be .50c + .90c (about 0.97c not 1.4c).

    Your reference point is from wherever you are making a measurement. You will always travel slower than the light leaving your house, as well as any other light. Since you can always measure your speed relative to yourself as 0 you will be able to accelerate forever. When you measure your speed relative to something else you will accelerate less and less, and never reach c.

  4. Feb 11, 2009 #3


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    It is very true you are traveling 0 mph with respect to your reference frame. The rest of the universe is in motion relative to your reference frame. No new science there.,
  5. Apr 29, 2010 #4
    Im having a little bit of a hard time understanding how time and speed are related if speed is only relative and not absolute.

    Say for example nothing else exists in the universe except for two people "A" and "B" . Relative to themselves they are traveling at 0mph but to each other 10,000 kmps away from each other for many years. Would one experience time more slowly then the other and age differently? If so why?

    This does not make sense at all to me because we say if someone were to depart earth and travel at extremely high speeds relative to the earth for years that time would be slower for that person and would be younger than if they would have stayed on earth. But isn't that all relative too? From the person on the spaceships perspective it could be looked at like the earth has been moving very fast since the departure and the spaceship has been standing still?

    If speed is directly related to time and ONLY relative speed exists...how can we even begin to measure what is fast or what is slow because it is all based on perspective correct?

    Any clarification on this would be greatly appreciated...thanks
  6. May 3, 2010 #5
    You might find it helpful to have a read of chapter 7 of Benjamin Crowell's online textbook Simple Nature. I always had a hard time understanding it too, but reading this when where it started to make sense for me.

    http://www.lightandmatter.com/html_books/0sn/ [Broken]

    If they always travel at the same velocity relative to each other, the situation is perfectly symmetrical. Each person will see the other's clock ticking slow compared to their own: A will see B's clock slow, B will see A's clock slow.

    Say A moves away from B at a constant velocity of 10 000 km per second, about three hundredths of the speed of light in a vacuum--or we could just as well describe this situation as B moving away from A at the same speed in the other direction. When 10 years have past according to a spacetime coordinate system in which A is not moving, then in that coordinate system, 9 years and 363 days will have passed for B. Likewise, when 10 years have passed according to a spacetime coordinate system in which B is not moving, then in that coordinate system, 9 years and 363 days will have passed for A.

    What do I mean by "for B" and "for A"? When, for example, A wants to measure how much time has passed on a clock carried by B over the course of 10 years, A needs some way of defining simultaneity: which events in the universe happen at the same time as each other. In other words, given an event at A's location, which event at B's location should A think of as happening at the same time as the first event? The natural way to decide which events are simultaneous is to make use of the constancy of the speed of light. If A sees two stars at the same distance explode simultaneously, A judges the explosions to have been simultaneous with each other. If A sees a star 100 light years away explode 100 years before a star that's 200 light years away, then A considers these two stars to have exploded simultaneously, and so on. This is the method A uses to compare readings on A's own clock with readings observed on B's clock. (A takes into account the time to takes for the image of B's clock to arrive; after 10 years, at this speed, they'll be about a third of a light year apart.)

    Obviously if two events happen in the same place at the same time in one coordinate system, they must happen at the same time in all reasonable coordinate systems, or there'd be a contradiction. Weirdly though, events that happen far enough apart in space and close enough together in time, so that light leaving one event doesn't have time to reach the other event, can happen in a different order according to coordinate systems moving at different velocities. No contradictions arise because such events can't effect each other, since nothing can travel faster than light and even light can't travel fast enough to get from one event to the other. In other words, given some event, i.e. a point in spacetime, each coordinate system defines its own "now", its own set of events elsewhere in the universe which are happening at the same time as that event. But from the perspective of a coordinate system moving at some other velocity, a different set of events are simultaneous with that event. There's no natural (non-arbitrary) way of chosing one absolute present moment that everyone can agree on no matter where they are or what velocity they're travelling with.

    The idea that even simultaneity is relative might seem even more confusing than relativity of time and spatial distance, but all three are necessary if the velocity of massive objects is relative and the speed of light the same for all observers, no matter what velocity they have relative to other observers. The easiest way to see how it all works out is to get stuck into the algebra. Read up on the basics and try out some examples. Spacetime diagrams--Minkowski diagrams are most widely used--are a good way to visualise it geometrically.
    Last edited by a moderator: May 4, 2017
  7. May 3, 2010 #6
    I appreciate very much for your detailed and informative reply to my question. I am presently trying to grasp some of the concepts that you presented. Thank you ever so much for pointing me in the right direction with this.
  8. May 7, 2010 #7
    you must see this like sun is also relative to you with 1000 mph. your speed then 0 mph is also correct if you consider sun and u both moving around the galaxy with same speed and your speed is zero relative to sun.
    I'm sorry to say but actually there is no new science in this.
    you should read Mr. tompkins in wonderland to understand relativity better.
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