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Hey gang, quick question.

  1. Dec 11, 2013 #1
    Ive been thinking about the movement of things recently. How we measure things here on earth but for some reason we don't add up all the variables... its kinda weird. So here is a general question that has been giving me a big ache. So, for instance, If particles can be accelerated to 0.99999999 the speed of light, and the earth traveling around the sun at 108,000 km/h and the solar system moves at around 675,000 km/h and our galaxy moves throughout space at around 900,000 km/h, than what is really going on here... maybe im missing something but would that mean we have already blown past the speed of light? I'm sure I can be proven wrong very easily haha.

    This is not homework or anything like that, just something that has been on my mind and i would love to hear what professionals and academics have to say about my silly observation!
  2. jcsd
  3. Dec 11, 2013 #2


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    You need to combine those velocities according the rules of relativity. You otherwise get nonsensical results.
  4. Dec 11, 2013 #3
    Ok, so things kinda only work in little bubbles? Thats really fascinating, its like science is organic. So when the cern guys do their relative speed test or whatever, they probably account for all of those variables or bassically they dont need to?
  5. Dec 11, 2013 #4
    For instance, if a particle from CERN got released and continued traveling at its speed in a straight line the base speed of it from earth the solar system and the galaxy dosent matter at all? It just stays relative regardless? Anyway, measuring those variables would be hard and they probably fluctuate depending on if earth is moving with or against the solar system in its rotation. Would science work differently if we could account for everything :P? No one has to answer me, these questions are very silly and im not trying to trivialize the wonders of science here, its just amazing for me thinking about it all and not knowing how to think about it quite yet...
  6. Dec 12, 2013 #5


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    A good place to begin is, whenever you say, "moving at NN speed", always add, "relative to X". This is easy and important to get into the habit of because there is no such thing as absolute speed. Swallowing that pill isn't easy. :yuck:

    In the case of multiple velocities as you've mentioned, the total velocity as measured by an observer will not be the sum of the pieces measured by other observers. There is an additive velocity formula that always keeps relative velocities less than 'c' (except for light, which is always measured locally at 'c').
  7. Dec 12, 2013 #6


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    They don't need to. They are irrelevant. All that matters to them is the speed of the particles relative to their starting point which is the same reference frame as their ending point --- the accelerator. Any motion that the accelerator has is irrelevant.
  8. Dec 12, 2013 #7
    I need some time to take this all in, this really has me thinking about some strange and interesting things. Im going to sleep on this. Thank you guys!
  9. Dec 12, 2013 #8


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    Definitely strange while learning relativistic concepts, and always interesting. You'll find a lot of "thought experiments" posted in the GR section. They're a good way to drill down to the nuts and bolts of any details you're trying to reconcile. It's best to keep thought experiments as simple as possible to focus on the area in question. Here's a link to a recent, somewhat light-hearted thread that's kinda' related to your post today - this might get you of to a good start.

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