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

I've gone faster than the speed of light, havent you?

  1. Apr 14, 2010 #1
    Okay, well I have in my thoughts anyway! Here I am bored out of my mind tonight watching TV when they mentioned the speed of light. Nothing can go faster than the speed of light. But, why? (and no, the answer to my "why" question is not because Einstien said so...). Theoretically, in my wandering thoughts, I dont see why a speed higher than that of the speed of light is impossible.

    For example my thought goes as such:

    Its the year 3000! Yay. Were in a spaceship traveling at precisely 670,616,640 MPH, or 186,282.4 miles per second, which is also the speed of light! :bugeye: Not too shabby, and a tad faster than your mother's Toyotal Prius, and certainly beats walking.
    Anyway, traveling at the speed of light in the year 3,000 is nothing spectacular. Of course it seems strange in the present year of 2010, but just imagine what someone 1,000 years ago would have thought about cellular phones, electricity, and jet aircrafts! Back on track. We're traveling merrily along and I am seated in the back of the ship. I get up and walk to the front of the ship at a pace of exactly 3 MPH. Now, in a fixed point relative to the outside of the ship my mass would be traveling in a linear direction at 670,616,643 MPH, uh oh, thats faster than the speed of light! :eek: Is this possible? According to the "laws" of physics this is not possible. If I am on-board this spaceship traveling at the speed of light is it impossible to move anywhere in the forward direction because if would exceed the speed of light, the speed at which nothing can exceed in all the known Universe?

    I will say I am definatly not a physics expert nor aficionado. Just a regular guy out there with a thought in my mind keeping me up tonight. I would love to hear some input on my theory here to help me better understand! My spaceship metaphor can be replaced with numerous other metaphor's if you will, but the originally question remains, is it absolutely 100% impossible to EVER exceed the speed of light? o:)
  2. jcsd
  3. Apr 14, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper

    Nope, if you are travelling at 670,616,640mph and add 3mph you don't get 670,616,643mph you get 670,616,640!
    Speeds don't add like that at high speed, in fact they don't at low speed but you mostly don't notice.

    The equation is actually = (v + u) / ( 1 + vu/c^2)
  4. Apr 14, 2010 #3
    Hmm, alright so mathematically it would be impossible. Thank you for the response! :wink:

    Now, I did have one more thought in my mind. The speed of light is obviously the speed at which a single photon of light can travel. Now lets say we're back on the "spaceship" traveling at the speed of light, and there is a light source at the front of the craft. Would the light, or that photon of light be able to travel in the forward direction? Because it seems that the photon would be traveling at the speed of light in the forward direction, on top of its original speed (which was already the speed of light in the forward direction).

    Mathematically it cant work since the speed of light cant be simply "added", but then what would actually happen? Or would the light simply be generated and continue with the existing speed of the craft because that was already going the speed of light?

    Or does a physical speed have no correlation to light speed?

    And one more thought:

    If im in a car going down the road at 10MPH and I throw a ball out the window straight ahead at 10MPH, the ball would have a total net speed of 20MPH and would continue at that velocity if wind resistance, gravity, etc. werent applicable. Perfectly within the laws of physics.

    So, what if Im theoretically traveling at 500,000,000 MPH and an object is propelled ahead from that at 500,000,000MPH. That would total a net speed of 1,000,000,000MPH. That would be way over the speed of light and against the laws of physics. I understand from your previous post that addition does not really apply. But if I can go 10MPH and propell and object from that at an additional 10MPH to net 20MPH, why cant it apply to a considerably higher number? Is it because when you "exceed" the speed of light it would require an infinate amount of energy? (which would be impossible?)
    Last edited: Apr 14, 2010
  5. Apr 15, 2010 #4
    WOW, you are the first person to have thought about that!!!


    This is the basis for the theory of relativity. If two ships at the speed of light are travelling in opposite directions, in YOUR theory, from your own frame of view you would be stoped and the other ship would come towards you at 2x the speed of light, correct? NO! Not correct!

    The whole difference in these calculations is caused by the distortion of time!

    really, you should at least try to read about it before coming here to say these things. Any Wikipedia article will already explain it...
  6. Apr 15, 2010 #5


    User Avatar
    Gold Member

    Actually, this is the basis for special relativity. The photon would travel at the speed of light in the spaceship's frame! It would also travel at the speed of light from the point of view of a stationary observer outside watching the space ship. Infact, the speed of light is completely independent of the observer, his speed, the source's speed, whatever.

    Use the velocity addition formula mgb_phys gave you. Let's use 10 m/s instead of 10mph for the sake of simpler calculation.

    [tex]v' = \frac{{10 + 10}}{{1 + \frac{{(10)(10)}}{{(3x10^8 )^2 }}}}[/tex]

    If you'll notice, the fraction in the denominator is so unbelievably small that it is basically 0 and you'll see the velocity addition becomes, at low speeds, what you'd expect in our everyday experience. Try using velocities closer to the speed of light such as 0.5c and you'll see that it doesn't simply add up to c.
  7. Apr 15, 2010 #6
    I think this video explains it well, check it out.
    Last edited by a moderator: Sep 25, 2014
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook