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Struggling with relativity

  1. Jan 27, 2009 #1
    ok, so i have watched tutorials on relativity, and other videos and such. looked at wikipedia but it has a lot of stuff i dont understand, and other places too. i could read through other posters questions, but im not sure how to really phrase my question and so i dont know what to look for, so i will try to say it here and hopefully somebody can make sense of the madness (my madness). so if a rocket is travelling the speed of light (assuming it could) and following an arbitrary path from the earth back to the earth where the elapsed time for the rocket people is X. time moves slower for him relative to people stuck on earth at a time i will call Xe, which is going to be less than X. Is that right? now why is this? i guess space and time are related and so i get all these videos and things telling me that all objects have mass and ca bend space and we can see that effect through time. So what? does that just mean that space and time are related? so then i guess that if you travel at light speed (hypothetically), then you move through space faster, so you would pass through time faster because it is the same medium, is that right? i mean is that the point of special relativity?
  2. jcsd
  3. Jan 27, 2009 #2
    You're right about space and time being related. You've probably heard the term space-time contunuum before, which is just a unification of space and time. You were also correct to say that moving through space causes your travel in time to differ, but moving doesn't cuase it time move quicker, but instead time slows down.

    This is called time dilation. Its kinda of easy to think of you, when moving, are diverging your motion through time, into motion through space.The effect isn't noticeable at low speeds, but when speeds are close to light the effect becomes extremely dramatic, also time ceases to flow at the speed of light. Objects with mass, or rest mass, can never travel at the speed of light.

    Now, I would like to stress that time dilation is a relative effect. You in the rocket ship passing Earth at a speed close to the speed of light, will consider yourself at rest if you are moving at a constant rate, and would say that Earth is moving past you at a speed close to the speed of light. You in the ship experience time the same no matter how fast you are going relative to the Earth, but measure Earth's time to be dilated, and slowed down. While people on Earth, who also consider themselves to be at rest, see time slowed down for you.

    So sadly, time dilation couldn't be used for a fountain of youth type of thing, but it could be used for something like traveling to the future. You may have heard this, but if you zoom off a way from Earth at close to the speed of light, you may think you have been traveling for, say, 30 years, but when you come back to Earth you find that something like 4 million years have passed on Earth!
  4. Jan 27, 2009 #3
    oh right, it slows down.. ok, so is that because it goes through space at a faster speed so the effects of time usually reserved for passing through that volume of space are negated? does it have to do with conservation of energy or something? im confused. why does time move slower?
  5. Jan 27, 2009 #4
    I'm afraid I can't give you a definite answer as to why time slows down. The best answer I can give you is that light travels at the same speed regardlas of your speed partly becuase of time dilation.

    Imagine you are riding around in your nuclear powered car that allows you to travel at 99% the speed of light. Now, as your moving around in your car at 99% the speed of light, you decide to turn on the headlights. Intuition would tell us that you would see the light move away from your vehicle at only 1% the speed of light, but instead you would see light zoom out of your headlights at exactly the speed of light, no less. The light moves away from your car at the same rate it would if you were still parked.

    So, because of time dilation, and a few other principles, light moves at the same speed in all frames of reference. To your friend, who is sitting still relative to you, you in your nuclear-powered car are trailing right behind that beam of light coming out of your headlights. But when you came back to him, the story you would tell would be very different. Contrary to what your friend said, you claim that no matter how hard you pushed your nuclear-powered engine the light escaped your car at the speed of light.

    That doesn't seem to important at first, but for all we know our galaxy could be moving through space at 99% the speed of light right now. Without time dilation's help, shining a light in whatever direction our galaxy may be moving in would be really frustrating! The light may take much longer to reach what you are trying to see. So I suppose time dilations only importance may be to keep all of the known laws te same in all frames of reference. Which is pretty convinient if our galaxy is moving around at 99% the speed of light.
  6. Jan 27, 2009 #5
    so why is the speed of light special? i mean einstein used it to produce this theory, so how did he incorporate its significance?
  7. Jan 29, 2009 #6
    Well, it all started with Maxwell's equations. Maxwell's equations predicted the speed at which electromagnetic waves, such as light, propagate. Which was a big no no. You see, all speeds are relative. Someone who is at rest relative to the street might say that the car is passing him at 65 miles per hour while someone moving at 50mph in the same direction as the first car will say that the first car is only moving at 15mph, because the second car considers himself at rest. For a car moving in the opposite direction of the first car at 50mph, the first cars speed is 115mph. So there is no definite number for somethings speed, it is all dependant on the eyes of the observer. Yet, Maxwell's equations predict an exact number for lights speed.

    Another issue was Galilean relativity, that says that when moving at a constant rate you can use no physical law to determine anything but relative motion. Like when flying in a plane, you cannot tell that you are moving, everything works the same, you can juggle on the plane as well as you can on the ground. When you look out the window and see the ground moving underneath you, however you cannot tell that you are moving you can only say that you and the ground have relative motion, either the plane is moving or the ground is, you cannot tell for sure. If light has this definite speed then you could determine that you were moving by shining a light. If you are moving at 99% the speed of light, you cannot tell, but if you shine a light in the direction of your motion it will travel from your flashlight at 1% the speed of light, so you could tell that you are moving at 99% the speed of light. And thats when physicists asked 99% the speed of light relative to what?

    So physicists came up with the ether, a medium on which light travels, like sound travels on air. The speed predicted by Maxwell's equations was the speed at which light propagated relative to the ether. Now the ether was a pretty wierd concept. You couldn't measure it. It is everywhere, including any vacuum of course since light travels through empty space. We can suck just about everything from a container on Earth but somehow the ether stayed there because we can still shine a light through the evacuated glass. The ether was also thought of to be at rest relative to the sun.

    So if Earth is moving through the ether, than light should appear to be slower in the direction of Earth's motion through the ether and faster in the opposite direction. By measuring the amount of speed change you could determine Earth's speed as it moved through the ether.

    That was the plan of Albert Michelson. His setup is somewhat complicated, so I won't go into it, but with it he could measure any speed change for a beam of light and determine Earth's motion through the ether. This is were all the drama begins. After performing the experiment several times at different times of the year, since Earth's direction changes, there was no speed change whatsoever. The beam of light traveled at the same speed regardless of its direction or Earth's direction.

    The results of the experiment, which is call the Michelson-Morley experiment if you would like to look it up, says that lights speed is the same in all frames. Like I said in my earlier post light is same speed for everyone, for all observers regardless of there motion. So if you were moving at 99% the speed of light and shined your flashlight there would be no speed change, you couldn't determine your speed so Galilean relativity stays intact.

    So this is when scientists are trying to figure out how could that be, how could lights speed be the smae for everyone. And then Einstein figured it out with his theory of relativity. He said light is the same speed because space and time are relative, dependant on the eye of the observer. Also, the concept of the ether was ignored by Einstein and I believe that most serious physicists also do not believe in its existence. Light travels with no medium whatsoever.

    You seem to have a genuine interest in the subject of relativity. I recommend that you read An Equation That Changed The World by Harold Fritzsch, I may have spelled the authors name wrong. I found it to be a great book and very good at explaining the concepts of special relativity:)
    Last edited: Jan 29, 2009
  8. Jan 29, 2009 #7
    There is actually some controversy as to whether or not Einstein even new of the Michelson-Morley experiments results. Either he did or he was a man too stubborn to believe in the ether and too stubborn to give up Galilean relativity and Maxwell's equations. Back then either Maxwell's equations were wrong or Galilean relativity was wrong, luckily Einstein came up with a way for us too keep them both:)
  9. Jan 29, 2009 #8
    Charlie G, thank you very much. I am grateful for your help and all the others who have reallly helped me. That does wonders for my understanding.

  10. Jan 30, 2009 #9
    No problem:)
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