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Thread: Einstein's Theory of Special Relativity

  1. #1
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    Post Einstein's Theory of Special Relativity

    It's all relative. How many times have you heard that? Well, when you're traveling close to the speed of light, everything really is relative; especially the passage of time. ...

    Read the full blog entry

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    If you travel to the nearest star (which is 4 llight years away) at the speed of light It would take 4 years. But is that 4 years ship time or 4 years for someone on the earth assuming you could know instantly when they had got there?

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    Quote Originally Posted by KimB View Post
    If you travel to the nearest star (which is 4 llight years away) at the speed of light It would take 4 years. But is that 4 years ship time or 4 years for someone on the earth assuming you could know instantly when they had got there?
    4 years for someone on earth. Travelling at c, the ship would experience no time for the trip (this assumes there is no acceleration or deceleration time, getting to and stopping from c.)

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    So, for the ship travelling 4 light years zero time speed is infinite... So doesn't that break the 'rule' that you can't travel faster than the speed of light?

    From the ship the earth is travelling (receding) at the speed of light... so no time passes there either (from the ship point of view). So when the ship gets to the star no time has passed on the earth???? (Now I'm confused.) Or is this another example of the lights on the train coming on at different times depending on your location.

    Kim

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    Quote Originally Posted by KimB View Post
    So, for the ship travelling 4 light years zero time speed is infinite... So doesn't that break the 'rule' that you can't travel faster than the speed of light?
    I didn't say infinite. Notice I also said you would have to ignore acceleration and deceleration time. Travelling at c, there will be no time elapsed for the ship, 4 years for earth.

    Quote Originally Posted by KimB View Post
    From the ship the earth is travelling (receding) at the speed of light... so no time passes there either (from the ship point of view). So when the ship gets to the star no time has passed on the earth????
    From the ship's point of view, that is correct. No time has passed. From the earth's point of view, 4 years have passed.

    Quote Originally Posted by KimB View Post
    (Now I'm confused.)
    Don't feel bad, it can be quite confusing. Relativity has quite a few subtle aspects that are very counter intuitive. As a result, there is a lot of confusion concerning some of those aspects. This is one of them.

    Quote Originally Posted by KimB View Post
    Or is this another example of the lights on the train coming on at different times depending on your location.Kim
    Well, it's not quite the same thing, but both of them are a consequence of relativity.

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    At one point you were being drowned out by some background music. Anychance you can hold off on the music until you have finished?

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    Quote Originally Posted by Sticks View Post
    At one point you were being drowned out by some background music. Anychance you can hold off on the music until you have finished?
    Huh?

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    Quote Originally Posted by Tensor View Post
    Huh?
    The podcast from 23:24 to 24:16

    It sounded like someone thought they had finished when they put the backing track on. It added nothing to the programme

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    I'm still a bit confused.

    I'll try to clarify my first question. Ignoring acceleration the ship is travelling at C. Time has slowed to a stop. Speed is measured in distance travelled in a given time (e.g. m/s). But travelling at C time stops, therefore the distance travelled (d) in a unit of time is infinite, d/0 (from the ship point of view). Star trek here I come. But I thought Einstein's theory excludes infinite speed?

    Maybe considering the return journey will help me understand what's going on.

    Let's say the spaceship is approaching earth now at C. Inside the spaceship there are no windows. The Ship is travelling at a constant velocity. Under these conditions, wouldn't it be impossible to tell if you were even moving let alone travelling at the speed of light?

    Now say from the ship, you could only see the earth. You couldn't tell if you were approaching the earth at C or the earth was approaching you at C. (Assuming you'd have time to think about it) If you can't tell the difference between these two scenarios, wouldn't it look from the ship like time on the earth was standing still and time on the ship was normal? But if this is true then the journey for the ship must take some amount time.

    what I'm trying to understand is a rule that works regardless of where you are. If that's possible.

    Thanks

    Kim

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    If you were traveling the speed of light, what would you see? wouldn't you and your ship be invisible to you, as you would be moving away from the light from that time frame at the same speed as it's traveling towards you? hmm...

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    Wink black hole inyour wake....

    Quote Originally Posted by Fazor View Post
    If you were traveling the speed of light, what would you see? wouldn't you and your ship be invisible to you, as you would be moving away from the light from that time frame at the same speed as it's traveling towards you? hmm...
    Fazor...for a speed boat, it's a water wave wake....for you it's a black hole. Seems to be a lot easier than aggregating sufficient mass in one place to create one. However, you have an if-then syllogism predicated on the fact that you...a material being in a supposedly material ship...are already traveling at the SR -forbidden velocity of c. Einstein spent a while thinking about your scenario. Pete.

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    For the most part the ToR makes sense to me. There is one thing that I've not quite got a good grasp on. Perhaps someone can explain it to me. Might even be a good question for a Q&A podcast. Here goes:

    We have two actors, Jerry and Jeri. Jeri gets onto a really fast motorcycle - naw better make that a space ship and accelerates away from Jerry. As Jerry observes Jeri, Jeri seems to be moving in slow motion as time is moving more slowly for her from Jerry's perspective. So what does Jeri see when she looks back at Jerry? It seems to me that she sees the exact same thing that Jerry saw looking at her, that is to say that from her perspective Jerry is moving very slowly because relative to her he is accelerating away.

    Is this correct and does it make sense? After all, this is relativity, that is to say that everything is relative to the observer.

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    I'll try to clarify my first question. Ignoring acceleration the ship is travelling at C. Time has slowed to a stop. Speed is measured in distance travelled in a given time (e.g. m/s). But travelling at C time stops, therefore the distance travelled (d) in a unit of time is infinite, d/0 (from the ship point of view).

    okay KIMB here we go.
    einsteins thinking about light speed motion is thus. The motion through space is totally relative to the observer(am i leaving earth or is earth leaving me?)
    So yes, the earth would appear to stand still from the vantage of the ship moving at C (the Cship). However, from the earth's vantage, the crew of the Cship would appear frozen in time.
    To each observer, however, their own time would remain CONSTANT and unnaffected, much in the way that you never notice time fluctuations here on earth despite our high velocity motions and accelerations through space(although they never even come close to the Cship's speed).

    The reason for this effect is that as we move through spacetime, our velocity is divided up between the 4 dimensions (up-down, left-right, back-forth, past-future) and all the motion through space is deflected away from the motion through time. So if ALL the motion is dedicated to moving through space, there will be none left over to move through time.
    The easiest way to understand it is to say that A BEAM OF LIGHT NEVER GROWS OLDER, hence the relative sense of "now" is relevant even when looking far into spacetime to the distant past. We see the sun there now, but really for the sun that was 8 minutes ago.

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    Quote Originally Posted by Fazor View Post
    If you were traveling the speed of light, what would you see? wouldn't you and your ship be invisible to you, as you would be moving away from the light from that time frame at the same speed as it's traveling towards you? hmm...
    No dude, thats the relative part of relativity, light would move away from your ship and or body at light speed compared to your motion and position....you cant beat light in a race (or even keep up) no matter how fast you go, so you would notice NO difference unless you looked OUTSIDE your ship. Everything behind you would appear frozen in time though, so that's pretty cool I guess.

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    Going back to KimB's question, if it takes 4 years for a photon to travel that distance, why doesn't it take the ship 4 years to travel that same distance? Wouldn't the ship act like a ridiculously huge photon? And being a part of that ridiculously huge photon why wouldn't you feel the passage of those four years?
    And I don't understand--Why is time stopping just because you are moving at the speed of light?

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    Quote Originally Posted by squid View Post
    if it takes 4 years for a photon to travel that distance, why doesn't it take the ship 4 years to travel that same distance? Wouldn't the ship act like a ridiculously huge photon? And being a part of that ridiculously huge photon why wouldn't you feel the passage of those four years?
    And I don't understand--Why is time stopping just because you are moving at the speed of light?
    Ok this is hard to explain but here's why, time and space are part of the SAME THING. moving through space IS moving through time. This effect is caused by the maximum speed limit of 'c', or the speed of light. The speed of light can also be called the speed of time. The reason for this is that two different observers viewing the same event from different locations will not be able to agree on the time at which it happened.
    Imagine a fireworks expert, who has two rockets (pointed away from each other like a 'V') that can be seen from a long distance away.If he lights both rockets at the same time, what will happen? Well, if you are EQUALLY distant from two explosions (as the engineer would be) you would see them go off simultaneously. However, an observer closer to one exploding rocket then the other will first see the one closest to him go off, and only later see the other. From the point of view of the fireworks engineer who lit the rockets, they both flew apart the same distance an went off at the same time. but from the viewpont of someone closer to one explosion the further one happened later in time. This would also be true no matter which rocket you are closer to, so two observers close to opposite rockets will both claim that theirs went off first. Which one is right? Both, thats the wierd part.

    when you 'move' close to the speed of light a similiar type of effect is percieved. Both the person 'moving' close to the speed of light and the "stationary" observer will claim that the other's watch is slowing down more and more as light speed gets closer. ('stationary' and 'moving' are words that can be used interchangeably, i.e. Is the astronaut moving away from earth or is earth moving away from the astronaut? again, Both is the correct response) so time does NOT in fact stop FOR YOU when moving close to light speed, but rather , everything outside appears stopped from your viewpoint, and other people looking your way would see that YOU appear frozen in time from their viewpoint... That's the relative part of relativity.
    Achieving light speed motion is not actually possible, but the time compression is observed thusly. As you move closer and closer to the speed of light, the distance gets shorter and shorter, even for the same trip. So going from here to the moon would be 400,000 km going at the speeds you and i are used to, but at 98% of light speed the trip would be much shorter in distance, maybe only a few hundred km, and thats why time slows when you go fast. If you could reach light speed all distances shrink to Zero size, no matter how far. That why you get there the same time you left(frozen time).

  17. #17
    I'm not buying that. Even if it was Einstein. Guy was smart but........

    If we had a clock that tells years. And some do. And we put it abord the
    new horizons space craft ( and it returned to earth), I'm putting my
    money on it taking 18 years for it to get to pluto and (from our perspective)
    and the clock aboard also saying 18 years had past.

    Even if it's traveling faster than we are. I'm betting the ship will not look
    brand spanking new either. And pretty much out of fuel. All of which take time.

    So .............

    At the speed of light. No matter what the speed is. Change it to whatever you want.
    (If we could change it). One year is still one year. To us and the ship. The ship would be either Farther or not as far. That's it.

    If not new horizons would be at pluto ( from it's perspective in less than nine years). But for us it would take nine years?

    So ..............

    Did the new horizons space craft take pictures of the Jupiter flyby even if to us it hadn't got there yet?

    Then again. Who's to say nothing can travel faster than the speed of light?
    Light is just the fastest thing we know of now. Then again how do we know light even travels. Maybe it just spreads or expands. Have we captured a light particle? or a light wave? or both (photon). What does it look like?

    Space itself could be light that just isn't bright but dark. And its spreading causing all of the red shift as it pulls the objects away from all the other objects just as a
    rubber duck gets pulled down stream by a water current. But because it was accelerated in the beginning it continues to accelerate until some other force stops it or decelerates it making it a constant speed or slower. Maybe that's why farther object appear to moving away faster than nearer objects.

    Always wanted to get that out.

    I still think the universe is infinitely large AND small.

    Every smaller particle is made up of smaller particles and ever Larger particle is made up of smaller particles. Infinitely ......................................

    Space either ends and something is encapsulating it or space is infinite.
    But..................

    Then the space encapsulating space itself either ends or is infinite as well.
    And so on. And so on.

    Now. If "Infinite" is not possible. Then there is a "REALLY GOOD" chance that "WE" do not even exist. "WE" just think "We" do.

    We may. When we die. Just see it all as a dream that was interactive.

    ok. Man I'm really scaring myself now. He....He.....He..........

    =)

    I got to stop now. Sorry.

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    Theory behind initial Speed-of-light Experiment

    There is one explanation in the show that I just don't understand:
    the theory behind the experiment with the perpendicular mirrors where they found out that the speed of light is constant (in the late 1800s I think)

    It's a bit hard to explain,
    so let me start with that image of the ball, thrown on a moving boat:

    If you throw a ball against a wall on a moving boat, and along the movement vector of the boat, someone on the shore in the same line of the movement vector of the boat, would see the ball move faster on the way to the wall (speed of the ball plus speed of the boat), and slower on its way ricocheting back from the wall (speed of the ball minus speed of the boat).

    Pretty straightforward, no problem so far.

    Now, if you throw the ball sideways (perpendicular to the movement vector of the boat),
    both you and the observer on the shore would see it moving at the exact same speed, both on its way to the wall and back, right?

    So far so good.

    However, an observer on the same boat with you
    would ALWAYS see the ball moving at the exact same speed,
    no matter what direction you throw it in, right?

    Since you AND the observer are both moving on the same boat, in the exact same direction and at the exact same speed, the movement of the boat would not at all affect the observed speed of the ball.

    So ONLY someone standing on the shore would observe a difference in the speed of the thrown ball, and ONLY if it is NOT thrown perpendicular to the movement vector of the boat.

    RIGHT?

    Now back to the experiment with the perpendicular mirrors:
    - the light is the ball thrown at the wall
    - the mirrors are the walls
    (one in the line of movement of the earth, the other perpendicular)
    - the earth is the moving boat

    But hey, all the observers are also on that moving boat (Earth),
    so the speed of the ball (light) shouldn't be affected,
    no matter what direction it is thrown in.

    Where is the 'independent' observer on the shore???
    Only a observer who is NOT on the same boat (earth) should observe any difference, RIGHT?

    So the fact that both light beams in that experiment traveled at the same speed, does not prove anything, does it???

    Sorry, this is a bit long, but I just can't figure it out...
    Can anybody help me?!

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    Quote Originally Posted by clint View Post
    There is one explanation in the show that I just don't understand:
    the theory behind the experiment with the perpendicular mirrors where they found out that the speed of light is constant (in the late 1800s I think)
    You have a slight misunderstanding of the reason for the experiment, and this may be why you are having trouble with this. At the time of the experiment, most physicists thought that light was made up of waves (it is now thought that EM radiation has both wave and particle aspects). The question was, what was waving. It was postulated that there was a medium and this medium was given the name "luminiferous aether".
    The experiment was an attempt to measure some of the properties of this aether. The reason for the perpendicular mirrors was one arm would measure the length of time each arm, traveling either with or against the motion of the aether past the earth. The other arm would not be moving either with or against the aether and it was thought that when the two light beams were brought together, there should be a shift of the interference patterns, indicating the different travel times. The mirrors could be rotated so the arms could measure the different shifts based on the rotation of the Earth, along with the expected differences due to the Earth's orbit around the Sun.
    As a result of the set up, the different paths should produce different travel times and this would show up as the shift of the interference pattern. The problem was, there wasn't any kind of shift at all (within the errors of measurement). There were serveral possible reasons for this, one, was the Earth was dragging the aether with it as the Earth moved. Another possible reason was the Earth, was, in fact, motionless. Another, possibility was that the speed of light would measure a constant speed, no matter what speed the test equipment was moving.
    Most of the possible reasons for the non shift would produce some kind of other observational effects, that weren't observed. In 1905 Einstein publish his Special Theory, in which he postulated that the speed of light was constant in all inertial frames of reference, along with the postulate that all laws of physics are the same in every inertial frame of reference.

    With just those two postulates, Einstein publish "On the Electrodynamics of Moving Bodies", which introduced SR. As you can see, the postulate that the speed of light was constant in all inertial frames of reference explains the reason for not finding any fringe shifts in the MM experiment. Coupled with the discovery that EM waves are transverse and don't need a medium, pretty much killed off aether (there continue to be some die hards who don't accept SR, and thus they keep looking for an aether or some other way to explain the observations and tests, without the need for SR.

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    The path of the ball is longer for the shore observer.

    Quote Originally Posted by clint View Post
    Now, if you throw the ball sideways (perpendicular to the movement vector of the boat),
    both you and the observer on the shore would see it moving at the exact same speed, both on its way to the wall and back, right.
    No, the observer on the shore will see the ball as moving faster, because the course of the ball to him is the vector sum of the boat's forward speed and the speed of the ball perpendicular to boat's motion.

    This is covered very well in one of the introductory texts on special relativity, and I'll go look it up and post the title. For light, where the observer on the boat and the observer on the shore must measure the same velocity, the clocks on the boat must run slower.

    Got it, the book is "Special Relativity" by A.P. French, and it is excellent. You can memorize his examples and astonish your friends.
    Last edited by John Mendenhall; 2007-Jun-01 at 07:27 PM. Reason: Addendum

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    (as stated before) Forget Acceleration on either end of the trip.

    At lightspeed, you would climb into the ship.... hit the "go" button and instantaneously arrive at your destination.

    This is where the movie "Contact" had it completely backward.

    Yes.. they used "wormholes' but the same principal was in effect. (I think)

    More time should have passed for the people waiting for Dr. Arroway... not the other way around.
    Last edited by EvilEye; 2007-Jun-01 at 08:35 PM. Reason: clarity

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    Huh?

    Quote Originally Posted by luckynate View Post
    As you move closer and closer to the speed of light, the distance gets shorter and shorter, even for the same trip. So going from here to the moon would be 400,000 km going at the speeds you and i are used to, but at 98% of light speed the trip would be much shorter in distance, maybe only a few hundred km, and thats why time slows when you go fast. If you could reach light speed all distances shrink to Zero size, no matter how far. That why you get there the same time you left(frozen time).
    You sure? Math backup?

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    I think luckynate got confused with "perception".

    On the ship you would percieve time as "normal" and that would make the distance seem very short.

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    Red face now thats funny!!

    Quote Originally Posted by spent View Post
    I'm not buying that. Even if it was Einstein. Guy was smart but........
    .

    gotta love that

    i like the thought that as light arrives at the same time it leaves
    this must mean light recognises no distance
    but i find it intresting that altho here and there are the same place if you are light i can tell if its here or there .......... *cough cough*

    inertia is a ***** as well

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    Red face now thats funny!!

    oops
    Last edited by damian1727; 2007-Jun-03 at 10:56 AM. Reason: computers are stupid

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    Quote Originally Posted by John Mendenhall View Post
    You sure? Math backup?

    yup as wierd as it seems..... known as


    Lorentz transformation (?lör?ens ?tranz·f?r?m?·sh?n)
    (mathematics) Any linear transformation of euclidean four space which preserves the quadratic form q(x,y,z,t) = t2-x2-y2-z2.
    (relativity) Any of the family of mathematical transformations used in the special theory of relativity to relate the space and time variables of different Lorentz frames.

    http://www-groups.dcs.st-and.ac.uk/~...s/Lorentz.html

    Lorentz is also famed for his work on the FitzGerald-Lorentz contraction, which is a contraction in the length of an object at relativistic speeds. Lorentz transformations, which he introduced in 1904, form the basis of Einstein's special theory of relativity. They describe the increase of mass, the shortening of length, and the time dilation of a body moving at speeds close to the velocity of light.

    ?

    as i understand it the spaceship gets shorter !!:surprised

    jets get a bit (tiny bit) shorter when going fast..

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    Quote Originally Posted by damian1727 View Post
    ?

    as i understand it the spaceship gets shorter !!:surprised

    jets get a bit (tiny bit) shorter when going fast..
    It would be more correct to say that distances appear to contract relative to the observer, while moving at velocities close to the speed of light.

    .

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    Quote Originally Posted by damian1727 View Post
    as i understand it the spaceship gets shorter !!:surprised

    jets get a bit (tiny bit) shorter when going fast..
    Jamini is right. Nothing gets shorter or heavier beacause it moves at speeds approaching the speed of light. It just appears to and that's weird enough...

    To the external observer, the spaceship might appear contracted whereas the astronaut on board would realize that everything is normal.

    To the astronaut the distance to the moon might appear to be very short while the external observer will realize that the moon is where it always used to be!

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    everything is under control!!!

    err if u say so

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    The appearance of a contraction is a artifact of the simplifying done to explain it. Most of the time, the explanation is only presented in one dimension. One dimensionally, the object would appear, to an observer, to contract. Realistically, if you were able to view an actual three demensional object traveling relativistically, it would appear to be rotated, not contracted. This is due to the combination of the speed and the different light paths from the front and rear of the object to the observer. "Spacetime Physics" by Wheeler and Taylor, has an excellent explanation of the effect.

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