Special Relativity - Time Dialation

http://www.youtube.com/watch?v=JQnHTKZBTI4

I have no credibility in the content of this video, but from this I gather there's more to concider on what it would look like that speed...

nikokapo said:

The cerenkov radiation article was very cool! didnt know anything could go faster than the speed of light :O

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Yeah light can actually break its own speed limit along with other things its been done many times, resulting in strange effects like bursts of light leaving a gas chamber before it had finished entering.

Diachi

Abray said:

OK, I don't know how easy this is going to be to understand, but here goes...

If light appears to always be traveling at 299 792 458 mps no matter how fast you, the observer, are going, then consider this instance. If one is traveling at 200 000 000 mps towards an object, and then a beam of light shoots by them towards the same object at 299 792 458 mps, the light would appear to be moving away from that person towards said object at 499 792 458 mps, correct?

An observer standing to the side of this "race" sees the light slowly pass the person. Would this also mean that to the person flying towards the object, the object would become illuminated before the observer located to the side of the race sees that object becomes illuminated? If so, do the laws of physics allow this?

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no matter what speed you go light will always travel away from/towards you at 299,792,458 m/s so say you projected a laser beam out of your spacecraft window and you were also travelling at 299,792,458 m/s, it would look exactly the same.

thats what I was told anyway

Diachi

Diachi said:

no matter what speed you go light will always travel away from/towards you at 299,792,458 m/s so say you projected a laser beam out of your spacecraft window and you were also travelling at 299,792,458 m/s, it would look exactly the same.

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Y'all seem to be confusing "speed" and "wavelength." How fast it gets here is NOT the same as what it looks like when it get here.

Peace,
dave

what I was meaning was that there would be no distortion, no bend or change in speed so the colour and the direction/straightness would be the same. but if it went faster than the speed of light it would most probably look completely different.

Diachi

no, the point of the question was the fact that the light would hit the object at two different times!

For the person traveling towards the object, the light would hit the object before the observer to the side would see that the light hit the object.

Actually, despite when the people SEE the light hit the object, the light has to be in two different places at the same time!

edit: Any comments on this? Thinking back to some of the other videos posted here, light makes a cone in the space time graph. It IS in a definite place at a specific time, it's just the axis on which the observer is on that makes the light appear to hit in different places relative to the other observer. Is that the explanation?

The essential flaw in this reasoning is that you think that time is a fixed feature in the universe. There is no such thing as "a specific time". A "specific time" would have to be measured by someone and their measurements would not agree with any other measurements made by anybody who was travelling at a different relative speed. If time is not fixed then speed is not fixed either, speed being the measure of distance travelled over time taken. To put it simply, in your OWN inertial frame of reference you can never move at or close to the speed of light. To you, light will always appear to travel at 3.0 x 10^8 m/s. Towards you, away from you, it doesn't matter. What will change is your perception of time compared to somebody who is moving at a different speed, relative to yourself.

The important thing to think about is that when you say this observer is moving close to the speed of light, you must specify what this speed is RELATIVE to. Relative to the Earth, the sun? Relative to Observer B? Speeds are always relative to something, and two observers moving relative to one another will measure the speeds of non-light objects to be different because they perceive time differently. Light, however, behaves in a different manner from this. Two observers in different frames of reference will always measure the same speed of light relative to themselves but if they meet later they will either disagree about how long the event took or the distance involved.

Example:

The observer A moving at any constant speed (his inertial frame of reference) will see the light pass him at the speed of light relative to his own speed, whatever that may be, and it will take the appropriate amount of time (to Mr. A) for the light to strike the object. Let's say Mr. A measures this amount of time to be 1 second, using his watch.

Observer B, standing to the side and moving at a different, but constant, speed (his inertial frame of reference) watches as Mr. A passes by him and measures that A is moving at close to the speed of light, compared to himself. He also sees the beam of light pass by him and measures that the beam of light is moving (of course) at the speed of light. He then checks his watch and sees that the light took 30 seconds to strike the object.

So, observer A and observer B now have the same measurements for the speed of light, but they disagree about how long the event took to happen. Who's right? Both of them are. The difference is that their measurements of time and distance will not agree, even though their measurements of the speed of the light beam are the same. Since speed = distance/time then simple math can show that you can have equal measurements for speed when your time is different ONLY if the distance travelled is also different. What this means is that while the observers agree with how fast the light was moving compared to themselves, they would both measure a different DISTANCE that the light travelled. This is length contraction.

On the other hand, if both observers would agree that the distance was the same (as is likely), then you would get the same result from the equasion only by showing that the TIME they measured was different, as in the example above with speedy Mr. A and pokey Mr. B. This is time dilation.

The practical upshot of all this is that ALL measurements of time and distance are PERSONAL measurements only. 1 second to me and 1 second to you are only the same if we both occupy the same inertial reference frame (i.e. if we're both travelling in the same direction and at the same speed). As noted in another thread, even a clock at the bottom of a mountain will show a different progression of time than a clock at the top of the mountain. Time passes differently at the top compared to the bottom and observers making measurements of speed, distance and duration using these clocks would find that none of their measurements would exactly line up EXCEPT for their measurement of the speed of light.

I hope that this makes some things clear as to why none of these theoretical observers would ever measure the speed of light to be anything but "c", compared to themselves. Time and space will bend to make it so.

*Edited for clarity*

I just remembered a great visualization for why different frames of reference result in different measurements of distance, time and speed.

Imagine that you are sitting on a train, which is moving at a constant speed. Let's say it's 10 m/s for ease of calculation. On this train, you bounce a ball. It falls 1 meter and then bounces and returns to your hand. (Yes, this is a magic ball with 100% elastic efficiency.) This event takes one second. So, to you, the ball has travelled 2 meters in one second and therefore has an overall speed of 2 m/s. Easy so far, yes?

As you do this, the train goes past a station where observer B (a busy fellow, apparently) is watching you bounce this ball. I suppose that the train is made of glass or observer B has a nifty pair of x-ray specs. Now, you (on the train) saw the ball move straight up and down and travel 2 meters and everyone else on the train would agree with you. Mr. B, however, sees the ball take a much different path. He sees the ball move up and down 1 meter, but because the train is moving at 10 m/s relative to him and the bounce takes 1 second, he also sees it move 10 meters to the side. Do you see where we're going with this?

Mr. B would say that the ball moved a total of (...hold on, finding trig calculator...) 10.19 meters in one second and therefore has a speed of 10.19 m/s. But on the train you measured that the ball moved 2 meters in one second and has a speed of 2 m/s. Who's right? In fact, you both are correct IN YOUR OWN FRAME OF REFERENCE and you're both wrong in the other guy's frame of reference. On Earth, we generally use the surface of the planet as a reference point for speed, but of course the Earth is hurtling around the sun at an enormous speed, the sun around the galactic center, and so on. In this way, you can see that speed and distance are relative measurements, NOT ABSOLUTES.

If you do the same thought-experiment with a beam of light bouncing in the train car you will come to the conclusion that, in addition to distance and speed, time is also a relative quantity and is NOT absolute.

LOL, my physics class just went over time dilation a few days ago. ;D