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# Hedos

Member Since 03 Oct 2002
Offline Last Active Feb 21 2012 11:00 PM

### In Topic: I have a problem with theory of relativity

02 November 2011 - 01:14 PM

Thank you for your answers. It is clearer to me now, even though I still don't understand it.

I have this one more related problem that concerns length contraction.

So, a ruler of length 100 cm moves at a relativistic speed and passes near another, stationary, 100 cm ruler.
My question is: Will the observers on both rulers see that both rulers have identical length? That is to say, will there be no length contraction?

In the frame of reference of ruler A, the ruler B will be shorter. In the frame of reference of ruler B, then ruler A will be shorter.

You might be interested in the ladder paradox for a conceptual application of this phenomenon.

I mean, is the length contraction just an illusion caused by light lag?

That's a very good question indeed. I am really not sure, but I would take the point of view that it is "real" (whatever that means), rather than just an illusion, because length contraction is analogous to time dilation which is clearly very "real". Moreover, the ladder paradox above suggests that length contraction IS real and not a mere illusion.

### In Topic: I have a problem with theory of relativity

02 November 2011 - 12:12 PM

This explains why the clock on the destination planet reads 20 years from the ship-based observer, even though he knows the clock must have only been ticking for 15 years since the ship left Earth. It's because from the ship's reference frame, the two clocks were not started at the same time and the clock on the destination planet has been started 5 years before the ship's clock was started. This part can be calculated using a Lorentz transformation to compare the space-time coordinates of an event in different inertial reference frames.

Confusing stuff... But you are saying that the guy left on the planet will read 15 years on the same clock?
What happens when you land your spaceship?
Will the decelleration make the clocks "synchronize", otherwise guy A on planet and B on the ship will have a big problem deciding on a time to meet eachother...
If you never land, but have some kind of way to communicate, would guy A and B start to argue about what time the clock is showing?
Or would communication be impossible?

The observer on the planet will read 15 years on the ship's clock, when the ship arrives, yes. This is the same time that the ship-based observer read on the ship's clock. The ship-based observer reads 20 years on the planet's clock and the planet-based observer reads 20 years on the planet's clock. The two observers are in complete agreement.

Whether you land the spaceship or not doesn't matter! Nothing would change at all. There is no synchronization involved and there is no contradiction either.

Time has indeed been shorter for the observer travelling on a spaceship, there is asymmetry in this situation (because the spaceship undergoes changes of velocity but the planet does not).

Keep in mind that when the spaceship has just been launched from Earth, then the reference frame of the Earth and the reference frame of the spaceship disagree about when the clock on the destination planet was started relative to when the ship-based clock was started. But that's because the ship-based clock and the destination planet's clock are physically separated. When the ship arrives at its destination, then because the two clocks are at the same location, it doesn't matter at all which frame of reference you choose, they will all be in agreement about the two clocks.

### In Topic: I have a problem with theory of relativity

01 November 2011 - 04:37 PM

At the time of launch, two stopwatch clocks are started simultaneously -- one on the spaceship and one on the distant planet.

How on earth is this synchronisation achieved (no pun intended)?

If you know the distance between the planets, you can easily achieve that (in theory). Suppose the distance is 1 light-year. Send a beam of light from planet A to planet B and wait 1 year. After exactly 1 year, immediately start the clock on the spaceship and launch the spaceship (assuming the spaceship can be given a high acceleration burst such that the desired velocity is almost instantaneously achieved). On planet B, start the clock exactly when the light beam is received from planet A.

This only synchronizes the watches in the reference frame of the planets, however.

### In Topic: I have a problem with theory of relativity

01 November 2011 - 04:35 PM

Okay, I think I understand your confusion. For simplicity, let's assume that the ship is going at speed 0.661c relative to the two planets, hence the time and space dilation factor is gamma = 3/4.

From the reference frame of the two planets, the ship is moving, hence the ship's clock is ticking slower. With a speed of 0.661c the clock on the ship will show approximately 15 years when it arrives at the destination planet due to time dilation. As others have pointed out, an observer on the planet will read the ship's clock as indicating 15 years too and an observer on the ship will also read the clock on the ship as indicating 15 years.

Now, from the reference frame of the ship, however, it is the destination planet that is moving. Hence, the clock on the planet is ticking slower. So at a speed of 0.661c, it is correct to say that the clock on the planet will only have ticked for 15 years throughout the journey and in the reference frame of the spaceship.

I think your mistake is to assume that the clock on the planet will read 15 years from the observer on the spaceship. That is wrong, the clock will read 20 years from the observer on the spaceship, as others have pointed out.

The problem lies with the notion of simultaneity of events. In relativity, there is no such thing as absolute simultaneity of events. So you cannot say without ambiguity that the two clocks were started at the same time. You need to specify in which frame of reference the clocks are being synchronized. So if the clocks were started at the same time in the reference frame of the planets, then in the reference frame of a spaceship that has just been launched from Earth, the two clocks were not started at the same time, in fact the clock on the destination planet was started earlier.

This explains why the clock on the destination planet reads 20 years from the ship-based observer, even though he knows the clock must have only been ticking for 15 years since the ship left Earth. It's because from the ship's reference frame, the two clocks were not started at the same time and the clock on the destination planet has been started 5 years before the ship's clock was started. This part can be calculated using a Lorentz transformation to compare the space-time coordinates of an event in different inertial reference frames.

### In Topic: Game programming vs Girls

31 October 2011 - 06:59 AM

Damn, Thanks guys , I know problem is by my side but i wanted to know what you guys did when you were at this problem. Anyways, I just talked with her, Ironically we're just friends, She doesn't know that i have a crush on her, Well soon i'd be telling that to her. ( I hope so )

P.S My hormones are raging.

Just a tip but why don't you ask her out instead? It's a lot less awkward and it achieves essentially the same goal. Moreover, you can make the date as casual as you like which puts a lot less pressure on both of you than telling her about your feelings before you even went on a first date! And think about it from her perspective, if you confess your "love" out of the blues, what is she supposed to respond?

Anyway, my advice is to try and go out with this girl as soon as possible so that you can gauge if there is any mutual attraction. So, just ask her out! It's really not worth your time obsessing about a girl if it's not meant to be anyway.

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