Questions about Special Relativity

[Naeman]

Physics is a lot more interesting than I ever imagined it to be, at least the special relativity aspects of it. I have a couple of questions. I mostly understand the theory, but a few questions which I can't find an answer to (at least not translated into non-expert-physicist terms) have been bothering me. (I know, they're the really amateur questions that people who don't understand it well ask a lot but....) First of all, if there's a spaceship moving at .9c, then relativity tells us that time moves slower because its moving, I.E. the twin paradox. However, couldn't the spaceship consider itself at rest and everything else moving slower through time? What is it that determines that the spaceship is specially the moving object? Also, the question of C being the same to all observers bothers me. How can this be? I understand that C is the same to all observers, just not how. You've probably all heard the "what if I'm travelling in a rocket at 1/2c and I shine a light, wouldn't it be going 1 1/2c?" question, so thats basically what I'm wondering. Again, I know galilean velocity additions don't apply, but it doesn't seem to register why it wouldn't go 1 1/2c. Is it because it would need infinite acceleration to surpass the speed of light? Thanks for listening.

[Kygron]

As a "non-expert-physicist" I'll consider myself qualified to answer

 

What you've missed is right in front of you. The name of the theory, "relativity".

 

You ask how you determine if the spaceship is moving, well, you only do that "relative" to something else. If you watch a spaceship moving past your planet, than the clock you can see through one of the windows will be "observed" to tick slower. As long as you use the words "relative" and "observed" than you can reverse it just fine. An "observed" clock on the surface of the planet passing by your spaceship will also tick slower.

 

But if there's no planet to measure your speed relative to, then there's no speed to worry about anyway!

 

If you shine a light from that rocket, it will move ahead at 1.0c, "relative" to you. Someone on the planet watching you will "observe" you moving at 0.5c, will "observe" your light moving at 1.0c. They'll also "observe" your ship shortened and your clocks run slow, so they'll excuse you if your math is messed up when you think the light is moving too fast.

 

There are formulas for adding speeds of different observers together, so that 0.6c + 0.6c < 1.0c, but keep in mind that these really are speeds of DIFFERENT observers and so they won't add up in the traditional sense.

[timo]

First of all, if there's a spaceship moving at .9c, then relativity tells us that time moves slower because its moving, I.E. the twin paradox. However, couldn't the spaceship consider itself at rest and everything else moving slower through time?

Actually it does.

But a sidenote here: The statement that clocks of moving observers move slower is a bit incomplete. Actually, observers moving with a relative speed to each other have a different understanding of what space and what time is. In the twins example the part of spacetime that the rocket considers time -it´s movement direction- is seen as a mixture of space and time by the twin that stays behind.

 

What is it that determines that the spaceship is specially the moving object?

That depends on the form of Special Relativity you´ve seen. The most common -and in your problem pobably sufficient- would be: The problem is not symmetric. The planet´s (or whatever is left behind) rest frame is an inertial frame for the whole process. The spaceship´s rest frame isn´t an inertial frame because it has to acceelerate to change it´s direction.

 

 

Also, the question of C being the same to all observers bothers me. How can this be? I understand that C is the same to all observers, just not how.

I doubt that anyone knows the "why". Afaik it´s just an experimental fact that our modern theories were built upon.

[Naeman]

Oh, I see now. I once saw an extremely complex version of the twin paradox explaining why my problem didn't happen before and after he accelerated, eliminating the problem that it wasn't constant. I didn't understand it, but it turned out that that it all balanced out in the end with the spaceship aging slower even though it started out faster or something....I don't know. This really answered my questions though, thanks.

[Naeman]

Oh, I just remembered something from a long time ago when I first learned and actually understood relativity. I believe the theory was that the fastest way to get somewhere (least aging) was to accelerate to the halfway point then decelerate at the same rate. Does this mean that time dilation doesn't apply to uniform motion? If I missed that, that would explain why I didn't get it for so long. Also, the physical length of a trip seems smaller to a ship that is a victim of dilation if measured, right? My for real, actually-last question about special relativity (or the basis of it anyhow) is about length contraction. Again, I understand that it happens, just not exactly how.

Again, thanks for listening, and thanks for the replies

[swansont]

Oh' date=' I just remembered something from a long time ago when I first learned and actually understood relativity. I believe the theory was that the fastest way to get somewhere (least aging) was to accelerate to the halfway point then decelerate at the same rate. Does this mean that time dilation doesn't apply to uniform motion? If I missed that, that would explain why I didn't get it for so long. Also, the physical length of a trip seems smaller to a ship that is a victim of dilation if measured, right? My for real, actually-last question about special relativity (or the basis of it anyhow) is about length contraction. Again, I understand that it happens, just not exactly how.

Again, thanks for listening, and thanks for the replies[/quote']

 

Time dilation applies to uniform motion - it's dependent on v. The dilation will be greater as you go faster. The scenario you describe may have other restrictions on it.

 

It all arises because c is a constant, and as a result, velocity addition must be nonlinear. This has ramifications on time and distance measurements.

[Naeman]

Okay, I think I understand most of what I asked. The only thing I don't get is why is a ship in uniform motion still going slower at that moment than the chose inertial frame? If it considers itself at rest since it is in uniform motion, why does the time dilation effect the ship and not the "planets" ? I think I'm just missing something, I guess what I'm wondering is what makes dilation apply to a ship thats not accelerating instead of the world outside of it. Is it because the ship is the one object that is moving in a different inertial frame than everything else?

[swansont]

An observer in an inertial frame will never see his own system affected by relativity. It's always the "other guy." The loophole is that the only way to compare, side-by-side, is for one person to undergo an acceleration, and that changes their coordinate system.

[J.C.MacSwell]

An observer in an inertial frame will never see his own system affected by relativity. It's always the "other guy." The loophole is that the only way to compare, side-by-side, is for one person to undergo an acceleration, and that changes their coordinate system.

 

It's kind of like "home ice advantage". You come to my rink and I'm the cagey veteran and you're the young punk. If I come to yours then You're older and I'm younger.

 

Apologies to relativists and hockey fans everwhere for a bad analogy,

[Naeman]

So then the reason that time goes slower for the ship's inertial frame is because its outside the "home ice" or the inertial frame for the rest of the universe that isn't moving, right? I think I get it now.

[Naeman]

Does anyone know? I can't find an answer that I can understand anywhere.

[[Tycho?]]

So then the reason that time goes slower for the ship's inertial frame is because its outside the "home ice" or the inertial frame for the rest of the universe that isn't moving, right? I think I get it now.

 

No, there is no universal frame of refference. This is a key part of relativity, as everything is relative to the observer.

[swansont]

Does anyone know? I can't find an answer that I can understand anywhere.

 

If A and B are in inertial frames and moving relative to one another, each will measure the other's clock as moving slow.

[Naeman]

If A and B are in inertial frames and moving relative to one another, each will measure the other's clock as moving slow.

 

See now, this I could understand, except what about this Twin paradox thing? How does time determine which object is in fact the one that receives slower time?

[Guest Dieleman]

'']No, there is no universal frame of refference. This is a key part of relativity, as everything is relative to the observer.

 

Since c is the only constant in the universe, doesn't it mean everything is relative to c?

[Johnny5]

Since c is the only constant in the universe, doesn't it mean everything is relative to c?

 

c is not the only constant in the universe.

[gaara]

hello,

what are other constants

an i have a question about lenghth contraction. whilst goin at relativsitic speeds does the distance to a destination decrease or what? thanks

[swansont]

an i have a question about lenghth contraction. whilst goin at relativsitic speeds does the distance to a destination decrease or what? thanks

 

Yes. That's how two different observers (stationary and moving) can agree that the same thing happened. Length contraction explains for the moving observer what time dilation explains for the stationary observer.

[Ophiolite]

what are other constants

There are around twenty. Here are some of them:

the ratios of the masses of fundamental particles

the fine structure constant

the electromagnetic coupling constant

the strong coupling constant

the gravitational fine structure constant

 

Dicussion here:http://www.answers.com/topic/fine-tuned-universe

[Johnny5]

If A and B are in inertial frames and moving relative to one another, each will measure the other's clock as moving slow.

 

This is impossible.

[Johnny5]

hello' date='

what are other constants

an i have a question about lenghth contraction. whilst goin at relativsitic speeds does the distance to a destination decrease or what? thanks[/quote']

 

Planck's constant for one.

 

If SR is correct, then the answer to your question is yes.

If SR is incorrect, and the Galilean transformations are correct, then the answer to your question is no.

 

Regards

[5614]

This is impossible.

 

Are you disagreeing with SR or is there another reason why you saying this?

[Johnny5]

Are you disagreeing with SR or is there another reason why you saying this?

 

The logic is correct, regardless of whether or not I disagree with SR.

 

Additionally, I disagree with SR.

 

Regards

[Johnny5]

If A and B are in inertial frames and moving relative to one another, each will measure the other's clock as moving slow.

 

This is impossible.

 

Why are you saying this?

 

Let there be two clocks in relative uniform motion, and let them be identical in every way.

 

Let them each be located at the origin of an inertial reference frame. (This means that Newton's laws are true in either frame of reference.

 

Additionally, let them be in deep space, free of all external forces.

 

I claim that if the two clocks are synchronous at one moment in time, they must remain synchronous.

[5614]

If A and B are in inertial frames and moving relative to one another, each will measure the other's clock as moving slow.

A and B are in inertial frames so they are not accelerating, however they are moving relative to one another, so e.g A is moving at 10mph and B at 20mph relative to me... so from A's frame B is moving at 10mph and from B's frame A is moving at -10mph.

 

Because of the speed difference and SR's time dilation A's clock and B's clock will run at different speeds.

 

Now Swansont said "each will measure the other's clock as moving slow" but surely B (moving quicker) clock would be going slower relative to A, consequently A's clock would be moving faster relative to B's clock.