# Earth Velocity has me in a spin?

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#### Fax_Monkey

##### Guest
OK I've lost the plot somewhere. Can somebody put me out of my misery with the following chain of ( probably faulty) assumptions.

1 - The Earth ambles round the Sun at 60,000 mph

2 - The Solar system belts round the galaxy at some scary speed

3- The Galaxy is moving through space away from universal centre at even more speed

4 - the speed of light ( C ) is a constant throughout the universe

OK my confusion is this - the earth as part of our galaxy must be moving at quite some speed from universe centre.
That velocity would have a value to a stationary observer watching our galaxy wizz by in space.
Enough velocity to suffer time dilation in comparison to the observer.
So on Earth we are always under a certain amount of constant time dilation ( as part of our galaxy).

So how can we say that we know the true speed of light - when every mesurement of distance over time would only be correct 'locally' The time function of the equation would surely be different to the stationary observer?

sorry for my stupidity in advance - fax -monkey

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#### ramparts

##### Guest
All good questions Let's look at your "( probably faulty)" assumptions. As it turns out, you only missed one!

Fax_Monkey":35iyzhhr said:
1 - The Earth ambles round the Sun at 60,000 mph

I don't know the number, but sure, something like that.

2 - The Solar system belts round the galaxy at some scary speed

Indeed!

3- The Galaxy is moving through space away from universal centre at even more speed

Not quite. Yes, the universe is expanding, and our galaxy has some speed, but it's not moving away from a "universal center." The common analogy (so common, in fact, I just used it in another thread!) is of blowing up a balloon. Imagine a 2-D universe on the surface of a balloon, with little galaxies painted on. If you blow the balloon up, this 2-D universe expands - every point starts to move away from every other point. Of course, there is no center to the surface of the balloon So neither is there a center to the universe (at least, not that we know of!).

This is important because it means that there's no stationary point (like a center) with which we can reference the Earth's velocity. And that brings us to....

4 - the speed of light ( C ) is a constant throughout the universe

Oh, right. That's also true. But the above point brings us to...

OK my confusion is this - the earth as part of our galaxy must be moving at quite some speed from universe centre.
That velocity would have a value to a stationary observer watching our galaxy wizz by in space.
Enough velocity to suffer time dilation in comparison to the observer.
So on Earth we are always under a certain amount of constant time dilation ( as part of our galaxy).

So how can we say that we know the true speed of light - when every mesurement of distance over time would only be correct 'locally' The time function of the equation would surely be different to the stationary observer?

We can't have a "stationary observer" So yes, the Earth is moving, and as it feels acceleration from various gravitational pulls, there is indeed some associated time dilation - but it's miniscule, really, a part in billions or more. But here's the important thing - that only happens because of our accelerating motion, like that around the Sun. Just going at a fast velocity doesn't mean anything because, as Einstein famously pointed out (well, more like Galileo, but who's keeping track), who's to say that you're not at rest and the "rest" observer isn't travelling quickly?

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#### chebby

##### Guest
Wait what about that example of getting into a near light speed spaceship example and coming back from a star trip thousands of years after all your relatives are dead? Can't you also view in that example that it's the spaceships that is at rest and it's earth that is moving away or accelerating away? So what determines which will have slow time and which regular time? i'm confused

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#### origin

##### Guest
OK my confusion is this - the earth as part of our galaxy must be moving at quite some speed from universe centre.
That velocity would have a value to a stationary observer watching our galaxy wizz by in space.
Enough velocity to suffer time dilation in comparison to the observer.
So on Earth we are always under a certain amount of constant time dilation ( as part of our galaxy).

So how can we say that we know the true speed of light - when every mesurement of distance over time would only be correct 'locally' The time function of the equation would surely be different to the stationary observer?

sorry for my stupidity in advance - fax -monkey

First - there is no center of the universe.

If there was an observer that was not moving with our galaxy watching us whizz by he would infact note a time dialation affect. But WE would also look at him and say he is experiencing a time dialation affect - relativity you know, that is he would seem to have a velocity relative to us - there is NO stationary position so there is only relative motion.

The reason that we know the speed of light is because it ALWAYS measure the same speed. If we are whizzing towards the sun and we measure the speed of the light coming to us it will measure at c. If we where whizzing away from the sun and we measured the speed of light coming from the sun it would measure at c. The speed of light always is the same independent of the motion of the source or the observer.

edited to add, it is not a stupid question at all, it is a good, fundemental question.

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#### MeteorWayne

##### Guest
Fax_Monkey":1pkphuvr said:
OK I've lost the plot somewhere. Can somebody put me out of my misery with the following chain of ( probably faulty) assumptions.

1 - The Earth ambles round the Sun at 60,000 mph

2 - The Solar system belts round the galaxy at some scary speed

Scary Speed= ~ 492,000 mph

[/quote]

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#### SpeedFreek

##### Guest
chebby":i47w8luq said:
Wait what about that example of getting into a near light speed spaceship example and coming back from a star trip thousands of years after all your relatives are dead? Can't you also view in that example that it's the spaceships that is at rest and it's earth that is moving away or accelerating away? So what determines which will have slow time and which regular time? i'm confused

Whilst the people on Earth remain in the same reference frame throughout, your spaceship has to constantly change reference frames as it accelerates from being at rest in relation to the Earth to a relativistic speed, when it decelerates or turns around.

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#### ramparts

##### Guest
chebby":6fmhkuf8 said:
Wait what about that example of getting into a near light speed spaceship example and coming back from a star trip thousands of years after all your relatives are dead? Can't you also view in that example that it's the spaceships that is at rest and it's earth that is moving away or accelerating away? So what determines which will have slow time and which regular time? i'm confused

Indeed - but as SpeedFreak says, when someone begins to accelerate, the equivalence of frames is lost. So if I'm moving at a velocity relative to you, I can very well say I'm at rest and you're moving. But if I start accelerating relative to you, well, then I can't say that anymore! Clearly I'm accelerating. So in the twin paradox you mentioned, the key is that when the spaceship leaves and comes back, it has to accelerate/decelerate. That's why the twin in the spaceship ages objectively less.

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#### chebby

##### Guest
Ah ok, that makes sense. I think my confusion stems from the fact that acceleration is usually defined as change of velocity over time, which is clearly the same for planet and spaceship here. But what sets spaceship apart is that is experiencing force (that accelerates it) while the planet does not.

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#### ramparts

##### Guest
chebby":3etgqjdd said:
Ah ok, that makes sense. I think my confusion stems from the fact that acceleration is usually defined as change of velocity over time, which is clearly the same for planet and spaceship here. But what sets spaceship apart is that is experiencing force (that accelerates it) while the planet does not.

Acceleration is always defined that way! In any kind of relativity, two observers at constant velocity can't tell who's at rest and who's moving. If your velocity is changing, though, then someone gets picked out.

Einstein's "equivalence principle" has a lot to do with this, actually. On one important level, it points out that an observer can't tell the difference between accelerating and being in the presence of a gravitational field. This is why you get things like gravitational time dilation - since we're on the surface of the Earth, time is actually a little bit slower here due to Earth's gravity than it is out in space!

An interesting tidbit copied from Wikipedia (since I'm too lazy to paraphrase!):

"Gravitational time dilation has been experimentally measured using atomic clocks on airplanes. The clocks that traveled aboard the airplanes upon return were slightly fast with respect to clocks on the ground. The effect is significant enough that the Global Positioning System needs to correct for its effect on clocks aboard artificial satellites, providing a further experimental confirmation of the effect."

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#### chebby

##### Guest
Yes, I read about "equivalence principle" in brief history of time (can't say I understood a lot of it.) This actually made me think, when scientists calculate how much fuel stars have, do they take into account that fusion reactions actually happen slowly there?

Ramparts, what does your avatar formula mean?

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#### Fax_Monkey

##### Guest
Thanks for clarifying it for me guys that's a great help - i guess i always had
this idea of the big bang being like a bomb blasting matter in all directions in
an already existing void - not realising that it was actually expanding void and matter together
I can see my understanding of time is incorrect as well - it's a lot more complicated than i thought!
In fact the whole universe seems more organic than clockwork to me now.
I'll get a few new books and try to get a better understanding of current thought.

Thanks again for the help, much appreciated

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#### ramparts

##### Guest
chebby":w8icli8k said:
Yes, I read about "equivalence principle" in brief history of time (can't say I understood a lot of it.) This actually made me think, when scientists calculate how much fuel stars have, do they take into account that fusion reactions actually happen slowly there?

Oh, they take into account most everything As I learned the somewhat-hard way this past semester... If you want a more in-depth answer, I'll take a stab at it, but I'm not entirely sure what you're asking (or how it's related to the the equivalence principle!).

chebby":w8icli8k said:
Ramparts, what does your avatar formula mean?

My avatar is the Einstein field equation; I put it up because it's probably the single most beautiful mathematical statement in all of physics. Unfortunately it's in a slightly condensed form, to be short enough to fit on the site, but it works. It's the main equation of general relativity. The left side - the G - is called the Einstein tensor, a mathematical quantity that measures the curvature of spacetime. The right side - the 8 pi T - is the energy-momentum tensor, which measures the energy and matter content at a given point, including pressure, density, etc. So it tells you how a certain distribution of mass curves spacetime, in turn creating what we see as gravity

It takes the place, essentially, of Newton's law of gravitation, F=GmM/r^2, which relates the gravitational force to mass and distance. The second important equation, called the geodesic equation, takes the place of F=ma. The first tells you what gravity is created, the second tells you how things move in response to the gravity.

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#### MetalMario

##### Guest
Wow, the Earth moves fast. But why is it, that if I jump up, I don't fly off the face of the Earth! The sheer speed should have some affect on my body.

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#### MeteorWayne

##### Guest
Because the strongest force you feel is the earth's gravity. All the atmosphere moves with the earth, so we all move together around the sun. If you jump in the direction of the moon, you'll jump an immeasurably small amount higher

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#### ramparts

##### Guest
Exactly - and don't forget that though the Earth is moving fast, you're on its surface... so you're moving fast, too! Don't believe me? Try jumping off the floor of a fast train some time. See if you get flung back since the train is moving so fast. People may look at you funny, but you won't move back an inch

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