Galaxies

[Wiccan_Child]

I hope all my rambling makes sense

Neither is orbiting the other. They are both moving.

As seen by an outside observer sitting some distance away. But for someone sitting on one of the planets, they only see the other planet moving. This isn't an illusion (e.g., "the sun just looks like it orbits the Earth, but it's really us who orbits it"), but a very real consequence of the lack of absolute motion: without a reference point, velocity makes no sense.

Think about it. What is speed? Speed is the rate of change of distance (or, more accurately, velocity is the time derivative of displacement, or v=dr/dt). But distance from what? Displacement from what?

That's why velocity requires two things: the thing moving, and the thing sitting still doing the measurement. Or rather, the reference frame (what is taken to be stationary).

Huh? Regardless of how we 'see' it, it is the Earth that orbits the Sun.

Nope. That's the whole point: we can 'see' it how we want, because what we see in each frame of reference (i.e., where we sit) is as valid as the next. Our brains have evolved to take the Earth (which is big, heavy, flat, etc) to be stationary, since it doesn't change (as opposed to the myriad of things on it which move about). This instinct follows over when we think about celestial movement, even though there is no real reason to think that the Earth is stationary (or, indeed, mobile).

Velocity is relative. You can't say "This thing has that velocity" without specifying what the measurer is doing.

Consider a car travelling at 30mph. What is that speed measured relative to? The road.
But now imagine that the whole thing is actually a treadmill. What is the speed of the car? Well, relative to the road, it's still 30mph: it's engine is running, its tyres are spinning, etc. It's just that, relative to the ground, the whole thing isn't moving (the forward motion of the car is counteracted by the backward motion of the road).

Consider you're in space, sitting on an elephant, travelling at some constant velocity. There are no stars, no reference points, no nothing. Just you, the elephant, and empty blackness.

Tell me: how would you measure your velocity?

Other than planetary moons everything does orbit the Sun.

True, everything else does. No matter which planet we choose to sit on, the rest obstinantly continue to orbit the Sun.

But the Earth isn't stationary.

From our point of view, it is. We're just used to thinking about it going round the Sun. We take the Sun to be stationary out of convention, instinct, and a little bit of logic (everything else goes round the Sun, after all). But as soon as we imagine the galaxy, suddenly the Sun's whizzing round the core (which has become stationary... what was it doing before?).

And someone said Quantum Mechanics didn't make sense

Give me quantum mechanics any day. I didn't sign on to deal with barn paradoxes!

 

[Chesterton]

You can't ask "How fast am I moving?" without first specifying the motion of the person doing the measurement. There's no such thing as absolute motion (except for light, but that's special...)

So can you use the speed of light as the absolute reference for the speeds of everything else? Or is everything else "standing still" relative to it?

 

[Wiccan_Child]

So can you use the speed of light as the absolute reference for the speeds of everything else? Or is everything else "standing still" relative to it?

Unfortunately, no. The speed of light is constant, regardless of how fast you're going. It's because of this that you get things like length contraction and time dilation (to keep the speed of light constant, space and time warp to accommodate it!).

You can't use it as an absolute reference because it's not a physical 'thing'. It's a velocity, not an object. It's like saying I have an acceleration of cheese .

 

[Allister]

Now, if that's too complex

Ragarth, thanks for the explanation and no, it itsn't too complex. I understand it. It makes sense.

From each other Car's POV the speed appears different, 70mph, 60mph, 10mph etc....

However... regardless of the Car's POV the cars MUST be travelling at some fixed speed. What is the 'real' speed, from an outside objective observer.

You're traveling towards it and your speedometer says 70mph(relative to the earth). The radar thing will, however, say 60mph because relative to it you are traveling 60mph.

True, but I am still actually travelling at 70mph. Only if I reduce my speed would I actually be travelling at 60mph.

I think I know where you're coming from. You assume that the universe is rigidly-shaped

Well, yes.

Imagine we am on board a scientific space ship sitting some few hundred light years (or whatever works) above (below) the Galaxy, using our mega telescopes and censors etc to scan the Galaxy. Everything would be moving and drifting and spinning simultaneously at their 'real' speeds. From this distance , using Miles Per Hours (knowing how long a mile is) we can see how quickly the Earth spins, orbits etc. The same with the Sun and the Galaxy arms and so on.

As seen by an outside observer sitting some distance away. But for someone sitting on one of the planets, they only see the other planet moving.

That is true. But it is an illusion.

Think about it. What is speed? Speed is the rate of change of distance (or, more accurately, velocity is the time derivative of displacement, or v=dr/dt). But distance from what? Displacement from what?

Isn't speed simply the rate by which you move from point A to point B or by the time it takes to travel a set distance, say a KM or a mile.

Consider you're in space, sitting on an elephant, travelling at some constant velocity. There are no stars, no reference points, no nothing. Just you, the elephant, and empty blackness.

Tell me: how would you measure your velocity?

I would drop something into space next to me and see how fast it retreats from me?

From our point of view, it is. We're just used to thinking about it going round the Sun. We take the Sun to be stationary out of convention, instinct, and a little bit of logic (everything else goes round the Sun, after all). But as soon as we imagine the galaxy, suddenly the Sun's whizzing round the core (which has become stationary... what was it doing before?).

Regardless of POV the Earth does spin and orbit the sun. Fact.
The sun moves through space and the solar system orbits with the galaxy.


I'm looking for what is really happening, not what it looks like is happening.

Nope. That's the whole point: we can 'see' it how we want, because what we see in each frame of reference (i.e., where we sit) is as valid as the next.

So the Earth doesn't orbit the Sun

 

[Chesterton]

Unfortunately, no. The speed of light is constant, regardless of how fast you're going. It's because of this that you get things like length contraction and time dilation (to keep the speed of light constant, space and time warp to accommodate it!).

You can't use it as an absolute reference because it's not a physical 'thing'. It's a velocity, not an object. It's like saying I have an acceleration of cheese .

But isn't having one thing in the universe (the speed of light) be a constant, isn't the upshot the same as if we had one thing which was motionless? It could serve as a universal reference?

light is travelling 670,616,700 mph (round number)

car is travelling 60 mph

so, car is travelling -670,616,640 mph (relative to a constant - the only constant)
​

Something ragarth said: "This is fluffed, speed is a scalar, so has no actual direction, by putting that negative sign in, it becomes a vector and so is technically now a velocity." Couldn't we say the car does have an absolute speed (or velocity?) of -670,616,640 mph? Or am I simply wrong in equating "constant" with "absolute"?

(My blatant attempt to derail this into Ethics. )

 

[Chesterton]

I'm looking for what is really happening, not what it looks like is happening.

Yep, aren't we all.

 

[Wiccan_Child]

However... regardless of the Car's POV the cars MUST be travelling at some fixed speed. What is the 'real' speed, from an outside objective observer.

There is no 'real' speed. There is only what a particular observer sees.

True, but I am still actually travelling at 70mph. Only if I reduce my speed would I actually be travelling at 60mph.

Nope. You're only travelling at 70mph relative to the road, which you hold stationary by convention.

Imagine we am on board a scientific space ship sitting some few hundred light years (or whatever works) above (below) the Galaxy, using our mega telescopes and censors etc to scan the Galaxy. Everything would be moving and drifting and spinning simultaneously at their 'real' speeds. From this distance , using Miles Per Hours (knowing how long a mile is) we can see how quickly the Earth spins, orbits etc. The same with the Sun and the Galaxy arms and so on.

Yep. But in that case, our observer is moving with the galaxy. Or, in other words, we're arbitrarily holding the galaxy as stationary, and calculating all the speeds based on that.

But you're getting the idea: we have to explain where we, the measurement-takers, are sitting. In this case, we have some spaceship looking down on the galaxy.

By jove, I think he's got it!

That is true. But it is an illusion.

... or not.
No, the whole point is that it's not an illusion. There's no 'real' or 'absolute' speed.

Isn't speed simply the rate by which you move from point A to point B or by the time it takes to travel a set distance, say a KM or a mile.

That's kinda the same thing. Ultimately, it requires two points. These could be two arbitrary points in space (A and B), or two more 'logical' points (your position and the position of the centre of your axes (aka, the Origin, [0,0,0])).

I would drop something into space next to me and see how fast it retreats from me?

Why would it retreat? You're in space, remember: no friction means that it wouldn't slow down (if it's moving at all). Regardless of how 'fast' a spaceship is orbiting the Earth, if they drop a pen, it will just float next to them. If they kick off from one end of the ship and let go of a pen in mid-flight, the pen will float along with the same 'speed' as them.

(Ignoring air resistance, of course! )

Regardless of POV the Earth does spin and orbit the sun. Fact.

Fiction.

The sun moves through space and the solar system orbits with the galaxy.

Only if you take the the galactic core as stationary. Why not take the Sun as stationary, and see the core orbit the Sun?

I'm looking for what is really happening, not what it looks like is happening.

The point is that there is no 'really', there is no 'absolute'. The whole point is that motion is relative: your velocity is relative to the observer.

So the Earth doesn't orbit the Sun

Nope. It's a common misconception that geocentricism was ever proven wrong. What was disproven was the idea that everything orbits the Earth. Even in a geocentric universe, the planets still orbit the Sun.

 

[Wiccan_Child]

But isn't having one thing in the universe (the speed of light) be a constant, isn't the upshot the same as if we had one thing which was motionless? It could serve as a universal reference?

light is travelling 670,616,700 mph (round number)

car is travelling 60 mph

so, car is travelling -670,616,640 mph (relative to a constant - the only constant)
​

Well, you could redefine your axes so that a velocity of zero is the speed of light, but it wouldn't really accomplish much. Since velocity is the rate of change of displacement (i.e., how quickly your distance from some point is changing), it makes sense to define 'zero' to be when that rate is zero.

That is, we define velocity in such a way that v = 0 is when your distance from some predefined point is a constant.

Something ragarth said: "This is fluffed, speed is a scalar, so has no actual direction, by putting that negative sign in, it becomes a vector and so is technically now a velocity." Couldn't we say the car does have an absolute speed (or velocity?) of -670,616,640 mph? Or am I simply wrong in equating "constant" with "absolute"?

You are indeed wrong . Speed does not take into account direction. A car can travel in a circle around some central point, and its distance from that point is a constant, yet it undeniably has some velocity (as viewed from the centre).

'Speed' is the absolute value of velocity, the length of the associated vector. 'Velocity' takes into account the direction in which you're going.

It's a distinction that only becomes important when doing funky things to mechanistic reactions.

 

[Allister]

There is no 'real' speed. There is only what a particular observer sees.

Isn't that the same argument as "if someone leaves the room and closes the door they no longer exist". Before there was life on Earth to observe the Universe it still moved and had speed.

Nope. You're only travelling at 70mph relative to the road, which you hold stationary by convention.

No. I am travelling at 70mph. If the speed remains constant I will travel 70 miles.

Yep. But in that case, our observer is moving with the galaxy. Or, in other words, we're arbitrarily holding the galaxy as stationary, and calculating all the speeds based on that.

We'll be outside of the Galaxies gravity but moving along so that the centre is stationary, which, if everything else orbits it, is. The speed at which the galaxy as a whole is moving can be worked out later. For now, we're dealing with the contents of the galaxy.

By jove, I think he's got it!

Don't speak too quickly

No, the whole point is that it's not an illusion. There's no 'real' or 'absolute' speed.

It's not an illusion that the sun orbits the Earth. Maybe they should re-write the science books.

If i'm driving at 70mph and you're in front travelling at 80mph, from your POV I am moving at 10mph. Yes?

But in reality I am actually going at 70mph, since I shall, in one hour, cover 70 miles, not 10. So myself travelling at 10mph is an illusion caused by your POV.

Why would it retreat? You're in space, remember: no friction means that it wouldn't slow down (if it's moving at all). Regardless of how 'fast' a spaceship is orbiting the Earth, if they drop a pen, it will just float next to them. If they kick off from one end of the ship and let go of a pen in mid-flight, the pen will float along with the same 'speed' as them.

OK. In a empty, starless, blacked part of space it wouldn't be possible to tell.

But the galaxy isn't any of the above.

Fiction.

How is that fiction?

Does the Earth spin?
Does the Earth orbit the Sun?

Only if you take the the galactic core as stationary. Why not take the Sun as stationary, and see the core orbit the Sun?

Because the Core of the Galaxy doesn't orbit the Sun.

I honestly think you've gone mad

So the Earth doesn't orbit the Sun

Nope.

 

[Naraoia]

There's no such thing as absolute motion (except for light, but that's special...)

Can light really be considered to be moving from its own POV? I mean, movement in my mental dictionary implies time, and AFAIK, time doesn't really exist for a photon

 

[Wiccan_Child]

Isn't that the same argument as "if someone leaves the room and closes the door they no longer exist". Before there was life on Earth to observe the Universe it still moved and had speed.

I think you're confusing the word 'observer'. It doesn't mean someone has to physically be there watching and measuring. It refers to a potential observer, someone who could be there.

No. I am travelling at 70mph. If the speed remains constant I will travel 70 miles.

You will travel 70 miles from your starting point, which has remained stationary.

We'll be outside of the Galaxies gravity but moving along so that the centre is stationary, which, if everything else orbits it, is. The speed at which the galaxy as a whole is moving can be worked out later. For now, we're dealing with the contents of the galaxy.

Exactly. For the time being, we've taken the galaxy as stationary, and calculated everything around it. Next, we take the Local Group as stationary, and calculate the galaxy's movement through it. But we always have to have some point defined as our origin, our 'zero'. Velocity is always how fast you're moving through from some reference frame.

It's not an illusion that the sun orbits the Earth. Maybe they should re-write the science books.

Maybe you should give up your monkey preconceptions . It is an illusion that the Earth orbits the Sun, insofar we hold the Sun as the centre merely by convention.

Mostly because it makes everything look so nice and easy to ponder.

If i'm driving at 70mph and you're in front travelling at 80mph, from your POV I am moving at 10mph. Yes?

Yes.

But in reality I am actually going at 70mph, since I shall, in one hour, cover 70 miles, not 10. So myself travelling at 10mph is an illusion caused by your POV.

No. Your 70mph is measured from the point of view of the road, which is no more special than my point of view.

There are three frames of reference in this scenario:

1. I see you moving backwards at 10mph, and the road moving backwards at 80mph.
2. You see me moving forward at 10mph, and the road moving backwards at 70mph.
3. The road sees me moving forward at 80mph, and you at 70mph.

Why is the road's point of view special? The answer: it isn't.

Asking for the real velocity is like asking for the real distance.

"How far am I?"
"How far you are from what?"
"No, just, how far am I?"
"... que?"

See?

OK. In a empty, starless, blacked part of space it wouldn't be possible to tell.

Bingo. There is no way to tell your speed without using a reference. You must have some explicit or implicit origin in your axes.

But the galaxy isn't any of the above.

Indeed. We can use it as a point of reference, we can say the core is our origin, and thus calculate our velocity.

How is that fiction?

Does the Earth spin?
Does the Earth orbit the Sun?

The Earth spins, that much we can say.

But it doesn't necessarily orbit the Sun. Ultimately, it's just bodies moving under gravity. Where they are in space depends on where your axes are: is your (0,0,0) on Earth, the Sun, the galactic core?

Remember, you can't ask "How far away am I?", you can only ask "How far away am I from point p?". Also remember that velocity is simply the rate at which this distance changes; if you don't specify a point, you can't have a distance, and thus can't have a velocity.

On the Earth, our distance from any other point on the Earth is a constant (i.e., it doesn't change with time), so our velocity is zero.

On the Sun, though our distance doesn't change, our displacement does (that is, our precise position in space relative to the Sun). So, though the numerical value of our distance is constant, we nonetheless have a velocity because our displacement is changing with time (an annoying formality that serves to confuse and confuse).

At the galactic core, our displacement is doing some funky things over time, and so our velocity isn't a nice constant number, but rather varies itself over time. In other words, we're accelerating.

But the point is that velocity only makes sense if you know what it is you're measuring distance to.

Because the Core of the Galaxy doesn't orbit the Sun.

I honestly think you've gone mad

To be honest, I've never had to explain this to anyway before (I surround myself with physicists in my Ivory Tower). I'm kinda surprised it's such a hard concept to get your head around.

I know, right?

The most archetypal and quintessential scientific revelation has, it seems, been proven wrong.
But don't worry, it's just an illusion .

 

[Chesterton]

Bingo. There is no way to tell your speed without using a reference. You must have some explicit or implicit origin in your axes.

It's like this grocery store I shop at. The computerized cash register spits out a receipt, and near the bottom it usually says something like "You saved $1.73 today!" If I didn't use coupons, and it doesn't mention any competitor's prices, it's meaningless. I'm thinking, "$1.73 relative to what?"

 

[Wiccan_Child]

Can light really be considered to be moving from its own POV? I mean, movement in my mental dictionary implies time, and AFAIK, time doesn't really exist for a photon

Oh, time exists for a photon, but in a funky way. Think about it: light still takes a finite time to travel from, say, the Sun to us.

The point of special relativity, though, is that the speed of light as seen by anyone (regardless of where they are or what they're doing) is a constant.

Though it hurts my head when I realised that you can calculate the speed of light using the laws of electromagnetism (it's (ε[sub]0[/sub]μ[sub]0[/sub])[sup]-½[/sup] ). What exactly are we calculating? The speed with respect to what?!

To the 'luminiferous aether'. Which we subsequently shown not to exist.

BADUH

 

[Allister]

I'll reply in full later WC.... in the meantime though...

To be honest, I've never had to explain this to anyway before (I surround myself with physicists in my Ivory Tower). I'm kinda surprised it's such a hard concept to get your head around.

Maybe that's why science gets such a bad Rep...

The Earth spins, that much we can say.

But it doesn't necessarily orbit the Sun.

The Earth doesn't necessarily orbit the Sun. <staff edit>

Since when?

OK, if it isn't necessarily orbit the sun what the <staff edit> is it orbiting?

Is the Moon orbiting the Earth, or is that an illusion too?


Can someone else verify what WC is saying?????

 

[gaara4158]

I'll reply in full later WC.... in the meantime though...



Maybe that's why science gets such a bad Rep...



The Earth doesn't necessarily orbit the Sun. <staff edit>

Since when?

OK, if it isn't necessarily orbit the sun what the<staff edit> is it orbiting?

Is the Moon orbiting the Earth, or is that an illusion too?


Can someone else verify what WC is saying?????

We're just trying to explain to you that there is no objective "default" frame of reference in space. The universe is expanding and we don't know what's beyond it. The universe itself could be revolving around something even bigger for all we know; in fact, evidence suggests that there are humongous bodies of matter beyond the edge of the universe. There is no absolute speed, that we can calculate.

 

[Naraoia]

Oh, time exists for a photon, but in a funky way. Think about it: light still takes a finite time to travel from, say, the Sun to us.

Yes, but that's from our POV. Isn't time supposed to slow down so much that it stops when you reach light speed? And that means that for the photon, it takes exactly zero time to travel any number of light years.

I guess zero qualifies as "finite"?

(Witness the biologist trying to understand wacky physics )

 

[Wiccan_Child]

Yes, but that's from our POV. Isn't time supposed to slow down so much that it stops when you reach light speed?

Only if you have mass, which a photon doesn't.

And that means that for the photon, it takes exactly zero time to travel any number of light years.

For the photon, well, I honestly don't know. Presumably, it experiences time just like the rest of us: it doesn't do everything simultaneously, but rather goes through the motions.

 

[ragarth]

Only if you have mass, which a photon doesn't.


For the photon, well, I honestly don't know. Presumably, it experiences time just like the rest of us: it doesn't do everything simultaneously, but rather goes through the motions.

I've interpreted relativity the same way as Naroia. Since a photon travels at or near the speed of light (Keep in mind that the speed of light changes based upon the medium and the frequency of the lightwave), then it has no actual perception of time even though we perceive time in the photon's movements. Let's do a thought expirement:

Our photon is now a yellow submarine. At the photon factory it's loaded up with a days worth of supplies and two people, and then it's launched out into the universe at the speed of light in a vacuum. Our yellow photon submarine will travel for eternity or until it's either absorbed by a particle or something happens to cease it's existence, and our people will never complete their first breath, first sip, or first drink of their stored supplies because time has effectively stopped for them. The photon continues to move, however, because even though time on the submarine has stopped, external time has not and so we will perceive it's motion even though the people on the submarine do not.

 

[Naraoia]

Only if you have mass, which a photon doesn't.

It doesn't have a rest mass. Does that automatically mean that it also doesn't have mass due to its movement?

Argh. I actually have a textbook on special and general relativity. And a good one at that, though my resolve to work through it didn't last very long. Maybe I should pick it up again.

 

[ragarth]

It doesn't have a rest mass. Does that automatically mean that it also doesn't have mass due to its movement?

Argh. I actually have a textbook on special and general relativity. And a good one at that, though my resolve to work through it didn't last very long. Maybe I should pick it up again.

Mass increases as you approach c as a function of your existing mass and the fraction of c that you travel. If you're mass is 0, then you have no mass either at 0c or 1c. I've always kind of assumed that it's massless nature is what allows a photon to both travel at the speed of light and not travel at a speed less than that (Keeping in mind that the speed of light varies based on the material your traveling through- hence why bose-einstein condensates allow you to reduce the speed of light to a few meters/second.).

One thing I've never understood is this:

E=mc^2

E=(0)c^2=0

So the energy of a photon = 0

If the energy of a photon equals 0, then how does it transfer radiant energy? With 0 energy, it should not be able to set up the molecular vibration that defines heat.