Moving at light speed

[Tormod]

You should read the entire article - it will reveal that the first task is indeed based on fallacious argumentation and illusion. I posted it to illustrate that explaining the concepts of time and relative motion is not easy to understand, even for teachers.

[Damo2600]

Hi Qfwfq,

 

I didn't quite understand your language clearly enough. I did state that I was happy with the explanation of the Lorentz Transformation. I have been assured that this is empirically correct. I don't have a problem with it. I am getting from your post though that time does not slow down. My clock ticks at the same rate yours does given we are travelling at different velocities. However the illusion created by this universe gives me the impression that events are taking place at different times than they are for you.

 

Is this correct?

 

Damien

[Damo2600]

Oh, I see, sorry Tormod! Eeeek

[Tormod]

Here are some interesting things when it comes to understanding the concept of time dilation:

 

A time dilation simulator (aimed at kids but still a neat page). Adjust the time and see how the clocks tick differently inside the spaceship and on the Earth.

http://www.walter-fendt.de/ph11e/timedilation.htm

 

Visualisation of travel at light speed - watch short movies showing how things appear to distort at high speeds.

http://www.spacetimetravel.org/index.html

 

Spacetime Wrinkles - a very good website by the Univ. of Illinois:

http://archive.ncsa.uiuc.edu/Cyberia/NumRel/NumRelHome.html

 

Hyperphysics: Time and length contraction. Harder stuff:

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/tdil.html

 

Wikipedia: Time Dilation

http://en.wikipedia.org/wiki/Time_dilation

[Buffy]

If you consider we travelled to the centre of the universe. From there we would see the centre of the expansion. Right? This point we would have to say is stationary.

This is actually why I was asking you about the Cosmological Principle which is one of the major foundations of cosmology. It basically says (see the link I posted above for more), that on "large scales" the universe is homogenous--roughly the same density--and isomorphic--looks the same in every direction--no matter where you are in the universe. One of the most illogical things about this is that it says there is no center of the universe, whereever you are, there is no center. The canonical way to explain this is to imagine a two dimensional universe that exists on the surface of an expanding balloon: everything is moving away from everything else, but there is no center. Since there is no center, there's no absolute reference frame for the universe, and therefore there's no edge, therefore the "galaxies furthest from the center" has no meaning and they can't therefore be shown to be moving at/past the speed of light.

 

Cheers,

Buffy

[Tormod]

My clock ticks at the same rate yours does given we are travelling at different velocities.

 

I'll try to explain. Your clock will tick differently assuming that you are moving relative to each other. Each observer will think he/she is moving at a normal rate, but the other appears to have slowed down. If you could see the other person's clock, it would tick differently than yours. When you meet up (ie, get together in the same frame of motion and can talk), your clocks will show different times and a different period of time will have passed for each of you.

 

However the illusion created by this universe gives me the impression that events are taking place at different times than they are for you.

 

No, it is not an illusion. Time dilation happens.

 

Here is very good page illustrating it:

http://www.phys.unsw.edu.au/~jw/time.html

[Damo2600]

A lot of this stuff gives incorrect assumptions. The kids rocket to Pluto is appalling. If I was a kid I wouldn't know 'what on earth' is going on (pardon the pun). The clock on the rocket is going at the same rate as the clock on earth. Is it not? The clock on the rocket *APPEARS* to be going slower if you were looking at it from Earth. But this is telling kids that if you fly in a rocket to Pluto it will take about two hours although you arrive 5 hours later. Come on. There is nothing in this to suggest that time is the same for all observers and time is only observably different.

 

The next one states that relative to the observer clocks tick slower. Hello: clocks don't tick slower, 'relative to the observer it appears to tick slower'.

 

Call me stupid but I do feel as though I have been had in a big way. I'm even questioning the spacetime thing.

 

Forgive me I'll go through your links.

 

Damien

[Damo2600]

So Buffy,

 

The impression most people get is that the universe started at a point. Right? This is the impression I get from most people with an interest in physics. The universe IS expanding from a point. Right? Then you tell me there is no centre of the universe. We cannot possibly ever know where this centre is right? But it does exist. Otherwise where are we expanding from?

 

The point I think you are trying to make is that there is no observable centre of the universe. These two points are absolutely not the same.

 

Damien

[Tormod]

The clock on the rocket is going at the same rate as the clock on earth.

 

Did you adjust the speed of the rocket and watch the changes in the tempo of the clocks?

 

There is nothing in this to suggest that time is the same for all observers and time is only observably different.

 

There is that in everything I have posted. Time is the same for all observers *within* their frame of motion. Only when we move in different frames, with relative speeds, does the time dilation effect kick in.

 

The next one states that relative to the observer clocks tick slower. Hello: clocks don't tick slower, 'relative to the observer it appears to tick slower'.

 

Sorry - the next one? Which next one?

 

Call me stupid but I do feel as though I have been had in a big way. I'm even questioning the spacetime thing.

 

No, you're not stupid. But just because you can't understand what is perhaps the last century's greatest discovery doesn't mean it is wrong! Remember, people are still being awarded Nobel prizes for proving Einstein right.

 

I think there are perhaps 2 people here at Hypography who can claim to have a very deep insight into relativity. I am not one of them.

 

You won't learn, nor understand, relativity in a day. Read up on some good books. For example, David Bodanis E=mc2 or Einstein's own Relativity.

[Tormod]

The impression most people get is that the universe started at a point. Right? This is the impression I get from most people with an interest in physics. The universe IS expanding from a point. Right?

 

This is where you're getting it wrong. The Universe did not expand from a point. The *whole point* expanded and became the universe.

 

Then you tell me there is no centre of the universe. We cannot possibly ever know where this centre is right? But it does exist. Otherwise where are we expanding from?

 

This assumes that the universe is a sphere with a center from which everything comes. But, as I said above: Space has expanded (and inflated!) into what the universe is today. It has not "grown out of a point". It is the whole point that has grown...

 

The point I think you are trying to make is that there is no observable centre of the universe. These two points are absolutely not the same.

 

No, Buffy's point (I hope) is the same as mine: There is no observable center for the very reason that the universe has no center. From our point of view it *appears* as if we are in teh centre of a huge sphere. But it looks the same from everywhere in the universe. There is no center.

[Queso]

blows my mind now that i finally understand that.

[Queso]

one thing i DON'T understand is it like this because of the other dimensions? or is it expanding and inflating in 3 dimensions? or 4? or 11???

[Tormod]

one thing i DON'T understand is it like this because of the other dimensions? or is it expanding and inflating in 3 dimensions? or 4? or 11???

 

Good question, Orb! Actually, there is currently no perfect answer (or more precisely my knowledge does not involve one).

 

We can also ask, "why 3 spatial dimensions? Why not 4?". When we try to look at what the universe would be like with more than 3 large dimensions it doesn't look pretty. So one response, firmly rooted in the anthropic principle, is that if space had more (or less) than 3 large spatial dimensions we would not be around to observe it.

 

But I don't think the expansion is due to the dimensions - rather, the dimensions are a property of our universe. So there might be other universes where there are 4 large dimensions, for example. We don't know...

[Queso]

so the universe itself IS in 3 dimensions, that's just not how we perceive it? (and obviously including the other small dimensions)

[Tormod]

Yes. The other dimensions are too small for us to perceive, and time is not a spatial dimension.

[Damo2600]

Orbsycli,

 

Sorry for pointing this out but you accepted that answer rather quickly. Aren't you curious how the universe (if I'm getting this right) began at a point (is this the size of a particle?) and then the point expanded. So if I can imagine this we are within this point. Yes/no?

 

Now all of the particles and anti-particles didn't explode out of a point: they exploded within the point. This makes some sense to me. The point grew extremely quickly as is my understanding. So there is no point within this point that is the centre. So I must assume you are saying that this point was filling with particles as it expanded. Kind of like one of those popcorn bags. It starts out small and as the popping starts the bag expands. However it didn't happen as slowly as this. Is it like a giant popcorn bag but it starts out as a single corn and within an instant the popcorn starts popping within this single corn.

 

So obviously the popping did take a certain amount of time.

 

Sorry I'm babbling to try to grasp this. So how did a single point fill with particles? What are the explanations? Did the particles enter the point or did they just happen within the point?

 

With regards to a black hole the matter apparently gets pushed into the particle point.

 

I'm well in over my head here I'll have to pull out of this conversation.

 

Sorry Damien

[Buffy]

No, Buffy's point (I hope) is the same as mine: There is no observable center for the very reason that the universe has no center.

Yup. There's the balloon analogy that some don't like, but it comes closest to making the point that if you live on the surface of that balloon, you can go anywhere on the surface and you'll see the same thing in every direction (albeit only if you're 2-dimensional).

 

Damo: As Tormod says, this stuff is unbelievably hard to grasp, but the more you learn about it the more you see the consistency of how these models explain how things work. As Feynman and others have said about quantum mechanics which is just as bizarre: "If you think you understand it, you obviously don't." Don't give up!

 

Cheers,

Buffy