Physics is all a hoax!

[Spec]

Sorry, slight over reaction on that title there...Realistically, i should edit the title to ''Stupid physics question'' but I'm way to lazy to do so, so I'll just leave it as is.





On the same train of thought, even if, according to Einstein, it's impossible to travel faster than the speed of light, we say that the observable universe has a radius of 46 billion light years, and yet, the big bang happened only 13.8 billion years ago.

What am I missing?

Spec.

[Ben Kenobi]

You're missing the fact that the universe expands in three dimensions. If X is moving from Y and X and Y are both moving at the speed of light away from each other and the original singularity - how fast will the radius - the distance between them expand?

You're assuming that - big bang travels in one direction and that it can only travel so far in so many years. However, it's really 3-D and the Earth is a point on that sphere - that moves along with space as it expands -increasing the distance faster than the light can travel from one edge to the other.

The whole concept of the 'observable universe' is defined as a sphere with the earth as the centre - gradually expanding outwards at the speed of light. This is different from a non-inertial frame of reference, centered on the initial singularity.

In a sense - we can only see stuff on 'our side' of the expansion', and we can't see stuff past it on the other side of the expansion. IE - the universe is always going to be larger than we can actually observe.

We gage the age of the universe - based on the Hubble constant, IE, how fast things are moving away from us and each other, and trace it back to a single point in the past ('the singularity'), and calculate from there. The way we calculate the hubble constant is to figure out how far galaxies with measurable redshifts are from us - by measuring the luminosity of certain stars that have a relationship between the period of their oscillations and their luminosities. These are called Cepheids - you can see one from earth in Polaris. They tend to be bright, and fairly luminous.

We gage the luminosity of Cepheids by comparing them to other similar stars much closer. Eventually we'll be able to use direct parallax measurements but we aren't quite there yet.

As for the geodesics of space in 4 dimensions and general relativity - you'll have to ask KH.

[Lorizael]

The universe gets most of its size from an event known as inflation that happened very early on in its expansion. I can't explain inflation very well (because no one can, and because I don't know much about it), but it did cause space to expand faster than the speed of light. The key here is that space itself is not prohibited from traveling faster than the speed of light, only the objects in space. Those objects got carried along with the expansion of space, but they themselves weren't moving faster than light.

[onodera]

Ignore Ben (duh). If two objects move in opposite directions as fast as physically possible, their relative velocities are still c.
You're missing the general relativity theory, which I have no capacity to explain. Basically, it's the scale of space itself that is expanding, not the galaxy clusters flying away from each other.

[MikeH]

Yeah, Ben's missed the point a bit. Lori has a good laymans summary I think.

Wikipedia is a good summary starting point but this is not a simple subject.

http://en.wikipedia.org/wiki/Inflation_(cosmology)

And there are lots of questions left unanswered by this model, so it might not be completely correct.

[MikeH]

This is probably the most awesome sentence.

"It lasted from 10^-36 seconds after the Big Bang to sometime between 10⁻³³ and 10⁻³² seconds"

[DaShi]

What if you are on a train going at the speed of light and walk from the caboose to the engine? Are you not going faster than the speed of light?

[Lorizael]

The answer is: trains can't go the speed of light. The answer also is: no, because of the Lorentz transformation. Speeds aren't really additive; they just approximate being additive at low speeds.

[Dauphin]

This is probably the most awesome sentence.

"It lasted from 10^-36 seconds after the Big Bang to sometime between 10⁻³³ and 10⁻³² seconds"

The inside out black hole metric is more appealing to me.

[Elok]

The answer is: trains can't go the speed of light. The answer also is: no, because of the Lorentz transformation. Speeds aren't really additive; they just approximate being additive at low speeds.

Doesn't the train also compress horizontally in the direction of motion, so that at the speed of light it doesn't have any distance within it for any passengers to move? I thought I read something like that in an oversimplified summary somewhere...

[Tuberski]

So you're telling me that if I jump up right before the elevator crashes into the ground, I won't be safe?


DAMMIT!!!!

ACK!

[Lorizael]

Doesn't the train also compress horizontally in the direction of motion, so that at the speed of light it doesn't have any distance within it for any passengers to move? I thought I read something like that in an oversimplified summary somewhere...

The train does contract, but only in the frame of reference of an observer watching the train go by. For the train and its passengers, everything is normal, but the universe in front of the train contracts. Relative to the train, it's the rest of the universe moving at the speed of light.

[DaShi]

Then we just need to contract the universe until Alpha Centauri is just a Sunday afternoon drive away.

[Ben Kenobi]

If two objects move in opposite directions as fast as physically possible, their relative velocities are still c.

From the point of view of the observer on one of the objects. From an outside observer, say perpendicular to either, the actual physical distance between the object would increase twice as fast. This is also why they tend to use a coordinate system that remains static by comparing the ratio of the distance between the two.

This is fr'nstance, how we can observe superluminal speeds, say, black hole ejecta travelling close to c.

[Ben Kenobi]

Wikipedia is a good summary starting point but this is not a simple subject.

No, I don't know enough about General relativity, but the explanation for spec's question can be found by looking up:

"Observable Universe"
"Co-moving distance"
"Proper distance"