h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

You may be right Xavius - it's a wonder how they manage to make physics so boring in school when it's in such excitement and turmoil everywhere else.

Well I have a pretty good physics teacher, but it still is made boring by the class. Nobody wants to just get on with it and learn the damn stuff.

Oh well.

My Physics teacher actually wanted to teach Chemistry, and is a bit miffed about it and consequently takes it out on his pupils.

For example, you know a thermistor? If you touch it, your hands heat it up, and its resistance decreases. We hooked up an ohmeter to check, and it worked for my friend, but when I touched it, the resistance shot up.
We called over the Physics teacher, who looked at me and said, "Have you heard of the Undead?"

Enough said.

~~A~~

hehe.

He really hates me.

~~A~~

Here's another confusing question that I'll not ask the Physics teachers...

If an object is travelling through space at, say, 2000m/s and passes a large star, it will accelerate towards the star while still moving at 2000m/s in its original direction. So overall its velocity has increased. What happens, then, if light passes a star? I thought that light can be attracted by large objects like stars, so surely you have light travelling faster than it originally was? Travelling faster than the speed of light?

I suppose I just like being annoying sometimes.

~~A~~

Aurora, your Physics teacher has not heard the phrase 'Cold hands, warm heart'.

Alji, of the Red Dragon (Swynwr y Ddraig Goch) (conducting a sun sign poll @ A712595)(Member of The H2G2 Guild of Wizards @ U197895 looking for wiz kids to join, though you don't have to be a wiz kid just know a bit about some subject that you think will be of interest to others or just bore the pants off them. This is an equal opportunities space open to all sexes, ages and abilities)

Thank you.

Ooh, I like your smiley. Is it one of the new ones?

~~A~~

I Like the mice too

Alji (Member of The Guild of Wizards @ U197895)

Answer to the "If an object is travelling through space at, say, 2000m/s and passes a large star..." question:

The object does not accellerate as such, it remains in free-fall. It just appears to accellerate to an observer. Imagine if you were that object, you would feel no force of accelleration whatsoever.
Light, likewise, would not accellerate. To an observer, however, the wavelength of the light would appear to decrease as a result of the increase in relative energy. (Actually, this is slightly incorrect - the wavelength would not actually change, but the effect would be the same.)

I go away for two years a look what happens...

We need to remind ourselves the difference between velocity and speed.

Speed is simply how fast something is moving.
Velocity is speed in a given direction.

When you exert a force on an object, you affect ist velocity. If that force is parallel to the object's vector (i.e. in the same direction), such as applying the brake or accellerator in a car, this change in velocity is equal to a change in speed. If the force is at right-angles, you changethe direction; any other angle affects a combination of both.

Just because you change something's velocity you don't necessarily change its speed. If you whizz something around on a bit of string, it's constantly accellerating but its speed remains constant.

...which brings us to an interesting point: the relationship between gravity and velocity. We've seen that if you exert a force on an object you change its velocity. What also seems true is that when you change an object's velocity you create an equal and opposite force to the force that was used to change its velocity. When you apply the brakes in your you feel a force wanting to throw you through the windscreen. Turn a corner very fast and you're thrown sideways. This is how we contrive to create artificial gravity.

A thought has popped into my head whilst writing this. It's long been explained that the "weightlessness" experienced in an orbiting spacecraft is due to the fact that everything is falling at the same rate, hence there is no perceived gravity within the spacecraft. The spacecraft is "falling" because the Earth's gravity is pulling inwards while it is trying to travel in a direction at right-angles to this pull. In other words, its velocity is constantly changing. Does this then mean that because gravity is causing this change in velocity, there is an equal and opposite gravitational force pushing outward, resulting in a net weight of zero?

My point is that, according to current theorey, gravity is not a force as such. The opposition of gravity as exerted by, say, the ground I am standing on, is a force - upwards in this case.

With no force applied, an object will travel in a perfectly straight line (as viewed by an external observer - from its perspective it will remain stationary. It's all relative).
HOWEVER, it is the straightest line in curved space and that may happen to be a circle around a planet.

Imagine a large sphere, planet sized. Any very small part of the surface will appear flat - two dimensional - any you can draw a straight line on it. However, the reality is that the surface is curved. The line, if drawn for a good distance, will appear curved to the 3D world.
A similar thing applies in curved space, the apparently straight line is actually curved with the contour of space.

The (relative) speed the object is travelling at affects the definition of what constitutes a straight line, but the same rule applies for all objects, including photons.

I hope this makes sense, it did when I wrote it. Then again, I know what I am trying to say!

No. The object accelerates towards the star because the star's gravity exerts a force on it. That is to say, its vector changes (and hence its velocity changes) but its speed does not.

Space isn't curved. It isn't anything. It's just space.

I think you need to read up on physics. Anything published in the last 20 years should do!

Yes, Aurora, it's one of the new ones.
There's also a and a which you might like.



And for Mina.

I have a full list in one of my recent journals.


I agree, space is a vacuum, it's light that curves, not space.

>Space isn't curved. It isn't anything. It's just space.

Having read much of the rubbish published over the last 40 years and a lot of the stuff before.
>Space isn't curved: The curvature of space is a mathermatical construct of non-Euclidian geometry in 4 dimensional space, time being the fourth. The earth moves around the Sun at 18.5 miles per second so the dimension T i.e. time, can't be a real dimension that we could choose to go forward or back to and still be on the earth.
The bending of light as it passes the Sun does not prove that space is curved just that the path taken by the ray of light is bent. I believe the electro-magnetic field around the Sun is responsible for the efect not the Sun's gravity.

>Space isn't anything: Space is full of energy, dark matter, electro-magnetic fields etc. Dark matter is supposed to make up 90% of the universe (btw, that includes the room you occupy).


>It's just space: Space is still a mystery - 9-Jul-2002 An analysis of 13.5 thousand million-year-old X-rays, captured by ESA's XMM-Newton satellite, has shown that either the Universe may be older than astronomers had thought or that mysterious, undiscovered 'iron factories' litter the early Universe.

http://sci.esa.int/content/news/index.cfm?aid=1&cid=1&oid=30255



Alji, of the Red Dragon (Swynwr y Ddraig Goch) (conducting a sun sign poll @ A712595)(Member of The H2G2 Guild of Wizards @ U197895 looking for wiz kids to join, though you don't have to be a wiz kid just know a bit about some subject that you think will be of interest to others or just bore the pants off them. This is an equal opportunities space open to all sexes, ages and abilities)

Whatever. This is boring so I am unsubscribing

Bye.

Back to the topic of faster than light.

Lets assume that the bird was instantaneously created inside the train, in the air (he suddenly poofed into existence flying in the air in the train). The bird would hit the back of the train (of course their would be air resistance, but were assuming this is a normal sized train car). So that whole bird argument doesn't make much sense.

As for the light thing, the speed of light is a constant as theorized by einstien and accepted by physicists then and today. So the light in the train is moving inside the train at the speed of light and it hits the front of the car going the speed of light.
What your saying is almost like saying, "lets fire one photon at another and since their both going the speed of light their net speed must be the speedoflightx2"

but it doesnt work that way.

The bird is travelling at almost exactly 90mph. This is because both speeds are very small relative to the speed of light, so the effects of relativity are tiny (negligible, in fact).

If you shoot a beam of light inside the train, the speed of the light is exactly the same relative both to someone on the train and somone standing by the track.

This is because space and time appear different to observers moving at different velocities (from Einstein's Special Theory of Relativity). A mathematical transform, called the Lorenz transform, defines the difference between each observer's perception of space and time.

The formulas derived from the Lorenz Transform show how 2 such velocities (i.e. bird and train) combine from the point of view of a 'stationary' observer. When the speeds involved are small compared to the speed of light, the formula approximates to simple addition (i.e. 70+20 = 90), but where at least one speed is significant proportional to the speed of light, this approximation breaks down.

If you're mathematically inclined, the formula for combining 2 velocities U and V is:

(U + V)/(1 + UV/C^2)

where C^2 is the speed of light squared.

Obviously, if U and V are small (70mph, 20mph) then UV/C^2 will be tiny (C is approx 670,000,000 mph!)

with these figures, the velocity of the bird is 90/1.000002 or 89.9998 mph.

What happens if you replace the bird with light travelling at C?

(U + C)/(1 + UC/C^2)
= (U + C)/(1 +U/C)
= (U + C)/((C + U)/C)
= (U + C) * C / (C + U)
= C

So the beam of light appears to be travelling at C even relative to the stationary observer.

OR:

If you took the more simple formula of 70+20=90, both these figures are so tiny compared to c that the speed of light would be very nearly the same for both observers. So close as makes no odds.

The speed of light shouldn't be looked at as some magical barrier, some mystical thing. It's purely and simply the speed at which light propogates. the formula (U + V)/(1 + UV/C^2) will still hold true with any arbitrary value of c that you choose, so long as it is sufficiently large compared to U and V. Whether or not the speed of light is constant will continute to be pure theory for a long time to come. To perform valid experiments you need observers to be sufficiently far apart, travelling at vastly different velocities, at which point their only means of communication is by some sort of electromagnetic radiation (e.g. radio). Trouble is: all EM radiation we know of so far only propogates at the speed of light!

Many apologies for reviving this thread, btw.