The Learn Physics Thread - Space and Time
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Yes, except the air in the carriage is also moving with the train so you'll probably still hear it, but it's a technicality."Plus he wore shorts like a total cunt" - Bob
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Bloody page turn.
RamSteelwood wrote:more good stuff. just so i check i'm understanding what you're saying right about sound being a constant too... we're both on a train, you're in the middle and i'm towards the front. the train is travelling at 1000mph. you throw a tennis ball to me, at let's say 20mph. i catch it normally cos it's relatively just at 20mph. you throw another ball out the window with a forward velocity of 20mph, it hits someone standing by the train track and kills them cos it hits with 1020mph force? when you throw the ball to me, you make a loud grunt, but i'll never hear it cos we're moving faster than sound and i'm in front of you on the train. another person standing by the side of the track does hear your grunt. are they all 'correct'?"Plus he wore shorts like a total cunt" - Bob -
Incidentally, if I had a gun and fired it directly up at the roof of the train, we'd both see the path of the bullet go vertically up, but what does a person on the ground see? They see the path as diagonal - the upwards velocity plus the velocity of the train, and the path is longer.
We're both right and the person on the ground is also right - all inertial frames are equal. We see a shorter path and that's called length contraction. If you were watching all this in a seat on the train and absent-mindedly tapping your finger at the same spot on a table, we should both consider this to be the same point in space because it IS! But the person on the ground doesn't agree.
It's tempting to think the person on the ground has the correct position on this, so imagine we're doing it instead in a space ship, and we're drifting past (the engines are off) an astronaut that happens to be floating by. It is perfectly correct to say the point on the table is the same position and the space ship is not moving - the observing astronaut is. This is how we abandon the idea of absolute space."Plus he wore shorts like a total cunt" - Bob -
This was beautifully demonstrated on mythbusters when they shot a ball from a cannon from the back of a moving truck.
The cannon was pointed backwards and they shot the ball at the same speed as the truck so the ball just fell to the ground from the perspective of someone standing on the roadside,but from the perspective of the truck the ball still shot out of the cannon at the correct speed.
You rang..... -
Nice. Imagine if they were doing that with a light gun instead of a cannon. EVERYONE sees light go at the same speed. The person on the ground measures the speed of the beam coming from the truck as exactly c, but so does the driver! This is when mad shit happens and we have to consider time. We'll get to that next."Plus he wore shorts like a total cunt" - Bob
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When velocities no longer stack, as in the case for light, we say light is invariant. It's speed does not depend on anything (like the velocity of the truck) and all inertial frames measure it at c (300,000 km per second)."Plus he wore shorts like a total cunt" - Bob
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No-one is sitting still anyhow - even with absolute space - it isn't valid to state that.
Have no formal physics education so didn't know about that defn of c. Interesting. Maxwell really was quite something.Holding the wrong end of the stick since 2009. -
If you're wondering how this can possibly be we'll get to it later."Plus he wore shorts like a total cunt" - Bob
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davyK wrote:No-one is sitting still anyhow - even with absolute space - it isn't valid to state that.
It's valid to say you are sitting still, because all your experiments work as per the laws of physics. And someone moving relative to you should also say they're sitting still. You're both right."Plus he wore shorts like a total cunt" - Bob -
The beauty of maths/physics is that wondrous connection.
When I first saw e^(i*pi) + 1 = 0 it had a real effect on me. Two irrational numbers, the square root of -1 combined with the two most important integers. Marvellous.Holding the wrong end of the stick since 2009. -
That is THE experiment!"Plus he wore shorts like a total cunt" - Bob
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Notice we're dealing with right-angled triangles in that vid. We can use Pythagoras to work out the time dilation."Plus he wore shorts like a total cunt" - Bob
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If we imagine Jim in the vid getting pushed at the speed of light, what path does the audience see getting traced out? Well all the velocity of the light must be going into the horizontal direction because it doesn't have any extra speed left over to go up, so the audience sees a straight line. The light ball never moves up or down - time for Jim has frozen (as the audience sees it)."Plus he wore shorts like a total cunt" - Bob
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But what does Jim see if he's moving at c? That's when it gets interesting. If he's on his way to the Moon, we see Jim traveling there with a frozen look on his face and his clocks not moving. From Jim's point of view he arrives there instantly. And Saturn, and the stars. But what does he see? He sees the moon and Saturn and the stars as the same distance, and that distance is zero. Jim is living in a flat universe! Photons don't experience distance or time. Crazy but true."Plus he wore shorts like a total cunt" - Bob
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Jim is omnipresent? -
He is in my head. I really like him.
"Plus he wore shorts like a total cunt" - Bob -
I know c is a constant but doesn't light travel at different speeds in different mediums?You rang.....
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Particularly slow within Mystic Meg -
Lurch666 wrote:I know c is a constant but doesn't light travel at different speeds in different mediums?
That's because it's absorbed by atoms and re-emitted which takes time. It goes between the atoms at c."Plus he wore shorts like a total cunt" - Bob -
The problem is, does anyone really check on all the physicists to make sure they're not just making all this up to keep themselves in a job?SpaceGazelle wrote:He is in my head. I really like him.
I reckon Einstein 'completed' physics so the rest of them got together and just made up quantum mechanics and these kind of experiments and you're not allowed near the equipment until you sign the nda and join the secret physics society....
"Like i said, context is missing."
http://ssgg.uk -
If they were going to make it up they'd go with something slightly more believable."Plus he wore shorts like a total cunt" - Bob
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How can we look back at light that's from the start of the universe? If lights the fastest thing going and we are expanding from the center how come all the light hasn't over taken us?
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Isn't it the light shining back at us from the edges of the universe (that has already "overtaken" us) that we see as cosmic background radiation, rather than light from the centre of the universe (which presumably isn't a thing that could ever be determined)?
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Get enough mass together and it starts shining. Atoms weren't formed for a few thousand years after the big bang anyway. The original light is the CMBR."Plus he wore shorts like a total cunt" - Bob
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GooberTheHat wrote:Isn't it the light shining back at us from the edges of the universe (that has already "overtaken" us) that we see as cosmic background radiation, rather than light from the centre of the universe (which presumably isn't a thing that could ever be determined)?
Yeah there's no 'middle'. Or you could say everywhere is the middle."Plus he wore shorts like a total cunt" - Bob -
SG will explain this better/correct me but as far as I understand it there isn't a centre. The big bang is a confusing name as it encourages ideas of an explosion but it was more a rapid expansion.
The classic way of showing this is blowing up a balloon partially and drawing dots on it to represent stars, then bllowing it up more. The dots move further apart from each other across the surface of the balloon rather than from a shared central point. This shows expansion in 2D but it happens in 3D in the universe.
Edit: beat me to it. -
We'll cover this in GR."Plus he wore shorts like a total cunt" - Bob
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It'd hard to imagine 3D space as being curved because it would seem like we've run out of dimensions to curve it into."Plus he wore shorts like a total cunt" - Bob
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