SPEED OF LIGHT

The non amateur stuff. Hawking, black holes, that sort of thing

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chris ide
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SPEED OF LIGHT

Post by chris ide »

HI ALL
WE ALL (MOST OF US)HAVE A LITTLE GRASP OF EINSTEINS THEORY OF RELATIVITY (APART FROM ME THAT IS ) CAN ANYONE IN VERY SIMPLE LAYMANS TERMS (AFTER ALL I AM ONLY A BRICKLAYER) TELL ME HOW , IF MY TWIN BROTHER WERE ABLE TO TRAVEL AT THE SPEED OF LIGHT FOR SAY 10 YEARS AND WHEN HE RETURNED HE WOULD BE YOUNGER THAN ME .
THIS WILL HAVE TO BE SIMPLE AS I GET CONFUSED WIRING A PLUG.

THANKS CHRIS
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Post by joe »

Ok Chris, I'll have a go but I can't guarantee it's efficacy

The simplest answer is that clocks tick more slowly when they are in motion. Or I should say that if you look at a clock that is moving, it appears to be ticking more slowly, from your point of view. If your twin accelerates off for five years, stops and returns for another five according to your clock on Earth, then his slow clock will show a much smaller amount of time having passed. He will be younger or he has travelled into your future. For clock read heartbeat, etc. He will have aged less.

Why does a clock run more slowly when in motion? If you imagine a "clock" as being a ball bouncing from floor to ceiling on a train, then when the train is stationary it takes one second to make one bounce. When the train is in motion, the ball is no longer travelling straight up and down, it is travelling horizontally as well. The ball has then a longer distance to travel and takes longer. One bounce, or tick, is one and a half seconds for example. Time appears to slow down. To someone on the train of course the bounce is still one second.

If you transfer to the twins to space then it is impossible to tell who is moving and who is stationary so why should one age and the other not? The clocks are interchangeable. One twin sees the other clock moving more slowly but the other twin sees his brother's moving slowly as well! This is the important point. In order to travel into someone's future, you have to move and to move you have to apply a force - acceleration. It is the acceleration to get moving, acceleration (deceleration) to stop after travelling some distance, acceleration to come back and acceleration to stop in order to compare clocks. The twin on Earth does not feel this force but the other does, so it is the acceleration that affects the balance. And in order to get near the speed of light you would have to apply a huge amount of force. That's why we don't see it in everyday life. We don't experience such massive forces.

If you know a little about Einstein's theories then you will probably know that gravity and acceleration are interchangeable. You therefore only need to subject a clock to a strong gravitational "force" to slow it down. Therefore a clock at the top of a mountain runs faster than one at the bottom. There is less acceleration up there.

Hope this helps a little. :?

Regards,
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chris ide
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Post by chris ide »

THANKS JOE,
I GUESS IT HELPS A LITTLE , I FIND IT TOTALLY FASCINATING AND MIND BOGLING THANKS AGAIN

CHRIS
cowait
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Post by cowait »

Hey joe, ( apologies to jimi hendrix)

The speed of light is constant. So we are to believe. So is it right that no matter what speed I am travelling at, the light from a lamp, for arguments sake would travel at the same speed irrespective of the direction the lamp is pointed?
For example: I am going at 90% of the speed of light. I flash a beam in the direction I am travelling. Said beam will travel away from me at 10% of the speed of light. If on the other hand I flash the light in the direction I have come from , what happens? Does the light stream away from me at 190% of the speed of light relative to me. ( I suppose relative to me it would seem just like any other beam of light) From an observers point of view what would be the situation? If the light does not travel away from me at 190% and only at 100% then taking off the 90% of the speed of light I am travelling at would simply make the light travel at 10%.

Another point. If time gives the appearance of slowing down as the speed of light is approached. Why does not it accelerate to an infinitely fast , over in the blink of an eye speed , as we come to rest?

cowait
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Post by joe »

Hi Cowait,

The speed of light is constant, 300000 km/s in a vacuum. No matter what speed you are travelling at it will always fly off at 300000km/s or "c". If you are standing still it travels at c and if you are moving at 99.999% of c it will still move away from you at c, in all directions. If you think about it Einstein said that no one has a special position or motion in the universe, it is all to do with frames of reference. If you are in space and see a spaceship flying past, it is impossible to tell if you are moving or they are moving or both of you, etc. It's only within a frame of reference (the vicinity of the Earth for example) that you make a judgement but in fact you only move relative to something. If the speed of light changed depending on your speed it would give you a special position and announce that you are travelling x amount of km/s east or west relative to the universe....which would be nonsense. Or put another way, that spaceship that was flying past, you have to see that light from it travels at c in all directions because, as was said before, there is no way of knowing that it or you is the one that is moving.

The slowing of time is a doolally of a subject and I'd better go and do some swotting before I attempt a proper answer. But there are two types, the appearance of someone elses time passing more slowly relative to your clock due to motion and time ACTUALLY running slower because the clock is subjected to a warping of spacetime... or gravitational field. ( I hope there are no physics professors reading this) In both cases time doesn't change locally, it is only relative to other time. As you move faster and faster, time moves slower and slower until you reach a point where it almost stands still, say 99.9999999% of the c. Light itself which does travel at c does not experience time AT ALL! Moving faster and faster means adding energy and it is this energy that warps spacetime. There is a famous description which goes something like... we are all of us, all things, "travelling" at light speed but we are doing it mostly through the time part of spacetime, ie. we are moving VEEERY slowly through space and not much of it. Light is travelling entirely through space and is not travelling through time at all. Going to lie down now.
:oops: :cry:
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Richard McC
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Post by Richard McC »

Hi all,
It's al irrelevant anyway because to travel at these fantastic speeds means you have to apply a force and as you speed up your mass increases exponentially until you get to 99.999% c.

S Hawking "For example, at 10% c. an objects mass is only 0.5% more than normal, while at 90% c. it would be more than twice its normal mass. As an object approaches the speed of light, its mass rises ever more quickly, so it takes more and more energy to speed it up further. It can in fact never reach the speed of light, because by then its mass would have become infinate. By equivalence of mass and energy, it would then take an infinate amount of energy to speed up an infinate mass."

This is the bit that confuses me though.......
"Only light or other waves that have no intrinsic mass, can move at the speed of light....."
Now I'm no particle physiscist, (I don't think I've even spelled it right), and I certaintly do not want to question the great Stephen Hawking, but surely light is BOTH a wave and a particle......so it must have a mass of some description. So what is the infinate power that propels that infinate mass (photon).

If the answer to this is simple then please don't make me look like a div by a sarcastic answer. This is a genuine query.
Go placidly amidst the noise and haste, and remember what peace there is in silence....(and true darkness, without cloud cover).
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Post by cowait »

Hi Richard,

I have read that photons do have mass. Have you seen those small devices that spin just because of the light falling on one side of a paddle. One side being white the other black. Presumably the light side reflects the light and the other absorbs much of it . The mass of the photons providing the force to drive it. Same as the "light sail" suggested for interstellar travel. Just a tad smaller mind you.
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Post by joe »

Hi Richard,

What you say about the impossibility of massive objects reaching c is true but particles with mass (tiny) can be accelerated up to 99.9999% of c and the increase in their mass measured as well as the time dilation. Of course they have to draw on half the electricity supply of Switzerland to do it. Photons have energy and this energy exerts a pressure on objects. Not quite sure how it all works, something to do with particles absorbing and emitting photons I think. It's similar I suppose to sound waves. Sound has energy but would you say it has mass?

Regards,
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paulestar
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Post by paulestar »

Richard McC wrote:I certaintly do not want to question the great Stephen Hawking, but surely light is BOTH a wave and a particle......so it must have a mass of some description. So what is the infinate power that propels that infinate mass (photon).

If the answer to this is simple then please don't make me look like a div by a sarcastic answer. This is a genuine query.
Hi Richard,

I came across this explanation of the wave/particle duality of light on a CD ROM about string theory...imagine looking at a pencil in 2 dimensions. If you look at the pencil down its long end (say, x, y), it looks like a circle. But if you look at the same pencil sideways on (say y,z), then it will look like a line - so the representation of a 3d object can appear differently when seen in a lesser number of dimensions, although it will still behave as both - the pencil can roll off the table; the pencil can be lodged behind your ear. The analogy is that light occupies more dimensions than the 3 we are familiar with, so when we see it in those 3, its characteristics may seem to be more like a wave or a particle.

I suspect...and I am probably way off the mark but anyway...that when we say light has a mass because it behaves as a particle, this may not be entirely true, because we need to take into account the fact that it also behaves as a wave - if we put a book on top of 2 pencils, we can push the book along in the z direction as the pencil acts like a circle, but it can't be pushed along in the y direction because the pencils behave like a stick. So when we try to accelerate light, maybe we're not accelerating the massive particles, but rather the massless waves?

Another point to consider, are there any other ways that light behaves (i.e. not as a particle or a wave) but as something else? Maybe that model of light would better serve our understanding of how light accelerates.
Paul
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joe
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Post by joe »

Light doesn't change its speed - accelerate or decelerate, in the normal usage of the words - it travels at a constant speed. It is a fundamental property of electromagnetic radiation. It has energy of motion which is affected by gravity but not inertial mass.

Regards,
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paulestar
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Post by paulestar »

Hi Joe,

Yes you're right! Sorry, my error - I was trying to get to Richard's question:

"what is the infinate power that propels that infinate mass (photon)."

so I should have written "propulsion" and not "acceleration". I think. I'm even confusing myself!
Paul
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Post by joe »

No need to apologise Paul, I'm trying to get my head around this too but like all matters concerning the quantum world it is worth looking at it a different way. Don't ask why light has been "propelled" to 300,000 km/s, this is a constant, ask why everything else has been slowed down.

:roll:
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BigGeoff
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Post by BigGeoff »

If it's any use to anybody, take a look here http://www.bartleby.com/173/. This is a translation of Einsteins theories of special and general relativity, and it starts off real slow. Unfortunately I have had a copy of this next to my bed for two years (sad person that I am), and I have read it twice but still get lost at the same point. Maybe others will find it useful.

It is interesting to note though, that Einstein himself had immense trouble when formulating his theories unless he had a picture in his head of what was happening, so we are not so different (well I like to think so anyway).

Geoff
Geoff

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Post by Gregger »

I have read that photons do have mass. Have you seen those small devices that spin just because of the light falling on one side of a paddle. One side being white the other black. Presumably the light side reflects the light and the other absorbs much of it . The mass of the photons providing the force to drive it. Same as the "light sail" suggested for interstellar travel. Just a tad smaller mind you.
Those gizzmos spin round the opposite way you'd expect form that theory I believe. There was some stuff in New Scientist about all due to convection currents I believe.

Going back to the Twins Paradox above, what confuses me is that the clock with the twin at the speed of light appears to go slower, but does he actually go slower. Isnt that why its relativity?

With the bouncing ball in the train. The ball still moves up and down in the train but appears to move horizontally to the outside observer. So isnt that the same as above.

The twin in the rocket feels normal but he thinks the man on the ground is moving slower/quicker. So I can understand a bit of the theory but cant translate that to real events. The theory seems to jump from clock appearing to go slow then accepting it has gone slow with little explanation.

Its probably "turtles all the way down" afterall!
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Post by joe »

If a clock is in the presence of a strong gravitational field it will run slower than a clock that is not. A clock up in a plane runs faster than one at the stronger gravitationally held ground. Fact. Acceleration is equivalent to a gravitational field.

Think about this example.

A rocket is going to travel to a star ten light years away and we will observe it all the way through a telescope. It should take ten years if it travels at the speed of light (I'm simplifying here) but if you consider that the light that leaves the clock when it is halfway takes five years to come to us then it is apparent that the clock on the rocket must be running slower because at this point ten years will have passed on Earth (five out and five for the light to reach us). When the rocket reaches the star we know that it should have taken ten years but the light/information from the rocket still has to travel for ten years to inform us that it has arrived. When we see the rocket reach the star our clock will read much more than ten years even though we know it has taken ten. So the ten year journey was only partially completed after our ten years had passed and therefore we must have watched a clock moving much more slowly. From the point of view of the rocket, the clock on board will read "ten" years when it arrives but it will see that the time passed back on Earth is much longer. (The maths here do not add up but it's only for a simple demonstration)

I would also add that much of the slowing of time is nothing to do with Einstein's Relativity but simply the doppler effect. Also the clock on the rocket will not read ten years when it arrives actually, it will be a smaller amount because when you travel at significant fractions of the speed of light, space dilates and therefore the distance is smaller!!
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