Speed of light....

[mrjaffa]

Well this isn't directly about the speed of light but I was discussing this with the wife and was explaining how it is independent from the source of the light....

What I tried explaining is that it always remains the same, even if you were travelling at 10mph and turned on a torch, it wouldn't then be travelling at the speed of light plus 10mph.

I was telling her how it is different if you were riding a bike at 20mph and there was a constant tail wind of 20mph then you would be travelling at 40mph. But she can't grasp this. She thinks that if you are travelling at 20mph and the wind is the same speed the wind won't affect you, I guess she's thinking the wind wouldn't catch you up. And if you were travelling at 20mph and the wind was 40mph then you would be travelling at 20 + 20 not 20 + 40.

I'm correct yes? But how can I explain this to her?

[rfdesigner]

time is not constant.

If you shine a torch in front of you, and you could observe the light 'flying away from you a 3x10^8 m/s' then your friend in their space ship at 0.5x the speed of light tries to catch the light, they will 'observe' that same light travelling away from them, not at 0.5x the speed of light but at 1x the speed of light because their time reference will be slower.... their watch will spend longer between each second compared to yours. They will still observe the light travelling at 3x10^8 m/s because they will observe the light as moving away from them at 1.5x10^8m in 0.5seconds... the same time that you see it move 3x10^8 m relative to you and your watch has moved 1seconds.

Hence the idea that you travel at 0.99x speed of light to alpha centuri, taking apparently only a few minutes, stop for lunch, then at 0.99x speed of light back and find everyone on earth has aged by 8 years and a couple of hours (it was a long lunch)

any help?

Derek

[gkec]

Well this isn't directly about the speed of light but I was discussing this with the wife and was explaining how it is independent from the source of the light....

What I tried explaining is that it always remains the same, even if you were travelling at 10mph and turned on a torch, it wouldn't then be travelling at the speed of light plus 10mph.

I was telling her how it is different if you were riding a bike at 20mph and there was a constant tail wind of 20mph then you would be travelling at 40mph. But she can't grasp this. She thinks that if you are travelling at 20mph and the wind is the same speed the wind won't affect you, I guess she's thinking the wind wouldn't catch you up. And if you were travelling at 20mph and the wind was 40mph then you would be travelling at 20 + 20 not 20 + 40.

I'm correct yes? But how can I explain this to her?

I see where you are coming from but the bike analogy doesn't work. If you are travelling at 20mph you are travelling at 20mph.

The wind direction helps you or hinders you. You just have to exert more or less energy.

A much better explanation is walking along a moving aircraft or train. So your speed is walking speed + vehicle speed.

But then you have to say, a la Who Wants To Be A Millionaire, but light is not like that and that's where it all gets confused.

[gkec]

Special relativity is all about thought experiments. You observe another frame of reference from your own and work out what is happening.

For example imagine that person A has a clock that shows seconds. If person B is travelling away from the clock then the light from one second to the next has to travel further so time appears to be slow.

Another phenomena is that an object travelling towards you will appear bent because it takes longer for light to travel from the far edges of the object than the middle. In effect you see the edges further back in time than the middle.

From this point of view you can derive the length contraction time dilation mass increase -- everything.

[nmoushon]

I see where you are coming from but the bike analogy doesn't work. If you are travelling at 20mph you are travelling at 20mph.

The wind direction helps you or hinders you. You just have to exert more or less energy.

A much better explanation is walking along a moving aircraft or train. So your speed is walking speed + vehicle speed.

But then you have to say, a la Who Wants To Be A Millionaire, but light is not like that and that's where it all gets confused.

If the bike analog thing doesnt work then how come if my scheduled travel time flying from London to New York is say 8 hrs on the nose. But when we get close to landing the pilot comes on and says looks like we had a good tail wind and we have arrived 20min early. How does that work then?

[RogerTheDodger]

Here's my take on this....

On a bike in the 20mph wind, if you are not putting energy in (pedalling) then assuming a perfect system the wind moves you at 20mph... if you now start pedalling and use the same amount of energy as you would to do to get to 20mph with no wind, you are now doing 40mph as your inertial reference frame is already +20mph... so I think nmoushon is right.

[ollypenrice]

Well this isn't directly about the speed of light but I was discussing this with the wife and was explaining how it is independent from the source of the light....

What I tried explaining is that it always remains the same, even if you were travelling at 10mph and turned on a torch, it wouldn't then be travelling at the speed of light plus 10mph.

I was telling her how it is different if you were riding a bike at 20mph and there was a constant tail wind of 20mph then you would be travelling at 40mph. But she can't grasp this. She thinks that if you are travelling at 20mph and the wind is the same speed the wind won't affect you, I guess she's thinking the wind wouldn't catch you up. And if you were travelling at 20mph and the wind was 40mph then you would be travelling at 20 + 20 not 20 + 40.

I'm correct yes? But how can I explain this to her?

The bike analogy isn't right. If you sit still on a level road on a stationary bike with a 20 mph tailwind you'll ... sit still! However if you ride at 20 with a 20 tailwind you will be riding in what appears to be still air. Maybe that's what you meant? A 20 mph tailwind won't add 20mph to your speed over the ground. It will add something, but not 20mph. There are many mechanical losses and a cyclist is not a perfect sail when it comes to harnessing the wind energy.

The case of the aircraft is more complicated because, unlike the bicycle, it has no mechanical relationship with the ground but is moving in moving air. I would expect it to gain more from the tailwind for this reason but there will be host of complications which a pilot would doubtless explain to us! I doubt that a 200mph tailwind really adds 200mph to the aircraft's speed over the ground but, as I say, I would expect an aircraft to gain more than a land vehicle.

Olly

[ArmyAirForce]

.....if my scheduled travel time flying from London to New York is say 8 hrs on the nose. But when we get close to landing the pilot comes on and says looks like we had a good tail wind and we have arrived 20min early. How does that work then?

Because aeroplanes fly in a fluid ( air ) that moves independantly. Same as swimming in a river. You have your speed plus or minus the fluid speed, depending on which direction you and the fluid are travelling.

[acey]

This is not a question about light, it's a question about a bike in a tailwind.

The original poster and his wife are both incorrect. If you ride a bike at 20mph relative to the ground in a tailwind of 20mph then you are travelling at 20mph relative to the ground and are at rest relative to the air. If you switch off the wind then you go a bit slower than 20mph, but we can't predict how much slower, unless we make numerous assumptions about air resistance and other forms of energy loss. The wind helps a bit - that's all we can say.

Similarly, if you ride at 20mph relative to the ground in still air, then switch on a 20mph tailwind, you will go a bit faster, but not 40mph relative to the ground.

There's a site that works it out (subject to assumptions) if you put in all the various parameters:

http://www.bikecalculator.com/

With the default parameters (including 0 headwind - mph or kph) you have a bike speed of 28.44. Putting in a windspeed -20 (i.e. tailwind) gives bike speed 41.88 which is an increase of 13.44.

There was an extensive discussion about the effect of tailwind on a bike in this cycling forum:

http://forums.bicycl...65/m/4801036357

My advice is that if you want to talk about special relativity then don't use a bike as an analogy. Boats on water are better (as long as it's not a pedal boat).

[Treeden]

If the bike analog thing doesnt work then how come if my scheduled travel time flying from London to New York is say 8 hrs on the nose. But when we get close to landing the pilot comes on and says looks like we had a good tail wind and we have arrived 20min early. How does that work then?

This has been answered already but just to flesh it out a little you could also say that the pilot could in fact adjust the power settings of his engines to allow for a head or tail wind. In this way he could arrive pretty much exactly when he is scheduled to, but as always, there are other things to take into consideration, the most important of these (after flight safety) is fuel consumption.

As aircraft spend the majority of their flight time cruising at altitude, and given that modern jet engines have a ‘sweet-spot’ where they deliver the best balance of power and fuel efficiency, airlines use this flight phase to maximise their fuel savings. If the pilot (or the flight computers via the autopilot) are constantly changing the power settings to hit a scheduled arrival time, then these fuel savings would be lost.

[Superdavo]

I recently thought of an analogy.

Say you were travelling in a car, with the windows shut. If you make a sound, because the air isn't moving relative to you, it appears to travel at the speed of sound relative to you, but it moves slightly faster relative to the outside. However, light behaves similar, but in time. If you look into the car, it seems like the sound is travelling faster, and the light would seem to travel faster in time. Strangely, light does the opposite, so it's as though the car is in reverse, and the light (or sound in the analogy) appears to have a lower velocity in a forward direction, and this difference counters the effect of the light gaining speed, by altering time instead.

David

[Scott]

I would have thought that if your speedo says 20mph then you're going 20mph. a tail or head wind just determines how easily this is achieved. if i'm sat on my bike not peddling with a tailwind of 2 miles an hr, I won't move. I assume this is because of friction ranging from the wheels touching the ground right through to the bearings in the wheels. I would also assume that the plane analogy differs because the plane attains it's thrust by pushing against the air where as the bikes forward motion is caused by the wheel "dragging" you along the surface.

[Watchman]

Yes, the bike analogy wrong as if you are going 20 mph, then you are going 20mph no matter what environmental factors may be affecting you. It may be that it was easier for you to reach 20mph due to the 20mph tail wind but that tail wind wont add to your original 20mph speed.

Light is a very funny thing and subject to subjectivity (that sounds so wrong but ...), for instance according to special relativity if you take the train example where a man in the centre of a carriage lights a lighter and a man at either end of the carriage (equal distance from the central man), the men on the train all percieve the light to arrive at the men at the ends of the carriage at exactly the same time. However, this is where it gets strange, if a fourth man was standing on a platform as the train went past, at the exact time the event took place and assuming he could see the light travelling along the carriage, he would see the light arrive first at the man at the back of the carriage and then arrive at the manat the front (as in the direction the train is travelling) of the carriage. This effect is due to the blue/red shift of light as observed from a subjective standpoint.

The speed of light (while precise) alters depending on a subjective or objective standpoint. Time dilation is a very hard thing for some people to get their heads around as it would make it appear we are all in fact living at slighly diferent times simply due to changing velocities. A quote I particularly like is that perception IS reality, subjectively speaking. What you percieve as being reality sat outside looking through a telescope is very very different from the reality percieved by a theoretical alien that see's with one eye in the ultraviolet spectrum, tastes smells with its feet and is travelling at 99% of the speed of light orbiting a super massive black hole near the centre of the milky way!

[Watchman]

Sorry if that makes aweful sense, I may have confused myself.. :S