Silly speed of light question..

Silly speed of light question..

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Discussion

TheLemming

4,319 posts

227 months

Monday 25th October 2004
quotequote all
ATG said:

TheLemming said:
Been trying to recall the answer to this one, I read one a while back that seemed rather effective.

IIRC doesnt it have something to do with the time dilation experienced as one travels closer and closer to the speed of light?



You don't experience anything odd. Inside your fast moving car everything is normal. When you look out the window at the people wizzing past you, they seem to be running in slow motion. As far as you are concerned their time is running very slowly.

However, when they watch you drive past them at enormous speed they think your time is running very slowly.

Which is odd.


I meant, given that speed = distance / time

Time is percieved differently for those travelling very quickly (ie an appreciable fraction of lightspeed) and the observers.

If that's the case, then light, which has a fixed speed, would cover a shorter distance in the shorter period of time to have the same speed?

So If I were at 50% of the speed of light, I would perceive light to be travelling out in front of me normally, at the speed of light.

This would be because the speed of light is a constant, speed = (distance / time) and the time has been shortened. Therefore the only way to reconcile it is for the distance travelled to grow.

As a result, it only LOOKS like its travelling faster than light, its actually not.

OR have I got the wrong end of the stick?

K.K.

397 posts

200 months

Monday 25th October 2004
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Ah but would you still be "caught" by the speed cameras?

havoc

25,447 posts

197 months

Thursday 28th October 2004
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distance = speed x time is Newtonian Physics.

Anything anywhere near the speed of light is Einsteinian Physics, probably Quantum! ( ;o) )

You can't mix your theories, now!!! Trouble is, most people only go as far as Newtonian Physics...so "Relativity" is a catch-all to the masses for "abandon all logic, ye who enter here!".

And even Einsteinian physics is guesswork and approximation...think of the Ian Stewart/Terry Pratchett concept of "lies to children" (All science taught at school is a gross simplification, therefore a lie. At GCSE, they're more sophisticated lies; at A-Level, more sophisticated still, but still lies; At Bachelor's degree, very sophisticated, and close to the truth, but no cigar!!!)

So I guess we're all lying to each other here!!!!

tvrgit

8,449 posts

214 months

Thursday 28th October 2004
quotequote all
havoc said:
So I guess we're all lying to each other here!!!!

Not at all, my Dyson really is a very good vaccuum.

swilly

9,699 posts

236 months

Thursday 28th October 2004
quotequote all
havoc said:
distance = speed x time is Newtonian Physics.

Anything anywhere near the speed of light is Einsteinian Physics, probably Quantum! ( ;o) )

You can't mix your theories, now!!! Trouble is, most people only go as far as Newtonian Physics...so "Relativity" is a catch-all to the masses for "abandon all logic, ye who enter here!".

And even Einsteinian physics is guesswork and approximation...think of the Ian Stewart/Terry Pratchett concept of "lies to children" (All science taught at school is a gross simplification, therefore a lie. At GCSE, they're more sophisticated lies; at A-Level, more sophisticated still, but still lies; At Bachelor's degree, very sophisticated, and close to the truth, but no cigar!!!)

So I guess we're all lying to each other here!!!!


Added to that is the fact that its all theoretical anyway.

Didnt Steven Hawking recently suggest his lifes work may be a load of bollocks????

vetteheadracer

8,267 posts

215 months

Thursday 28th October 2004
quotequote all
If you were travelling at the speed of light you would not be able to reach forward and turn the lights on as the G forces would be pinning you to the seat
Also your eyes would have exploded due to the excessive speed and therefore you wouldn't have been able to see anything past about 10,000 mph

chris71

21,500 posts

204 months

Thursday 28th October 2004
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wayne_uk said:
Right,your car is having a nice little trundle along at the speed of light ( trying to keep it car related here.. ) , you turn you lights on..

Does your lights go twice the speed of light,or don't they come on ?,or do they come on,but stay still..ie it the car does 5 mph slower will you see light travelling at 5mph ?


beer...,stop...,now....



Simple - the light will not move relative to the headlamp, however fast it starts off it cannot move faster than 186,000 miles/sec. Erm, that is assuming their aren't any non-einsteintonian physcists out here.

Like my new word? :-)

tvrgit

8,449 posts

214 months

Thursday 28th October 2004
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em... have we moved back in time too?

just that I got this feeling of deja vu

jimbro1000

1,617 posts

246 months

Thursday 28th October 2004
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vetteheadracer said:
If you were travelling at the speed of light you would not be able to reach forward and turn the lights on as the G forces would be pinning you to the seat
Also your eyes would have exploded due to the excessive speed and therefore you wouldn't have been able to see anything past about 10,000 mph


Only if you were accelerating - if you were at the speed of light you would be not be changing speed which means no artificial g forces at all

On the other hand the increase in mass to get there would likely have spread you and your car over a very large area as the gravity field generated would be noticible to say the least...

On a more serious note - wasn't there something about all this in the papers last month, some mad scientists actually trying to prove the foundations of Einsteins theories (which incidentally are *just theories*)

Along side that I'm sure I remember reading something about quantum spin being responsible for mass which means light could easily have mass in some situations but also that other recognised "massive" particles could equally have their mass removed...

z_chromozone

1,436 posts

211 months

Thursday 28th October 2004
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paolow said:

alexkp said:


gh0st said:



groucho said:

Speed of light is not possible as for reasons that thebluemonkey explained.
But who knows? light travels at that speed, so why not other things.

Grouch.





Because a photon has no mass therefore does not need infinate energy to achieve the speed of light.




That is most probably true - but some physicists now believe that a photon may have mass, and if true it may have quite profound implications. There are also theoretical particles even smaller...

Anyhow, I'm kinda reaching the limits of my knowledge here. To go further would require some very long posts and someone much more informed than I.



if solar winds exist and solar sails are theoretically feasible, surely then the photons do have a mass to exert pressure?



The solar wind is mostly very fast moving protons not light.

Z

love machine

7,609 posts

197 months

Thursday 28th October 2004
quotequote all
swilly said:

havoc said:
distance = speed x time is Newtonian Physics.

Anything anywhere near the speed of light is Einsteinian Physics, probably Quantum! ( ;o) )

You can't mix your theories, now!!! Trouble is, most people only go as far as Newtonian Physics...so "Relativity" is a catch-all to the masses for "abandon all logic, ye who enter here!".

And even Einsteinian physics is guesswork and approximation...think of the Ian Stewart/Terry Pratchett concept of "lies to children" (All science taught at school is a gross simplification, therefore a lie. At GCSE, they're more sophisticated lies; at A-Level, more sophisticated still, but still lies; At Bachelor's degree, very sophisticated, and close to the truth, but no cigar!!!)

So I guess we're all lying to each other here!!!!



Added to that is the fact that its all theoretical anyway.

Didnt Steven Hawking recently suggest his lifes work may be a load of bollocks????



Yes, I believe he has also changed his career now.

www.mchawking.com

swilly

9,699 posts

236 months

Thursday 28th October 2004
quotequote all
Does light always travel at the speed of light?

When light is reflected it changes direction. Does it therefore need to decelerate in the intial direction and accelerate in the new direction?

TheLemming

4,319 posts

227 months

Thursday 28th October 2004
quotequote all
swilly said:
Does light always travel at the speed of light?

When light is reflected it changes direction. Does it therefore need to decelerate in the intial direction and accelerate in the new direction?



This is one of those wonderfull discussions where all answers have a caveat and even the experts dont really understand the subject. God knows how the rest of us muddle along...

As I understand it, light travels at a constant speed (distance covered over time). Therefore even if it bounces off something, its still covering the same distance over the same period of time and as a result doesnt slow down.

Having said that, I seem to recall an article a few months back about some research succeeding in slowing light down. Dont know how they did it...

lunarscope

2,894 posts

204 months

Thursday 28th October 2004
quotequote all
TheLemming said:

swilly said:
Does light always travel at the speed of light?

When light is reflected it changes direction. Does it therefore need to decelerate in the intial direction and accelerate in the new direction?




This is one of those wonderfull discussions where all answers have a caveat and even the experts dont really understand the subject. God knows how the rest of us muddle along...

As I understand it, light travels at a constant speed (distance covered over time). Therefore even if it bounces off something, its still covering the same distance over the same period of time and as a result doesnt slow down.

Having said that, I seem to recall an article a few months back about some research succeeding in slowing light down. Dont know how they did it...


Not only did the researchers slow light down, they actually stopped it for a short period and then let it continue on it's way.

esselte

14,626 posts

229 months

Thursday 28th October 2004
quotequote all
swilly said:
Does light always travel at the speed of light?

When light is reflected it changes direction. Does it therefore need to decelerate in the intial direction and accelerate in the new direction?




Isn't acceleration a change in velocity?Velocity has 2 elements speed and direction .So can you accelerate/decelerate without changing speed (just direction)?

>> Edited by esselte on Thursday 28th October 16:38

swilly

9,699 posts

236 months

Thursday 28th October 2004
quotequote all
esselte said:

swilly said:
Does light always travel at the speed of light?

When light is reflected it changes direction. Does it therefore need to decelerate in the intial direction and accelerate in the new direction?





Isn't acceleration a change in velocity?Velocity has 2 elements speed and direction .So can you accelerate/decelerate without changing speed (just direction)?

>> Edited by esselte on Thursday 28th October 16:38


You can accelerate but stay at the same speed by travelling in a circle (constantly changing direction therefore accelerating in a new direction).

TheLemming

4,319 posts

227 months

Thursday 28th October 2004
quotequote all
swilly said:

esselte said:


swilly said:
Does light always travel at the speed of light?

When light is reflected it changes direction. Does it therefore need to decelerate in the intial direction and accelerate in the new direction?






Isn't acceleration a change in velocity?Velocity has 2 elements speed and direction .So can you accelerate/decelerate without changing speed (just direction)?

>> Edited by esselte on Thursday 28th October 16:38



You can accelerate but stay at the same speed by travelling in a circle (constantly changing direction therefore accelerating in a new direction).



Are you sure?

I was under the impression that accelleration was a change in speed occuring over time.

if your distance / time (which is speed) is constant, you cant be accellerating?

havoc

25,447 posts

197 months

Friday 29th October 2004
quotequote all
Acceleration is delta-v, or, for the mathematicians, dv/dt (Velocity differentiated with respect to time, i.e. THE RATE OF CHANGE OF VELOCITY)

Velocity has a directional component, speed is just a non-directional measure of velocity.

e.g. You're doing 60 down a country road, in a straight line. Both speed and velocity are constant. You go around a corner, still at 60 on the speedo. Your speed has not changed, but your velocity has as you are now pointing in a different direction.

So, the SPEED of light is constant (well, maybe, but lets not confuse things now), but clearly it's velocity will be anything it wants, depending on direction, but always with the same speed.

So light reflected in a mirror has constant speed, but, at the point of reflection, a massive delta-v of approaching twice the speed of light (as the incidence angle approaches 180-degrees).


Re: the squashed flat point - sorry, but no, you only get squashed flat by acceleration, NOT by speed/velocity. Think - Evo FQ-900 at full acceleration pushes you back hard in your seat, but tops out at 155mph. Yet a 747 gets up to 3x or 4x that speed (500+mph), but you don't feel much acceleration at all once you've left the ground...

So travelling at the speed of light won't squash you...trying to accelerate hard to get to that speed will! But if you can accelerate constantly at 1g, you can walk around quite happily on the "back wall" of your spaceship all the way to the speed of light, although it'll take you ~350 days to get to that speed.

ATG

17,532 posts

234 months

Friday 29th October 2004
quotequote all
swilly said:
Does light always travel at the speed of light?

When light is reflected it changes direction. Does it therefore need to decelerate in the intial direction and accelerate in the new direction?


No, it doesn't always travel at "the speed of light". Light slows down when it passes through any tranparent medium. In fact this is how lens work. The measurement of how much light is slowed down is the "refractive index" of the medium. The degree to which light is slowed varies with the frequency of the light (leading to the prism effect, chromatic aberation in cameras, etc.) (You can even make optical "sonic booms" by firing high speed electrons into glass. As they pass through the glass, until they hit something, they are travelling faster than "the speed of light in glass".)

Light happens to have no rest mass, so in a vacuum it happens to travel at "c". Referring to "c" as "the speed of light" in the context of relativity isn't very helpful. When we say "the speed of light", what we really mean is "the asymtopically reached maximum possible relative speed of two objects in our Universe", or something along those lines. I.e. the speed "c" is a fundamental property of space-time, it is a fundamental property of the geometry of the Universe. "c" also happens to be the speed of light in a vacuum, but that is of secondary importance.

Pigeon

18,535 posts

208 months

Friday 29th October 2004
quotequote all
ATG said:
(You can even make optical "sonic booms" by firing high speed electrons into glass. As they pass through the glass, until they hit something, they are travelling faster than "the speed of light in glass".)

Cerenkov radiation... perhaps more familiar as the blue glow surrounding nuclear fuel elements in water, which is caused by neutrons travelling faster than the speed of light in water. Neutrons, being dense and uncharged, tend to go a long way before they hit something, so you get a blue haze extending quite a long way from the neutron source.