Headlights at the speed of light.....

[Monocock]

I'm not a frequent visitor to this section but somebody in Private Flying suggested I asked my question here.

To tie up an old debate I have been having with a friend can somebody give me the UNDOUBTEDLY CORRECT answer for the following?

The driver of a car travelling at night time at the speed of light turns his headlights on, do they light the way ahead of him?

[russelld]

if you were sat immediately above the lights then no, you wouldnt see anything because the car arrives on the surface taht would reflect your lights immediately, but say you were 1.5m behind the lights (approx. driving position) then your brain would have 0.0000000025 seconds to interpret the signal and see the light, which i believe is a bit too quick

[ddraig]

the UNDOUBTEDLY CORRECT answer:

(if the current understanding of the universe is undoubtedly correct)

Yes

no matter what speed you are travelling at, the speed of light is constant relative to your motion.

ie the light still travels away from you at the speed of light, even though you are travelling at the speed of light.

... that's what relativity is all about


Hence the lights would light the way ahead of you... (from your perspective at least).


Mind you, travelling at about 670 million miles per hour but only lighting the road a few hundred metres ahead might not be much use after all...

[Eckhard]

As an object's speed increases, so does its mass until at the speed of light the mass is infinite. Therefore you would be unable to:

a) Lift your hand to reach the light switch;
b) Move the light switch; or
c) Open your eyelids to observe the results!

However, I do agree that the speed of light is constant, relative to the observer, irrespective of the observer's speed.

[inbalance]

ddraig wrote:

"the UNDOUBTEDLY CORRECT answer:

(if the current understanding of the universe is undoubtedly correct)

Yes

no matter what speed you are travelling at, the speed of light is constant relative to your motion.

ie the light still travels away from you at the speed of light, even though you are travelling at the speed of light.

... that's what relativity is all about"

Thats wrong, like Einstein tells us, the speed of light is 300000 km per second.
That´s a fixed speed.
So if you are travelling with lightspeed, it will remain dark in front of you.

[sss]

got any more like that, its quite intresting watching the answers

[Monocock]

Thanks for the response although I must admit I'm still none the wiser.....

I posted this thread in Private Flying too and the response was just as mixed. Perhaps there is no definitive answer but at least it makes us wonder.

How about.....

If a marksman fell from an aircraft at the speed of a bullet from a gun and he fired his gun up at the aircraft............would it leave the barrel or would it stay in the barrel???????

[inbalance]

take a look at that:

http://www.pbs.org/wgbh/nova/einstein/hotsciencelight/

http://www.amnh.org/exhibitions/einstein/light/constant.php

[ddraig]

yes, of course, travelling at the speed of light is not possible, but the question still has its merits. It is useful to help in the understanding of relativity.

Ignoring the (minor!) problem of it being impossible, i stand by my earlier response...
... let's say you're travelling at 0.99*c (c: the speed of light) to make it just practically impossible.

Einstein's theory tells us that the speed of light is constant relative to any inertial (that is, non-accelerating) frame of reference. It does not say that the speed of light is constant relative to just one inertial frame as suggested. That is what newton's laws were all about.

It seems that the common misconception with this type of problem is as follows:
If you are travelling at .99c, and the light is travelling away from you at speed c, then the surely the light has speed 1.99c. This is not the case.....

Relative to you the light has speed c, and relative to a stationary observer the light also has speed c, not 1.99c... Even if you could travel at 1000c, the speed of light relative to you would still be c.


So according to Einstein's theory of relativity the definitive answer to the question...
it's not possible.. but if you could get to that speed, then yes.. the road would be lit...

[777AV8R]

Another one, you say?

OK, this one used to keep us entertained for 15 hrs as we trundled across the North Atlantic in a C-130...we still don't have the answers....so here goes.....

A C-130 takes off with a load of Budgie birds. When loaded, it was kind of cool, so the birds were at rest and content! Therefore, when the birds were weighed on the scales, the total of their weight came to 30,000 lbs! Huge birds!!!

Now, the aircraft gets airborne and strikes off to it's destination. The birds become excited and start flying around in their huge cage....The question now becomes:

Does the aircraft still have the weight of 30,000 lbs with the birds all flying around, or do the birds still exert their weight on the aircraft?

[jtr]

first off, that is about 450,000 budgies. Cramming them into the back of a c-130, I doubt they will have the room to fly.

Assuming the a/c is sealed, then yes, they will weigh the same flying or not, due to the downdraft effect.

[Golden Rivet]

If you stick a Camaeleon on a mirror what colour will he go ?

[russelld]

heres another great one, i know theanswer, but lets see what you lot think...

a man stood in a train facing the direction of travel fires a rifle at a target sat at the opposite end of the carriage.

another person on the train times the interval between him pressing the trigger (exactly) and the bullet hitting the target.

The train is moving at 120mph say.

Another observer, stood on the ground beside the track as the train goes past also times the interval between the chap pulling the trigger and the bullet hitting the target.

Are the times the observers record the same?

(Hint: This helps to prove that time is not after all constant!!)

[bookworm]

To about 1 part in 10 to the 13, the times are the same. How good's your stopwatch? :)

[Slasher]

To answer the original question, everything that you (the driver) could see around you (including the sides and back) would be concentrated into a infinitley compressed small bright point just in front of your eyeballs. Your headlights, the road, stop signs, frightened roos etc and your CD-player lights as well as your rear lights would all be in that bright point. So basicaly your stuffed as far as seeing anything ahead. Yep the headlights are in fact shinin on the road ahead of you, and the last thing a goanna would see before you took him out would be your headlights hittin his eyes at exactley C. Also the time taken to get wherever you were goin to at such a great rate of knots would be zero as measured by your watch. If you drove to Proxima Centauri (nearest star) youd be there in an instant but we back on Earth would measure 4.25 years in real time (as against apparant time which would be 7 years near enough).

Since C is a constant to observers, then time is the variable. The road-side garage bloke would see you wizzin past at C but hed definatley see your headlights (and your rear-plate rego too!).

[Lu Zuckerman]

Or possibly Han Solo. And what about Warp Drive?

:rolleyes:

[White Knight]

Well it has to be said that although your mass "increases" as you approach the speed of light "c" it's all RELATIVE. ie your eyelids will still weigh the same to you at c as they do to you at human speed. so you will be able to open them and see.......I think.

[Kestrel_909]

What car does the speed of light and at night time to? Little boy racer lol. I think the correct answer is the one that it would, because it is travelling from its source at a constant speed, and if the source (light on the car) is moving foreward at the speed of light, then the light will stil travel foreward.


As for the marksman, if he is falling to the ground from FL350 at the speed of a bullet, and fires a bullet from a gun, what ever good that would do to save him, the bullet will leave the gun at the normal bullet speed so to speak.So would it not exit the rifle at the usual speed, but because the rifle is doing bullet speed, it would appear to exit at twice the speed but only be doing its normal speed, it appears as two times faster as it is going x m/s that way < and we are going this way > at bullet speed?

Just a thought, keep these coming, very interesting to ask and pass time.

[timzsta]

When standing on the platform at an underground station as the train approaches there is an onrush of air ahead of it. If the train was travelling at the speed of sound would there be an onrush of air ahead of it (I know the answer!)?

[m&v]

you fly an aircraft once around the globe(great circle),what part of the aircraft travels the furthest??
Cheers:rolleyes:

answer tomorrow!!

[russelld]

flying around the globe, all parts of the aircraft travel the same distance. their displacement is exactly the same, since if say it lands at the exact same spot, the noses it back where it started, and the tail is back where it began. providing it's altitude was constant. the actual distance above the earths surface (which would differ as you flew over mountains/sea etc.) wouldnt have an effect since you are effectigvely in free space.

re: the bullet question, don't forget all objects have a terminal velocity. a bullet only travels at the speed of a bullet because 'g' is a variable in working out it's acceleration, so free falling at the speed of a bullet only means the speed (i presume) that a bullet travels horixontally, so it would accelerate from the gun at a great rate, and then maintain that relative speed

heres another load;

1)

1 costs £10, 2 costs £10 and 17 costs £20.

what are they?

2)

a fly hits your card windscreen at 5mph. you are traveling at 50mph. at a split second the fly is stationary, yet your speed does NOT alter atall. how come?!

3)

the Earth's upper atmosphere is radiated with high energy particles from the sun (solar wind) creating a steady stream of positive ions, which, rain down onto the Earth's surface. But the earth's total average surface charge does not change. Why?

more soon if i can remember them

[foghorn]

timsta

No - it's the same principle as when develop on an airframe when the local flow speed is greater that the speed of sound.

M&V

Assuming that the aircraft stays upright during the full journey, the tip of the tailfin will travel the furthest distance (very slightly), as it's always the outermost point of the circle that the aircraft flies around the centre of the Earth.

russell

2) Momentum is conserved (if they were perfectly inelastic indestructable objects which they're not but let's not get into that OK ;)) As there's just a huge difference in mass between the fly and the car, therefore the change in the car's speed is not appreciable.

[pinkie]

Assuming that you had an infinitely deep hole, and an infinitely long piece of rope, what is the longest piece of rope that you could lower in-to the hole?.
(clue. The answer is not an infinitely long piece):confused: :confused: :confused:

Will give the correct answer later...

[compressor stall]

WRT the aircraft, what is the reference for "furthest". Are we talking a distance in free space (as was explained above) or a distance relative to the airport of departure/arrival?

[Burger Thing]

I think it would be quite dark ahead. Apparantly the speed of light is the maximum velocity in our universe. So, if someone would be travelling with the speed of light, emerging light beaming ahead would mean, that the beamlight is travelling faster than the "normal" speed of light relative to other objects

Look at the light of the stars. Most of the positions between our earth and the stars are not fixed. Either because of the movement oif the earth around the sun, or the movement of our solar system in the Galaxy itself, or the movement of other star systems, or because of the spiraling movement of our Galaxy. Or better, take the movement of another Galaxy compared to our Galxaxy (expanding universe).

A star, which has a relative movement towards us, is emitting the light exactly at the same speed as a star, which has a relative movement away from us.

Why? Because the speed of light is fixed. It is the maximun possible speed. So, if an object would travel at light speed already, the light of the headlight wouldn't be able to accelarate away form that source.

[Tricky Woo]

The UNDOUBTEDLY CORRECT answer is:

While travelling at the speed of light, time dilation would approach zero. This means that the headlight switch would take longer than the lifetime of the universe to click from 'off' to 'on'.

Better put the kettle on, mate.

Furthermore, space dilation would make you look a bit thin from the outside. A bit like an anorexic, you'll think yerself as fat as normal in the mirror, but the non-speed of light onlookers will see you contract to a length approaching zero.

TW

[HEP-PPL]

Hello everyone, this is my first post to PPRUNE. I'm a private pilot,
however during the day I'm a particle physicist, so feel qualified to
respond.

Firstly, as has alread been stated, it will not be possible to move at EXACTLY the speed of light. The inertial mass of an object (that is the mass that you think of when you calculate momentum, kinetic energy, etc) increases with velocity and as a result an infinite amount of energy is required to accelerate an object to exactly the speed of light. Please note that this is the mass of the "car" as viewed by a stationary external observer. From the point of view of the driver, the car is stationary and so the mass has not changed at all. Hence, arguments about being unable to open your eyelids are not valid. I will return to time dilation, length contraction, etc later.

As has already been stated, the argument can still be made for a hypothetical car moving at 90%, 99%, etc of the speed of light.

One of the important points of Special Relativity is that the speed of light is constant irrespective of the frame of reference, so from the point of view of the car, the light is moving at the speed of light. From the point of view of an observer at the side of the road the light is STILL moving at the speed of light.

So, for our car at 90% of the speed of light, the light will indeed strike the road and some fraction of it will be reflected back to the driver.

The driver will then see this light, which has been reflected by the road which to the driver is racing towards him at 90% of the speed of light. As a result, the light will be "blue-shifted" and depending on the speed of the car, it will arrive with a much higher frequency and thus not be visible to the driver's eyes.

To return to the comments about time dilation and length contraction. It should be remembered that these are what an external observer sees when an object travels close to the speed of light.

So, the driver in the car, looks at his/her watch and sees the second hand moving at the same speed as before. To the external observer, however, the clock will appear to be running very slowly. This is the origin of the twin paradox (a google search should find this easily enough for those who have not encountered it).

The length of an object appears to be contracted IN THE DIRECTION of that object's motion.

So from the point of view of an external observer, the car will appear to get shorter. The driver (sitting in their seat) will be the same height, but their arms reaching to the steering wheel will appear (to this external observer) to be shorter.

From the point of view of the driver, they have not changed one bit, however the road, which is rushing towards them at 90% of the speed of light, will appear to be shorter.

These results of Special Relativity are confirmed (and indeed
used) by physicists such as myself in our day-to-day work.

To bring this into a slightly aviation-related context, a large fraction of the cosmic rays that are produced in the upper atmosphere and arrive at the Earth's surface are elementary particles called muons.

A muon is like a heavy electron, except that its lifetime is 2 micro-seconds!

A quick calculation shows that the muon should thus travel (on average) 600 metres. This cannot possibly be the case as the muons are produced in the very upper atmosphere.

So, from the point of view of an observer on earth, you can say that the muon, travelling close to the speed of light, travels further becaus its internal clock is slowed down, and thus it does decay after 2 microseconds, but that 2 microseconds takes a lot longer to occur for the observer on Earth.

From the point of view of the muon, the Earth is rushing towards it at close to the speed of light, and so the distance it has to travel is contracted. The muon thinks that is has lived for 2 microseconds, but that the size of the entire atmosphere has shrunk to less than 600 metres!

I hope this helps.

[m&v]

foghorn absolutely RIGHT-the tip of the fin transscribes a wider circumference..An olde Sailing question!!
Cheers :rolleyes:

[Self Loading Freight]

HEP-PPL--

Did you read the New Scientist article about (or do you already know of) that Italian chap who thinks special relativity is past its best? I'm not sure I understand it, but he seems to say that different energies of particle have their own maximum speed, and consequently c has been decaying since the very energetic start of it all... and so length can be invariant regardless of frame.

Um.

Anyway, the combination of time dilation and the various frames in classic special relativity does seem to mean that you could see the road ahead, although what you see as the road and what an observer outside your frame would see as the road would be somewhat different. You see your photons leave your headlamp at c, impinge on the road and return, and interpret the short travel time as a short road: someone outside your frame would interpret the short travel time as you getting to the returning photon almost as soon as it reflected. Is that right?


I'm down with that... but I've never understood what the photon thinks of it all. A photon travels at the speed of light and thus has no time, yet we see them as objects that travel at a finite speed and can be manipulated predictably (and how!) in real time. Could the photon's lack of time explain some of the rather spooky quantum effects of action at a distance?

R

(oh, and I Never Knew That about atmospheric muons. Pprune's a joy, innit)

[HEP-PPL]

SLF --

I recently read the New Scientist article, but I'm afraid that's the
limit of my knowledge of this theory.

I'm not aware of any consequences of this theory that could be
tested in my field, but if they exist I'm sure they will be checked.
Up until now Special Relativity has proven to be a very accurate
and useful theory. On an aviation note, I think that GPS uses
corrections for both Special and General Relativity.

As to the photon's view of the universe. Being the only
massless particle (in our current understanding anyway) it is
of course travelling exactly at the speed of light. This means that
from its point of view, the universe is contracted into a plane,
perpendicular to its direction of travel. And of course, time
completely stops, so while to us the Universe is 4D, the photon
sees it as 2D.

These results are purely Special Relativity and would apply to any
theoretical object that you could make with no mass.

The "spooky action at a distance" stuff is just a result of Quantum
Physics.

A lot of these odd behaviours do not need zero mass, indeed its
possible to reproduce (for example) the interference effects that
you see with light, using electrons. You can perform the two slit
experiment with light and electrons and in both cases you will
see a diffraction pattern, you can even choose where/how to
measure the electron and see either wave or particle behaviour.

There are several examples in particle physics of the
superposition of two quantum states to produce a particle that
we can then observe. One consequence of this superposition and
a mass difference is that the type of particle we observe will
change depending on where we try to measure it. The most
recent example of this has been the measurement of Neutrino
Oscillations.

Entaglement of states can in principle (although so far I don't
think in practise) be measured at the experiments running in
America and Japan at the so-called "B factories". These
experiments collide an electron and positron (anti-electron)
at just the right energy to produce a B0 and its anti particle.
This B0 is a particle that contains the second heaviest known
quark, the b quark.

Anyway, these two particles are produced coherently in an
entagled state and then travel some distance apart before
decaying. The decay products are then measured in the detector.

The experiments are actually looking for something called CP
violation, but I won't go into that unless asked!

[captchunder]

Blimey! Thats cleared that one up then??:eek:

Anyway, the chameleon: it will change to the colour of the ceiling, assuming the mirror is on the floor.

My brain hurts...

[Look_Up]

HEP-PPL:
The experiments are actually looking for something called CP violation, but I won't go into that unless asked!

Please could you go into this, at least a little way. It's good to hear someone responding with authority on this subject! :D

[Slasher]

HEP-PPL the question was a car traveling at speed C. While I dont question your argument of the effects at sub-light yet near-C speeds, I cant see why Q.P. would disallow speculative figures of a theoretical mass at C itself, using Plancks constant and the equations of elementary length and time:

Length:

r0 = h*/M0 x c0
= 6.63 x 10-34J x s/1.6 x 10-60kg x 3 x (10-8 m/s)
~
= 1.4 x 10-102m

(h = Plancks Constant, M0 the total matter/energy of the Universe and c0 the velocity of light in vacuum)

Time:

t0 = r0/c0
~
= 4.7 x 10-111s

Elementary length and elementary time define an absolute space interval and an absolute time interval. So its understood that any observers, independant of their relative movements will measure the same value of elementary length and elementary time. This can be seen from the equations as theyll all measure the same velocity of light and the same Planck Constant. If one plays around with the Lorenz Transformation equations for a vectored moving frame (where y' = z in this case) with a "Galilean twist" then a speculative "whats it like at C?" result of sorts could be obtained.

I hope to hell that makes some sort of bloodey sense. I just mashed 2 million brain cells figurin it out!

[HEP-PPL]

Lets see if I can muddy the waters a little further :D

Firstly, I think I should go back and discuss Relativity and Quantum Physics a bit more.

Relativity deals with the mechanics of objects that are moving at a significant fraction of the speed of light. If you are dealing with masses that
are moving very slowly, the maths is then the same as the Newtonian description.

Quantum mechanics cares about the very small, and how its behaviour is different from the normal scale of the Universe that we are used to.
If you talk about car/person sized objects then it is not relevent to think of QM effects as they are infinitessimally small.
This is similar to the way we don't worry about Special Relativity when dealing with objects that move at "every day" speeds, such as cars, trains, etc...

So it is possible to study the Quantum mechanics of a system involving a very small mass that is not moving close to the speed of light.
This would then be pure Quantum Mechanics and Relativity would not play any part.

It is then possible to extend QM to include the treatment of very small masses moving at close to the speed of light,
and you get Relativistic Quantum Mechanics.

So, firstly Slasher, while your maths may indeed be correct (I haven't checked it, we Particle Physicists are lazy and use a
system of coordinates in which c and h/2pi are both 1!) it does not apply to the car at speed of light question.
The car is very much a macro object and so any QM effects will be practically zero.

Relativity states that you would require an inifinite amount of energy to accelerate the car to the speed of light which is
why I say that the car will never reach the speed of light exactly.

Now, Look_Up:

There are three basic symmetries that apply in the interactions of
the fundamental particles:

C - Charge Conjugation
P - Parity Conjugation
T - Time Conjugation

If a process conserves one of these symmetries then it means that if you take an interaction, look at it, and then take the same interaction
after applying the symmetry you should get the same outcome.

As an example, C means changing the charge, in other words turning matter into anti matter. So if I have an interaction involving matter
particles it should behave exactly the same as an interaction involving the anti-matter particles.

The Parity symmetry involves flipping all 3 axes, that is X -> -X, Y -> -Y, Z -> -Z

So far I've mentioned interactions, without defining what I mean.

There are 3 fundamental forces that apply to subatomic particles (and then the fourth force is of course gravity).

The Electromagnetic force is what makes an electron repel another electron, it is also responsible for the forces that we experience everyday,
as well as for electromagnetic fields, light...

The Weak force results in nuclear decay, allows processes such as the fusion that goes on in the Sun and causes the decay of the muons that I refered to earlier.

The Strong force is what holds the quarks together into what we know as protons, neutrons, etc... It is the residual of this force that holds
the protons and neutrons together in the nucleus of an atom.

OK, so the point is that it was discovered that Weak interactions do not conserve P or C.

It is still believed that the combination CPT (that is change matter to antimatter, reverse all coordinates AND reverse the flow of time!) is
conserved in all interactions, which leads people to wonder if the combination CP is.

On first looking, it appeared that CP was conserved but on closer inspection it has been discovered that in the interactions of certain particles,
CP is violated at a VERY small level.

Now, why do we care about this?

The reason is that the Big Bang theory says that the Universe was produced in a "Bang" of energy, the Universe expanded, cooled and matter was produced.

The matter was produced through the production of matter/anti-matter pairs from the photons that constituted the original energy.

In the same way that if you take a proton and an anti-proton they will annhiliate and give you energy (ie photons), the reverse process is also possible (under certain circumstances).

So, the theory says that the energy was turned into matter/anti-matter in equal amounts.

But of course, we are in a matter universe, at least locally, so what happened to all the anti-matter?

A consequence of CP violation is that it results in matter behaving ever so slightly differently to anti-matter and so what we see as the Universe today is the tiny excess of matter that was left over after the matter and anti-matter in the early universe was formed, interacted and annihilated!

The goal now is to try to measure enough CP violation to explain the observed excess of matter over anti-matter in the early universe, and so far we come up WELL short.

[cwatters]

...and in case anyone missed last years news...

http://www.research.ibm.com/quantuminfo/teleportation/

http://www.its.caltech.edu/~qoptics/teleport.html

http://abcnews.go.com/sections/science/DailyNews/teleport981022.html

Today a photon, tomorrow...

[PA38]

1 costs £10, 2 costs £10 and 17 costs £20.

what are they?

House numbers;)

But if you stood on the south pole (magnetic) which way would your compass point??

[compressor stall]

In theory if you were able to stand exactly at the south magnetic pole (which is near impossible as it moves around so quickly over a couple of hundred metre radius) AND your compass was weighted correctly, your compass would point towards the Zenith.