# Thread: Light Speed question

1. Established Member
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Forgive my ignorance here, lol.

If I am in my car traveling at or just above the speed of light and turn on my headlights, will they even make a difference? I mean, will the light coming out of them be traving at the speed of light+the vehicles speed?

Sorta along the same lines, driving my car at the speed of sound and honk the horn, what happens?

I know if I get hit with a basball thrown from a moving vehicle, it will have a stronger impact becase it is traveling at the base speed of the platform + whatever speed it gained from throwing. Same true for slight and sound?

Again, forgive the childish questions, but I assure you I have actually pondered this over the years, lol.

Thanks&#33;
Jerry

2. Hi Jerry, welcome to the forum.

You&#39;ve asked several questions:
If I am in my car traveling at or just above the speed of light and turn on my headlights, will they even make a difference?
Your car won&#39;t be travelling at the speed of light or faster. Special relativity guarantees that. Because the situation is not possible physically we can&#39;t say what would happen with your headlights, but at that speed, you&#39;d better hope nothing is in front of you.
driving my car at the speed of sound and honk the horn, what happens?
You should try it and see. The sound waves add to the sonic shock front going forward, but are audible behind the horn.
if I get hit with a basball thrown from a moving vehicle, it will have a stronger impact becase it is traveling at the base speed of the platform + whatever speed it gained from throwing. Same true for slight and sound?
The sound would be higher frequency, and the light would be bluer.

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Yeah... What Antonsieb said....

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Hi Jerry,

Welcome also -

Not such a daft question :blink: - this one comes up a lot

Pretty well answered above but for a bit more info and a few more anecdotes / points about this (including the consoling fact that this is just the kind of question that got Einstein started&#33; ) - go here: http://math.ucr.edu/home/baez/physics/Rela...headlights.html

Mild

5. Guest Guest
Wow, thanks guys (for not laughing at me, lol) The whole light speed thing has always perplexed me. I&#39;m still working towards the speed of sound, but have been having troubles finding a straight away long enough......

6. It&#39;s important to realize that the sound and light work differently.

When a moving vehicle makes a sound, the sound waves travel at the speed of the vehicle + the speed of sound.

When a moving vehicle shines a light, the light waves travel at the speed of the light regardless of the speed of the vehicle. It&#39;s the frequency of the light that changes in this case (moving towards the red or blue end of the spectrum depending on whether the vehicle is moving away or towards you)

7. :huh: You wont need to concern your self about speed camra traps, they wont see you till you have gone. Your lights will be of no use to you at all. and you would need to have very good eye sight, so as to avoid hitting anything. Or ya could just close your eyes and hope for the best. Thats what i would do....

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An addition to the light thing:

We all know (or have heard) that the light speed &copy; has is a constant in vacuum. When you would be driving your car at near c, the speed at which the light from your headlights are also going c and not c+[whatever speed your carr is driving].

This is due to the Lorentz transformations. Lorentz made a formula with which to calculate speeds when you would "add" them.

His formula:

Vt=((V1 + V2)/1+V1·V2)/c^2)

Or, written differently:

Vt = V1 + V2
.......1+V1·V2
..............c^2

Vt = Total speed
V1 = Speed 1
V2 = Speed 2
c = The speed of light

Normally you would say Vt = V1·V2
That is why a normal person would say c + c = 2c.

When we use the formula given by Lorentz, we get a different answer. We will use a car that is at lightspeed with headlights on. (Practically it can&#39;t be done but we&#39;re being theoretical). V1 is the speed of the car, V2 is the speed of the light from the headlights.

V1 = c
V2 = c

Vt = c + c
.......1+c · c
..............c^2

Vt = 2c
..........2

Vt = c &#33;

This is not only with light, but always&#33; It just is more obvious at speeds nearing c.

When you are in a bus that is speeding at 50 km/h and you walk towards the driver at 5 km/h, you think you walk at 55 km/h, but this is not the case. Using the formula you would actually be walking at 54,99999985 km/h. B)

9. Originally posted by Sp1ke@Sep 9 2004, 10:51 PM
When a moving vehicle makes a sound, the sound waves travel at the speed of the vehicle + the speed of sound.
Please bear in mind that not every post in this forum is correct. We all are wrong sometimes, and sp1ke is wrong this time. Sound waves move at the speed of sound, plus the speed of the wind.

10. (Q)
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If I am in my car traveling at or just above the speed of light and turn on my headlights, will they even make a difference? I mean, will the light coming out of them be traving at the speed of light+the vehicles speed?

The speed of light is not dependant on the source but is dependant on the permeability and permittivity of space. Therefore, the source emitting the light can move at any speed and the light emitted will always be moving at c.

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Holy cow, guys. Good stuff.

O.k. a little more please. Sound has to travel through something, right? Like atmosphere, or matter or something? How about light? Argh, I just don&#39;t get it. Somebody draw me a friggin picture.

I know light can be affected by gravity, can sound waves? My buddy tells me light is not a wave, but a particle, but how come different spectrums of light are reffered to by it&#39;s wavelength (i.e. infrared, etc)

Can anyone point me towards some good literature that can talk down to me on this subject?

Jerry

12. StarLab Guest
Hmm...well, this query is a bit different from your initial one...google it, or search on yahoo or amazon.

I believe it has something to do with density (or maybe presence of complex elements). Sound exists on earth, on planets, and in vaccuums it exists in nebulae (because of the density and high atomic numbers, I think).

Sound waves are matter waves, from my knowledge, rather than energy waves. So, yes, they&#39;d be affected by gravitational effects.

Each segment of the E/M spectrum has a different characteristic or usage. Frequencies above infrared are essential for radio broadcasts and microwaves, to name a few. Infrared is the section where heat is an essential characteristic of the E/M spectra. Next is light, which is not radioactive to us and is visible to our eyes; below that are UV Frequency waves, which is the segment of wavelenths that can effect the DNA in our epidermis when released by the sun; below are x-rays, which are used for x-rays, even further down is the assorted gamma ray arrangement which comes into play around high-energy near-death stars.

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Sound is a distribution of mechanical energy, radiating out from its source. If I explode a firecracker at the corner of two solid metal walls, there are three effects taking place as the impetus is communicated outward. The tightly-packed metal of the walls will conduct the radiating concussion very quickly and if the metal is like that of a bell, the sound will travel fairly effortlessly as the metal conducts the event. But if it is packed like the metal on armor shielding of a navy warship, the concussion will be dampened and die off quickly.

Meanwhile, there is the similar wave of mechanical energy that radiates out from the explosion through the air. The ripple of air molecules bumping air molecules, bounced into motion by their neighbor behind, ricocheting off their neighbor ahead and fairly losing its forward momentum or bouncing backward in rebound. The wave spreads out at a speed that is a factor of the atmospheric density, which increases when cold and decreases when hot.

Then there is the third, a complex event of harmonics wherein the wave energy is sustained by the wave flowing and echoing through the metal wall and bounces off of one wall against the other in the backflow of the forward shock. That is why a firecracker sound is not just a very short chirp or crack when nearby but an aggregate of echos and reverberations, though at a distance the sound will be sharper because the local effects don&#39;t come as strongly into play.

The photons waving through the transparent material patterns of your headlight glass and the air in front of it, until it strikes some obstruction, something opaque. Those photons flying through air are not striking the next air molecule, being absorbed but with the energy and momentum transmission to in turn bump off still more photons in the same direction. The photons are zipping through the molecules, winging past the obstructing matter in a waving, oscillating fashion as they weave through matter. The oscillations are a factor of their energy--their wavelength.

Those mechanical interactions that accompany sound have a cost--their collision and momentum energy exchanges. This mechanical process is comparatively very slow, yet very dependant upon the medium through which it travels. The photons fly at a rate established by particle physics at that quantum moment of emission. Again, it doesn&#39;t need the chain reaction transferrance to be transmitted over a distance. So that, in a general sense, is the long and short of it from my petty perspective .

14. Originally posted by antoniseb@Sep 2 2004, 02:15 AM

The sound would be higher frequency, and the light would be bluer.
Are you referring to "Doppler effect" by this statement?

15. nak(Yokohama,Japan) Guest
Basic Doubts About Light

I opened a web-site about subject above. See if you please.
http://www.geocities.co.jp/Technopolis/2561/eng.html

16. When a moving vehicle makes a sound, the sound waves travel at the speed of the vehicle + the speed of sound.
Not so. Sound waves emmitted by a moving vehicle travel at the speed of sound with respect to the air, at least globally. Locally, all sorts of weird things happen.

The fact that light propogates in a vacuum and not a medium makes any sort of description of going "faster" a bit problematic (especially since mass and energy are equated using it). (No medium to move faster with respect to)

Just as sort of a Sci-Fi flight of fancy, suppose that your vehicle is somehow capable of travelling faster than light. (And, just as a person using sonar to observe a sonic world would be baffled by supersonic phenomena, someone using light to observe your craft would have similar observations). Defining reference frames in terms of the speed of sound would yield similarly absurd results with respect to supersonic phenomena.
I wonder if your spacecraft would generate an electromagnetic bow shock, just as supersonic craft generate pressure bow shocks. Cerenkov (spell check?) radiation can be viewed as the electromagnetic turbulence generated by radiation travelling faster than light in a medium. Of course, light in that medium is slower than light in a vacuum.

17. Here&#39;s one for you:

Two cars going at two-thirds the speed of light towards each other. Their relative speed is one-and-a-third times the speed of light. Since there is no absolute direction or position in relativity, we could consider one as fixed and the other would be travelling faster than the speed of light toward you.

Any thoughts?

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LOL&#33; This is the kind of thing that makes relativity resemble my uncle Frank--pretty strange.

19. :huh: Just in case it isént so obviouse to all of you, the descusion relating the speed of light in a vacume.. Is light something,? yes. So its not a vacume anymore. OK that sounds silly but it makes my point that even deep space is not so empty. The more of it there is the slower it will go. And as for sound.. compleatly diferent set of rules aply. Sound is the vibraiting of particals. like a precusion, compresion, shock wave,.. Cant happen out of atmosphear or other medium. ( water ). No particals, no sound. Space is a very quiet place... The question regarding two objects passing each other at near light speed. Yes there relative velosiety&#39;s would appear to exeed light speed. But only becouse there&#39;s no referance point to properly measure the true speed. We are told that the energy requiered to move an object at the speed of light would exeed the mass of the object. So it should not be posible. I&#39;m not so sure. Time may prove me wrong. I&#39;m not wright so often... :P and would like to be right about this. We need to be able to exeed light speed in order to properly explore the cosmoss. Live long and prosper.

20. As for them travelling towards each other at near the speed of light, I don&#39;t think any faster than light phenomena are supposed to be observed from any reference frame under SR. Under SR, from either reference frame, the other object merely appears to be coming on at a speed closer to lightspeed (but never equal or greater), with associated distortions in time/space/mass/energy/charge/magnetism/ect. From a middle perspective, they are both moving together some fast speed. It doesn&#39;t matter what the original speeds are, just so long as they are below lightspeed.

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Cerenkov Radiation, for all who need a picture at this point.

The faster than light in a medium particles are slower than light in a vacuum, as I understand it.

Light (photon) presumably, are absorbed by atoms, raising the electrons orbital to a higher level, until that electron releases the photon and returns to its apropriate orbital, this light is slower through matter.

It is still an energy though, not matter.

The only space for this C+ theory is in travelling vacuums, according to relativity. as if you had two high pressure systems colliding and sound waves from two accompanying cars tooting their horns

A sutble dopplar shift, no big crash in relativity.

But moving vacuums? It is a question of inertia and what was the state of the preceeding vacuum before the big bang I guess. If there were a big bang that didn&#39;t create time as it exploded, then there was a vacuum before, that would contribute to a big crunch as it is pushed outward then returned to its pre big bang state??? :blink:

Or perhaps there is a whole lot of fracturing in the pre-bang vacuum that gives us the chaotic ordering of the supergalactic structure? :blink:

Anyone know how to colide vacuums? I guess it would be like trying to manipulate High and Low pressure systems in the atmosphere?

These are the sort of questions I ask myself as a result of accepting that light has a finite velocity, relative to its communicative conditions.

22. Theres all sorts of crazy weirdness when you start thinking about relativity. My Physics teacher told me they&#39;re thinking now that a complete vacuum exerts a pressure somehow&#33; Anyone know how this is possible?

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Theres all sorts of crazy weirdness when you start thinking about relativity. My Physics teacher told me they&#39;re thinking now that a complete vacuum exerts a pressure somehow&#33; Anyone know how this is possible?
H Gavwvin (cool name by the way),

Yes there sure is "all sorts of crazy weirdness" in relativity. But most of it is caused by not understanding exactly how the component parts of the universe are related to each other. One of the big Qs in modern science is the search for a Grand Unifying Theory which realy will connect everything together. But there is still a long way to go. We can see through the crazy weirdness a bit by learning the relationships between things like time and gravity, mass and light speed, etc. But you have to read slow and think lots&#33; Get a popular book like R. Feynmans "Six Easy Pieces" which explains a whole range of physics subjects.
Ask your teacher to explain the Vacuum thing and tell him/her if you don&#39;t get what he/she&#39;s saying. He/She&#39;ll love the chance to spout some theories&#33; And teachers love an appreciative audience&#33;
Cheers
Ferg.

24. Some of the crazyness is in here and its mine, but if I can raise a question, create a dought, encourage the science. Then I have done well and feel encouraged. Do go and read good books. or use the web. Join your local astronimy association. Look for the ramblings of Stephen Hawking. Its all good advice. The News page of this web site is one of the best places to learn the cutting edge of science. Visate an observatory, meat the people. one of them could be me.
B) Lets hope that people like us can find a way to bend the rules and find the magic needed to exeed the speed of light by ten fold. Just exploring the local group of stars would be so ecxiting... enticing and just plain old interesting.

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gavwvin

I know that vacuum exerts a structural stress, but pressure? The only pressure I can recall discussion about was upon the walls and gaskets of an apparatus producing the vacuum. I remember someone talking about a "metal-rich" vacuum once because metalic atoms would pop into vacuum from the walls of the small metal tube he was trying to build a vacuum in to help in some supercold experiment a friend of mine was doing.

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I can&#39;t believe I can even follow some of this crazy talk, lol. I need to get out more often.

So, I can surmise that -perhaps- the speed of light may indeed not be a constant?

27. (Q)
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Two cars going at two-thirds the speed of light towards each other. Their relative speed is one-and-a-third times the speed of light. Since there is no absolute direction or position in relativity, we could consider one as fixed and the other would be travelling faster than the speed of light toward you.

Not really, it&#39;s a matter of reference frames.

Both cars are in two separate reference frames hence Lorentz tranformations must be applied in order for them to agree on velocities and clocks, but since both are moving at the same velocity, clocks should tick at the same rate.

But before that, they must first agree on the measurements taken which cannot be applied from their moving reference frames but must first be agreed upon together from the stationary frame. Again, more transformations are required from the stationary frame.

From these applications we derive the relative addition of velocities in which all results are fractions of c. Hence each car will measure the others relative velocity less than c.

28. I see. So basically its oversimplifying to just add their velocities as you would in a Newtonian situation. There&#39;s other "crazy weirdness" going on, as there usually is with relativity.

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gavwvin

It gets stranger and weirder still in the nanotech and quantum chemistry/physics stuff. That&#39;s why the math gets wacked-out derivative with enormous banks of nearly incomprehensible formulas to represent steps and facets of phenomena. I remember telling my former science teacher father about charmed quarks several years ago, the characteristics of congruent and incongruent spins. He thought I was trying to sell him a public park or famous bridge somewhere. He used to teach that as an excited atom received energy its electron would simply and slowly rise up to a higher valence orbit, quantum leaps were too weird for words, though quantum physics has gone far, far beyond that early discerned characteristic. The "hard sciences" today assume things today that were pure fantasy half-a-century ago. Hang with some of these geeks and nerds and they might show you how some of the curves work in our non-linear, beyond simple arithmetic universe. This dinosaur wishes you good luck.

30. Yes- we had a good laugh in our Physics class about the strangeness of the strange quark as opposed to the charm of the charm quark. Who names these things?&#33;

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