A thought provoking question

[Cepheus]

Let\'s say that you are riding on a beam of light, and therefore traveling at the speed of light. (ignoring what it\'ll take to get YOU there, of course) You fire a laser at a target. Will the laser ever reach the target? (this is also implying that you ARE able to fire the laser in time, no matter how improbable it is.)

Let us also suppose two situations: you fire the laser in front of you, and you fire the laser behind you.

[ping111]

I don\'t understand this question, but it gives me another.

If you are at travelling at C, if you fire a never-dispersing beam at a target behind you, where does the light go?

[Guest Flixxbeatz]

L I G H T C E P T I O N

[Phoxtane]

I don\'t understand this question, but it gives me another.

If you are at travelling at C, if you fire a never-dispersing beam at a target behind you, where does the light go?

Easy. You leave a trail of light, much like in Tron.

[Altair1371]

Easy. You leave a trail of light, much like in Tron.

Not so easy. What he\'s saying is that your are heading at C, or speed of light.

You fire a flashlight behind you, which travels at -C

C+-C=0

[Awaras]

Light always (ALWAYS! No exceptions, no substitutions) travels at the speed of light, the only thing that changes when the object that emits light increases its speed is the wavelength of the emitted light (so called red/blueshifting). So if you fire a beam directly ahead of you while moving at the speed of light (which you can\'t, because you would have infinite mass and TIME WOULD STOP COMPLETELY so you would be frozen but lets say you can), it will just pile up in front of you blueshifting to gamma radiation - no matter how long you keep the beam on, to an outside observer it would appear as a single, infinitely short burst of gama radiation - more powerful the longer you kept the beam on. If you fired the beam directly behind you, it would get redshifted far far far into the ULF frequency radio waves, probably so far that it would become undetectable (so C+-C=0 is kinda right), but it would still be there, as faint as it is, and still travelling at C.

[Awaras]

Let\'s say that you are riding on a beam of light, and therefore traveling at the speed of light. (ignoring what it\'ll take to get YOU there, of course) You fire a laser at a target. Will the laser ever reach the target? (this is also implying that you ARE able to fire the laser in time, no matter how improbable it is.)

Of course it will reach the target, its just that (if the target is directly ahead of you) it will reach it at the same time as you.

[Cepheus]

What if you fire it behind you?

I need to read more.

[Awaras]

What if you fire it behind you?

If you fired the beam directly behind you, it would get redshifted far far far into the ULF frequency radio waves, probably so far that it would become undetectable (so C+-C=0 is kinda right), but it would still be there, as faint as it is, and still travelling at C.

[HarvesteR]

Interesting question this one... I would agree with the light piling up in front of you explanation, and the \'trail of light\' for beaming backwards. It makes sense, because you\'re already riding ON a beam of light, so it\'s more of a question of creating a second beam as you go by.

But a proper explanation of the effects will require answering a more fundamental question first, and one that\'s been bugging me for a while lately. In space, everything is relative, and that includes velocities. That is, to measure a velocity, you must first have a point (or frame) of reference to measure it from. But in space, everything is also moving, including your frame of reference.

See where I\'m going with this? If you measure the speed of an object from the perspective of someone standing on a planet\'s surface, you have to factor in the velocity of the planet into the calculation. But then, the planet\'s sun is also moving, so factor that in as well. And the galaxy itself is also moving, and so on and so forth.

What we need to know then, is by what reference frame is C calculated, given that there is no truly inertial reference frame?

If C is absolute, that implies the universe has an absolute coordinate system, therefore a center (or origin), and then, the explanations above make sense. But without a reference frame, C would become relative, which I admit sounds pretty weird, but I wouldn\'t believe in an absolute measurement without being given an absolute frame of reference to go by.

If any theoretical physicists are lurking around, now would probably be a good time to jump in.

Cheers

[Arrowstar]

Let\'s say that you are riding on a beam of light, and therefore traveling at the speed of light. (ignoring what it\'ll take to get YOU there, of course) You fire a laser at a target. Will the laser ever reach the target? (this is also implying that you ARE able to fire the laser in time, no matter how improbable it is.)

Light travels at the speed of light regardless of reference frame, so yes.

[Arrowstar]

If C is absolute, that implies the universe has an absolute coordinate system, therefore a center (or origin), and then, the explanations above make sense. But without a reference frame, C would become relative, which I admit sounds pretty weird, but I wouldn\'t believe in an absolute measurement without being given an absolute frame of reference to go by.

As I just said, the speed of light is constant in all references frames. There does not need to be an inertial frame to make this so, and in fact General Relativity has as one of its outcomes a distinct lack of inertia frames. Now, how is the speed of light constant in all reference frames? Frankly, it\'s the way the universe works and as an engineer we didn\'t get into the philosophy of the subject too much in my relativistic dynamics course. Unless you\'re willing to go through the tensor calculus yourself to demonstrate the basic principles of GR, however, you might as well take my word on it.

EDIT: In short, your premise is incorrect: a constant speed of light does not imply an absolute frame.

[Awaras]

Put simply, and not going into details, things like time dilation, lenght constriction and other relativistic effects 'conspire' to keep a beam of light moving at C no matter what reference frame you observe it from.

[HarvesteR]

As I just said, the speed of light is constant in all references frames. There does not need to be an inertial frame to make this so, and in fact General Relativity has as one of its outcomes a distinct lack of inertia frames. Now, how is the speed of light constant in all reference frames? Frankly, it\'s the way the universe works and as an engineer we didn\'t get into the philosophy of the subject too much in my relativistic dynamics course. Unless you\'re willing to go through the tensor calculus yourself to demonstrate the basic principles of GR, however, you might as well take my word on it.

EDIT: In short, your premise is incorrect: a constant speed of light does not imply an absolute frame.

I didn\'t say a constant speed of light implied an absolute frame, I said an absolute speed of light implied an absolute frame.

I\'m not questioning the constancy of C here, I\'m trying to figure out if C is C in relation to something, and if so, what is that something?

If you say C is the same regardless of reference frame, then one could argue that a beam shot from a moving planet would move in a different velocity than one shot from the planet\'s star. That would make C constant in, but at the same time relative to, the reference frame it was shot from.

It seems like a paradox, but I reckon I\'m reckoning this without thinking about time dilation. Time dilation could explain all of it I think, if you think of C as only a speed, and not a velocity... My head is starting to spin with this... =P

EDIT: Ninja\'d. Yep. Time Dilation should compensate I think.

Cheers

[Entroper]

Light always (ALWAYS! No exceptions, no substitutions) travels at the speed of light, the only thing that changes when the object that emits light increases its speed is the wavelength of the emitted light (so called red/blueshifting). So if you fire a beam directly ahead of you while moving at the speed of light (which you can\'t, because you would have infinite mass and TIME WOULD STOP COMPLETELY so you would be frozen but lets say you can), it will just pile up in front of you blueshifting to gamma radiation - no matter how long you keep the beam on, to an outside observer it would appear as a single, infinitely short burst of gama radiation - more powerful the longer you kept the beam on. If you fired the beam directly behind you, it would get redshifted far far far into the ULF frequency radio waves, probably so far that it would become undetectable (so C+-C=0 is kinda right), but it would still be there, as faint as it is, and still travelling at C.

This is basically the right answer. You can\'t travel at the speed of light. If you did, because of time and length dilation, the universe would contract to a single point, and you would travel infinitely far in zero time. There would be nothing to point your laser at, and you wouldn\'t have time to fire it.

[Guest Flixxbeatz]

Tachyons?

[Awaras]

Tachyons?

You mean the completely theoretical, never observed, doubtfully real always-faster-than-light particles? What about them?

[Asmosdeus]

Hypothetical.

An object travelling at the speed of light has an infinite angular mass from which light can not accelerate away from, only be left behind.

The beam will try to come out the back of the laser, not the aperture.

[epicphoton]

So, I don\'t think I can find the reference, but I saw a video somewhere where a physicist explained that when something is traveling at the speed of light, due to time dilation effects, an observer on a ship/object traveling at the speed of light would perceive the journey as instantaneous. When the object hits C, time slows down infinitely, and when the object drops below C, time speeds up just enough to be perceivable, so the journey from point A to B is instant, at least to the passenger on the C object.

It\'s kind of a hard concept to imagine, but it gets rid of a lot of the problems with the whole question. You wouldn\'t be able to do anything between A and B, because to you, no time would pass. I\'m not sure what would happen just below a speed of C, but AT C, the question becomes irrelevant.

(From my limited understanding.)

[Blooner]

If C is absolute, that implies the universe has an absolute coordinate system, therefore a center (or origin), and then, the explanations above make sense. But without a reference frame, C would become relative, which I admit sounds pretty weird, but I wouldn\'t believe in an absolute measurement without being given an absolute frame of reference to go by.

If any theoretical physicists are lurking around, now would probably be a good time to jump in.

Cheers

I just so happened to be reading 'A Brief History of Time' by Stephen Hawking at the moment, and can therefore answer your question quite easily.

The question you posed is exactly the question that Albert Einstein posed, following his famous equation E=MC^2.

His answer led to the conclusion that time is relative.

You see, c (the speed of light) is reliant on two variables; distance and time. Obviously, as it is the measurement of the distance light travels in a particular measurement of time.

Your proposition that C is an absolute value therefore proves that time is the variable value.

Time is thusly different for everybody, dependent on location and our speed of travel. The faster we travel, the \'slower\' time goes for us.

I\'m sure I\'m explaining it poorly, and I might even be explaining it incorrectly.

If you are at all interested in the topic at hand, I can HIGHLY suggest reading 'A Brief History of Time' by Stephen Hawking.

Or go for 'A Briefer History of Time' if you want a more compressed and easy to understand version of the book.

Greetings,

Blooner

[Hypocee]

Exactly. The question is nonsensical because there\'s no room for events at c. It only makes sense if you drop it to 'very close to c', in which case it becomes trivial. I\'ve said it here before, but this carries the rather delightful implication that from their own point of view, photons and similar massless particles don\'t exist, having no time to exist in; the emitter and receiver simply 'touch' somehow in the overall quantum event. Even if that event happens to span 13 billion years of slow stretching down to CMB.

I\'m just now reading Lawrence Krauss\' Quintessence about the history of dark matter, and among its general excellence it points out that special relativity has a second line of reasoning, implicit in Maxwell\'s equations 20 years(?) earlier - the speed of an electromagnetic wave is dependent on two quantities that are quite measurable in the laboratory and affect all sorts of stuff. If c were not fixed for all inertial observers it wouldn\'t be a matter of a subtle wiggle in a cunning experiment; serious laws of physics - chemical reactions, atomic cohesion etc. - would break down.

Anyone who likes to play with relativity, you may enjoy Poul Anderson\'s hard sci-fi novel Tau Zero. It\'s about a Bussard ramjet that gets trapped into going faster and faster 'forever', and gives gentle but detailed explanations about the mathematics of the relativistic velocity factor tau, and how the ship\'s ever-increasing speed affects the universe around it - fighter-style realtime dodging between star systems on the fringes of a whirling galaxy sparkling with rainbow supernovae is a rather memorable image.