It's an impossible situation - you can't travel at the speed of light.
When I give that answer, I'm usually faced with the follow-up, "But, if you could..." The problem with that is, if you could, the rules of the universe would have to be completely different than what they are, so it's impossible to make a prediction using the rules of our universe for this nonexistant universe. *************************************************************** 08-08-05, 09:52 AM gerry But if you could travel near the speed of light, then your flashlight would turn on and send a light beam forth at the speed of light, which would light up objects in front of you just as if you (and the flashlight)were standing still and the objects were approaching you at near light speed. You wouldn't see much, however, due to time dilation, length contraction, and frequency shift. You're clock would only tick seconds as perhaps near the entire universe passed you by in a dazzling display.
08-08-05, 10:01 AM DorianGreyed I think that the headlights would overtake the beam of light (actually, the speed wouldn't allow the beam to leave the bulb), and the lamps would overheat, causing the battery to explode. That's my story and I'm sticking to it.
11-13-05, 04:04 AM bunkboy The light will appear to travel from the source to your optic nerve at twice the speed of light (assuming you're behind it).
Reason:
If you are traveling at the speed of light, your light is traveling with you at the same speed, and your speed forward plus the light from the headlight backward to you is the result. This is because the wire inside the bulb is omnidirectional.
To an observer outside your "vehicle", the light will travel at the speed of light toward them as it passes. If it is truly visible for only a tiny fraction of time because of the movement, the light will be visible but imperceptible by the brain.
(Also, there will be a measurable "red shift" from the light source, due to the doppler affect of the movement of your "vehicle", but no observer will notice this, either.)
And as an added bonus, you will likely appear in "Car and Driver" for having the coolest, fastest car on the market.
11-13-05, 07:52 AM methos No, the speed of light, as measured by you, will be the speed of light.
11-13-05, 11:26 AM frankvan
quote: This is because the wire inside the bulb is omnidirectional.
Meaning what???
11-13-05, 12:34 PM babthrower
quote: Originally posted by Matiqua: If I'm travelling at the speed of light and turn on headlights, would anything happen?
Good lord, you were travelling at the speed of light and you hadn't yet turned on your headlights? Eek
Methos is right. Hypothetical situations are tough enough to deal with
"If Kennedy hadn't been assassinated, would we have had World War III? "
without dealing with hypotheses which are impossible.
Some purists won't recognize any question that has a hypothetical basis.
11-13-05, 03:34 PM Professor I also agree it's difficult to deal with hypothetical contrafactuals. In formal logic, all deductions from false premises are equally valid. That is, if p and q are statements, and p is false, then p --> q (the arrow means "implies") is true for all q. If pigs had wings, they could fly to the moon!
But Gerry and Methos are correct in stating that the observer always measures the same speed for light in a vacuum in his or her frame of reference. All experiments have confirmed this.
Matiqua should be heartened to know that this paradoxical question was originally asked by young Einstein himself, leading eventually to his formulation of special relativity.
11-13-05, 05:24 PM babthrower That site is, um, very enlightening, Perf. Thank you.
Isn't it the case that in traditional formal logic the result would be undetermined? And that the somewhat counter-intuitive
If p then q Not p therefore (if p then q) is true
is a result of the desire to on the part of some logicians to make modern logic binary, and consequently capable of being computerized?
(Hollerith was contemporary with Frege)
Or am I reaching?
11-13-05, 07:20 PM DorianGreyed I still say the lamps would overheat, causing the battery to explode. Going the speed of light and then turning on the headlights should only be done by a professional driver on a closed course. Please do not try this at home.
11-14-05, 09:36 AM methos I believe bunkboy meant that the light was emitted from the wire omnidirectionally, so that some of it would be coming at you at the spped of light. He then adds your speed (nearly lightspeed) to the speed of light and concludes that you'll see the light as coming at you at twice lightspeed. This is not how relativity works - you will see the light coming at you at lightspeed.
11-14-05, 02:51 PM Professor Babthrower: On the stuff about formal logic: This is standard fare from Boolean algebra, which describes binary logic. Every variable has only two states - 1 or 0, true or false, on or off, etc. My point really wasn't very profound -- just restating a definition of implication (p implies q) as a Boolean operation between two variables, p and q. It can be defined by a so-called truth table:
If you assign meanings 0=False, 1=True, it yields what I said. I'm hoping we'll drop this off-topic digression into symbolic logic, because if you ask more questions I'll have to go off and actually read up on it! Wink
11-17-05, 08:14 AM FlyingHellfish
quote: Originally posted by methos: No, the speed of light, as measured by you, will be the speed of light.
This is correct. Special transformations occur as you yourself approach the speed of light in such a way that the light emitting from the bulb is still moving away from you at the speed of light. Even if you somehow manage to attain a speed 1mph less than the speed of light, you will never be able to see the "front end" of that light ray.
The speed of light isn't just a constant; it has many different inherent properties that make it the limit of most known physical properties in some way or another.
01-26-06, 02:39 PM coldfuse My story, and I'm sticking to it, is that the headlamps would burn up along with the entire vehicle well before light speed - maybe around Mach III or so. If the car has the old Firestone tires they used to put on Explorers it will start to disintegrate around 71 mph.
01-26-06, 03:00 PM DorianGreyed "If the car has the old Firestone tires they used to put on Explorers it will start to disintegrate around 71 mph."
Well, I knew that something would explode. (I still think it's the battery.)
01-26-06, 11:55 PM coldfuse Does anybody know if Mat's question is serious? If you've driven from Calgary to Banff then you have been passed by a driver attempting warp speed. The only driving close to that is on back roads in rural counties just north of Charlotte - of course, half the drivers around there are named Earnhardt, Johnson, Petty or Allison and the other half work for them.
01-27-06, 01:36 AM babthrower I think Matt's question was serious, but it was answered by Methos. As for the rest of us loonies, I'm sure Mat knows better than to take us seriously. He's been around these threads for a while. Smile
01-28-06, 09:06 PM babthrower I felt guilty about being flippant so I thought I'd flesh out the information Mat was looking for a bit.
There is real-world evidence that the speed of light is always the speed of light. A physicist named Willem de Sitter (1872-1934) spoke of the light coming from double stars, which orbit around their common center of gravity.
Observations using a spectroscope did not show that the light, as the orbiting star either advances toward earth or retreats from earth, show the Doppler effect (a change in wavelength of the light into the red, as it moves away, or into the blue, as it approaches).
So he concluded that the light which the star sends to Earth as it moves towards Earth must have the same velocity as the light the star sends to earth as it moves away from earth; the velocity of light does not depend on the motion of the source of light.
Those experiments were done years ago. Hubble has a spectroscopic telescope. I don't know if they've reveiwed De Sitter's evidence yet. It's not a trivial thing to analyse, so backyard telescope won't do it, even with handy spectroscopic attachment.
Question for Methos: The doppler shift has two components: the red shift due to the expansion of space between us and it, and the shift due to the motion of the star you're observing in its local space. But if the speed of light is independent of the speed of the light-emitting object, why do we see the red shift? Confused
01-28-06, 11:20 PM Professor The Doppler effect shifts the frequency of light (redshift or blueshift) just as one would expect for a wave source moving toward or away from the observer. At relativistic speeds there is an additional relativistic doppler effect owing to time dilation of the source object -- even if it's moving sideways to the observer!
The expansion of the universe, as described by the current standard model (big bang), gives rise to an additional cosmological redshift arising from the expansion of space itself, stretching the light waves. According to Hubble's Law the cosmological redshift increases linearly with distance.
Willem De Sitter collaborated with Einstein on basic cosmologic models. Wikipedia describes the 1913 De Sitter double star experiment , replicated in 1977, which confirmed the constancy of c and was another triumph of relativity.
De Sitter was looking for evidence that c is not constant everywhere, which would have resulted in an "image that was scrambled and out of sequence" (quoting link above).
Babs, if you're saying that the constancy of c implies that no Doppler shift should be observed, I think that's incorrect.
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The Doppler effect shifts the frequency of light (redshift or blueshift) just as one would expect for a wave source moving toward or away from the observer. At relativistic speeds there is an additional relativistic doppler effect owing to time dilation of the source object -- even if it's moving sideways to the observer!
The expansion of the universe, as described by the current standard model (big bang), gives rise to an additional cosmological redshift arising from the expansion of space itself, stretching the light waves. According to Hubble's Law the cosmological redshift increases linearly with distance.
Willem De Sitter collaborated with Einstein on basic cosmologic models. Wikipedia describes the 1913 De Sitter double star experiment, replicated in 1977, which confirmed the constancy of c and was another triumph of relativity.
De Sitter was looking for evidence that c is not constant everywhere, which would have resulted in an "image that was scrambled and out of sequence" (quoting link above).
Babs, if you're saying that the constancy of c implies that no Doppler shift should be observed, I think that's incorrect.
Posts: 2039 | Location: U.S. | Registered: 06-03-02
Babs, if you're saying that the constancy of c implies that no Doppler shift should be observed, I think that's incorrect.
No, I'm not saying that, because I don't know whether there should or should not be a shift if the speed of light is an absolute.
I'm asking the question
- given that the absence of redshift in the observation of the binary star system gave the impetus to the theory of relativity
- given that relativity theory is unequivocal: c is absolute
how does the theory explain the redshift that Hubble first observed?
I understood that the Hubble redshift is caused by the recession of the source of light.
I'm trying to understand how the absence of a redshift in one observation (two observations, counting the 1977 one, thanks for that information) proves that c is constant yet the presence of redshift (in countless observations since) is consistent with the theory that c is constant. *********************************************************** 01-29-06, 03:32 PM methos Relativity can really give you a headache, eh?
First, there are actually three redshifts - the two you mentioned plus a third relating to loss of energy by light overcoming a gravitational field (lower energy = longer wavelength).
Second, the cosmological* redshift is not due to the source of the light moving away from us. Rather, it is due to the space in which the light is contained expanding. The light expands with everything else, lengthening (redshifting) its wavelength. * I use the term cosmological rather than Hubble, because so many people's names have been variously associated with it, including de Sitter, who predicted it before Hubble observed it - Hubble refers to "the de Sitter effect" in his paper anouncing his experimental results - a paper which also included experimental results by Slipher who had observed, even before de Sitter's calculations, that a majority of galaxies were moving away from the Earth.
As for de Sitter, are you sure that you have the details right? Unfortunately, my astrophysics text doesn't mention the experiment and I don't have a text giving anything more than a cursory treatment of relativity handy at the moment. My general physics text does, however, give data for a similar experiment - a 1.8-nm difference in wavelength for light emitted from two sides of a rotating galaxy. *Wikipedia's article on de Sitter's experiment makes no mention of a red or blue shift measurement. Rather, it talks about the pattern of the light reaching us not being scrambled as it would be over such a large distanc if some of the light were slow and some were fast. * DG would mention that Wikipedia isn't always right.
01-29-06, 03:39 PM DorianGreyed
*DG has learned from ColdFuse that Wikipedia is a much better resource than originally thought. DG may learn slowly, but he does learn.
01-29-06, 04:05 PM Professor There's tantalizingly little to find about this with Google (many sites quote the same Wikipedia article), and I'm away from my personal library where I keep a few textbooks of astrophysics (really Wink ). So here's what I infer from the article above, and some basic knowledge:
If a double star orbits its companion at velocity V with its orbital plane aligned with Earth, then (prior to Einstein) it should be expected to emit light with velocities c+V and c-V. De Sitter reasoned that the faster light emitted during approach could overtake the slower light emitted earlier at recession. He calculated that the "scrambled" signal would not look like a simple Keplerian elliptical orbit as seen with closer objects.
Failure to find such an effect helped confirm Einstein's postulate that light is emitted at exactly velocity c regardless of the star's motion.
That said, the star's light still undergoes redshift while receding and blueshift while approaching. This effect is added to other sources of redshift mentioned in my previous post, so the observations might in reality consist of varying degrees of redshift.
But (I am asserting) the ordinary Doppler effect still applies to light. Not all redshift is due to expansion of space or other exotic cosmological effects -- some of it is quite ordinary and local.
I'll keep looking for an authoritative source to settle this.
01-29-06, 07:23 PM frankvan I probably don't understand the problem, but it seems to me not to be a contradiction but inevitable that a velocity c that never varies would have to produce a red shift from a receding source. If the speed of recession or approach were to add or subtract from the speed of light, I would expect the wavelength at the observer to be unchanged. On the other hand if the speed of vibration or pulsations arriving at the observer are lower in frequency, wouldn't that imply that the speed of propagation was probably constant? I fully expect to be shot down in flames. Smile
01-29-06, 09:02 PM Professor The Doppler effect for sound waves is familiar to us as drop in pitch as a police siren or train horn passes by. This typically occurs when we are standing still and at rest with respect to the atmosphere (no hurricanes in this scenario Wink ), and the sound source is moving. Thus we hear the effect under conditions where the speed of sound is constant.
Of course light doesn't propagate through 'ether' the way sound propagates through air. (That was settled 100 years ago.) Does that ruin the analogy between sound and light for doppler effects? Well, in both cases the speed of propagation is constant with respect to the observer, so it seems valid to me. In the case of a sound source in an elliptical orbit about a center -- it will "warble" synchronously with its orbit.
frankvan, I think your intuition is correct about a constant c implying doppler shifts. However I'm not sure that the converse is true.
I think there is confusion between variations in frequency of light with variations in velocity. The variable-velocity scenario is truly weird -- imagine sound waves coming at you at a variety of velocities.
01-30-06, 09:05 AM methos There is a Doppler effect with light (the wavelength changes depending on the relative motions of the emitter and observer). If there isn't, I wasted a lot of time correcting for it in my latest publication. I'm dealing with moving atoms rather than moving stars, but the effect doesn't care. That is why I think there was some misunderstanding involved in the description Babs gives for de Sitter's experiment (not necessarily her understanding, it may have been been a mistake on the part of the person explaining it to her, or further back). I read the Wikipedia article, the only decent source I've found so far since my Quantum and Relativity text doesn't mention the experiment either, the same way that Professor does (faster light overtaking the slower light producing a scrambled image).
01-30-06, 11:41 AM babthrower I'm not arguing that the doppler effect operates (or doesn't operate) as far as light is concerned.
My real issue is the apparent contradiction between what Einstein concluded as the result of the de Sitter work, (which was seminal to (Einsteins's) theory), and what theory is today.
Here is (an English translation of) what Einstein said he drew from de Sitter's work:
"By means of similar considerations based on observations of double stars, the Dutch astronomer De Sitter was also able to show that the velocity of propagation of light cannot depend on the velocity of motion of the body emitting the light. The assumption that this velocity of propagation is dependent on the direction “in space” is in itself improbable."
I guess it's easier to understand de Sitter's position when you consider that it was in response to the theory of Walter Ritz who in 1908 published that "the velocity fo light is c + v" , that is, the velocity of a light source is vectorially additive to the velocity of the light emitted by it. (Not true in earth's atmosphere.)
This is what de Sitter responded:
"The addition of the velocity of a visible component of a binary star to the velocity of its light emitted in the direction of an observer would allow slower light (c - v) from one side of the orbit (when the component was traveling away from the observer) to be overtaken by the faster light (c + v) from one half orbit later (when the component was traveling toward the observer). At the right distance this effect could cause the visible component to periodically be seen at two different locations simultaneously and generally would lead to apparent observational departures from Keplerian motion."
The article goes on to say, "Contrary to the de Sitter claim and to other arguments advanced more recently, John Fox found that visible binary stars do not offer evidence against the Ritz theory."
and
"In 1987 Vladimir Sekerin of Novosibirsk, Siberia, in an article titled Gnosiological Peculiarities in the Interpretations of Observations (For Example the Observation of Binary Stars)(4), showed that when we consider the distances (binary-to-observer) required for de Sitter's "whimsical" images effect to manifest themselves that the angular resolution of our best telescopes (1987) are insufficient for us to resolve them."
The 1977 tests that Prof referred to used x-rays to avoid the proplem of the extinction of light due to interstellar dust particles.
Here is some interesting stuff from page 2 of the article, having to do with the view which de Sitter opposed:
"For a non-accelerating source which is moving with respect to a given inertial reference frame in a genuine c + v arena (no ether medium) an observer who is stationary with respect to that reference frame would not detect a Doppler related wavelength change. The frequency would be changed by a factor of 1 + v /c, but the wavelength would remain constant.
Ritz expressed this idea (about the unchanged wavelengths) by saying that a material point (or a star in our case) moving at a constant speed emits concentric spherical wave fronts, each of which is centered on the source at its latest position. There is no bunching up of wavefronts in the forward hemisphere and no spreading out of those behind."
Clearly this is not the mainstream view. But are the objections important enough to consider the issue unresolved?
01-30-06, 12:05 PM methos "I'm not arguing that the doppler effect operates (or doesn't operate) as far as light is concerned. My real issue is the apparent contradiction..."
But the apparent contradiction you originally stated was no Doppler effect seen by de Sitter and the presence of a Doppler effect in other experiments. The problem is, I still don't see where the Doppler effect comes into de Sitter's work at all. He talks about the consequences of slower and faster light but does not mention wavelength at all in anything quoted from him so far, so I'm not seeing what the contradiction is.
01-31-06, 03:07 AM babthrower Watson: "Is there any point to which you would wish to draw my attention?"
Holmes: "To the curious incident of the dog in the night-time."
Watson: "The dog did nothing in the night-time."
Holmes: "That was the curious incident."
I guess I was wondering why de Sitter did not introduce the doppler effect into the discussion of the behavior of the light from the binary star. The pre-conditions were there: the star was advancing, then receding relative to earth. And certainly de Sitter knew all about the Doppler effect, (described in 1842) and de Sitter wrote in the earth 20th century.
Well, if nothing else, this point may well help identify my points of confusion!
01-31-06, 07:30 AM methos Ah.
There are likely two reasons de Sitter didn't bring the Doppler effect into the discussion. It wasn't that he wasn't aware of or interested in red and blue shifts, he was just a few years away from anouncing a model for the universe that predicted the sort of universal redshift from expansion that Hubble would later observe. More likely, he didn't see it as particularly interesting - the Doppler effect had been seen in astronomy before. More than that, though, he probably didn't see it. We aren't talking about large galaxies spinning at high speeds. The motions of the stars were probably pretty slow relative to each other, so the Doppler effect would have been very small. Even for the motion of galaxies, it takes a fairly sophisticated (especially in that day) bit of instrumentation to observe. Even today, telescopes tend to be outfited with a handful of filters, providing resolution in the hundreds of nanometers, not the single nanometer necessary for the galaxy I mentioned above or the likely far less than that necessary for the stars de Sitter studied.
So basically, he probably didn't see it because it was small and his instruments weren't good enough, and he probably didn't care because it wasn't anything new.
01-31-06, 08:49 AM methos I should mention that he also probably wasn't looking at a narrow atomic line but rather the broad blackbody radiation that is most of the light from a star and it far too broad to notice the small shift caused by the Doppler effect, particularly since its peak wavelength will vary with temperature. Atomic lines are used for Doppler or other wavelength-shift measurements because they are narrow (making changes in position relatively easy to see) and because their wavelengths aren't effected by many things.
01-31-06, 10:30 AM babthrower Hmmm. This is very interesting. I guess it's a weird analogy of Doppler effect - I'm looking back with a narrow focus at events that happened a long time ago, and the foreshortening spoils my perspective.
Thanks for the information, Meth.
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Posts: 6538 | Location: British Columbia, Canada | Registered: 06-11-02
========================= No man can travel with speed c=1. Only Light quanta can do this. To think about c =1 travellimg for man is abstract. Therefor I say that SRT is theory only about Quantum of Light.
Posts: 122 | Location: israel | Registered: 12-05-05
