Puzzled

[Stumblebum]

Can somebody tell me where I went wrong? Here's the scenario....

 

I'm standing outside looking up into the night sky. Unbeknownst to me there is a spacecraft travelling towards me at 90% c. Ten light seconds from Earth's frame of reference the pilot turns on the headlights. I will eventually see the headlights, determine them to be 10 light seconds away. After I first see the lights come on shouldn't I see the spaceship go by 1 second later? Will this create an illusion that the spacecraft is travelling faster than c? Will my eyes be absorbing photons at a higher than normal rate? Ok guys and gals...where am I going wrong? I dreamed of this one night after thinking about tachyons and whether or not they could be illusory.

[J.C.MacSwell]

Can somebody tell me where I went wrong? Here's the scenario....

 

I'm standing outside looking up into the night sky. Unbeknownst to me there is a spacecraft travelling towards me at 90% c. Ten light seconds from Earth's frame of reference the pilot turns on the headlights. I will eventually see the headlights' date=' determine them to be 10 light seconds away. After I first see the lights come on shouldn't I see the spaceship go by 1 second later? Will this create an illusion that the spacecraft is travelling faster than c? Will my eyes be absorbing photons at a higher than normal rate? Ok guys and gals...where am I going wrong? I dreamed of this one night after thinking about tachyons and whether or not they could be illusory.[/quote']

 

You basically went wrong when you assumed that you were going wrong. You will "see" (be exposed to) 1 second of very blueshifted light. I hope the headlights are not too strong.

[Stumblebum]

So I basically see 10 seconds of light crammed into 1 because wavelengths are shorter? I think what I was trying to get at is this...if I first see spaceship 10 lightseconds away and in the next second its upon me then could I not assume it travelled 10 lightseconds in 1 second and as a result think that the spaceship must have been travelling faster than c? I was thinking tachyons might be something similar to this effect. Thanks for response.

[bascule]

What you see is time relative to the ship condensed into time from your frame of reference. In the ship's frame of reference, nothing is askew, but for you the whole ten light seconds of its voyage is condensed into one in a massive blue shift. The ship would not appear to exceed the speed of light, it would appear to travelling .9c, but from your frame of reference that's rather fast.

[swansont]

In the ship's frame of reference, nothing is askew

 

In the ship's frame, it is not 10 light-seconds away, due to length contraction.

[J.C.MacSwell]

So I basically see 10 seconds of light crammed into 1 because wavelengths are shorter? I think what I was trying to get at is this...if I first see spaceship 10 lightseconds away and in the next second its upon me then could I not assume it travelled 10 lightseconds in 1 second and as a result think that the spaceship must have been travelling faster than c[/b']? I was thinking tachyons might be something similar to this effect. Thanks for response.

 

You cannot assume that. You must allow for the speed of light and the speed of the ship in your reference frame to make conclusions about what is happening in your reference frame.

[geistkiesel]

So I basically see 10 seconds of light crammed into 1 because wavelengths are shorter? I think what I was trying to get at is this...if I first see spaceship 10 lightseconds away and in the next second its upon me then could I not assume it travelled 10 lightseconds in 1 second and as a result think that the spaceship must have been travelling faster than c? I was thinking tachyons might be something similar to this effect. Thanks for response.

 

 

In the ship's frame of reference' date=' nothing is askew [/quote']

 

 

 

In the ship's frame, it is not 10 light-seconds away, due to length contraction.

 

 

Is this an example of the equivalence of inertial frames where in one frame a light switch is turned on and seen for ten secinds; the receiver of the light sees the same light for 1 second?

 

Besides the 9 secinds of reality, what else is missing here?

 

I find it difficult to rationalize the shorter wavelength od teh photons, I do see that teh wavelength would pass through your cornea faster, so the measured frequency would increase. But to squish the wavelengths shorter requires the application of force on the material comprising the light, which would have a very local affect of decreasing the currently affected wavelength segment as it shortened. And this requires a force be applied. Look at Doppler from the point of view that he wavelength of the emitted light remains invariant throughout and only the frequency of the measured components will vary as the motion of the observer is included in the measurement of the light, but here also, the SRT profs say to negate the motion of the observer and consider the light wavelength shortened by some force - ignore what you observe, like motion for instance, is the mantra of the SRT crowd.

 

Stumblebum, how could you see the space ship and determine it is ten seconds away? Would not the only thing you see is the light the ship turned on 10 of your seconds earlier, that you didn't know about? But you would not know this occured would you? All you see is the light that was turned on. Me thinks the responses to your posts were hurriedly assembled. If we assume some kind of standing wave for the lights turned on, the oncoming ship would continuously reflect that light it came in comtact with.

 

Shine a laser beam intio the sky, you can see it from the side correct? Even lasers have side reflections/radiations. It is what,

impossible

(at least it would reqruire serious technological attention) to hide the side lobes ways and backways pieces of light? If the observers on the ship were looking they would see the light reflected in sidelobes and back and could measure the energy density of the emitted light as they collected it from their motion and then compare that energy density with with some v = 0 stationary standard, hence the ship's crew can determine their speed (velocity) wrt the emitted photons, known to move at 3x10^5 km/sec.

 

But one doesn't learn this from SRT professors who negate the possibility of measuring a relative motion of frame and photon other than c.

 

Unless later some one informed you of what was claimed to have happened all you would see is the pulse of light and some brief residual reflections and radiations. You would shrug your shouklders and . . .

Geistkiesel