MadSci Network: Physics

Re: What would you really see if you were traveling faster than light?

Date: Wed Sep 1 19:33:32 1999
Posted By: Layne Johnson, Undergraduate
Area of science: Physics
ID: 936002173.Ph

Hello, Greg!

Imagine you're sitting right next to the back straight-away at the 
Indianapolis Motor Speedway.  The cars rocket by at 220 mph.  The crowd 
roars, the bleachers you're sitting on shake, and the Doppler effect
Dopples, er, Dopplegates.  Doppelizes?

Well, the Doppler effect works.  What that means is the sound waves, which 
travel at about 700 mph, are compressed as the source of the sound (the 
cars' engine) approaches you.  The faster the car approaches, the greater 
the frequency of the sound wave.  In other words, the peaks and troughs of 
the sound wave reach your ears more times a second than they would if the
car was racing its engine in pit row.  If the car is moving away from you,
the peaks and troughs of the sound waves have less frequency, or they
reach your ears fewer times a second.

It is the Doppler effect that causes the cars' engine to make a higher 
pitched sound as it approaches you, and a deeper pitch as it goes away.  
It's much more noticeable at Indy speeds than at freeway speeds.

At Mach 1, the speed of sound, something different would take place.  You 
would not hear the approaching car at all.  The sound waves that radiated 
form the engine a mile away would reach you at the same time the sound 
waves radiating from the car 5 feet away hit you, and so would all the 
sound waves from all points in between.  A sonic boom would occur.

Travel at light speed is very similar to travel at Mach 1.  The Doppler 
effect holds true for light waves as well as sound waves.  When the 
frequency of the light wave is compressed, it appears more blue.  When we 
are moving away from something at speeds approaching light speed, the 
object appears more red.

So the first noticeable effect of light speed travel would be a REDSHIFT of 
things directly behind you, and a BLUESHIFT of things before you.  The 
spectrum shifts would be less pronounced the more you gazed to the side, 
and in an imaginary plane perpendicular to the direction of travel, there 
would be no spectrum shift at all.

Sometimes I've wondered if the redsifts and blueshifts at light speed would 
be so pronounced that objects in front of us and behind us would be 
invisible to our eyes, since we can't see infra-red or ultra-violet.  I 
don't know the answer, but I know that the objects would appear darker.  If 
the color shift is great enough, then what we would see at light speed 
would be a black circle in front of us, with violet objects radiating from 
it.  These objects would then work their way through the color spectrum, 
turning blue, then green, then bright yellow as they passed near the 
perpindicular plane.  As we pulled away from them, they would turn orange, 
then red, and finally fade from sight into a black circle behind us.

Even if the spectrum shifts weren't great enough to to cause infra-red 
blind spots behind us, we couldn't see anything directly behind us for 
another reason.  The light coming from it would never catch us.

Another point to ponder is the altered appearance of things.  Imagine you 
flew over the north pole at light speed and looked down.  You would see the 
45th north parallel.  At any speed, you see things as they were when the 
light you see left the object.  You see Alpha Centauri as it was 4 years 
ago, because it's four light years away.  At light speed, you've travelled 
so far by the time the light reaches you, you're seeing the object from a 
different perspective by the time its light reaches you.  At the moment you 
crossed the north pole, the light reflecting off the 45th parallel would be 
observed at such an angle that it would appear as near to you as the pole. 
I'm not sure exactly how that would appear.  It's difficult to imagine.

Would time stop?  That depends on where you are.  If you're in the vehicle 
and I'm here on earth timing your trip, time wouldn't stop for me.  But it 
would slow down for you.  I might be an old man when you came back two 
minutes later.  Yes, technically time would stop on earth, but not on your 
vehicle, and only from your point of view.  That's a long subject.

Other than the above mentioned effects, I think the screen savers have it 
pretty correct.  Closer objects would appear to move faster than distant 
objects.  One thing's certain, it would be more fun than a drive in an Indy 

Layne Johnson

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