# My Problem Understanding Special Relativity

1. Dec 2, 2008

### DaveShepherd

If light is not accelerated past the maximum speed proposed by Einstein then why do speeding cars seem blurred when they drive past you at a crossroads. If light is not affected by the speed of the cars then why does it seem distorted contrasted to when you are driving at the same speed as the car you are watching.

Also what would happen if a spaceship travelling at near the speed of light turned its headlamps on? Why is light not afftected by motion?

I would appreciate it if all answers were in laymans terms. Thanks.

Last edited: Dec 2, 2008
2. Dec 2, 2008

### Staff: Mentor

This blurring has more to do with how your brain processes images than about anything to do with light. Take a picture of a speeding car using a high-speed camera and you won't see the blurring.
The speed of light is not affected by motion, but the apparent direction of the light is. The light from the headlamps will appear to be concentrated in the forward direction. This is called the "headlight effect". Here's an applet illustrating this: The Headlight Effect

Last edited: Dec 2, 2008
3. Dec 2, 2008

### neu

Re: My Problem Understanding General Relativety

Firstly, our eyes only resolve images at an effective rate of 24 frames per second. Hence our eyes will not be able to resolve things cleary which move a significant amount in 1/24th second, relative to us.

I dont know what you mean in your example of speeding cars, I cant imagine how that would happen apart from engine and suspesional vibrations at high speed.

Now, w.r.t the headlights on a spaceship:

There are 2 fundamental assumptions in special relativity:
1 The speed of light in a vacuum is a constant.
2 The laws of physics remain the same in all inertial frames of reference.

If you ever get confused aout special relativity start from these points. All phenomena involving special relativity can be explained using these points as your starting point.

So for your spaceship example there are 2 scenarios; one in the spaceships reference frame and one in an arbitrary reference frame separate to the spaceship, lets say it's at rest.

Both reference frames see the speed of light to be the same THEREFORE the static observer must observe the spaceship to dilate (elongate or stretch) in space and time to compensate. The degree of spacetime dilation is dependent on the motion of the spaceship relative to the observer.

The fundamental principle of special relativty is that everything is relative to the observer.

I hope that helps

4. Dec 2, 2008

### Naty1

Re: My Problem Understanding General Relativety

As noted already it's a detection phenomena rather than a characteristic of light. And perhaps in part due to background reflections. An analogous effect could be observed in earlier versons of LCD TV's when a football, for example, when thrown might appear blurred on the TV screen. Newer TV's have "120hz technology" which I assume means a faster refresh rate versus the prior 60Hz rate, to reduce motion blur.

Eye limit at 24 frames seems a bit slower than I would have expected but about the right order of magnitude....

Wikipedia ("human eye") says:

5. Dec 2, 2008

Staff Emeritus
Re: My Problem Understanding General Relativety

Also, this is actually Special Relativity. Maybe a mod can fix it.

6. Dec 2, 2008

### DaveShepherd

Re: My Problem Understanding General Relativety

Thanks I now "see" where my thinking was going wrong.

If light can be bent by gravity then could a really heavy object produce a sling shot effect on light? If so why would this not accelerate the speed of light past its "maximum" speed?

7. Dec 2, 2008

### Staff: Mentor

Re: My Problem Understanding General Relativety

While gravity does bend light, it doesn't speed it up. (Now you're talking about general relativity.)

8. Dec 2, 2008

### neu

Re: My Problem Understanding General Relativety

Gravity doesn't bend light as such, it warps spacetime such that light which is travelling in a straight line in spacetime appears to bend in observation from another reference frame. I'm not big on General relativity so please correct me, but I think that's the general layman explanation.

9. Dec 2, 2008

### DaveShepherd

Thanks, So why exactly will light not travel any faster than a set speed? Is there some sort of counterforce?

Can light slow down?

10. Dec 2, 2008

### neu

The fundamental consituent (quantum) of light is the photon. Light is only made of photons. Photons are massless and so they permeate spacetime at the maximum speed allowed.

Think of the speed of light as just another arbitrary fixed constant of our universe (of which there are abotu 26). All things that are massless travel at the speed of light.

In the mathematics of special relativity you can think of the speed of light as telling you how time and space are related, e.g. how a metre relates to a second.

Light does effectively travel at slower speeds in different media, e.g. light travels slower in glass than in air. But this is due to atomic interactions. The inter atom speed is that of a vacuum, but the photons are continually absorbed and re-emmitted giving an apparant slower speed (Hand waving).