# The Naked Scientists Forum

### Author Topic: Why does light go "straight"  (Read 3053 times)

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##### Why does light go "straight"
« on: 09/05/2012 20:51:54 »
Hi everyone,

Please forgive a question from a non-scientist but I just can't wrap my head around this.  I understand that the velocity of the light source is not added to the velocity of light.  But if this is so, why does light go "straight."

For example, if I toss a baseball on a moving train, the ball appears to go straight from my point of view because the velocity of the train is added to that of the ball so it stays in my train-frame with me.

But how does this work for light?  What happens with a laser or a single photon.  If I bounce a beem of light back and forth between mirrors on a smoothly moving train, why don't the mirrors immediately move away from the light?  (I understand the train would have to be going very very fast.)

Thanks!

#### Phractality

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##### Re: Why does light go "straight"
« Reply #1 on: 09/05/2012 22:39:36 »
Light follows straight pathes (in a vacuum) because the space that other things move in is defined by the motion of light. Distances between objects are determined by how long it takes for light to travel from one object to the other. The time it takes for the light to get there is also determined by the speed of light. Clocks at both objects are synchronized by the definition of simultaneity, which says that it takes equal times for light to get from the middle to both ends. So we measure both distance and time according to how long it takes light to move between clocks, and the clocks are synchronize with reference to the light.
In general relativity, the path of light is actually the defintion of a straight line. So instead of gravity bending light, it warps the fabric of space-time. Diagrams that show light bending around a gravity well are an attempt to show how space-time is warped, but in general relativity those curved light pathes are actually straight lines by definition.

#### CliffordK

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##### Re: Why does light go "straight"
« Reply #2 on: 09/05/2012 23:42:58 »
I think you are right.
I assume you mean shooting the beam of light crossways on the train.

Consider if you put a mirror on Pluto.
And, then shot a laser towards the mirror.
To hit the mirror, you would have to aim at where Pluto & the mirror will be in about 5.5 hours, not where you see them, nor where they are at at the moment.

Likewise, for the reflected beam to be picked up on Earth, it would have to be directed towards where Earth will be in 5.5 hours after it is reflected.

Light, of course, is going straight, but one is just moving out from under the path.

If you observed the beam of light from a third moving object synchronized with the first two in the plane above.  You could draw a line between Earth and Pluto, and the beam would appear to curve similar to the Coriolis effect.

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##### Re: Why does light go "straight"
« Reply #3 on: 10/05/2012 03:12:35 »
Thank you both for the answers.  I think I'm getting there.  My question is that unlike the situation where I am trying to aim a laser at Pluto, if I am aiming the laser at a mirror opposite me on the moving train, I don't have to anticipate where the mirror will be when the light hits it.  (I realize I am assuming a very large train).  What I don't understand is why.  In the case of tossing a baseball, I know that I am imparting the velocity of the train to the velocity of the baseball.

I'm going to take a stab at answering my own question.  The speed of light is constant, but not the velocity which has a directional componant.  The motion of the train does not affect the speed of light but it does affect the direction.  I suppose its futile to ask HOW the movement of the source could affect the direction of light but I think that is what is troubling me.

Thanks again.

#### lightarrow

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##### Re: Why does light go "straight"
« Reply #4 on: 12/05/2012 22:39:27 »
For example, if I toss a baseball on a moving train, the ball appears to go straight from my point of view because the velocity of the train is added to that of the ball so it stays in my train-frame with me.
But how does this work for light?  What happens with a laser or a single photon.  If I bounce a beem of light back and forth between mirrors on a smoothly moving train, why don't the mirrors immediately move away from the light?  (I understand the train would have to be going very very fast.)
Try to imagine what you would see from the ground, outside of the train. You will see a beam which is no more perpendicular to the train's direction, but it forms an angle less than 90°.
This proves that the beam's velocity *does* add to the train's velocity. This because they are not parallel.

In general, for an object (included a light's beam) moving with speed v1, perpendicularly with respect a frame of reference FR1 (the train, in our case) which moves with speed v2 with respect to the frame FR2 (the ground, in our case), the composite speed V is:

V = sqrt(v12 + v22 - v12*v22/c2)

Note that if v1 or v2 or both are equal to c, that equation gives V = c. That is, the "speed" of light in your example doesn't change; the "velocity" of light *does* change (it changes direction).

--
lightarrow

#### wolfekeeper

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##### Re: Why does light go "straight"
« Reply #5 on: 13/05/2012 17:55:38 »
You can look at this as space-time distorting, but you can also look at it slightly more simply; all your clocks get messed up by the motion, and all your rulers get distorted in a particular way.

The result of this messing about is that the speed of light you can measure is always the same.

This happens because all your clocks and rulers are held together with electromagnetism; all the atoms are held together by electromagnetic forces; the electrons are held in place by the positive charges of the protons as are the molecules, and the shape of the force fields change when you accelerate; all the molecules become a slightly different shape, and they don't jiggle around so fast, chemical reactions are slowed, mechanical movements; everything.

In effect, your rulers are held together by light, and if the relative speed of light varies, the ruler changes shape, you change, and your clock changes, perfectly, so you can't see it!

So you don't have to aim off- the distortions automagically do it all for you.

In the particular example you have, you don't have to aim anywhere different, but it takes longer to get to the mirror because the light travels a longer distance, but you don't notice that either because your clock has slowed down!
« Last Edit: 13/05/2012 18:00:11 by wolfekeeper »

#### lightarrow

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##### Re: Why does light go "straight"
« Reply #6 on: 14/05/2012 18:49:28 »
My question is that unlike the situation where I am trying to aim a laser at Pluto, if I am aiming the laser at a mirror opposite me on the moving train, I don't have to anticipate where the mirror will be when the light hits it.
Maybe you will be surprised but in this case there is no difference between a beam of light and a baseball
You are inside the train and you aim a mirror opposite you with a laser beam, in a direction perpendicular to the train's velocity v. You see (or measure) that the beam's path is perpendicular to v. What sees someone on the ground, outside the train: the beam is bent of an angle related to v/c (if I remember well, tg () = v/c).

Quote
(I realize I am assuming a very large train).  What I don't understand is why.  In the case of tossing a baseball, I know that I am imparting the velocity of the train to the velocity of the baseball.
The motion of the train does not affect the speed of light but it does affect the direction.  I suppose its futile to ask HOW the movement of the source could affect the direction of light but I think that is what is troubling me.
I have made several problems of this kind and I discovered a trick: see it in terms of little balls (photons) emitted from the laser as if they were bullets emitted from a gun. In this case (not in others) it works: there is no difference. The only difference is in the numerical value of the ball's speed...

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lightarrow

#### yor_on

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##### Re: Why does light go "straight"
« Reply #7 on: 16/05/2012 01:40:59 »
Heh :)
And now for mine definition.

It has to do with what we call frames of reference. Sounds almost magical that one, doesn't it?

If you let a light bounce on that train between two fixed mirrors, the mirror facing furthest away (B) in the direction of the trains motion will see a red shifted light (photon) coming at it at the speed of, eh, light :) 'c' that is.

The other mirror (A) won't see that redshift as I think of it, now assuming that it magically could follow that 'lights propagation', but as the light bounce back from the second (B:s) mirror towards it (A) again it will find the light to be blue  but coming at it at 'c' according to its ruler and its own 'wrist watch'.

So the energy of that light will blue (energetic), respective red (weak) shift depending on the mirror measuring relative the frame of that train.

Which could be one way of defining it, but in this particular case would be totally wrong as I see it. Both mirrors share a common fixed reference frame mounted, unmovingly, to the floor of that train, assuming the train to have a constant uniform motion (and ignoring gravity for this). To assume that it could red, relative blue, shift here would in fact introduce an idea of a 'absolute motion', which light then would 'use' as a measure, bouncing.

But motion, and 'energy' is always relative the observer, and when sharing the same 'frame of reference' (the train) there will be noting 'weird' to be seen.

If you instead assume that one mirror (B) move from the other mounted one (A), on the train then? Well, then the 'frame of reference' can't be the same for both and both will find a redshift.

So what frame of reference would be able to see that red, respective blue, shift from the mirrors light bouncing? Now, that's a question of geometry, and 'frames of reference'.

What would someone see if standing 'still', on the side of the bank, watching that train pass by?

Another thing that is really important to understand is that what an observer finds to differ always will be a definition from comparing 'frames of references'. 'Locally', using your wrist watch and your ruler nothing never change for you, no matter where you are or how fast you move. Your esteemed life span and length will always be the same according to those. What changes is the relation your 'local frame of reference' finds itself to have relative another frame of reference.

And if we look at a length contraction then the universe must 'shrink' in a acceleration for you. The problem here is where to define that local 'frame of reference', but very loosely speaking you might assume that your 'space craft' is one undifferentiated same 'frame of reference' for this. So the universe shrinks, but you stay the exact same.

And that is the truth,the whole truth and nothing but the truth, so help me, eh, Einstein?

:)

Can you see what I'm getting on here? That the thing defining your universe will be your ruler and your clock. Assuming that there exist a 'conceptually same universe' in fact becomes a solely conceptual exercise, not the reality you experience.

So now the question becomes, what would you define as more 'real'?
What you expect from preconceptions, grounded since birth, or what experiments tells you?

To find repeatable experiments assume a 'same universe', or, it assume a 'locally same universe', then as defined relative 'your ruler' and 'your clock'. That locally same universe is defined through constants. And 'c' is the one creating both time dilation's and LorentzFitzgerald contractions. And 'c' is also the information carrier telling you what there is to 'observe'. It's your 'curtain of light', painting your 'reality' constantly.

And 'c' doesn't care about motion, as I see it.

#### Geezer

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##### Re: Why does light go "straight"
« Reply #8 on: 16/05/2012 07:05:32 »
Well gosh! What a lot of complicated stuff from many members.

But isn't the the answer simply;

"Because it's easier than changing direction"

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##### Re: Why does light go "straight"
« Reply #8 on: 16/05/2012 07:05:32 »