# What do I see if I was a particle at speed moving by a spaceship?

1. Jan 17, 2012

### fbs7

Say I became a particle and I'm moving at 0.9c towards a spaceship. My course is parallel to the ship, and I'm seeing it bow first.

So the entire ship is initially in front of me, seems quite short and is highly blue-shifted, right? So any crew would seem to be quite blue and is moving around really fast. Also say I can see one clock at the bow, another at the stern, and both show the same time.

Then as I move past its bow, the bow of the starship moves to my back and becomes red-shifted, so any crew member in the bow now became redder and is much slower; yet, the stern is still blue-shifted, and the crew members there are still moving around like crazy. Eventually I move past the whole ship and everything is now red-shifted and everybody is moving about much slower.

So is that right? That's how I should see it?

If so, here's the part I'm stuck: the whole spaceship is in one reference frame (that's moving past me at speed), so the time-dilatation effects should be the same for the whole ship... how come the ship shows at one point in time two different time-dilatation effects - one for the red-shifted bow, and another for the blue-shifted stern?

Also, because I saw the clock at the stern blue-shifted more time than the one in the bow, it should show a time slightly ahead as the one in the bow... but that doesn't seem correct at all, as initially both clocks seemed to have the same time, now that I'm past they seem to have different times...

Last edited: Jan 17, 2012
2. Jan 17, 2012

### ghwellsjr

In Special Relativity, you always have to make a distinction between what an observer sees and what is actually happening according to a specified frame of reference. This is because what you can see is affected by the propagation of light which adds delay and so makes things appear later than the frame says they are. And, because of the relative motion, this delay can be decreasing (blue shifted) or increasing (red shifted). This effect is called Relativistic Doppler. So this explains the effects that you have described.

3. Jan 18, 2012

### alexg

It is. From the reference frame of the ship.

But you are in a reference frame which is changing with regards to the spaceship frame. Time dilation effects are relative to your frame.

4. Jan 18, 2012

### fbs7

That's awesome... so if a galaxy was passing by us at speed, and half of it had passed already, then half the galaxy would be red-shifted and seem to spin very slowly, while the other half would be blue-shifted and seem to spin very swiftly?

What an odd sight that would be!

5. Jan 18, 2012

### ghwellsjr

Not really, because it would happen so fast that you wouldn't actually see much spinning at all. Galaxies don't spin very fast.

Also, when we talk about something approaching us being blue shifted and immediately changing to being red shifted as it passes us, we mean it is directly in line with us. When it is off to the side, which I would hope a galaxy would be, the transistion is more gradual. Furthermore, when you're looking at a real three-dimensional object, you have to take into account the varying light transit times for every point that is a different distance away so it causes a lot of distortion in the visual image. It is very hard to calculate the precise image of a rapidly moving object, spinning or not.