## K10    Measuring and Seeing in the STR

Measuring and seeing are not the same thing. To take a measurement typically means to 'stop' time using a clock at the location of an event, that is, to capture a moment at a given location. Vision is a process by which at a given moment and a specific location we register all of the optical signals, which arrive from various places and which originated at various times. Seeing is therefore comparable to photography. In an astronomical photograph of the new moon with an exposure time of 0.1 seconds, we see how it looked about a second ago; the planet Mars, as it was a quarter of an hour ago; and for Saturn we see the light which was reflected about an hour ago! If we increase the exposure (a few minutes is sufficient), then perhaps we can even detect a galaxy, revealed by photons which started their journey millions of years ago!

Thus it is possible to see things that no longer exist. A supernova in the Large Magellanic Cloud, a small companion galaxy of our Milky Way, occurred about 163,000 years ago, when we observe it today. The travel-time of the light must also be considered when thinking about how an object appears which is moving very quickly. Consider the following diagram from [25-85]:

Photons leave the corners A and B of the rail car. However, the photons from B cannot reach us.

Only now (after time Δt) are the photons from the front corners C and D sent, which reach our eye simultaneously with those from A.

The car would appear thus if Lorentz contraction did not exist!

We actually see the car thus, since the distance between the corners C and D has shrunk due to the high velocity v.

That is precisely the view of the car we would have, if he had turned away from our line of sight at the angle α, where sin(α) = v / c.

The fact that the car appears to have turned away is only due to our 3D interpretation of the 2D image we have of the situation!

It would be equally correct to say that the car has been tilted and Lorentz contracted. However, we are not used to this interpretation of a visual impression. But it would fit much better to the transformation of the density!

Of course, this comment is not from the author of [25].

Meanwhile, there is a group within the physics community, which is using the computing power available today to show how, for example, a flight through the Brandenburg Gate at 0.95 • c would be visually experienced. Sometimes even the change in color due to the optical Doppler effect is taken into account. Clearly, one must play the whole thing back in slow motion, so that it does not run too fast for a human observer. A good location for such visualizations is www.tempolimit-lichtgeschwindigkeit.de

Ute Kraus of the University of Tübingen is the person behind this address. Her group realized for the Einstein Jubilee of 2005 relativistic bicycle tours through the old towns of Bern and Tübingen. The exhibitions in Ulm and in Bern even offered the visitor to mount a real (stationary) bicycle and pedal away. The pedal speed was then processed by a computer which amplified the effects of 30 km/h to 300,000 km/s. To make it possible to navigate the narrow streets (and still be able to see something!) the houses passed by at the unamplified velocity.

If you delight in such visualizations, then visit the website recommended above. You will also find references and other related material about the theories of relativity of Albert Einstein.