Monday, 16 January 2012

Physics question

I have only a layman's comprehension of physics. Despite that I'm boldly plunging into a new piece of writing about physics and have started to read around the subject as background. Naturally I'm stumped by the material, but there's one thing that really bugs me in my ignorance and I'd be very grateful if anyone could unravel my confusion for me. But only with recourse to words please! Any mathematical proofs will be completely over my head. Besides, equations and novels don't often make comfortable bedfellows.

Einstein's thought experiments that led to his two theories of relativity, were posited around the speed of light. The 'fixed', unvarying vantage of an observer was shown to be false at speeds approaching the speed of light. If an observer was actually travelling along a beam at light at its speed of 186,000 mps, then all sorts of strange things happen to space and time within his/her observations at that speed.

But my question is, if the observer is travelling at the speed of light, so that everything becomes the present, with no discernible past, then what happens to the light itself? If there is only the present, then is the light actually moving? Would the light not just be present everywhere simultaneously? And yet we know that light is an energy source, it originates from some star burning fuel somewhere and that the energy moves through space and time. If the observer is travelling on the light beam, then relative to him/her the light isn't moving at all?

If anyone can help sort out this muddle in my mind I'd be terribly grateful and you'd get an acknowledgement in the novel too!

Thanks in anticipation


James Everington said...

I think the confusion is about the idea that time is appears different to the person travelling at the speed of light. I'm not sure, but think this isn't the case.

There's a famous example about two twins, one remains on earth, the other blasted into space at near speed of light. When the second comes back he is say 1 year older and the one who remains is 50 years older. But for both of them, time appeared to pass normally (1 year and 50 respectively) - it's only when he comes back and slows down that the difference is noticed.

I think. I might be wrong.

Sulci Collective said...

Thanks James. I knew of that type of example, and while I can get my head around the time aspects of relativity, it's what happens to light itself and its speed that confuses me. When everything else is determined and defined by this one fixed unity, it can't undergo any changes or relativity itself. And yet logically it must do?

James Everington said...

Well I'm a bit out of my depth here (a lot actually) so I'll await other my learned replies!

Chris said...

Time dilation when travelling at speeds approaching the speed of light is derived from what mathematically should happen given that the speed of light is a constant. As such, they start to sound nonsensical if the speed of the vehicle in the thought experiment in question approaches that of light - however these effects have been experimentally measured and as such are very real. Like quantum effects they are effectively undetectable and very counterintuitive in the real macro- and brady- worlds we are used to…

The usual analogy starts with throwing a ball along a train carriage.

If a train is travelling at 100mph and a ball is thrown along the carriage in the direction of travel at 5mph, then observed from a field beside the railway track the ball travels at 105mph, adding together the speeds of the train and the ball.

However if instead of a ball our experimenter switches a laser pointer on at one end of the carriage and shines it in the direction of travel, it’s a very different matter.

In classical physics you would expect that when observed from a field beside the railway track the photons in the beam of light from the laser would be measured to be travelling at the speed of light + the speed of the train.

But this is not so. Nothing can travel faster than the speed of light. To fix this discrepancy, we have to change the rate at which time passes inside the train.

From the outside it appears that the photons have to travel a greater distance than the length of the carriage because the carriage is moving. For the purpose of demonstration let’s call this length 10 metres.

At a fixed speed it will take light a fixed amount of time to travel 10 metres. Lets call this 10 timebits.
However inside the train carriage, measured relative to the carriage interior, the length the beam of light has to travel is only 9 metres. At a fixed speed it will take light a fixed amount of time to travel 9 metres. Let’s call this 9 timebits.

However, viewed as a whole, the beginning and the end of the experiment are at the same points in time. Somehow people on board the train have only experienced 9 timebits whilst their colleagues observing from within the field have experienced 10.

Time on board the train has therefore slowed down.

The faster the vehicle is travelling, the slower time inside passes. At 0.9 of the speed of light it would take a starship approximately five years to travel to Alpha Centauri but only two and a half years would pass on board.

Travelling at the speed of light, time would stop on the ship and the crew would have no experiences. Time spent at the speed of light is non-existent which means that it would be impossible for a ship to function whilst travelling at the speed of light. Or a laser pointer.

The whole concept of things "working" whilst at the speed of light is meaningless. The words "happening" and "working" imply the existence of a temporal axis which disappears at the speed of light.

An aside - the same equations also predict that at the speed of light mass increases to infinity and length to zero, so it's not just time that goes wonky. These are all demonstrations of why it is probably impossible to accelerate to the speed of light.

Chris said...
This comment has been removed by the author.
Chris said...

PS photons are unaffected by these transformations because they have zero mass or volume and don't "experience" time.

From the photon's "point of view" whilst its back end is touching the surface of the sun its front end is touching your retina! The journey appears to take no time at all to it.

Dan Holloway said...

Chris' explanation is very well put. It makes sense only to say that from the point of view of the guy travelling at light speed nothing happens because he's travelling in what could best be described from a sensory perspective as a single packet of sense-experienceb (like - metaphorically - a particular packet of light forms a skin around him that doesn't shift), but from the point of view of the guy watching it unfold things do happen because that packet of light is moving relative to him.

Have you read Aggie? There's a load of pseudo-physics guff in there about near light speed travel and preserving the moment of death.

What interests me most as a thought-experiment is the obvious one. Chris, can you help - what happens to the guy travelling at light speed and time if he shuts his eyes? In other words, how do internal and external time relate?

Chris said...


"Chris, can you help - what happens to the guy travelling at light speed and time if he shuts his eyes? In other words, how do internal and external time relate?"

He'd have to shut his eyes just before reaching light speed. Once at light speed they remain shut but there is no time so if he reopens them again when the ship drops below light speed again as far as he is concerned he blinked. Even if the ship was travelling at light speed for what the outside universe considered to be centuries...

At the speed of light there is no time - his brain was frozen in an instant so no subjective internal time passed whatsoever.