General Relativity question/problem

[Niffty951]

The internet may be the source of all knowlegde but humans still provide better search engines than AI to complex questions and I have a question. I'm asking here because Pistonheads is my internet home and I'm not an active forum member anywhere else.

The theory of general relativity bothers me no matter how many times I cross it. So I have a problem/question that I hope will offer some enlightenment if answered.

If I'm driving my car travelling at 99.5% the speed of light and I flash the headlights, then due to the constancy of the speed of light, I as the observer in the car would see the light illuminate objects in the path in front of me. Lets say I have very good headlights and they illuminate an object 10 light seconds in front of me.

If I'm stood in the road as the car approaches and I see the headlights flash. Would I have 10 seconds to move out of the way?

I think what I cannot accept is, how the light would get to the pedestrian ahead of the car. If time effectively stops at the speed of light then from the perspective of the light is all travel instantaneous or there are no distances to travel?

Secondly, as I walked out of the road to avoid the car would the speed of my movement appear normal to the driver of the car?

[Niffty951]

Excellent. However, according to the reference frame of the car it is my understanding that the instant the light leaves the vehicle it will travel away from the vehicle at the speed of light? As it cannot travel faster than the speed of light I would have agreed that time for the pedestrian would be 100%-99.5% but how can this be right to the observer in the car?

You mentioned an increase in frequency. This feature I'm aware of but why does its frequency increase? What is happening in the space between the pedestrian and the vehicle?

[Niffty951]

Simpo Two said:

Niffty951 said:

You mentioned an increase in frequency. This feature I'm aware of but why does its frequency increase? What is happening in the space between the pedestrian and the vehicle?

It's the Doppler effect.

This is good, like the 5 why's of an 8D analysis in Engineering, you're helping me dig down to the issues that are bothering me here.

So with the doppler effect of sound waves that is obvious. The origin emiting the pulse is moving so the wave in front has not travelled as far away from the origin before the next pulse so there is less distance between the two peaks, hifher frequency.

Yet light is made up of photons. I understand that it is the energy level of the photon that determines the frequency (and therefore colour) not the quantity of photons released (that would be brightness/intensity)? If a photon is travelling at the speed of light then the source emitting the next photon can never catch it and nor can the next photon emitted by the source? So how can the movement of the source affect the frequency - transfer energy - change colour?

What effect causes this energy change in the photon?

As a side note if energy is added to the photon by the movement of the origin then the source must lose energy that it gives to the photon? Does this slow it down? A photon does not have mass so it's not a Newtonian relationship?

[Niffty951]

ash73 said:

WatchfulEye said:

Energy is not conserved between reference frames. In the same way in which observers in two different reference frames may disagree about the passage of time, they may also disagree about the energy content of a system.

Makes things interesting if you consider the example of a spaceship with solar panels approaching a star at relativistic speeds, an observer on a nearby planet would disagree about how much energy is reaching the panels, but if they had a telescope which could see the dial on the console they would see what the spaceship sees.

Well I tried to keep it on thread but the string is now unravelling faster than a Higgs boson.

That is a superbly imaginative and a brain teasing scenario, even for someone with a brain the size of a planet to solve.

As to the above, I'm not convinced that cat was alive when you hit it. Thanks all for the input. There was a hint of usefulness to ponder in all that. Not bad for Pistonheads thread on physics

[Niffty951]

P.s. I'm still not convinced by relativity as it stands. It may be good enough for Google's GPS but I think there's a logical alternative out there more convincing than tired light.

I do like the tension in the fabric of 'space-time' caused by mass though. That has weight to it in my book.

Edited by Niffty951 on Tuesday 11th September 20:30

[Niffty951]

James_B said:

Niffty951 said:

P.s. I'm still not convinced by relativity as it stands. It may be good enough for Google's GPS but I think there's a logical alternative out there more convincing than tired light.

I do like the tension in the fabric of 'space-time' caused by mass though. That has weight to it in my book.

Edited by Niffty951 on Tuesday 11th September 20:30

What alternative is that then?

Do ‘t You think that you are just reacting to the fact that you’ve not studied the subject? Relativity is just some rotations in space-time. The “clever”thought experiments that people come up with to find a flaw in it can never work, it’s like looking at a map with two towns ten mi,es apart and trying to come up with a convoluted route between them that proves that they aren’t.

I would love to do a physics degree to form a learned opinion but as I'm too old and busy earning bread to support society to take another 3 years of my life out I'm trying to learn it from books and questions.

In answer to the above I could break a convoluted route down into vectors and logically prove the two locations. What I can't seem to do with general or special relativity is easily break down the end result into logical steps to see the workings.

Edited by Niffty951 on Wednesday 12th September 07:07

[Niffty951]

Morpheus, I'm beginning to believe.

I understand there is significant evidence for the existance of such distortion beyond the realms of our perception. Gravitational waves for one, although that doesn't help me picture the fabric of space time in 3 dimensions sadly.

What was the light bulb moment for you?

[Niffty951]

Thank you for those last responses. I've been thinking on the matrices comment, that is something I can picture in 3 dimensions. Useful to solve the problem mathematically. The 2D example equally.

Feynman is also my favourite lecturer, I love his passion, depth of knowledge and lack of ego. It's all about the beauty of nature for him.

Speaking of which, I think this is where I get tied up. Any other problem can be tackled by logic, mathematics and imagery. They usually compliment each other.

Even the invisible nature of magnetics and the nature of atoms can be pictured on a macro scale and the maths involved can be linked back to those images. It completes the picture.

With relativity I can picture the vectors you mention but I can't relate it to any other physical representation.

Could it be related to refraction of light through different densities? Not that this provides a great leap in my understanding as I don't understand what happens to the extra energy of the light when it loses ..velocity? No? Frequency? How does the constancy of the speed of light hold here? Different frequencies take different paths but they all cover the same distance in the same time? What happens to the lowest frequency? If frequency is an energy level what happens when it drop to zero and what happens to the energy it loses? Mass? (I'm losing all knowledge here, I'm sure much of this is simpler than I'm currently muddling)

Getting very tangental now but I wonder if by using your vector analogy and having sufficient mass in a black hole you'd get a 'change in angle' such that light does not propogate in xyz but instead moved in...time? (what is the dimension I'm missing here for a 4th dimension?

Sorry, I'll go in search of more Feynman lectures until I understand. I find it hard to jump in and out of his lectures (and without worked examples) as you do with youtube.

Edited by Niffty951 on Wednesday 26th September 22:05

[Niffty951]

ash73 said:

Read Feynman's QED book, it's quite short and easy to follow, and is the best explanation of the paths taken by light.

Book arrives tomorrow. Thank you & Amazon