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?

[annodomini2]

Niffty951 said:

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?

1. No because of time dilation.
2. No because by the time the light reached the pedestrians eyes you would have moved much closer.
3. The light from the headlights is travelling 0.5% faster than the car so it will reach the pedestrian before the car, but not with sufficient time to react.
4. No the pedestrian would appear to move very fast, due to time dilation.

[Toltec]

Observer would need to be able to see in the UV range too if you have a modern car with LED or HID lamps.

[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?

[Simpo Two]

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.

[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?

[WatchfulEye]

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.

From the perspective of the driver a visible photon was emitted, and a visible photon was absorbed. From the frame of the pedestrian, an ultraviolet photon was emitted and an ultraviolet photon was absorbed.

In the more familiar system of a moving mirror causing doppler shift (for example, a doppler radar speed camera), then the photon when it is reflected exchanges momentum and kinetic energy with the mirror, losing or gaining energy as a result. This is a different process to the issue of relativistic reference frames.

[Simpo Two]

Niffty951 said:

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?

I'm not a nuclear physicist but, as far as I was taught as an undergraduate... light is part of the electromagnetic spectrum and so has a frequency. Think of waves on the sea. If you stand still they hit you at speed X. If you move towards them, they hit you faster - even though they're going at the same speed.

[bearman68]

To my way of thinking, you all have it wrong (Because I have a brain the size of a planet I'm absolutely correct, it's just the maths hasn't caught up yet).
I think there is a tendancy to think of time as a fixed thing. But in fact, as someone has mentioned, there's time dilation.
So the car emits light, Light hit the object.
Because light speed is a constant, the object will see the car and the light at the same time, and if the object moves out of the way, the car will appear in the new place at the same time the object gets there.

This is absolutely proven with insurance claims. I moved over, but the car came out of nowhere, and hit me before disappearing. Observation evidence that Newtonian laws are not accurate at near light speed conditions.

And then schroeder's cat get's run over as well, thus solving that problem.

See, this quantum physics is easy.

[Toltec]

bearman68 said:

And then schroeder's cat get's run over as well, thus solving that problem.

Ahh, but does it?

[Dr Jekyll]

Toltec said:

bearman68 said:

And then schroeder's cat get's run over as well, thus solving that problem.

Ahh, but does it?

It does, it's Schrodinger's cat I'm not sure about.

[bearman68]

Toltec said:

Ahh, but does it?

[Toltec]

Dr Jekyll said:

Toltec said:

bearman68 said:

And then schroeder's cat get's run over as well, thus solving that problem.

Ahh, but does it?

It does, it's Schrodinger's cat I'm not sure about.

The spelling made it uncertain.

[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

[bearman68]

Toltec said:

The spelling made it uncertain.

It was uncertain in principle but the spelling was accurate (I checked), except for the little " on top of the o. (I don't know how to do those)

[James_B]

Niffty951 said:

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?

What’s your General Relativity question then? You’ve posted one on Special Relativity as a teaser, but where’s the real one?

[James_B]

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.

[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

[Toltec]

Niffty951 said:

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

Relativity is not easy to grasp intuitively based on human experiences of reality, as a, very, loose analogy is like the concept that the shortest route between two points is a straight line, however if you live on the surface of a sphere it is in fact a great circle line.