B How is that light from the sun takes about eight minutes to reach us

[Ibix]

Imagine two stopwatches, the first one on Earth and a second one attached to a photon of light at the Sun

It's impossible for an object with mass to travel at the speed of light. So it's impossible - even in principle - to "attach a stopwatch to a photon". The photon has no perspective; attempting to describe one in relativity is self-contradictory. This has been pointed at least twice in this thread already.

 

[Drakkith]

Imagine two stopwatches, the first one on Earth and a second one attached to a photon of light at the Sun.

The stopwatch that was riding on the photon of light would show that zero seconds have elapsed.

This is a common misconception. You cannot attach a stopwatch to a photon. And I don't mean that you can't physically attach it (which is true regardless), I mean that you can't assign a reference frame to light because one of the postulates of special relativity is that light travels at c in all inertial frames of reference. Since light doesn't accelerate this means that you can't treat it as a non-inertial frame, but you also can't treat it as an inertial frame. In fact, the amount of time dilation cannot even be calculated for an object traveling at c. There is a $\sqrt{1-\frac{v^2}{c^2}}$ term in the bottom of a fraction in the equation. When $v=c$ you end up with $\sqrt{1-1} = 0$ in the denominator. This is a division by zero error. It means that if $v=c$ the amount of time dilation is undefined, not infinity. In other words, when $v=c$ the equation cannot be used. It's meaningless.

 

[chmasy]

Yes you both are correct, however the gentleman is trying to understand what is going on. Sometimes you need to use the imaginary images to explain the concept. You need to help people to first understand then introduce them the higher concepts.

 

[Drakkith]

Yes you both are correct, however the gentleman is trying to understand what is going on. Sometimes you need to use the imaginary images to explain the concept. You need to help people to first understand then introduce them the higher concepts.

Then it helps to mention in your post that what you're explaining is a direct violation of a fundamental concept of relativity, but for the other person to just accept it for the moment and look up the concept later. Otherwise you risk confusing the person later on down the road and looking to others like you didn't know about this fact.

 

[Ibix]

Simplifications are one thing, but I fail to see the value in teaching things that are explicitly wrong. Especially when it's fairly straightforward: time is not defined along the paths followed by light, only for the paths followed by massive objects.

 

[chmasy]

Then it helps to mention in your post that what you're explaining is a direct violation of a fundamental concept of relativity, but for the other person to just accept it for the moment and look up the concept later. Otherwise you risk confusing the person later on down the road and looking to others like you didn't know about this fact.

Yes, I am new to forum and will remember that in the future
Thank you for advising

 

[chmasy]

Drakkith, would this response be more appropriate for the forum, Or am I still way out of the park?Sunny,

From the question that you asked. Imagine two stopwatches, the first one on Earth and a second one traveling at 99,999/100,000 of the speed of light. When that Stopwatch passes by the sun, both stopwatches are simultaneously pressed to start and record the elapsed time on each stopwatch. When the 2nd stopwatch passes by the earth, both stopwatches are simultaneously stopped and the second stopwatch slows down and returns to earth. We then compare the stopwatches. The one that has been on the Earth would show around 8 minutes have elapsed. The 2nd stopwatch that passed by the sun and Earth would show that very little time had elapsed. So yes, from our perspective on the earth, an object moving close to the speed of light would take around 8 minutes to travel from the sun to the earth. And yes, from the perspective of the 2nd stopwatch traveling pass the sun and Earth would show that the faster an object is travels the slower time elapses for that object.For sake of argument here the sun and Earth are fixed in space with zero velocity in the x, y, and z directions.

 

[Ibix]

The first half is fine. From the perspective of the "fast-moving" stopwatch, however, it sees itself as stationary and behaving normally. Stopwatches on the Earth and Sun tick slowly because they are the moving ones. But that means that the distance between them is length contracted - which is why the "moving" stopwatch doesn't think it should take much time for Earth to reach it. It explains the Earth clock showing 8 minutes because, in this frame, it was incorrectly synchronised.

 

[chmasy]

The first half is fine. From the perspective of the "fast-moving" stopwatch, however, it sees itself as stationary and behaving normally. Stopwatches on the Earth and Sun tick slowly because they are the moving ones. But that means that the distance between them is length contracted - which is why the "moving" stopwatch doesn't think it should take much time for Earth to reach it. It explains the Earth clock showing 8 minutes because, in this frame, it was incorrectly synchronised.

Thank You

 

[Herman Trivilino]

When that Stopwatch passes by the sun, both stopwatches are simultaneously pressed to start and record the elapsed time on each stopwatch.

You can easily know when it's time to start one of those stop watches because you can be present when it happens. But you cannot be present for the starting of the other one, so you will need some way of knowing when you should start it. Saying you'll start it "simultaneously" glosses over how you'll go about establishing simultaneity. However you do go about establishing it for yourself, when someone co-moving with that other stop watch applies the same method he'll get a different result. That is, he won't agree that the stop watches were started simultaneously.

In order to conclude that light takes 8 minutes to reach Earth from the sun, you need to know what time it was on the sun when the light left. In other words, you need to establish what it means to say that something at a distant location is happening "now". That notion of what is happening "now" at a distant location is not something that's absolute. It depends on how fast you're moving parallel to a line that passes through the two locations.