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Seeing things in their past? You are full of beans!

TrueBeliever37

Well-Known Member
The answer depends on whether you use your original frame (where the star is 30 million light years away and the travel time is 30 million years) or the 'frame' of the photon, which is found by taking a limit of frames with velocity <c. In this limiting frame, the distance is 0 and the time of travel is 0.

When you ask if the photon gets across the gap, you are automatically using a frame where it takes time to cross that gap. But, in the 'limiting frame' of the photon, there is no gap, so the 'travel' is instantaneous.

Please acknowledge that you understand that there are *two* reference frames here: one in which the distance is 30 million light years and the travel time is 30 million years and *another* frame where the distance is 0 and the time is 0. Also, please acknowledge that you understand that these two frames describe *exactly* the same set of events.

I do acknowledge you are trying to use two reference frames, but one is actually nonexistent.

In what way is the photon traveling at C, if the time and distance are both 0?
 

Subduction Zone

Veteran Member
I do acknowledge you are trying to use two reference frames, but one is actually nonexistent.

In what way is the photon traveling at C, if the time and distance are both 0?

You do not seem to understand what an inertial frame of reference is. An observer in an inertial frame of reference is not "traveling". The observer feels that he is stationary. When you are traveling in a car both you and a passenger in that car are at rest relative to the world. It looks like the world is "traveling". Let's scale up a bit. If you are standing on the ground it always feels as if you were not "traveling" Yet someone outside the Solar System could see both the Earth traveling around the Sun and perhaps the Sun traveling in the galaxy. As one pulls further and further back what is a "rest" keeps changing. Zooming back in if one is moving at just below the speed of light in a space ship relative to the Earth one still feels that one is stationary and it is the Earth zooming past, or more worrisome zooming towards you.
 

james blunt

Well-Known Member
You do not seem to understand what an inertial frame of reference is. An observer in an inertial frame of reference is not "traveling". The observer feels that he is stationary. When you are traveling in a car both you and a passenger in that car are at rest relative to the world. It looks like the world is "traveling". Let's scale up a bit. If you are standing on the ground it always feels as if you were not "traveling" Yet someone outside the Solar System could see both the Earth traveling around the Sun and perhaps the Sun traveling in the galaxy. As one pulls further and further back what is a "rest" keeps changing. Zooming back in if one is moving at just below the speed of light in a space ship relative to the Earth one still feels that one is stationary and it is the Earth zooming past, or more worrisome zooming towards you.

An inertia observer feels stationary but is travelling at ~ 1000 mph and they also are experiencing an acceleration of 9.82 m/s . Feelings are nothing.
 

TrueBeliever37

Well-Known Member
You do not seem to understand what an inertial frame of reference is. An observer in an inertial frame of reference is not "traveling". The observer feels that he is stationary. When you are traveling in a car both you and a passenger in that car are at rest relative to the world. It looks like the world is "traveling". Let's scale up a bit. If you are standing on the ground it always feels as if you were not "traveling" Yet someone outside the Solar System could see both the Earth traveling around the Sun and perhaps the Sun traveling in the galaxy. As one pulls further and further back what is a "rest" keeps changing. Zooming back in if one is moving at just below the speed of light in a space ship relative to the Earth one still feels that one is stationary and it is the Earth zooming past, or more worrisome zooming towards you.

You replied but didn't answer my question.

when I am in a car I am traveling whether I feel like I am stationary or not.
 

Subduction Zone

Veteran Member
You replied but didn't answer my question.

when I am in a car I am traveling whether I feel like I am stationary or not.

Actually you aren't. The problem is that you can't come up with a consistent definition of "traveling" that is not refuted by reality at some point.

What is "traveling"?
 

Polymath257

Think & Care
Staff member
Premium Member
Are you guys saying that one equation produces a limit for time = 0? If so, is the limit for what you call the proper time or observer time?

Wouldn't it be observer time the way the equation is set up?

Both proper time and observer time appear in the formula. The limit is not as time goes to 0, but as v goes to c. When that happens, the proper time (but not the observer time) goes to 0.
 

james blunt

Well-Known Member
Both proper time and observer time appear in the formula. The limit is not as time goes to 0, but as v goes to c. When that happens, the proper time (but not the observer time) goes to 0.
No it doesn't , or there would be no velocity measured. You can't have velocity without t/d.
 

Polymath257

Think & Care
Staff member
Premium Member
Right, but their equation says observer time = proper time/ something. He said location of the observer didn't matter. I am trying to show that it has to matter, if different answers are obtained when the observer is in a different location.

But different answers won't be obtained from different locations as long as the two observers are at rest with respect to each other again, we are ignoring gravity--where position *is* important because of curvature).

It isn't the different location that gives different observer times, it is relative motion.
 

Polymath257

Think & Care
Staff member
Premium Member
Ok I will ask the question differently. Does the photon get from the star to the planet instantly or does it take time?

For every reference frame, there is an elapsed time for the motion of the photon. The *proper time* for the motion of the photon is 0. Choose whether you want the proper time of the clock time in a reference frame.

If instantly, then why does it take 8m and 20s for us to notice it getting from sun to earth?

The 8 minutes, 20 seconds is the *observer time* for the reference frame of the earth. The *proper time* is not the same, in this case, as any observer time.
 

Polymath257

Think & Care
Staff member
Premium Member
I do acknowledge you are trying to use two reference frames, but one is actually nonexistent.

In what way is the photon traveling at C, if the time and distance are both 0?

There is no frame in which the distance and time are 0. However, we can still talk about the proper time and for a photon, that proper time is 0.

BTW, proper time is ONLY defined for events in the light cones of each other. For events that would take 'faster than light' to get between, the proper time is not defined.
 

Polymath257

Think & Care
Staff member
Premium Member
An inertia observer feels stationary but is travelling at ~ 1000 mph and they also are experiencing an acceleration of 9.82 m/s . Feelings are nothing.

if you feel an acceleration, you are NOT in an inertial frame. Now, an acceleration of 9.8m/s^2 is small enough that it doesn't make much of a different locally. But it does if you don't stay local.
 

Thermos aquaticus

Well-Known Member
I was trying to say that no matter what valid reference frame was used, that to the observer it appears that the photon always takes twice the time to go twice the distance. Which I equate to being the same thing as the photon experiencing twice the time (in that reference frame).

Thank you for the clarification! Science is one of those fields that requires very precise language so there can be some confusion at times.
 

Thermos aquaticus

Well-Known Member
You replied but didn't answer my question.

when I am in a car I am traveling whether I feel like I am stationary or not.

You don't feel velocity. You feel acceleration. That's the point we are trying to make.

If we had a perfectly smooth and straight track and a car with really soft suspension and blacked out windows you wouldn't be able to tell if you were sitting still or travelling at a constant 60 mph. The only forces you would detect is when we accelerate the car to 60 mph or decelerate (i.e. hit the brakes) to slow down from 60 mph. Any frame with constant velocity is the same in physics. All of the physical constants and laws are the same for all frames of reference with constant velocity.
 
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