High School Traveling through space at zero km/s

[mike9731]

TL;DR

Is it possible to travel through space at zero km/s relative to the speed of light and therefore be perfectly still?

In sci-fi when an author is talking about space travellers or describing the movement of galaxies they will say something like “movement in space only means anything in relation to another object”. Examples of this would be, a space ship moving away from earth at 100 km/s, or 2 galaxies moving towards each other at one light year per century. I think it would make it easier to describe movement in space if we had three axis that we all agree on and we used 0 km/s relative to the speed of light as a frame of reference for all movement in space. If an object travelling at the speed of light in a straight line and it’s speed is reduced by the speed of light would it now be at absolute zero for movement? If a cloud of hydrogen in space became dense enough for fusion to begin occurring and at a point in time it began emitting light in all directions, forming a sphere of light the radius of which would grow at the speed of light, this is sometimes referred to as an event horizon, the event is the beginning of fusion and light is at the edge of the spere. So if the new star is in motion it will not stay in the centre of the sphere but if the star is perfectly still it will always stay in the centre of its own event horizon. Does this concept exist in physics?

 

[Ibix]

“movement in space only means anything in relation to another object

This is correct, and you will find it in physics textbooks too.

0 km/s relative to the speed of light as a frame of reference

This, however, is impossible. If you attempt to construct a frame of reference in which light is at rest in a relativistic universe you will find yourself requiring two lines that don't point in the same direction but also do point in the same direction. This follows fairly directly from Einstein's second postulate, that the speed of light is constant in all inertial frames of reference.

 

[Herman Trivilino]

I don't understand what you mean by "relative to the speed of light"? We refer to objects, not speeds, when we speak of relative motion. For example, when my car's speedometer reads 60 mi/h it means I'm moving at that speed relative to the road.

I might try to measure speed relative to something like a sound wave. Then I might be able to say I'm at rest relative to the sound wave, or at any other speed relative to the sound wave.

But I could never do such a thing with a light wave in a vacuum. The light wave always has the same speed relative to me. And to everything else, too.

 

[mike9731]

This is correct, and you will find it in physics textbooks too.

This, however, is impossible. If you attempt to construct a frame of reference in which light is at rest in a relativistic universe you will find yourself requiring two lines that don't point in the same direction but also do point in the same direction. This follows fairly directly from Einstein's second postulate, that the speed of light is constant in all inertial frames of reference.

Thanks for the reply I am still trying to wrap my head around relativity.

 

[mike9731]

I don't understand what you mean by "relative to the speed of light"? We refer to objects, not speeds, when we speak of relative motion. For example, when my car's speedometer reads 60 mi/h it means I'm moving at that speed relative to the road.

I might try to measure speed relative to something like a sound wave. Then I might be able to say I'm at rest relative to the sound wave, or at any other speed relative to the sound wave.

But I could never do such a thing with a light wave in a vacuum. The light wave always has the same speed relative to me. And to everything else, too.

Thank you for your response I am still trying to understand relativity.

 

[kuruman]

Is it possible to travel through space at zero km/s relative to the speed of light and therefore be perfectly still?

Think this through.

If you are traveling at zero km/s relative to the dog you are walking, this means that you are moving at the same velocity relative to the ground as the dog.
If you are traveling at zero km/s relative to a jogger, this means that you are moving at the same velocity relative to the ground as the jogger.
If you are traveling at zero km/s relative to an airplane, this means that you are moving at the same velocity relative to the ground as the airplane.

Question
If you are traveling at zero km/s relative to light, this means that you are moving at the same velocity relative to the ground as what?
(a) The dog
(b) The jogger
(c) The airplane
(d) Light

 

[mike9731]

Think this through.

If you are traveling at zero km/s relative to the dog you are walking, this means that you are moving at the same velocity relative to the ground as the dog.
If you are traveling at zero km/s relative to a jogger, this means that you are moving at the same velocity relative to the ground as the jogger.
If you are traveling at zero km/s relative to an airplane, this means that you are moving at the same velocity relative to the ground as the airplane.

Question
If you are traveling at zero km/s relative to light, this means that you are moving at the same velocity relative to the ground as what?
(a) The dog
(b) The jogger
(c) The airplane
(d) Light

I think the answer is d. light.

 

[Herman Trivilino]

Thank you for your response I am still trying to understand relativity.

The thing that makes Einstein's relativity unique is the Second Postulate: Light in a vacuum always moves at the same speed regardless of the speed of the source.

 

[jbriggs444]

And not just regardless of the speed of the source. Regardless of the speed of the observer doing the speed measurement.

No matter how fast you go, a pulse of light behind you will be measured to overtake and pass by you at the speed of light.

No matter how fast you go, a pulse of light ahead of you will be measured to approach and pass by you at the speed of light.

This idea seems unreasonable at first glance. How could two observers in relative motion measure the same relative speed for the same pulse of light? Resolving that conundrum involves the relativity of simultaneity.

 

[kuruman]

I think the answer is d. light.

Correct. Putting this together with what @Ibix already mentioned, namely that light travels at the speed of light in all reference frames, if you travel at zero km/s relative to the speed of light, there can be no reference frame in which you will be "perfectly still." Does this answer your question?