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# Constant speed of light

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### #1 SouthernSky

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Posted 19 December 2013 - 03:22 AM

Hi, we all know the speed is constant from all observers regardless of the reference speed of the observes. My question is: if an object is travling at the speed of light, how will it see the light traveling at the same direction? Or will it be able to see the light at all? Please ignore the reality factory such as it is impossible to achieve speed of light for object with any mass.

Thank you.

### #2 FirstSight

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Posted 19 December 2013 - 09:11 AM

The speed of light will seem the same because time becomes dilated for an object as it is accelerated toward the speed of light.

More particularly, where:
t(1) = time elapsed as seen from accelerated object
t(2) = time elapsed as seen from non-accelerated object
u = velocity of accelerated object
x = one-dimensional position of accelerated object relative to non-accelerated object
c = speed of light

THEN:
t(2) = (t(1) + ux/c^2) / (1 - u^2/c^2)^(1/2)
i.e. t(2) becomes much larger than t(1)

### #3 shawnhar

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Posted 19 December 2013 - 10:20 AM

Hmmmm, wouldn't no time pass from the observer's pov?
Wait, you didn't see it, let me do it again...

### #4 PeterR280

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Posted 19 December 2013 - 10:42 AM

The transformation equations blow up, I.e. They become zero or infinite at the speed of light so the answer becomes meaningless. Time becomes infinite and lengths become zero.

### #5 PeterR280

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Posted 19 December 2013 - 10:57 AM

Just to further explain what the answers mean if they are zero and infinity, if something were traveling at c and emitted light you wouldn't see anything because time will appear to have stopped for it and any wavelength would be infinite, or the frequency of the light would be zero.

### #6 GregLee1

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Posted 20 December 2013 - 02:43 AM

My question is: if an object is travling at the speed of light, how will it see the light traveling at the same direction?

It should try to arrange to travel through a dusty region which reflects toward it some light originally traveling in parallel. When it detects the reflected light, it can make inferences about the light that was not reflected, but continues on a parallel course with it. (In much the way we can see a car's headlights shooting out in front of it because of bugs and such which are caught in the headlight beams.)

### #7 FirstSight

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Posted 20 December 2013 - 08:58 AM

My question is: if an object is travling at the speed of light, how will it see the light traveling at the same direction?

It should try to arrange to travel through a dusty region which reflects toward it some light originally traveling in parallel. When it detects the reflected light, it can make inferences about the light that was not reflected, but continues on a parallel course with it. (In much the way we can see a car's headlights shooting out in front of it because of bugs and such which are caught in the headlight beams.)

That's not really the issue - see the post above yours for an explanation. You're making the false assumption that because the light source is traveling at near the speed of light, any light emanating from it is necessarily polarized in the direction of travel. Any reflections still won't help with the issue of the frequency approaching infinite length.

### #8 PeterR280

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Posted 20 December 2013 - 09:47 AM

The universe would appear to have been squeezed to a zero depth. There would be nothing in front or behind you to travel to.

### #9 GregLee1

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Posted 20 December 2013 - 10:14 AM

You're making the false assumption that because the light source is traveling at near the speed of light, any light emanating from it is necessarily polarized in the direction of travel.

But the question, which I quoted, was about "light traveling in the same direction". So how can it be a false assumption that the light is traveling in the same direction as the object? That's what is asked about. And the question doesn't say that the object traveling at the speed of light is the source of the light it is trying to see. I didn't understand the relevance of the other answers to the question that was asked.

### #10 PeterR280

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Posted 20 December 2013 - 10:24 AM

There would be nowhere for the light to travel to.

### #11 GregLee1

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Posted 20 December 2013 - 11:09 AM

There would be nowhere for the light to travel to.

I don't understand that.

Let the object traveling at light speed be represented by a photon traveling in a certain direction, and suppose there is a beam of many photons alongside it traveling in that same direction. When shall we say the object (a photon) sees the light traveling in the same direction? It seems reasonable to say it's when photons from the beam intersect and penetrate the object. Then the object will never see the parallel beam of light directly, because their paths will never intersect.

But the object can see the light indirectly, because a few photons from the beam can be reflected into the path of the object so as to intersect it. I don't see anything impossible in this, and I don't see why time rates or any relativistic reasoning needs to be involved.

So that's how the object sees the light, and that is what was asked.

### #12 PeterR280

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Posted 20 December 2013 - 11:54 AM

An object with non-zero rest mass is not a photon.

### #13 GregLee1

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Posted 20 December 2013 - 02:06 PM

An object with non-zero rest mass is not a photon.

We may assume the object has zero rest mass, or else it makes no difference that it doesn't have, since the OP says, "Please ignore the reality factory such as it is impossible to achieve speed of light for object with any mass."

Besides, I didn't say the object was a photon.

### #14 PeterR280

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Posted 20 December 2013 - 03:18 PM

Again, the universe would appear to be contacted to a zero depth in the direction of motion. Where are the photons going to travel to?

### #15 PeterR280

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Posted 20 December 2013 - 03:59 PM

To give a perspective, protons in the LHC are traveling just 3 meters per second slower than c. At that speed, the Lorentz factor is about 7500, so that the visible universe of 13 billion light years would appear to be contracted to about 1.7 million light years. Distance to the Sun would be contracted to 12,400 miles.

### #16 GregLee1

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Posted 20 December 2013 - 04:19 PM

Again, the universe would appear to be contacted to a zero depth in the direction of motion. Where are the photons going to travel to?

I have no idea. Does your observation somehow answer the question that was asked? Are you saying that the object can't see light traveling in the same direction?

### #17 PeterR280

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Posted 20 December 2013 - 04:35 PM

I am saying it's an impossible situation since the answers a zeroes and infinities.

### #18 shawnhar

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Posted 20 December 2013 - 04:44 PM

Well if we are ignoring the reality factory, we need to know if it is an African or European photon, then we can determine it's unladen velocity.

### #19 Xiaoding

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Posted 06 February 2014 - 07:02 PM

"Again, the universe would appear to be contacted to a zero depth in the direction of motion. Where are the photons going to travel to? "

The same place they are traveling to now, I would suppose, since that is the exact situation they are in.

### #20 Xiaoding

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Posted 06 February 2014 - 07:13 PM

Hi, we all know the speed is constant from all observers regardless of the reference speed of the observes. My question is: if an object is travling at the speed of light, how will it see the light traveling at the same direction? Or will it be able to see the light at all? Please ignore the reality factory such as it is impossible to achieve speed of light for object with any mass.

Thank you.

I am going to make a very uninformed guess.

I take it to mean, as the object is accelerating, there is light also traveling along the same path? So how will it see that light? I think the answer is, very slowly.

The speed of light will appear to be the same, in the objects frame of reference, right? But, that traveling light, IS NOT in that same frame of reference.

So, that is a complicating factor. For me, anyways. It seems, more a question of time, than of distance, or speed.

For instance, would not quasars, speed up, as you increase speed to C? Since time is "slowing down" for you. Thus providing a way of determining your relativistic speed, in relation to the rest of the universe.

So, my guess would be, that the object would, indeed, see the traveling light, but the photon counter would slow way down.

### #21 PeterR280

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Posted 06 February 2014 - 07:32 PM

If you were traveling at c, no matter how far away you were from something you would hit it in zero seconds. Also, your kinetic energy would be infinite so the collision would destroy the entire Universe, so this isn't something you should try at home.

### #22 llanitedave

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Posted 06 February 2014 - 09:49 PM

Darn. And that was going to be one of my next projects, too.

### #23 genethethird

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Posted 07 February 2014 - 11:24 AM

Peter's comment points up for me how trying to imagine the implications of doing something that is mathematically and physically impossible is, well, impossible.

### #24 Pess

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Posted 08 February 2014 - 09:39 AM

If you were traveling at c, no matter how far away you were from something you would hit it in zero seconds. Also, your kinetic energy would be infinite so the collision would destroy the entire Universe, so this isn't something you should try at home.

Let me take a stab at paraphrasing this.

If YOU were traveling exactly at 'c', then time for you would stop.

So you could 'perceive' nothing about the outside Universe.

Pesse (The faster I go, the more behind I get) Mist

### #25 maugi88

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Posted 08 February 2014 - 10:20 AM

If you were traveling at c, no matter how far away you were from something you would hit it in zero seconds. Also, your kinetic energy would be infinite so the collision would destroy the entire Universe, so this isn't something you should try at home.

So protons don't have any mass? Must not, or the universe could not exist. Gamma rays must have no mass either right?

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