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Constant speed of light Topic Page: 1 2

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.

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)

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

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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.

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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.

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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.)

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Quote:

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.

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

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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.

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There would be nowhere for the light to travel to.

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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.

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

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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.

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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?

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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.

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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?

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

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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.

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"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.

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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.

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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.

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

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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.

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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

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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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Protons have rest mass. Gamma ray photons dont have rest mass. Protons can't travel at c. The LHC accelerates protons close to c but can't reach c.

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Protons have rest mass. Gamma ray photons dont have rest mass. Protons can't travel at c. The LHC accelerates protons close to c but can't reach c.

Sorry I meant photons. Light. Oops didn't notice that misprint. Thanks though, that's what I was asking.

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That leads me to another question and forgive me if it is off topic. If light photons have no mass, how would reflective energy push a solar sail? Doesn't force require mass of some kind? I mean a blast wave can push something but only because of the mass of air or gas that it is propagating thru. Yes? Sound cannot propagate in a vacuum but of course light and radio waves can. Does a blast wave? Or is that sound? Forgive my ignorance.

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Photons have energy so when they hit something they create forces on the particles absorbing the electromagnetic radiation.

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OK thanks

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It's actually an interesting topic because traveling at c for anything with non zero rest mass is impossible since it would require infinite energy to get to c. However, black holes have characteristics that are equally paradoxical at the event horizon and black holes do exist. The equations of General Relativity blow up at the event horizon. You get zeros and infinities as answers. The black hole paradox is a real puzzle to physicists. I know Hawking has come out recently with some sort of solution. I don't know all the details.

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

(Corrected quote)
As others stated photons have no rest mass. Photons do have energy given by e = hv

where h = Planck's constant & v = frequency.

To convert energy into equivalent mass use: m = E/c^2
Since photons are NEVER at rest and always travel at c, this could be considered
"virtual" mass.

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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.

Yeah, might as well begin the question "assume the tooth fairy exists..."

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It's actually an interesting topic because traveling at c for anything with non zero rest mass is impossible since it would require infinite energy to get to c. However, black holes have characteristics that are equally paradoxical at the event horizon and black holes do exist. The equations of General Relativity blow up at the event horizon. You get zeros and infinities as answers. The black hole paradox is a real puzzle to physicists. I know Hawking has come out recently with some sort of solution. I don't know all the details.

It seems in laymen's terms he is saying Black holes don't exist because they are grey. Instead of the event horizon being a crisp clearly definable place it is fuzzy with information and matter smashed and mixed all over the place and a lot of it escapes.(his words not mine) I think it has to do with the gamma ray bursts at the poles or something. Some of the matter and light that goes in does not in-fact get devoured but spewed out. Making the BH not black but grey. Which I thought was part of the perodox.(nothing can escape but huge amounts of energy does) Wait, the burst originate outside the EH right. Its the stuff that cant quite get gobbled fast enough? Like a drain overflow? Anyway, Hawking is obviously going thru a midlife crisis and needs to rethink his place in the history books. I have always agreed with his, information is forever lost stance but now its forever fuzzed.??????? Black holes exist for sure, the nature of them is not for me to say. Over my head, witch isn't all too lofty for sure. These things are fun to think about, great thread OP.

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Quote:

Quote:

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

(Corrected quote)
As others stated photons have no rest mass. Photons do have energy given by e = hv

where h = Planck's constant & v = frequency.

To convert energy into equivalent mass use: m = E/c^2
Since photons are NEVER at rest and always travel at c, this could be considered
"virtual" mass.

Cool. If I have one regret, (I have more than one) it would be that I did not take high school as seriously as I should have. This stuff is fascinating and I wish I would have went to college and majored in physics. Could have made a go at this for a career and been very happy. Not to say I am not happy now, I love my family and would not give them up for anything. Physics is very enjoyable, you people in the know make it so.

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Anyway, Hawking is obviously going thru a midlife crisis and needs to rethink his place in the history books.

Mid-life crisis????

The dude is 71 years old and by all rights should have been dead decades ago. I suspect he's already gone through his existential crises and is working strictly on the theory at hand at the moment.

His place in the history books is secure, whatever happens in the future.

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Quote:

Anyway, Hawking is obviously going thru a midlife crisis and needs to rethink his place in the history books.

Mid-life crisis????

The dude is 71 years old and by all rights should have been dead decades ago. I suspect he's already gone through his existential crises and is working strictly on the theory at hand at the moment.

His place in the history books is secure, whatever happens in the future.

Of course your right. It was a term to try and explain why he has changed his mind. You will notice that I did reference the history books. Would not have done that if I did not belief he deserved to be in them.

Was it not Hawking that first starting taking black holes seriously? The subject was taboo at the time, no? He was one of the founders of modern black hole science was he not? Why the 180? Or is it a 90 deg turn? It is very hard to show emotions in text. I am not trying to degrade his legacy. He has all my admiration. Jeez I cannot say anything without offending someone.

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I'm not offended. I just very seriously doubt that Hawking's change of mind is due to anything but the ramifications of the theory combined with the data available -- data in this case also including the ramifications of other related theories.

Maybe he's lost his skill for the math involved. Probably not. Maybe he's considering other factors he hadn't considered previously. Also remember that Hawking is not working in a vacuum. Even in his condition, he's not an isolated brain in a vat. There are other, very talented physicists working on the same problems, and he's almost certainly keeping up with their results and being influenced by them. That's another aspect of good science.

What everyone needs to keep in mind that this is not about Hawking. It's about nature. We're trying to figure it out. Sometimes it forces us to change our point of view.

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I'm not offended. I just very seriously doubt that Hawking's change of mind is due to anything but the ramifications of the theory combined with the data available -- data in this case also including the ramifications of other related theories.

Maybe he's lost his skill for the math involved. Probably not. Maybe he's considering other factors he hadn't considered previously. Also remember that Hawking is not working in a vacuum. Even in his condition, he's not an isolated brain in a vat. There are other, very talented physicists working on the same problems, and he's almost certainly keeping up with their results and being influenced by them. That's another aspect of good science.

What everyone needs to keep in mind that this is not about Hawking. It's about nature. We're trying to figure it out. Sometimes it forces us to change our point of view.

Very good points Dave, I hadn't thought about outside influences. He no doubt has brilliant colleagues that don't have the influence he does. Not saying for one minute that Hawking doesn't have an original thought. He probably did and asked others to refute it before publishing. I feel like I may be digging another hole here. It's certainly possible I don't understand the jist of his new theory. I am sure someone can explain it better.

Sincerely grateful for the input.

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Hawking has top notch PhD level physicists and mathematicians that work with him.

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