Why can light "push" but not be "pushed?"

[owl]

This question has bugged me for a long time. Sunlight "pushes" against 'solar sails.' They call it "solar wind."

 

Laser light "pushes" against the lasers which fire it. They call it laser recoil.

 

Light trapped in a box of mirrors "pushes" against the inside of the box, giving it more static inertia as if it were added mass.

 

Yet the classic thought-experimental spaceship traveling at a significant fraction of lightspeed can not "push" a light shining ahead any faster than its speed limit, 'c.'

 

It seems that the ship must travel into the tail end of that light, absorbing the difference between its velocity and 'c.'

 

And it seems that its momentum acts like mass traveling past massive objects.

 

I don't know. Just asking.

[D H]

Light can be "pushed". Just not in speed.

 

Suppose you shine a laser at a mirror moving toward you. The reflected light is blue shifted. It has more energy and more momentum than does the light moving toward the mirror.

[swansont]

Also the very fact that the light is reflected represents a push, since the momentum changes direction.

 

Light that is out ahead of a spaceship is not interacting with the spaceship. There's no way to exert a force on it.

[owl]

Light can be "pushed". Just not in speed.

 

Suppose you shine a laser at a mirror moving toward you. The reflected light is blue shifted. It has more energy and more momentum than does the light moving toward the mirror.

 

I see. The velocity of the approaching mirror gives reflected light more energy and momentum without more speed. But I'm not sure how that constitutes a "push." Maybe its just that "push" is not a specific enough scientific term.

 

It's obvious with a bullet fired ahead from a moving gun, "pushing" it to its regular velocity plus the gun's velocity. But, of course that doesn't work for a light shone ahead of a high speed ship, as above. In that case it must be, as I said, "... that the ship must travel into the tail end of that light, absorbing the difference between its velocity and 'c.' Yes?

 

Also the very fact that the light is reflected represents a push, since the momentum changes direction.

 

In the 'box of mirrors' experiment, the light is clearly "pushing" against the mirrors inside the box. It is hard to see how the mirror above reverses that, i.e., how does its velocity reverse the physics of what is pushing against what? I'm still not clear on the concept. Surely there is mutual "pushing" in that case, light against mirror and mirror against light. No?

 

Light that is out ahead of a spaceship is not interacting with the spaceship. There's no way to exert a force on it.

 

How is the above situation different than light "pushing" against a laser gun as in laser recoil when fired? Certainly the light is "interacting" with (pushing against) the gun... why not with the ship? (I'm talking a steady beam here, not just a single pulse, to be clear.)

 

Edit; Ps: In all possible cases of light pushing or being pushed, the question for me still remains, "How is it that mass-less light act like mass? Or, "how does mass-less momentum "push" at all with no 'substance' (so to speak) to impact whatever it pushes on?"

Edited June 13, 2012 by owl

[D H]

I see. The velocity of the approaching mirror gives reflected light more energy and momentum without more speed. But I'm not sure how that constitutes a "push." Maybe its just that "push" is not a specific enough scientific term.

You're the one with the fuzzy terminology. How does adding energy and momentum not constitute a "push"?

 

 

It's obvious with a bullet fired ahead from a moving gun, "pushing" it to its regular velocity plus the gun's velocity. But, of course that doesn't work for a light shone ahead of a high speed ship, as above. In that case it must be, as I said, "... that the ship must travel into the tail end of that light, absorbing the difference between its velocity and 'c.' Yes?

No.

 

You have to surrender your Newtonian point of view. A Newtonian POV is valid in our everyday world experiences, but that's only because a bullet fired from a gun is moving at many, many orders of magnitude less than the speed of light. The difference between what relativity and Newtonian mechanics say will happen is immeasurably small in this domain. Newtonian mechanics is only approximately as speed increases. For very high speeds, they aren't valid at all.

 

This is the heart of your problem in this thread (and elsewhere). Stop insisting the universe is Newtonian. It isn't.

[owl]

You're the one with the fuzzy terminology. How does adding energy and momentum not constitute a "push"?[/Quote]

 

Yes, "push" was my term, and I acknowledge that it may be too "fuzzy" in this context.

 

Me:

'In that case it must be, as I said, "... that the ship must travel into the tail end of that light, absorbing the difference between its velocity and 'c.' Yes?"

You:

"No."

Please explain how the above is a false assumption.

You have to surrender your Newtonian point of view. A Newtonian POV is valid in our everyday world experiences, but that's only because a bullet fired from a gun is moving at many, many orders of magnitude less than the speed of light. The difference between what relativity and Newtonian mechanics say will happen is immeasurably small in this domain. Newtonian mechanics is only approximately as speed increases. For very high speeds, they aren't valid at all.

 

This is the heart of your problem in this thread (and elsewhere). Stop insisting the universe is Newtonian. It isn't.

Could we please leave Newton (and the history of science) out if for the moment as we discuss the topic here as per the inquiry.

My "bullet" example was intended to contrast cumulative velocity with the established fact that constant 'c' requires that there is no cumulative velocity for light shone ahead from a high speed ship. What is wrong with my guess as to why not?

Please don't make it personal.

[D H]

My "bullet" example was intended to contrast cumulative velocity with the established fact that constant 'c' requires that there is no cumulative velocity for light shone ahead from a high speed ship. What is wrong with my guess as to why not?

Your bullet example is implicitly assuming a Newtonian universe. The universe isn't Newtonian. The local speed of light in all frames of reference is 'c'.

 

But, of course that doesn't work for a light shone ahead of a high speed ship, as above. In that case it must be, as I said, "... that the ship must travel into the tail end of that light, absorbing the difference between its velocity and 'c.' Yes?

The ship does not "travel into the tail end of that light, absorbing the difference between its velocity and 'c.'" Once a photon is emitted it is gone. There is no further interaction with it. And to be blunt, what you are saying doesn't quite make sense. Travel into the tail end of that light? What does that mean.

 

It's hidden here, but you are assuming there is some absolute frame of reference in which an object's true velocity can be specified. There isn't.

[swansont]

In the 'box of mirrors' experiment, the light is clearly "pushing" against the mirrors inside the box. It is hard to see how the mirror above reverses that, i.e., how does its velocity reverse the physics of what is pushing against what? I'm still not clear on the concept. Surely there is mutual "pushing" in that case, light against mirror and mirror against light. No?

 

Newton's third law: if A pushes on B, B must also push on A. The forces will be equal in magnitude and opposite in direction. If a photon pushes on a mirror, the mirror will push on the photon. There is no reversal of physics going on. You had said there was no pushing on the light, but here you recognize that there is. However, the pushing is limited to when there is an interaction, i.e. during the time in which the photon is reflected.

[owl]

Newton's third law: if A pushes on B, B must also push on A. The forces will be equal in magnitude and opposite in direction. If a photon pushes on a mirror, the mirror will push on the photon. There is no reversal of physics going on. You had said there was no pushing on the light, but here you recognize that there is. However, the pushing is limited to when there is an interaction, i.e. during the time in which the photon is reflected.

Most of my questions are still left unanswered. But I will go with what you did say above.

 

I don't think that the forces will be equal in magnitude.

 

My question was about pushing vs being pushed. The velocity of the approaching mirror will effect how much it pushes on the reflected light. But we know that the light has the momentum of lightspeed. Its pushing on the mirror will probably dwarf the force of the mirror, at whatever sub-light speed, “pushing” on the light beam.

 

I don’t know if light can be pushed, however trivially... not yet measured. That’s why I asked the question.

 

My reply to this post just disappeared. Who knows why?

I will write it again.

 

Your bullet example is implicitly assuming a Newtonian universe. The universe isn't Newtonian. The local speed of light in all frames of reference is 'c'.

 

My bullet example assumed no such thing. It was an example of physics demonstrating the contrast with constant 'c', as I said already.

 

The ship does not "travel into the tail end of that light, absorbing the difference between its velocity and 'c.'

 

Once a photon is emitted it is gone. There is no further interaction with it. And to be blunt, what you are saying doesn't quite make sense. Travel into the tail end of that light? What does that mean.

 

Is the first statement above just your opinion or do you have a reasoned argument to back it up?

 

As I said, I was positing a constant light emission, not a pulse of photons which are then "gone.'

 

"Travel into the tail end of that light? What does that mean."

 

It means that the light which the ship is shining ahead is traveling at 'c' no matter how fast the ship is traveling. So if it is going 1/2 'c', the reason that the beam is not boosted to 1&1/2 'c' is that the ship is traveling into the beam at 1/2 'c.'

 

It's hidden here, but you are assuming there is some absolute frame of reference in which an object's true velocity can be specified. There isn't.

 

There is nothing intentionally hidden here. I am not "assuming there is some absolute frame of reference in which an object's true velocity can be specified."

 

We all know that velocity is always relative to whatever it is measured in reference to. (See all of the above.) This seems to derail the topic.

 

My questions remain unanswered.

[swansont]

Most of my questions are still left unanswered. But I will go with what you did say above.

 

I don't think that the forces will be equal in magnitude.

 

They will be. Momentum conservation has a very sound basis in physics and stems from physics being the same from place to place (i.e. translation symmetry). They are equivalent to each other.

 

My question was about pushing vs being pushed. The velocity of the approaching mirror will effect how much it pushes on the reflected light. But we know that the light has the momentum of lightspeed. Its pushing on the mirror will probably dwarf the force of the mirror, at whatever sub-light speed, “pushing” on the light beam.

 

I don’t know if light can be pushed, however trivially... not yet measured. That’s why I asked the question.

 

Any change in momentum is the result of a push, and every push results in a change in momentum. That's basic physics. F=dP/dt

[D H]

My bullet example assumed no such thing. It was an example of physics demonstrating the contrast with constant 'c', as I said already.

Your bullet example assumes velocities add as 1+2=3. That is how things work in Newtonian mechanics, but not in relativity. Your Newtonian mindset is pervasive. You have to get rid of that mindset or you will not understand things.

 

 

"Travel into the tail end of that light? What does that mean."

 

It means that the light which the ship is shining ahead is traveling at 'c' no matter how fast the ship is traveling. So if it is going 1/2 'c', the reason that the beam is not boosted to 1&1/2 'c' is that the ship is traveling into the beam at 1/2 'c.'

 

Imagine that the ship sends out a single photon. There is no interaction between the spaceship and the photon after the spaceship emits the photon. That photon is gone, moving away at the speed of light. Now imagine the ship sends out a stream of photons. There is still no interaction. There is no traveling into the beam of light. It doesn't make sense.

 

Finally, imagine that the ship sends a photon in the opposite direction. I'll leave that thought up to you.

[owl]

Your bullet example assumes velocities add as 1+2=3. That is how things work in Newtonian mechanics, but not in relativity. Your Newtonian mindset is pervasive. You have to get rid of that mindset or you will not understand things. [/Quote]

I already said twice that my bullet example was an illustration of the difference between a mass gaining velocity as shot from a moving gun ("cumulative velocity") and the fact that light does not gain velocity in a similar manner. So far you have missed that point.

I do not have a "Newtonian mindset." It was an example of the difference between the former and the latter. I asked you nicely not to make this personal. I ask for moderation here to keep this an impersonal scientific Q& A.

 

Imagine that the ship sends out a single photon. There is no interaction between the spaceship and the photon after the spaceship emits the photon. That photon is gone, moving away at the speed of light. Now imagine the ship sends out a stream of photons. There is still no interaction. There is no traveling into the beam of light. It doesn't make sense.

 

If the ship sent out a pulse of light, your first statement would remain true. Even a single photon pulse would be gone.

 

But I already "imagined" and suggested a steady beam being shone ahead, as in your second example.

You then say, "There is still no interaction. There is no traveling into the beam of light. It doesn't make sense."

 

Am I to take this on your authority or because of your lack of making sense of it or do you have an argument against my assumption?

 

As a steady beam projected ahead at 'c' from a ship traveling at 1/2 'c', we know that the beam remains constant at 'c'. One explanation of the fact that the velocities are not cumulative is that the ship is traveling into the beam it is projecting. Rather than just saying 'not so' how about explaining how it can not be so?

 

Finally, imagine that the ship sends a photon in the opposite direction. I'll leave that thought up to you.

 

That photon travels at 'c' too. 'C' remains constant regardless of the velocity of the light source... in any direction (and also independent of any other observational frame of reference.)

 

So please address my inquiry as stated ... or quit the thread if it's just another lecture on constant 'c.'

[iNow]

Your question has been answered more than adequately. The fact that you have chosen to dismiss those accurate answers does not mean your question remains open.

[md65536]

As a steady beam projected ahead at 'c' from a ship traveling at 1/2 'c', we know that the beam remains constant at 'c'. One explanation of the fact that the velocities are not cumulative is that the ship is traveling into the beam it is projecting. Rather than just saying 'not so' how about explaining how it can not be so?

 

How is a "steady beam" different from a stream of individual photons?

 

 

From the perspective of the ship, the ship is at rest, and not traveling into the beam. That's one explanation for how it can't be so. If the light were traveling through some "ether frame" then you could describe an absolute velocity of the ship relative to that frame, but that isn't so. When discussing the movement of light relative to the ship, the frame in which the ship is at rest is as valid as any other frame.

[ACG52]

As a steady beam projected ahead at 'c' from a ship traveling at 1/2 'c', we know that the beam remains constant at 'c'

 

The beam remains at a constant c with respect to the ship. No matter how fast the ship is moving with respect to something else, the beam of light is always moving away from the ship at c, when measured from the ship. So to talk about the ship flying into the beam is nonsensical.

[swansont]

If the ship sent out a pulse of light, your first statement would remain true. Even a single photon pulse would be gone.

 

OK. Let's call this case 1.

 

But I already "imagined" and suggested a steady beam being shone ahead, as in your second example.

You then say, "There is still no interaction. There is no traveling into the beam of light. It doesn't make sense."

 

Am I to take this on your authority or because of your lack of making sense of it or do you have an argument against my assumption?

 

As a steady beam projected ahead at 'c' from a ship traveling at 1/2 'c', we know that the beam remains constant at 'c'. One explanation of the fact that the velocities are not cumulative is that the ship is traveling into the beam it is projecting. Rather than just saying 'not so' how about explaining how it can not be so?

 

The beam is comprised of individual photons. For each photon, apply case 1.

 

Your assumption is unphysical. D H has already explained why.

[Spyman]

As a steady beam projected ahead at 'c' from a ship traveling at 1/2 'c', we know that the beam remains constant at 'c'. One explanation of the fact that the velocities are not cumulative is that the ship is traveling into the beam it is projecting. Rather than just saying 'not so' how about explaining how it can not be so?

A steady beam of light is not like a solid steel beam, it's more akin to a rapid stream of bullets from a machine gun.

 

If a spaceship with a machine gun mounted in the front would continuously fire a stream of bullets ahead of the ship, then the ship would not catch up with the bullets and as such not travel into the stream it is fireing either. Photons in the beam are also increasing their distance from the ship.

[rigney]

A steady beam of light is not like a solid steel beam, it's more akin to a rapid stream of bullets from a machine gun.

 

If a spaceship with a machine gun mounted in the front would continuously fire a stream of bullets ahead of the ship, then the ship would not catch up with the bullets and as such not travel into the stream it is fireing either. Photons in the beam are also increasing their distance from the ship.

 

Just a question adding to this speed thing and may not belong here at all. But if a lazer moving at c is fired forward, will there even be a beam emitted? I can understand not catching up to a beam fired from a lesser speed. But at the same speed as c, what happens? Edited June 14, 2012 by rigney

[D H]

A laser can't move at c. No massive object can.

[rigney]

A laser can't move at c. No massive object can.

 

I understand the inadequacy of my question, but even Einstein liked hypotheticals, such as: looking into a mirror moving at c. Would there be a mirror image, reflection? Edited June 14, 2012 by rigney

[D H]

Basically you are asking "what do the laws of physics say would happen if we violated the laws of physics?"

 

There's a huge difference between a nonsensical question and a hypothetical question. Yours falls in the first category.

[rigney]

Basically you are asking "what do the laws of physics say would happen if we violated the laws of physics?"

 

There's a huge difference between a nonsensical question and a hypothetical question. Yours falls in the first category.

 

What's nonsensical about something Einstein theorized long before you were out of knee pants.?

http://voices.yahoo.com/paradoxes-einsteins-relativity-556787.html

Edited June 14, 2012 by rigney

[D H]

You are apparently talking about Einstein's musings when he was 16. About a decade later he realized that these were nonsensical musings. This realization is what led him to publish his 1905 paper on relativity.

 

Asking what happens when light hits a mirror going at the speed of light is nonsensical. A mirror cannot go at the speed of light. The structure of the universe makes such an event impossible.

[rigney]

You are apparently talking about Einstein's musings when he was 16. About a decade later he realized that these were nonsensical musings. This realization is what led him to publish his 1905 paper on relativity.

 

Asking what happens when light hits a mirror going at the speed of light is nonsensical. A mirror cannot go at the speed of light. The structure of the universe makes such an event impossible.

 

You're saying that Einstein was sixteen when he was "musing", or theorizing the train ride incident?

 

In the theory of Special Relativity, Einstein pictured himself riding on a train, moving at incredibly high speeds. He envisioned himself staring at a mirror when the train reached the speed of light threshold.

Edited June 14, 2012 by rigney

[D H]

rigney: I suggest you look for some more reputable source, either an expert in the field or a respected science writer. You have found the ramblings of some random nut job who doesn't know what he's talking about. The internet is full of net jobs, crackpots, and charlatan, and you are very good at finding them.