Science Forums

# Relativistic Law Of Reflection

## Recommended Posts

Yes, the angle does change according to the observer in the reference frame that is in relative motion to the mirror. Earlier, you said the angle remains constant, I’m glad to see you are now in agreement on that point. But there is no need for the mirror to have a frame* in order for this to happen (where frame* is not referring to a frame of reference but something like a picture frame)

I am not. Length contraction is an effect for forms of matter in motion.

In the graphic, only the segment d is affected by lc, not the space on either end.

This is true for the entire diagonal length of the mirror. The mirror gets thinner!

If the mirror was a prism, with a vertical back face, then the prism would change front to back with the greatest effect at the bottom, resulting in a change of the forward moving face.

• Replies 74
• Created

#### Popular Posts

No such thing as length contraction.

Which is what he said in his first post. This could be an interesting example to solve using time only as there's no such thing as length contraction.    PS. Considering time only, this is the same as

Your question is good because it highlights the thing most misinterpreted about relativity. If the light hits the eyepiece in any frame, it must hit the eyepiece from any perspective. Reality is not s

#### Posted Images

Here is a video at 11:30 for sluggo and popeye explaining length contraction does not involve a physical contraction of matter or space but is the result of relativity of simultaneity.

A 15 ft pole will fit into a 10 ft barn not because of physical length contraction but because the stationary perspective clocks the pole ends to be within the barn at the same stationary time when it's clear that from the pole's perspective the time  when the front of the pole is within the barn is not when the rear of the pole is in the barn. The pole's simultaneity is that when the back of the pole is in the barn, the front has already moved out of it. The pole has not shrunk from either perspective but it's length has been timed differently from 2 different perspectives.

Now if relativity explains length contraction is due to differences of the relativity of simultaneity of clocks, it cannot also hold that length contraction is a physical phenomenon because both explanations would be concurrent to cause double length contraction. He goes on to explain in a subsequent video a permanent physical double Y2 length contraction would occur only in non-constant relative motion if the pole was stopped within the barn. This is equivalent to the twin paradox and how permanent age difference occurs except it's now permanent length contraction.

In the past I've said length contraction is not real because there is no permanent length difference equivalent to the permanent age difference that comes out of the twin paradox. Now it looks like not only will the twins age differently but the one coming back will indeed be much thinner or crushed thinner. I'm saying this can't be true because length contraction doesn't even exist as anything but relativity of simultaneity. I'm going to have to show Greene's video on the physics stack exchange and see if anyone believes this claim.

##### Share on other sites

One of my difficulties with relatavistic length contraction (as a physical process) is understanding...kind of what you allude to. I mean, that would require physical change at the atomic (or lower) level correct? Are protons, neutrons, electron orbital shells getting packed in more denser? Then expanding back out afterwards (non relativistic velocites)? What other atomic (or elementary) particle characteristics are thought to occur? Temperature increase/decrease? If length contraction does occur...is it a, "Kiss of death" for living organic matter?

Edit (unfinished thought): On the, "Kiss of death" comment. If true, it's almost as if the universe is teasing us while striving for relativistic speeds/travel. For if death is a certainty, my hopes for mankind pollinating the Milky Way galaxy become much more challenging:(

Just thinking out loud. Appreciate thoughts/information for those points if available.

TX

Edited by Kardashev6
##### Share on other sites

One of my difficulties with relatavistic length contraction (as a physical process) is understanding...kind of what you allude to. I mean, that would require physical change at the atomic (or lower) level correct? Are protons, neutrons, electron orbital shells getting packed in more denser? Then expanding back out afterwards (non relativistic velocites)? What other atomic (or elementary) particle characteristics are thought to occur? Temperature increase/decrease? If length contraction does occur...is it a, "Kiss of death" for living organic matter?

Just thinking out loud. Appreciate thoughts/information for those points if available.

TX

The space itself shrinks thus it doesn't change anything besides everything is smaller by the same amount. The object doesn't truly change the size of space does in the presence of the velocity that the energy represents.

Edited by VictorMedvil
##### Share on other sites

The space itself shrinks thus it doesn't change anything besides everything is smaller by the same amount. The object doesn't truly change the size of space does

That helps point me in a direction to pursue more depth of detail (education). TX. It would be improper for me to reject something without having enough thorough understanding of the issue.

My education travels continue:)

Edited by Kardashev6
##### Share on other sites

Well, Victor, you just didn't read my last or any posts. You just spew out your flash cards like they're ever on topic.

##### Share on other sites

Well, Victor, you just didn't read my last or any posts. You just spew out your flash cards like they're ever on topic.

I read all your posts but I don't necessarily have a opinion about the subject. I believe the universe is as it is and there is no change of theory that can change that, basically the universe follows its rules and we just need to learn and understand them, that's all we can really do. To me the universe is just a composite of rules governed by correct equations that predict the rules, no cleaver words or equations can change what experimentally has been shown to happen.

Edited by VictorMedvil
##### Share on other sites

Here is a video at 11:30 for sluggo and popeye explaining length contraction does not involve a physical contraction of matter or space but is the result of relativity of simultaneity.

A 15 ft pole will fit into a 10 ft barn not because of physical length contraction but because the stationary perspective clocks the pole ends to be within the barn at the same stationary time when it's clear that from the pole's perspective the time  when the front of the pole is within the barn is not when the rear of the pole is in the barn. The pole's simultaneity is that when the back of the pole is in the barn, the front has already moved out of it. The pole has not shrunk from either perspective but it's length has been timed differently from 2 different perspectives.

Now if relativity explains length contraction is due to differences of the relativity of simultaneity of clocks, it cannot also hold that length contraction is a physical phenomenon because both explanations would be concurrent to cause double length contraction. He goes on to explain in a subsequent video a permanent physical double Y2 length contraction would occur only in non-constant relative motion if the pole was stopped within the barn. This is equivalent to the twin paradox and how permanent age difference occurs except it's now permanent length contraction.

In the past I've said length contraction is not real because there is no permanent length difference equivalent to the permanent age difference that comes out of the twin paradox. Now it looks like not only will the twins age differently but the one coming back will indeed be much thinner or crushed thinner. I'm saying this can't be true because length contraction doesn't even exist as anything but relativity of simultaneity. I'm going to have to show Greene's video on the physics stack exchange and see if anyone believes this claim.

So length contraction cannot be measured in the object's rest frame, but only in a frame in which the observed object is in motion. ... It was the negative result of a famous experiment, that required the introduction of length contraction: the Michelson–Morley experiment (and later also the Kennedy–Thorndike experiment).

Edited by VictorMedvil
##### Share on other sites

More random stream of consciousness connections. Do you look up length contraction and just pull out random paragraphs? There's a discussion going on here. Do you wish to join in or is your next flash card the one with the light cone again?

##### Share on other sites

A and B have a mirror M on the end of a rod of length d.
In the A frame, the length of the rod is ct=d.
The red hyperbola, aka calibration curve, denotes the same A-time where it intersects the time line of a moving observer. Gamma=g.
Left:
Bt=2gt at the return signal, event D. B assuming a pseudo rest frame (because there is no absolute rest frame) perceives his signal to M as equal out and back, giving d=gct, greater than d.
Right:
With length contraction, B measures the B rod as ct=d.
A measures the length of the B rod as d/g.
B measures the length of the A rod (on the Bx axis) as d/g.
The results are reciprocal only if the B frame is affected by lc.

##### Share on other sites

I am not. Length contraction is an effect for forms of matter in motion.

In the graphic, only the segment d is affected by lc, not the space on either end.

This is true for the entire diagonal length of the mirror. The mirror gets thinner!

If the mirror was a prism, with a vertical back face, then the prism would change front to back with the greatest effect at the bottom, resulting in a change of the forward moving face.

hypo-reflection.gif

Your diagram makes sense. If lc occurs in either x or y, surface defection angle remains constant. In addition, if lc occurs in both x and y (equal amounts), again surface defection angle remains unchanged. Angle appears to possibly change only when x and y are altered by differing values.

Do I have this correct?

##### Share on other sites

Your diagram makes sense. If lc occurs in either x or y, surface defection angle remains constant. In addition, if lc occurs in both x and y (equal amounts), again surface defection angle remains unchanged. Angle appears to possibly change only when x and y are altered by differing values.

Do I have this correct?

Yes. Length contraction only occurs in the direction of motion, typically labeled as x.

##### Share on other sites

A metal rod can be heated, and measured before and after, to show the length of an object is variable. There are no rigid bodies. The metal craftsmen knew this thousands of years ago.

The composition of atoms and molecules is regulated by chemistry, the physics of the electron cloud. The em interactions are mediated by photons. The light clock can be modified with the mirror and detector replaced with two particles, with the oscillation in the x direction. If the particle pair is moving, the interaction requires more time. The additional time is equivalent to a greater distance between particles, and thus a weaker exchange force. If the composite object (a system of particles) is accelerating, the weaker force allows compression of particles for the duration of the acceleration.

The revelation after 1900 was that motion altered measurement and perception. The excessive abstraction of the 4D theory has transformed physics into fantasy physics. In one scenario, the anaut was launched toward an object 4 ly distant. Since he moved at .6c, and arrived after 5.3 yr, the anaut concludes the distance contracted to 3.2 ly. Astronomers did not notice any changes that required modification of their star charts, so, where's the physics? How can the anaut's motion change the dimensions of the universe? Reason says it can't. Since his conclusion is based on his experience, his experience/perception was altered. Specifically his time, biological, mechanical, etc. The anaut would have studied "Relativity Theory" as part of his training, and is aware of the options. His altered perception doesn't require any new physics.

What about reciprocity requirements of SR? If A and B pass each other in identical ships with a relative velocity, each shouldobserve/measure the other ship to be lc.

In the first case, anyone observing the moving anaut would detect lc of his ship. The anaut

however observes lc of all objects and the space between them. The relation is not symmetrical. Add to, this 'moving in time', and we have 'warp speed' physics.

How prophetic that phrase, 'may the farce be with you'.

##### Share on other sites

No errors in the Brian Greene video. He does introduce length contraction first, before considering simultaneity.

##### Share on other sites

Thank you for that Sluggo. I had thought of you a parrot but you're anything but. You definitely have your own interpretations of physics. Length contraction is a time phenomenon arising out of the measurement of length in relative motion using the relative simultaneity of clocks. Your take is that it's an actual physical contraction of matter( not the line of space in motion) . I'm having an interesting discussion about this with myself (since they all seem to be morons) on the physics stack exchange. My question has a lot of interest but no valid answers ( except for my own). Here's a transcript:

In the twin paradox does the returning twin also come back permanently length contracted flatter than the twin on Earth?

This video from Brian Greene suggests this is so: https://www.youtube.com/watch?v=2sZUNud6rRw&list=PLj6DWzIvBi4PFDXCCV1bNhVUgDLTwVbFc&index=60

It shows if you stop a pole in the barn (ignoring all the obvious engineering challenges of doing so) it will end up permanently length contracted just like the returning twin will end up permanently younger than her earth bound twin in the twin paradox. Ignoring the practical problems with infinite deceleration, she stops when she turns around and that causes her permanent age difference but does she also end up permanently flatter? Again just consider the relativistic math and not all the physical impossibilities this example entails.

Relativity allows a frame jump without deceleration, it's called a clock handoff in the twin paradox. Since a clock is used to measure length for length contraction, a clock handoff could also keep a record of both permanent age difference and permanent length contraction when the twin hands off her clock readings to a ship passing her to return to earth. There's no physical crunching of the pole in a clock handoff.

So does relativity sanction permanent length contraction along with permanent age difference in the clock handoff twin paradox?

I slept on it and came up with a partial answer. As I said I spoke with Don Lincoln years ago on a forum and he impressed into my brain that space and time are two sides of the same coin; whatever problem you can solve using the time phenomena of relativity you can also solve using its symmetrical counterpart space. So I decided Greene's example was the space equivalent of the muon example.

A spacetime path must begin and end with the participants co-located. The muon's path does not begin co-located with earth, so the clocks are not sync'd, and there is no frame jump (only constant velocity) in that path. Hence, even though the muon ends up co-located with earth, there is no permanent twin paradox age difference. In fact any clocks that co-locate must have the same clock reading unless a frame jump has occurred.

So in the classic twin paradox example of Alice going and returning at .6c 3 ly out, Bob on earth will age 10 yrs and Alice only 8 when they re-unite. That age difference will be seen in a spacetime diagram. In the muon example, since the clocks did not start co-located and no frame jump occurred, the two clocks will not indicate that the muon aged less a la twin paradox but that its time dilated from an unknown start time. The spacetime diagram should have no difference between the clocks at co-location. Please don't confuse permanent age difference of 1 participant resulting from a frame jump with the reciprocal time dilation of both participants due to constant relative velocity.

Greene's example is very similar to the muon. The pole starts out unsync'd to the barn clocks so there can be no discernible permanent length difference when it's stopped in the barn. Just like the muon, there is a crash at the end but that doesn't constitute a frame jump when the clocks are co-located. A frame jump has to occur at an appreciable distance for there to be permanent effects on either the moving frame's time or space relative to the stationary frame which is chosen to be the barn in this case.

Greene's analysis may be wrong but what if he had properly done it for a true twin paradox example with a valid spacetime path. He'd have to start the pole and barn together, the pole then goes out and returns to the barn for a valid spacetime path. The pole would have to be permanently length contracted at the turn around point, not at the barn. One thing he did get right was the length contraction is not caused by a physical crushing of the pole but by the incremental relative simultaneity of the clocks as the pole is being stopped by subsequent hands.

Like I said, the clock handoff scenario of the twin paradox involves no crushing of the pole only the effects of relative simultaneity on measuring the pole's length. In the clock handoff, Alice going out meets Charlie returning the pole's measurements (not a physical pole) for her. Neither Alice or Charlie are affected by a frame jump but the data passing between them is.

Charlie coming in from deep space towards earth has no sync'd clock to earth's clock. He is the muon example incarnate. He sync's his clock to earth's when he gets Alice's clock data. Since neither Alice nor Charlie experience a frame jump, they do not experience a permanent age difference to earth, they just experience reciprocal time dilation relative to earth's clock. What does experience the frame jump is the data. So the data doesn't really represent that either Charlie or Alice have aged less during their journeys. All Charlie has really done is drag a distant time reading into the co-located present with earth when he reaches it. He has aged normally and earth can't really tell he has aged slower, only that his inherited clock data from Alice has aged slower.

So the question is has time and space itself been warped to cause the clock to take real measurements of time and length or has the info of time and space been warped by the delay of distance and the effect velocity has on rate of returning information. For example if a clock face were receding from you, the info from the clock face would have a rate of delay which you could easily misinterpret as time itself being slowed. If only the info is getting distorted, is there any real permanent age difference and permanent length contraction that comes out of a frame jump? Those who answer only permanent age difference is real are forgetting that there's a symmetry between time and space and their position breaks that symmetry. Relativistic effects can only be calculated for either space or time. The muon doesn't cross the atmosphere length contracted AND in a dilated time, it's one or the other. So if permanent age difference does exist according to the twin paradox, then the twin can't also return concurrently flatter as well, it's one or the other. That's my answer interpreting relativity but I'm now personally leaning to the belief that neither occur just as neither really occurs in the clock handoff example.

PS. No I'm wrong in my last paragraph. The muon example proves that it's not just the information of its clock that velocity affects, it's time itself that the muon's clock measures. Otherwise the muon would not be able to really make it to earth.

Edited by ralfcis
##### Share on other sites

Ralf,

Could you elaborate on what, "clock handoff" means or how you are referring to it please? I am struggling with applying in in my mind.

TX

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.