Speed of light and dark matter

mfarineau
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First off, I'm not a physicist (as I'm sure is evident by my multi-part question), but I'm hoping someone will be kind enough to explain this in basic terms.

Part the first: I'm wondering if someone can explain to me in layman's terms why the speed of light is essentially the universal speed limit?

Part the second: Why does dark matter have to respect the speed of light speed-limit (and how do we know that it does?)

Thanks!
 
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The simple answer is that it would take an infinite amount of energy to speed up anything to the speed of light (relativity theory). This applies to all matter (dark and ordinary).
 
the speed of light is a universal speed limit because no one could measure the speed of anything more than that of light, not even of the light.
 
ravikannaujiya said:
the speed of light is a universal speed limit because no one could measure the speed of anything more than that of light, not even of the light.
That isn't true. Simply set up some synchronized clocks a known distance apart and check the time on each as the object goes by. That will give you the speed regardless of if it is slower or faster than light.
 
mfarineau said:
First off, I'm not a physicist (as I'm sure is evident by my multi-part question), but I'm hoping someone will be kind enough to explain this in basic terms.

Part the first: I'm wondering if someone can explain to me in layman's terms why the speed of light is essentially the universal speed limit?

The way you're thinking about this is a little inside out. The universal speed limit is a property of the geometry of spacetime. The fact that light travels at the speed limit really is telling us something about light, not something about the speed limit. What it's telling us is that light is massless. We can see this by considering how energy works in special relativity. And object of mass m moving at speed v will have energy given by
E = \frac{mc^2}{\sqrt{1-\left(\frac{v}{c}\right)^2}}.

If we examine this equation, we can see that, when v=c, the energy becomes infinite unless the object's mass is 0. And, we can also see that the energy of an object with m=0 will be 0 unless v=c. In the case where v=c and m=0, this equation is actually indeterminate, which turns out to happen because massless objects can actually have any amount of energy without it changing their speed.

So, it isn't that something about light affects how everything else moves. It's that a property of the way the universe works both determines how fast light moves and, for the same reason, restricts how everything else moves, as well.

Part the second: Why does dark matter have to respect the speed of light speed-limit (and how do we know that it does?)

Thanks!

Dark matter has to obey the speed limit for the same reason that everything else does - the speed limit is an underlying property of spacetime itself.
 
@ DaleSpam
As far as I know the concept of synchronization of clocks is lost at the relativistic speed and we can not measure the absolute time and distance so measuring the speed is not simply division of distance and time but we need to take account of frame of reference, time delation and the whole special relativity. And it says that nothing can move faster than the light. for simplicity just try to do a thaught experiment:
let's say two observer O1 and O2 are close to each other and they synchronised their clocks. Now they go away from each other with their clock at a distance 300000 km. for which light takes almost 1 sec. to travel. Now if they are able to see the clocks of each other, observer O1 would find that the clock of O2 is 1 sec behind of his own clock and same would go for observer O2.
Now suppose, an object is moving with infinite speed, i.e. a particle reaches instantenously to observer O2 whenever it is realsed by the observer O1. It will take no time but as the clocks are showing the time difference of 1 sec., so, for both the observers, it will take 1 sec to travel the distance as they have their clocks synchronised locally. Thus both the observer will find the speed of the particle is same as the speed of light.
 
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ravikannaujiya said:
@ DaleSpam

It will take no time but as the clocks are showing the time difference of 1 sec., so, for both the observers, it will take 1 sec to travel the distance as they have their clocks synchronised locally. Thus both the observer will find the speed of the particle is same as the speed of light.

But it is not possible to measure infinite speed. But i agree with DaleSpam is that we can measure something below infinite speed.If something travels faster than light,it will take less than 1 second and both observer will agree on it as their clocks go at the same rate. You used the context of infinite speed,we cannot measure infinite speed as we can't know what is the answer for 1 divided by 0.
 
ravikannaujiya said:
@ DaleSpam
As far as I know the concept of synchronization of clocks is lost at the relativistic speed and we can not measure the absolute time and distance so measuring the speed is not simply division of distance and time but we need to take account of frame of reference, time delation and the whole special relativity

You have misunderstood Dalespam's answer. Synchronization is only lost between clocks that are moving relative to one another. It's OK to have two synchronized clocks one kilometer apart and not moving relative to each other; they will remain synchronized and one km apart for as long as I need them, and that's what Dalespam is describing.

If I'm standing next to one clock when it reads zero, and I shoot a bullet at the other clock... The bullet hits the other clock and stops it. I then slowly walk over to the other clock and look at it, and see that it read one second when it was stopped by the bullet. I know that the speed of the bullet was 1 km/sec in a frame of reference in which I and the clocks are at rest, and I've used my synchronized clocks to discover this.
 
@ash64449 and Nugatory
First I would like to address that the synchronisation is lost when no observer is moving. for this I take example of three spaceship at rest observing a star:
Untitled.jpg

Suppose we have three spaceship A, B, and C. The spaceship is not at the same distance from star as shown in the figure above. Every pilot has synchronised his clock with others. One night they observed an abnormal event on a star. Next day, they met with each other and discussed the events took place on the star. They agreed on everything except timing of the event (we can easily figure it out why it is so). And I see that the synchronisation is lost anyway.
Now take the example of bullet. At the speed of bullet we never take relativistic correction. But when we talk about the velocity of light we must take care of relativity.

Untitled1.jpg

In case of bullet the distance between the target and firing position is remain same, i.e. X=X0. But when bullet would be moving comparable with the speed of light then,
X=X0*√(1-(v2/c2). Now the distace between the two point is function of the speed. So if you say that we can measure the speed of a particle (we are mesuring because the velocity is unknown) that is heigher than or cmparable to the speed of light, then I doubt how we can find the speed when we do not know the relativistic distance between them as it is a function of velocity itself. The same we can have if we measure the time between the two events, i.e. it also needs relativistic correction.
 
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  • #10
ravikannaujiya said:
Next day, they met with each other and discussed the events took place on the star. They agreed on everything except timing of the event (we can easily figure it out why it is so). And I see that the synchronisation is lost anyway.

Not if they have the least understand of physics. The light from the star will reach them at different times. Of course all along they have slightly different measurement of the distance to the star, and so the differing times of observation for them are understood in terms of that different distance for A, B, and C so they come up with the same time for the initial event, before its light was sent to each of them. What you said is true only if they imagine the speed of light to be infinite.
 
  • #11
ravikannaujiya said:
@Next day, they met with each other and discussed the events took place on the star. They agreed on everything except timing of the event (we can easily figure it out why it is so).

No, they still agree about the timing of the events on the star. The light from the events on the star hits their eyes at different times, but because they know about light travel time, they also understand that the the events happened sometime before the light hit their eyes. So the guy who is four light-minutes away from the star sees something happen at four minutes past midnight, the guy who is two light-minutes away from the star sees something happen at two minutes past midnight, and they both agree that it really happened at the same time time, midnight - as long as they are at rest relative to one another so can maintain clock synchronization.

Time dilation, length contraction, relativity of simultaneity are what's left over after you've allowed for light transmission times.
 
  • #12
ravikannaujiya said:
In case of bullet the distance between the target and firing position is remain same, i.e. X=X0. But when bullet would be moving comparable with the speed of light then,
X=X0*√(1-(v2/c2). Now the distace between the two point is function of the speed. So if you say that we can measure the speed of a particle (we are mesuring because the velocity is unknown) that is heigher than or cmparable to the speed of light, then I doubt how we can find the speed when we do not know the relativistic distance between them as it is a function of velocity itself. The same we can have if we measure the time between the two events, i.e. it also needs relativistic correction.

Now you're making an easy problem difficult. Speed is defined to be distance traveled divided by time of travel (using a reference frame in which the observer is at rest - speed is a frame-dependent quantity even in ordinary classical physics). The relativistic effects of the bullet's speed are irrelevant unless we want to calculate how things look to the bullet.
 
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  • #13
1977ub said:
Not if they have the least understand of physics. The light from the star will reach them at different times. Of course all along they have slightly different measurement of the distance to the star, and so the differing times of observation for them are understood in terms of that different distance for A, B, and C so they come up with the same time for the initial event, before its light was sent to each of them. What you said is true only if they imagine the speed of light to be infinite.

Nugatory said:
No, they still agree about the timing of the events on the star. The light from the events on the star hits their eyes at different times, but because they know about light travel time, they also understand that the the events happened sometime before the light hit their eyes. So the guy who is four light-minutes away from the star sees something happen at four minutes past midnight, the guy who is two light-minutes away from the star sees something happen at two minutes past midnight, and they both agree that it really happened at the same time time, midnight - as long as they are at rest relative to one another so can maintain clock synchronization..

if the speed of light were infinite, the time of observing the event would be same as it reaches to A, B and C simultaneously irrespective to their distance from the star.
you guys again assuming the distance to be known. :)
 
  • #14
Nugatory said:
Now you're making an easy problem difficult. Speed is defined to be distance traveled divided by time of travel (using a reference frame in which the observer is at rest - speed is a frame-dependent quantity even on ordinary classical physics). The relativistic effects of the bullet's speed are irrelevant unless we want to calculate how things look to the bullet.

Relativity matters only when speed of the bullet is comparable to the speed of light whether we see how things look like to the bullet or to our position as the bullet is moving w.r.t to us and we are moving w.r.t. to the bullet with the same speed.
 
  • #15
ravikannaujiya said:
Relativity matters only when speed of the bullet is comparable to the speed of light whether we see how things look like to the bullet or to our position as the bullet is moving w.r.t to us and we are moving w.r.t. to the bullet with the same speed.

This is incorrect. Do you use relativistic addition all the time? After all, according to an observer in another galaxy, you are moving very, very, very fast!

If the bullet is not moving relative to you, no matter how fast it looks from another frame, that bullet isn't relativistic to you.

Zz.
 
  • #16
ravikannaujiya said:
if the speed of light were infinite, the time of observing the event would be same as it reaches to A, B and C simultaneously irrespective to their distance from the star.
yes, of course. And then the correction for light travel time would be even easier because the travel time would always be zero. But I'm not sure of the significance of this point because no one in this thread is suggesting that the speed of light might be infinite.

you guys again assuming the distance to be known. :)
Of course we are... Because in principle the distance can always be measured.
 
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  • #17
ravikannaujiya said:
if the speed of light were infinite, the time of observing the event would be same as it reaches to A, B and C simultaneously irrespective to their distance from the star.
you guys again assuming the distance to be known. :)

I see my error regarding infinite speed. The issue is that people do not make determination of when an event occurred without knowing how far away something is.
 
  • #18
If A, B, and C have synchronized their clocks, they presumably have measurements regarding how far away they are from one another. If they all agree to send out pulses of light at time t1, each will receive the others' pulses at different times. This won't confuse them about their clocks being in synch because they know the light has to travel to get to them.
 
  • #19
ZapperZ said:
This is incorrect. Do you use relativistic addition all the time? After all, according to an observer in another galaxy, you are moving very, very, very fast!

If the bullet is not moving relative to you, no matter how fast it looks from another frame, that bullet isn't relativistic to you.

Zz.

yes .. if the speed of the bullet is high enough (comparable to the speed of light), relativity is always there to play a role. But when speed is very low even 1000km/s, there is no need to take relativistic corrction. You can see when we deal with the nutrinos or high energy particles from outer space falling on the atmosphere of the earth, we take relativistic correction.
 
  • #20
ravikannaujiya said:
yes .. if the speed of the bullet is high enough (comparable to the speed of light), relativity is always there to play a role. But when speed is very low even 1000km/s, there is no need to take relativistic corrction. You can see when we deal with the nutrinos or high energy particles from outer space falling on the atmosphere of the earth, we take relativistic correction.

But what does this have anything to do with what I was responding to. You clearly stated that it doesn't matter as long as something is moving close to c in any frame. I'm saying it does! If the bullet is moving at 0.9 c in some frame, but I am in the same ref. frame as the bullet, I see the bullet is not moving, and I do not have to use any relativistic correction of any kind!

Zz.
 
  • #21
ZapperZ said:
But what does this have anything to do with what I was responding to. You clearly stated that it doesn't matter as long as something is moving close to c in any frame. I'm saying it does! If the bullet is moving at 0.9 c in some frame, but I am in the same ref. frame as the bullet, I see the bullet is not moving, and I do not have to use any relativistic correction of any kind!

Zz.

yes ... if the bullet is moving at 0.9 c and observer moving with the bullet with same speed, then both are at the rest with each other and obsly why one would take relativity when observer and observable are not moving with each other. but I am talking when there is relative speed.
 
  • #22
ravikannaujiya said:
yes ... if the bullet is moving at 0.9 c and observer moving with the bullet with same speed, then both are at the rest with each other and obsly why one would take relativity when observer and observable are not moving with each other. but I am talking when there is relative speed.

Then maybe what you wrote earlier is not what it turned out to be:

ravikannaujiya said:
Relativity matters only when speed of the bullet is comparable to the speed of light whether we see how things look like to the bullet or to our position as the bullet is moving w.r.t to us and we are moving w.r.t. to the bullet with the same speed.

Zz.
 
  • #23
Wow ravikannaujiya, that is an impressive amount of misinformation in one post.
ravikannaujiya said:
As far as I know the concept of synchronization of clocks is lost at the relativistic speed
Not true. The concept is not the lost, it is just that different frames disagree on whether or not two specific events are synchronized. Relativistically moving clocks can still have a well-defined notion of synchronization.

ravikannaujiya said:
and we can not measure the absolute time and distance so measuring the speed is not simply division of distance and time
The premises are correct (we cannot measure absolute time and distance) but the conclusion is incorrect and does not follow from the premises. Speed is simply the division of distance and time. The premises show that it speed is not absolute, not that the forumla is any different.

ravikannaujiya said:
but we need to take account of frame of reference, time delation and the whole special relativity. And it says that nothing can move faster than the light.
No matter or information can move faster than light, but that doesn't place a limitation on speeds that are measurable. For instance, you can measure the superluminal speed of a "laser dot" sweeping across some known distance by using synchronized clocks with a photosensitive trigger. No matter or information is superluminal, but the measured speed of the dot is.

ravikannaujiya said:
for simplicity just try to do a thaught experiment:
let's say two observer O1 and O2 are close to each other and they synchronised their clocks. Now they go away from each other with their clock at a distance 300000 km. for which light takes almost 1 sec. to travel. Now if they are able to see the clocks of each other, observer O1 would find that the clock of O2 is 1 sec behind of his own clock and same would go for observer O2.
True, but irrelevant. All of the SR effects are what remain after accounting for the finite speed of light. O1 and O2 would know that their clocks are still synchronized precisely because they are 1 light second away and the visual image they see is 1 second late.

ravikannaujiya said:
Now suppose, an object is moving with infinite speed, i.e. a particle reaches instantenously to observer O2 whenever it is realsed by the observer O1. It will take no time but as the clocks are showing the time difference of 1 sec., so, for both the observers, it will take 1 sec to travel the distance as they have their clocks synchronised locally. Thus both the observer will find the speed of the particle is same as the speed of light.
No, this is silly. The time that O2 sees the release by O1 has nothing to do with the speed of the released particle. The particle will reach O2 before the image of O1 releasing it, thus O2 will immediately know it was FTL, just like getting hit by a bullet before hearing the shot is a sure indication that the bullet was supersonic.
 
  • #24
ZapperZ said:
Then maybe what you wrote earlier is not what it turned out to be:



Zz.
hmm that bold letters were my fault...
 
  • #25
@ daleSpam, then pls clarify me
If we can not measure absolute time and distance in relativistic case, how two observer can have their clocks synchronised with each other moving in different frames of reference? If I consider they had their clocks synchronised before entering into their frames of refernce, why should they have their clocks remained synchronised? As we can see the twin paradox, where twins had their clocks synchronsed before leaving the Earth but when one of them returnes their clocks are no more synchronised so their age.
If they ar not moving but very far away from each other, is there any kind of observation that could show that their clocks are still synchronised? And I think for both of them it would be like looking into past whenever they observe the clocks of each other. If you say why they need such observation, Then I think the very nature of science is to prove your previous data with current experiments. If you say that they know their distance between each other so they can find the exact time deleting all posiblity of misconception. For this you must assume the speed of light is fixed and cannot exceed the value of c if it exceeds then again c becomes unknown and to find the exact time is out of hand. If you say anyother particle may exceed the speed of light, then the particle would not be observable until light reaches to our eyes and it shows that the light came before the particle because that was the light which made the particle observable, again destroying the idea that particle moved faster than light. this thing goes with Cherenkov effect where light reaches before the particle instead particle moved faster than light.
If there is no limitation to measure the speed and we can measure the speed of an object more than that of light, what would you say about second postulate of special theory of reativity?
If the formula for speed is simple distance upon time, why do we need to take relativistic correction when we tackle with the particles with very high speed? Why the Newtonian physics becomes irrelevant while it gives the formula for speed as distance upon time?
 
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  • #26
ravikannaujiya said:
If you say that they know their distance between each other so they can find the exact time deleting all posiblity of misconception. For this you must assume the speed of light is fixed and cannot exceed the value of c if it exceeds then again c becomes unknown and to find the exact time is out of hand

Yes, we must assume that the speed of light is the same for all observers. This is the second of Einstein's two postulates; all of relativity theory is based on this assumption. Of course Einstein didn't just pull this assumption out of thin air; both experiment (Michelson-Morley) and various theoretical considerations pointed that way, and subsequent experiments have vindicated it.
 
  • #27
Nugatory said:
Yes, we must assume that the speed of light is the same for all observers. This is the second of Einstein's two postulates; all of relativity theory is based on this assumption. Of course Einstein didn't just pull this assumption out of thin air; both experiment (Michelson-Morley) and various theoretical considerations pointed that way, and subsequent experiments have vindicated it.

yes...and that's why speed of light becomes cosmic speed limit.
 
  • #28
ravikannaujiya, please consider spending a little more effort on your posts. Instead of just asking everything that comes to your mind, narrow it down to one or two key points. Then present those points using paragraphs to organize your thoughts in a logical manner.

ravikannaujiya said:
@ daleSpam, then pls clarify me
If we can not measure absolute time and distance in relativistic case, how two observer can have their clocks synchronised with each other moving in different frames of reference? If I consider they had their clocks synchronised before entering into their frames of refernce, why should they have their clocks remained synchronised? As we can see the twin paradox, where twins had their clocks synchronsed before leaving the Earth but when one of them returnes their clocks are no more synchronised so their age.
None of this is relevant to the topic at hand. For measuring speed, the two synchronized clocks are at rest wrt each other a known distance apart.

ravikannaujiya said:
If they ar not moving but very far away from each other, is there any kind of observation that could show that their clocks are still synchronised?
Yes, the Einstein synchronization method, or any equivalent method.

ravikannaujiya said:
For this you must assume the speed of light is fixed
Yes, of course we assume this. It is the second postulate. A postulate is an assumption. The postulate is well-confirmed experimentally so we can be quite confident making that assumption: http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

ravikannaujiya said:
If you say anyother particle may exceed the speed of light, then the particle would not be observable until light reaches to our eyes and it shows that the light came before the particle because that was the light which made the particle observable, again destroying the idea that particle moved faster than light.
This is simply not correct. Suppose there is some tachyon that moves at 10 c which we can detect as it passes through some detector block which emits a flash of light as the tachyon passes through.

Now, suppose that we have a tachyon emitter aligned with two detector blocks. The detector blocks are separated by 1000 ft (in the x direction) and 1 ft away (in the y direction) from each detector block is a clock and a high-speed camera which can see both blocks and the time on the other clock. The clocks are synchronized using Einstein's convention (c=1ft/ns).

The following observations are made: at t=1ns the near camera records a tachyon flash in the near block, at t=101ns the far camera records a tachyon flash in the far block, at t=1001ns the far camera records a flash in the near block when the near clock read 1ns, and at t=1101ns the near camera records a flash in the far block when the far clock read 101ns.

From those observations the experimenters can tell that the tachyon hit the near block at t=0ns and the far block at t=100ns. Then they determine its speed as 1000ft/100ns=10c.

ravikannaujiya said:
If there is no limitation to measure the speed and we can measure the speed of an object more than that of light, what would you say about second postulate of special theory of reativity?
I would say it is well-confirmed experimentally. The second postulate is a postulate about the speed of light, not a postulate about a limitation of measuring devices.

ravikannaujiya said:
If the formula for speed is simple distance upon time, why do we need to take relativistic correction when we tackle with the particles with very high speed? Why the Newtonian physics becomes irrelevant while it gives the formula for speed as distance upon time?
I am not aware of a relativistic correction for speed. Can you please provide a mainstream scientific reference for the correction you are thinking about?
 
  • #29
you are saying two contradictory things at the same time. Special relativity and exisitance of tachyon. First if second postulate is correct, then there would be no tachyon. If there is tachyon then the second postulate is incorrect. If there is tachyon, then the case is no more relevant to anykind of light detector which is the fastest way of detecting by now, and there must be tachyon detector. If we put light detector anyway then the detectors would only recognise the light first, before coming the tachyon as Chereknov effect.
 
  • #30
ravikannaujiya said:
you are saying two contradictory things at the same time. Special relativity and exisitance of tachyon. First if second postulate is correct, then there would be no tachyon. If there is tachyon then the second postulate is incorrect.
This is not true. You can have any two of tachyons, relativity, and causality. So tachyons do not by themselves contradict relativity.

However, that is not the point. The point is that our measuring devices are not limited to only being able to measure subluminal speeds as you incorrectly claimed in post 3. I demonstrated a set up where, IF there were a tachyon traveling at 10 c, the superluminal speed could be correctly measured. The fact that nothing we know of goes at 10 c does not stem from a limitation of our measuring devices to measuring v<c.

ravikannaujiya, if you believe that it is impossible to measure a superluminal speed due to some fundamental limitation of all measuring devices then please post a mainstream scientific reference which describes that fundamental limitation of measuring devices. Note, this is not the same as a reference that nothing can travel at v>c, but a reference that no device can measure v>c.

If you cannot post such a reference then further comments to that effect are deliberate personal speculation.
 
  • #31
"Speed is only a number." All one would need to do in order to detect a speed greater than c is make two measurements. Note the locations and the times in your IRF. Then do simple division of the distance change divided by the time change
and that is the speed. There is *no* specific measurement apparatus that needs to be to used calculate speed.

[edited had initially left out *no*]

\frac{\Delta d}{\Delta t}
 
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  • #32
Read first three chapters of "On Time: An Investigation into Scientific Knowledge and Human Experience" by Michael Shallis and First chapte of "Concepts of Modern Physics" Arthur Beiser.
about tachyon and SR you can read first few lines on Wikipedia by typing Tachyon whether they contradict or not.
 
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  • #33
Trying to *cause* tachyon radiation and *use* it in a meaningful way would violate relativity and generate potential causality violations.
 
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  • #34
ravikannaujiya said:
Read first three chapters of "On Time: An Investigation into Scientific Knowledge and Human Experience" by Michael Shallis
That is a pop-sci book, not a valid scientific reference.

ravikannaujiya said:
and First chapte of "Concepts of Modern Physics" Arthur Beiser.
I didn't see anything there that said that our measuring devices could not detect any v>c. You are inserting your own personal speculation into your reading and not learning what the author is actually saying.

ravikannaujiya said:
about tachyon and SR you can read first few lines on Wikipedia by typing Tachyon whether they contradict or not.
Nothing there supports your position of post 3 either.
 
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  • #35
ravikannaujiya said:
you are saying two contradictory things at the same time. Special relativity and exisitance of tachyon. First if second postulate is correct, then there would be no tachyon. If there is tachyon then the second postulate is incorrect. If there is tachyon, then the case is no more relevant to anykind of light detector which is the fastest way of detecting by now, and there must be tachyon detector. If we put light detector anyway then the detectors would only recognise the light first, before coming the tachyon as Chereknov effect.
that's what wikipedia says:
A tachyon (pron.: /ˈtæki.ɒn/) or tachyonic particle is a hypothetical particle that always moves faster than light. The word comes from the Greek: ταχύς or tachys, meaning "swift, quick, fast, rapid", and was coined by Gerald Feinberg.[1] Most physicists think that faster-than-light particles cannot exist because they are not consistent with the known laws of physics.[2][3] If such particles did exist, they could be used to build a tachyonic antitelephone and send signals faster than light, which (according to special relativity) would lead to violations of causality.[3] Potentially consistent theories that allow faster-than-light particles include those that break Lorentz invariance, the symmetry underlying special relativity, so that the speed of light is not a barrier.
what you have said:
 
  • #36
what you say
DaleSpam said:
This is not true. You can have any two of tachyons, relativity, and causality. So tachyons do not by themselves contradict relativity.
you can read below the animation of right side of this page on wikiapedia
http://en.wikipedia.org/wiki/Tachyon
now I would not say anything.
 
  • #37
ravikannaujiya said:
that's what wikipedia says:
...which (according to special relativity) would lead to violations of causality.[3] Potentially consistent theories that allow faster-than-light particles include those that break Lorentz invariance, the symmetry underlying special relativity, so that the speed of light is not a barrier.
Which agrees exactly with what I said in post 30 and does not support in any way what you said in post 3. The first bold says that if you have relativity and tachyons then you violate causality. The second bold says that if you have tachyons and causality then you violate relativity. As I said, you can have any two of tachyons, relativity, and causality.

You still have been completely unable to produce any reference even remotely supporting your comments of post 3. Do you now rescind those comments?
 
  • #38
I am still on the third post. But when you are trying to misinterpret the written word, then what can I say.
if you are saying that tachyon and relativity can exist together but then causality will be voileted.
tachyon and causaltiy can exist together but the relativity will be voiletd. Isn't it silly. when relativity always lead to causality.Outcome of relativity is cuasality. If you had any sense you can think that relativity and causality belong to one set of theory. One leads to another and have to satisfy each other . and you say tachyon may be with one of them and not with the other at the same time.
I gave you reference. The books I told you is a authentic science book, a book based on qualitative physics and you can not discard it by just saying "not a valid science reference". The book talks abou underlying philosophy behind the natural sciences. And if you disregard that, it does not mean that the book has no value.
and if you are right why we do not measure the speed of light (V+c) moving against the direction of light, instead we are just able to measure c.
 
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  • #39
ravikannaujiya said:
I am still on the third post. But when you are trying to misinterpret the written word, then what can I say.
if you are saying that tachyon and relativity can exist together but then causality will be voileted.
tachyon and causaltiy can exist together but the relativity will be voiletd. Isn't it silly. when relativity always lead to causality.Outcome of relativity is cuasality. If you had any sense you can think that relativity and causality belong to one set of theory. One leads to another and have to satisfy each other . and you say tachyon may be with one of them and not with the other at the same time.

This is simply not correct. At its most basic, all that special relativity states is that the spacetime interval between any two events is the same in all inertial frames of reference, where the spacetime interval is defined to be
s = (c\Delta t)^2 - |\Delta \vec{x}|^2.
That's it. This only implies causality if you add the presumption that nothing moves faster than c. It does, however, tell you that nothing can move slower than c in one frame and faster than it in another; but, that does not preclude objects which move faster than c in all frames. And, in fact, the math allows for that possibility.

I will also note, in reference to several previous posts, that one does not, in fact, need to presume the invariance of the speed of light. In the formulation above, c is just a conversion factor between units of time and those of space. That it is the speed at which light moves, in this way of looking at things, can be derived from the masslessness of light.
 
  • #40
Parlyne said:
This is simply not correct. At its most basic, all that special relativity states is that the spacetime interval between any two events is the same in all inertial frames of reference, where the spacetime interval is defined to be
s = (c\Delta t)^2 - |\Delta \vec{x}|^2.
That's it. This only implies causality if you add the presumption that nothing moves faster than c.
Yes, it implies not only causality but this is also a Lorentz invariance which is the symmetry underlying special relativity. If Lorentz invariance breaks down, I don't think SR would work. The simple reason is that the symmetry is broken.
Parlyne said:
It does, however, tell you that nothing can move slower than c in one frame and faster than it in another; but, that does not preclude objects which move faster than c in all frames. And, in fact, the math allows for that possibility.
If we are talking about only math then fine. But physics would not allow to do it, if laws of physics are same everywhere.

Parlyne said:
I will also note, in reference to several previous posts, that one does not, in fact, need to presume the invariance of the speed of light. In the formulation above, c is just a conversion factor between units of time and those of space. That it is the speed at which light moves, in this way of looking at things, can be derived from the masslessness of light.

Pls just think of consequences in nature and laws of physics if speed of light need not to be invariant. Masslessness of the light resides in the nature of light itself, as it has zero rest mass but has momentum given by hλ or h*neu/c.
if you use relativistic mass formula, then
realtivistic mass mv= rest mass/(√1-(v/c)2)
Now you can find what kind of mass you get if you put v=c or v > c.
for any patricle if you put v=c, you get that is not defined in mathematics.
If you choose V > c, you get that is not permisssible for mass in real world of physics.
 
  • #41
ravikannaujiya said:
I am still on the third post.
OK, please return and post when you believe you have some support for your incorrect claims of post 3.

ravikannaujiya said:
But when you are trying to misinterpret the written word, then what can I say.
if you are saying that tachyon and relativity can exist together but then causality will be voileted.
tachyon and causaltiy can exist together but the relativity will be voiletd. Isn't it silly. when relativity always lead to causality.Outcome of relativity is cuasality. If you had any sense you can think that relativity and causality belong to one set of theory. One leads to another and have to satisfy each other . and you say tachyon may be with one of them and not with the other at the same time.
I disagree, but it is irrelevant.

The point isn't whether or not tachyons can exist, the point is that our measuring devices could, in principle, measure their speed if they did. I.e. our measuring devices are not inherently limited to measuring v<c, as you incorrectly claimed in post 3. We can, in fact, construct devices such that speeds > c can be accurately measured, regardless of whether or not anything actually exists which can travel that fast.

ravikannaujiya said:
if you are right why we do not measure the speed of light (V+c) moving against the direction of light, instead we are just able to measure c.
Because in the reference frame of the measuring device the speed of light "against the direction of light" is c, not V+c.

Notice how your logic fails if you go "with the direction of light". In that case Newtonian physics predicts the speed of light to be c-V, and our measuring devices can clearly measure c-V. So, if it were merely a limitation of the device that is causing the non-Newtonian measurement then measurements not subject to the limitation should be Newtonian. But they are not, because in the reference frame of the measuring device the speed of light "with the direction of light" is c, not c-V.
 
  • #42
Yes our measuring device cannot give different value for speed of light irrespective of wherever we move.
But anything that moves faster than light could not be measured until and unless light from that object reaches to the measuring device as it would be simply unobservable object. That is why chereknov effect comes. When any particle moves faster than light, it produces a light waves (Chereknov radiation) that come before the object to measuring device. If light waves reach before the object how one can conclude that object moved faster than light.
 
  • #43
ravikannaujiya said:
But anything that moves faster than light could not be measured until and unless light from that object reaches to the measuring device as it would be simply unobservable object. ... If light waves reach before the object how one can conclude that object moved faster than light.
See my post 28 for a detailed example.
 
  • #44
the example of Tachyon that you took do not consider the Chereknove effect produced by it. Wikipedia also says that, "Because a tachyon would always move faster than light, we would not be able to see it approaching. After a tachyon has passed nearby, we would be able to see two images of it, appearing and departing in opposite directions. The black line is the shock wave of Cherenkov radiation, shown only in one moment of time. This double image effect is most prominent for an observer located directly in the path of a superluminal object (in this example a sphere, shown in grey). The right hand bluish shape is the image formed by the blue-doppler shifted light arriving at the observer—who is located at the apex of the black Cherenkov lines—from the sphere as it approaches. The left-hand reddish image is formed from red-shifted light that leaves the sphere after it passes the observer. Because the object arrives before the light, the observer sees nothing until the sphere starts to pass the observer, after which the image-as-seen-by-the-observer splits into two—one of the arriving sphere (to the right) and one of the departing sphere (to the left).
 
  • #45
ravikannaujiya said:
the example of Tachyon that you took do not consider the Chereknove effect produced by it. Wikipedia also says that, "Because a tachyon would always move faster than light, we would not be able to see it approaching. After a tachyon has passed nearby, we would be able to see two images of it, appearing and departing in opposite directions. The black line is the shock wave of Cherenkov radiation, shown only in one moment of time. This double image effect is most prominent for an observer located directly in the path of a superluminal object (in this example a sphere, shown in grey). The right hand bluish shape is the image formed by the blue-doppler shifted light arriving at the observer—who is located at the apex of the black Cherenkov lines—from the sphere as it approaches. The left-hand reddish image is formed from red-shifted light that leaves the sphere after it passes the observer. Because the object arrives before the light, the observer sees nothing until the sphere starts to pass the observer, after which the image-as-seen-by-the-observer splits into two—one of the arriving sphere (to the right) and one of the departing sphere (to the left).

Whether true or not, none of the above is in any way relevant to measuring the speed of an object, whether superluminal or not. I don't need to see the object coming, I don't need to see the object leaving, I don't need to be present at the arrival and departure events. All I need is an arrival time, a departure time, and a distance, and I can calculate the speed after the fact and at my leisure.

The procedure that I described in #8 and that DaleSpam has described above works this way.
 
  • #46
ravikannaujiya said:
the example of Tachyon that you took do not consider the Chereknove effect produced by it.
Irrelevant, as Nugatory pointed out.

I still await a reference supporting your post 3, or your admitting that post 3 was wrong.
 
  • #47
After a tachyon has passed nearby, we would be able to see two images of it, appearing and departing in opposite directions.

If such a thing were to happen, make a note of the time. When it happens to a confederate, he will do so as well. You'll have two sets of location & time coordinates. Voilà - velocity
 
  • #48
ravikannaujiya said:
Yes, it implies not only causality but this is also a Lorentz invariance which is the symmetry underlying special relativity. If Lorentz invariance breaks down, I don't think SR would work. The simple reason is that the symmetry is broken.

Who said anything about violating Lorentz invariance? The simple statement I gave not only entails Lorentz invariance, it also requires translation and time translation invariance, giving the actual symmetry group of SR - Poincare invariance. Tachyons do not violate this. They only violate causality, which is not inherent in either this formulation of SR or that in terms of Einsteins original postulates.

If we are talking about only math then fine. But physics would not allow to do it, if laws of physics are same everywhere.

Again, tachyons do not violate Poincare invariance; so, you are simply wrong here.

Pls just think of consequences in nature and laws of physics if speed of light need not to be invariant. Masslessness of the light resides in the nature of light itself, as it has zero rest mass but has momentum given by hλ or h*neu/c.
if you use relativistic mass formula, then
realtivistic mass mv= rest mass/(√1-(v/c)2)
Now you can find what kind of mass you get if you put v=c or v > c.
for any patricle if you put v=c, you get that is not defined in mathematics.
If you choose V > c, you get that is not permisssible for mass in real world of physics.

I think you misunderstood my statement. The parameter c is a frame-invariant constant no matter how you formulate SR. My statement was simply that it only corresponds to the speed at which light actually travels because photons are massless. Were they not massless (and, strictly, we don't actually know that they are exactly massless - only that their mass is smaller than 10-18 eV) their speed would vary with energy; but, in no frame would it be equal to c.

As for relativistic mass, it is a deprecated concept in the practice of physics, as it adds nothing of any use, but tends to lead to a great deal of confusion. The problem is that relativistic mass is really nothing more than energy divided by c2. But, if we think about it that way, we can see that there is a perfectly consistent way to look at the kinematics of tachyonic particles if we just allow their masses to be imaginary valued. Then, since the Lorentz factor will also be imaginary valued, energy and momentum will be real valued and quite sensible.
 
  • #49
1977ub said:
If such a thing were to happen, make a note of the time. When it happens to a confederate, he will do so as well. You'll have two sets of location & time coordinates. Voilà - velocity
You also can refer to wikipedia on that or any so-called reliable science reference.
DaleSpam said:
Irrelevant, as Nugatory pointed out.

I still await a reference supporting your post 3, or your admitting that post 3 was wrong.
I know I am right whether you accept it or not. I gave you the book and if you do not rely on it, I have shown you some lines on wikipedea and you find it so called Nugatory, leave the matter apart. You can measure speed of anything more than that of light theoreticly or practically. But I don't see it anyway simply when nothing can move faster than light then how can I measure it.
Parlyne said:
Who said anything about violating Lorentz invariance? The simple statement I gave not only entails Lorentz invariance, it also requires translation and time translation invariance, giving the actual symmetry group of SR - Poincare invariance. Tachyons do not violate this. They only violate causality, which is not inherent in either this formulation of SR or that in terms of Einsteins original postulates.



Again, tachyons do not violate Poincare invariance; so, you are simply wrong here.



I think you misunderstood my statement. The parameter c is a frame-invariant constant no matter how you formulate SR. My statement was simply that it only corresponds to the speed at which light actually travels because photons are massless. Were they not massless (and, strictly, we don't actually know that they are exactly massless - only that their mass is smaller than 10-18 eV) their speed would vary with energy; but, in no frame would it be equal to c.

As for relativistic mass, it is a deprecated concept in the practice of physics, as it adds nothing of any use, but tends to lead to a great deal of confusion. The problem is that relativistic mass is really nothing more than energy divided by c2. But, if we think about it that way, we can see that there is a perfectly consistent way to look at the kinematics of tachyonic particles if we just allow their masses to be imaginary valued. Then, since the Lorentz factor will also be imaginary valued, energy and momentum will be real valued and quite sensible.

When you say Causality is voileted but not the SR, speed would vary with energy, allow masses to be imaginary then you can imagine anything you want. But I do not agree with you just because I don't find nature voilating some fundamental principle of causality but not the others. And you also need not to be agree with me... :)
 
  • #50
ravikannaujiya said:
When you say Causality is voileted but not the SR, speed would vary with energy, allow masses to be imaginary then you can imagine anything you want. But I do not agree with you just because I don't find nature voilating some fundamental principle of causality but not the others. And you also need not to be agree with me... :)

What makes you think that causality is fundamental in the first place?

More to the point, though, all I've been trying to point out throughout this discussion is that causality is a separate issue from the validity of SR, as SR is simply about Poincare invariance. I personally suspect that both are true (well, in terms of relativity, what really should be true is local, rather than global, Poincare invariance, as shows up in GR); but, that doesn't change that either could be violated independently of the other.

All that said, it's certainly not true that "you can imagine anything you want." The kinematics of tachyons are just as constrained as those of ordinary objects (or, bradyons, as the terminology would have it).
 

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