Can We Travel Faster Than Light?

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SUMMARY

The discussion centers on the possibility of traveling faster than light (FTL) and the implications of special relativity. Participants argue that while relativity prohibits mass from exceeding the speed of light, phenomena such as laser spots and wormholes suggest scenarios where FTL concepts can be explored. The Alcubierre drive and tachyons are mentioned as theoretical constructs that challenge conventional understanding. Ultimately, the consensus is that while certain abstract points can appear to move faster than light, they do not violate the principles of causality or relativity.

PREREQUISITES
  • Understanding of special relativity and the Lorentz factor
  • Familiarity with concepts of wormholes and the Alcubierre drive
  • Basic knowledge of quantum chromodynamics and tachyons
  • Comprehension of spacetime events and their implications
NEXT STEPS
  • Research the Alcubierre drive and its theoretical framework
  • Study the implications of tachyons in quantum physics
  • Explore advanced concepts in quantum chromodynamics
  • Investigate the relationship between causality and superluminal speeds
USEFUL FOR

Physicists, science enthusiasts, and anyone interested in theoretical physics and the implications of faster-than-light travel on our understanding of the universe.

  • #31
vin300 said:
The problem with Dr.Greg's explanation is that he thought light from the new point reaches 1sec after the last view of the original point so it moved at a superluminal speed.
That isn't what is happening When the original point is last viewed the light has just reached the new point and has to travel equal distance back
That proves it does not move at superluminal speeds

That's just false. You continue to view a dot on the original left hand side for 59 seconds after the dot has actually reached the new point on the right.

The example involves light traveling a distance of one light-minute to the screen, and one light-minute back. Since it goes at the speed of light, you see a dot two minutes after you shine the beam to initiate a dot. Hence you see the dot on the left, and then a second later on the right, but all one minute after the dot sweeps across the screen.

Therefore you see the dot sweeping across the screen between 12:02:00 and 12:02:01.

The dot actually sweeps across the screen between 12:01:00 and 12:01:01.

Put numbers on it to check. You say that "When the original point is last viewed the light has just reached the new point". That's silly. You've been given a precise description of where the laser is aimed. What is that time you think that the original point is "last viewed" by the observer? The answer there is 12:02:00.

What is the time that you think the light actually reaches the the right hand side? The answer there is 12:01:01.

You've got a complete description of the experiment. Put numbers on your answers.

Cheers -- sylas
 
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  • #32
sylas said:
You continue to view a dot on the original left hand side for 59 seconds after the dot has actually reached the new point on the right.
No it takes one second more than the period you see the dot on the left side for it to actually reach the right.
If this is the basic misconception, then for the new set of photons to reach there, it takes only one second while the distance is one light minute.What about what you said in this para below?
Look at the third quote, It should be clear then
involves light traveling a distance of one light-minute to the screen, and one light-minute back. Since it goes at the speed of light, you see a dot two minutes after you shine the beam to initiate a dot.
Right
Hence you see the dot on the left, and then a second later on the right, .
No you don't You see the dot on the left for two minutes after you deviate the laser then you see it sweeping for another one minute, if you realize the diagonal formation for one minute after you deviate the laser. The last photon that hits the original point followed by lagging new photons forms a diagonal of length one light minute and one light sec which reflect back in a pattern such that you see the whole one minute and one sec span between the original and final spot.Is it fine now?
It may be clear this way- youshoot the photons in different directions- such that every new photon is diagonally behind the previous by a particular distance(which sum up to one light minute and one light second)or a particular period (which sum up to one minute and a sec)

Therefore you see the dot sweeping across the screen between 12:02:00 and 12:02:01.

The dot actually sweeps across the screen between 12:01:00 and 12:01:01.

Put numbers on it to check. You say that "When the original point is last viewed the light has just reached the new point". That's silly. You've been given a precise description of where the laser is aimed. What is that time you think that the original point is "last viewed" by the observer? The answer there is 12:02:00.

What is the time that you think the light actually reaches the the right hand side? The answer there is 12:01:01.
All that can be resolved
 
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  • #33
vin300: You seem to be quite stuck on this one. DrGreg's example is perfectly correct, as is sylas's commentary. Did you read the link I gave earlier?

vin300 said:
No you don't You see the dot on the left for two minutes after you deviate the laser then you see it sweeping for another one minute, if you realize the diagonal formation for one minute after you deviate the laser. The last photon that hits the original point followed by lagging new photons forms a diagonal of length one light minute and one light sec which reflect back in a pattern such that you see the whole one minute and one sec span between the original and final spot.Is it fine now?
No, not fine. The spot takes 1 second to sweep across. (Of course you "see" the 1 second sweep two minutes after you first fired the laser.)
All that can be resolved
Nothing to resolve.
 
  • #34
Vin... do you understand the experiment described? You seem to be the only person not getting this. Make sure you read and understand the account and indicate the times at which events occur.

There is a screen, one light minute away. It is two light seconds wide. You have a laser aimed at the left side of the screen. OK?

At 12:00:00 you swing the laser to point towards the right side of the screen, taking one second to do so.

Hence, at 12:00:01 the laser is aimed at right side of the screen.

One minute after this, the photons which left the laser between 12:00:00 and 12:00:01 arrive at the screen.
  • Photons leaving at 12:00:00 arrive at the left side of the screen, at 12:01:00.
  • Photons leaving at 12:00:00.383 arrive at 12:01:00.383..., a point 0.766 light seconds from the left side and 1.234 light-seconds from the right side.
  • Photons leaving at 12:00:01 arrive at the right side of the screen, at 12:01:01.

There is a a dot of light (not a particle) which moves from the left to the right over the time span 12:01:00 to 12:01:01 --- one second.

To SEE the dot, the light has to get back to your eye again... which takes another minute.

Hence at 12:02:00 you see the last view of the dot on the left side of the screen. At 12:02:01 you see the dot on the right side of the screen. Between 12:02:00 and 12:02:01 (one second) you see the dot sweeping over the screen... one minute after it actual did sweep over the screen.

vin300 said:
No it takes one second more than the period you see the dot on the left side for it to actually reach the right.

That's not really coherent... the "period" we see the dot on the left side is indefinite. The experiment says that the laser has been trained on the left side, so you've been watching it there for some time.

You see the dot leave the left side two minutes after you cease pointing at the left side. You see the dot arrive at the right side two minutes after you start pointing at the right side. Therefore you see the dot leaving the left side one second before you see it arriving at the right side. You see it traversing the screen for that one second.

Put times on it. The correct answer is that you see the dot leave the left side 12:02:00 and you see it arrive at the right side at 12:02:01.

No you don't You see the dot on the left for two minutes after you deviate the laser then you see it sweeping for another one minute, if you realize the diagonal formation for one minute after you deviate the laser. The last photon that hits the original point followed by lagging new photons forms a diagonal of length one light minute and one light sec which reflect back in a pattern such that you see the whole one minute and one sec span between the original and final spot.Is it fine now?

No. You are still completely wrong.

You deviate the laser at 12:00:00. At 12:00:01 is it now pointing to the right, because the example involves turning the laser over a duration of one second.

The photons which leave the laser at 12:00:01 are directed to the right side of the screen. They arrive at the screen at 12:01:01, making a dot. You see the dot, on the right, at 12:02:01.

You are correct that you see the for on the left for two more minutes, up until 12:02:00.

Note that you see it on the right one second after seeing it on the left. Not one minute later. It only takes two minutes for light to get to the right side of the screen and back; NOT three minutes.

It may be clear this way- youshoot the photons in different directions- such that every new photon is diagonally behind the previous by a particular distance(which sum up to one light minute and one light second)or a particular period (which sum up to one minute and a sec)

All that can be resolved

Everyone so far has resolved it except you. OF COURSE you shoot different photons in different directions, but because they are different direction, what the heck are you summing them for?

The line of photons approaching the screen are in a line that is almost, but not quite, parallel to the screen, and they are all moving almost perpendicular to that line.

Think. At 12:00:00 to 12:00:01 you twist the laser.

Ten seconds later at 12:00:11, the photon headed for the left is 11 light seconds from the laser, and the photon headed for the right is 10 light seconds from the laser. They are spead out over a span of about one third of a light second, from the leftmost to the right most, as they all continue to advance at c towards the screen.

Cheers -- sylas
 
  • #35
OK I got it my stupid mistake
 
  • #36
what the heck are you summing them for?
The total time lag and distance lag between the two extreme spots
 
  • #37
OH! I have started to get headache reading this, First sorry to all for late reply, I didn't had any access to internet for these days. :frown:
I hope you understand. :redface:
 
  • #38
Okay, so I don't understand how to start exactly, cause it's grown faster than I expected.
I'll try to start with simple thing(s), than I may (try to) complicate 'em.
 
  • #39
HallsofIvy said:
"The spot" not being an actual physical entity, of course.

What do then you consider as "physical entity"?
 
  • #40
DaleSpam said:
The whole point about wormholes etc. is that you always have a timelike worldline (ie v<c always). Wormholes are about taking a shortcut, not about going fast.

So what about this?

http://img188.imageshack.us/img188/9195/wormholetimetravels1.jpg
 
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  • #41
DaleSpam said:
You can have any two of the three: relativity, causality, and FTL. Right now it looks like relativity and causality, not FTL.

No, its not like this,
http://img240.imageshack.us/img240/1262/478pxlightconesk1.png
Here AB is 'timelike' , so that there is a frame of reference in which A & B occur in same location, but just separated in time, and if n that frame A precedes B, than A precedes B in all frame(s).

But this is not true for spacelike separated events as are A & C, there is a frame in which A and B occur simultaneously separated only in space, but there are also frames in which A precedes B (as the given) and frames in which B precedes A, so that causality may not be maintained.
 
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  • #42
Hermit said:
Special relativity explains that an information cannot travel at speed greater than c, because the Lorentz factor would become a complex number ; but there are situations where "something" goes faster than c. For example, imagine a laser pointed forward a planet ; then, you turn the laser forward another planet ; this action during one second. If there is a distance of one light-year between the two planets, the spot will have a speed of one light-year per second.

If we turn our laser from planet 'A' to 'B' the light's displacement will be in tangential plane, but not the motion, which would be rotational and hence the velocity would be changing, ( though we all know that speed will be const. ) hence there'll be acceleration, so this cannot be treated taking just SR into consideration.
 
  • #43
  • #44
sylas said:
Vin... do you understand the experiment described? You seem to be the only person not getting this. Make sure you read and understand the account and indicate the times at which events occur.

There is a screen, one light minute away. It is two light seconds wide. You have a laser aimed at the left side of the screen. OK?

At 12:00:00 you swing the laser to point towards the right side of the screen, taking one second to do so.

Hence, at 12:00:01 the laser is aimed at right side of the screen.

One minute after this, the photons which left the laser between 12:00:00 and 12:00:01 arrive at the screen.
  • Photons leaving at 12:00:00 arrive at the left side of the screen, at 12:01:00.
  • Photons leaving at 12:00:00.383 arrive at 12:01:00.383..., a point 0.766 light seconds from the left side and 1.234 light-seconds from the right side.
  • Photons leaving at 12:00:01 arrive at the right side of the screen, at 12:01:01.

There is a a dot of light (not a particle) which moves from the left to the right over the time span 12:01:00 to 12:01:01 --- one second.

To SEE the dot, the light has to get back to your eye again... which takes another minute.

Hence at 12:02:00 you see the last view of the dot on the left side of the screen. At 12:02:01 you see the dot on the right side of the screen. Between 12:02:00 and 12:02:01 (one second) you see the dot sweeping over the screen... one minute after it actual did sweep over the screen.



That's not really coherent... the "period" we see the dot on the left side is indefinite. The experiment says that the laser has been trained on the left side, so you've been watching it there for some time.

You see the dot leave the left side two minutes after you cease pointing at the left side. You see the dot arrive at the right side two minutes after you start pointing at the right side. Therefore you see the dot leaving the left side one second before you see it arriving at the right side. You see it traversing the screen for that one second.

Put times on it. The correct answer is that you see the dot leave the left side 12:02:00 and you see it arrive at the right side at 12:02:01.



No. You are still completely wrong.

You deviate the laser at 12:00:00. At 12:00:01 is it now pointing to the right, because the example involves turning the laser over a duration of one second.

The photons which leave the laser at 12:00:01 are directed to the right side of the screen. They arrive at the screen at 12:01:01, making a dot. You see the dot, on the right, at 12:02:01.

You are correct that you see the for on the left for two more minutes, up until 12:02:00.

Note that you see it on the right one second after seeing it on the left. Not one minute later. It only takes two minutes for light to get to the right side of the screen and back; NOT three minutes.



Everyone so far has resolved it except you. OF COURSE you shoot different photons in different directions, but because they are different direction, what the heck are you summing them for?

The line of photons approaching the screen are in a line that is almost, but not quite, parallel to the screen, and they are all moving almost perpendicular to that line.

Think. At 12:00:00 to 12:00:01 you twist the laser.

Ten seconds later at 12:00:11, the photon headed for the left is 11 light seconds from the laser, and the photon headed for the right is 10 light seconds from the laser. They are spead out over a span of about one third of a light second, from the leftmost to the right most, as they all continue to advance at c towards the screen.

Cheers -- sylas

Really I didn't understand much of this what you posted before but this one was really helpful. Thanks!
 
  • #45
Hermit said:
Special relativity explains that an information cannot travel at speed greater than c

What do you consider as "information"
 
  • #46
In many of the posts it is mentioned that the things (as the contact point of scissors, dots, shadows etc.) and all other things that travel at faster than light are not "physical entities".
I'll give you two examples of "physical entities, one being 'imaginary' and other being 'real' that travel ftl.
For the hypothetical physical entity, we have tachyons as I mentioned in OP.
for real physical entity, consider the following case,
We have galaxy (now don't misunderstand my word "have"), in there we have sun, and then earth, which revolves around sun at speed 1 revolution per year, our sun in turn, revolves around our galaxy at a speed of 1 revolution per 230 million yrs, ( I may be a bit up or down in stats but its nearly the same ), so according to principle of relativity we can consider our sun ( or http://calgary.rasc.ca/howfast.htm#galactic" ) is at rest and milky way is revolving around the sun ( or as well as Earth ), and in this case the revolution speed comes to be thousands of light yrs per day.
 
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  • #47
Can time dilation have any effect on information? I mean "sending (or transfer) of information"?
 
  • #48
velocity would be changing, ( though we all know that speed will be const. ) hence there'll be acceleration, so this cannot be treated taking just SR into consideration.

No velocity will be changing The same photons do not change direction
 
  • #49
aaryan0077 said:
but there are also frames in which A precedes B (as the given) and frames in which B precedes A, so that causality may not be maintained.
No, this is incorrect. There is no frame in which B preceeds A. You can derive that directly from the Lorentz transform.
 
  • #50
aaryan0077 said:
in this case the revolution speed comes to be thousands of light yrs per day.
This "speed" is the coordinate speed in a non-inertial coordinate system. It has no physical significance whatsoever. In that reference frame the speed of light is not c.

The worldline of the galaxy is timelike (slower than light) in all reference frames, even non-inertial ones. It never becomes spacelike (FTL). This concept of timelike, lightlike, or spacelike is the one with a coordinate-independent physical significance.
 
  • #51
aaryan0077 said:
Here AB is 'timelike' , so that there is a frame of reference in which A & B occur in same location, but just separated in time, and if in that frame A precedes B, than A precedes B in all frame(s).

But this is not true for spacelike separated events as are A & C, there is a frame in which A and B occur simultaneously separated only in space, but there are also frames in which A precedes B (as the given) and frames in which B precedes A,...
Did you make a typing error here? In the second paragraph, did you really mean C, not B? If so, that is correct
aaryan0077 said:
...so that causality may not be maintained.
but that isn't. "Causality" refers only to the ordering of timelike-separated events (like A and B) or null-separated events.
 
  • #53
Hermit said:
Special relativity explains that an information cannot travel at speed greater than c

Okay, if its info that can't travel faster than light, this means that any other thing. which is not info, or does not contain any info could indeed go ftl.
then, consider the time of creation, at instant of BB (big bang) we had nothing, no particles antiparticles, any any other stuff, just energy, but I still don't understand that if there was nothing than how come, means in which form energy existed?
Has it got something to do with GUT or what?
Anyways, its not the case here,
So all quarks, anti-quarks and all that stuff was created much later, when supersymmetry was broken. But considering just that time in which there was nothing, the expansion should have been much faster than speed of light, and not like the bubble nucleation.
But since there was nothing to expend, then what expended?
Was that a "perfect spacetime" that expended devoid of any "physical entities", or something else, or it was nothing?
I think I am really confused. :confused:
 

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