HK physicists prove single photons do not exceed the speed of light

Jul 24, 2011
HK physicists prove single photons do not exceed the speed of light
Prof Shengwang Du, Assistant Professor in HKUST's Department of Physics, and his research team have published their study in Physical Review Letters recently

(PhysOrg.com) -- Hong Kong physicists say they have proved that a single photon obeys Einstein's theory that nothing can travel faster than the speed of light -- demonstrating that outside science fiction, time travel is impossible.

A group of physicists at the Hong Kong University of Science and Technology (HKUST) led by Prof Shengwang Du reported the direct observation of optical precursor of a single and proved that single photons cannot travel faster than the speed of light in vacuum. HKUST's study reaffirms Einstein's theory that nothing travels faster than light and closes a decade-long debate about the speed of a single photon.

Prof Shengwang Du, Assistant Professor in HKUST's Department of Physics, and his research team have published their study in recently. Co-authors include three postgraduate students Shanchao Zhang, Jiefei Chen and Chang Liu, Chair Professors Michael M T Loy and George K L Wong. The paper was selected as editors' suggestion for reading. This research was also highlighted as a Physics Synopsis by American Physical Society with a title "Single photons obey the speed limits."

Prof Du's study demonstrates that a single photon, the fundamental of light, also obeys the traffic law of the universe just like classical EM waves. Einstein claimed that the speed of light was the traffic law of the universe or in simple language, nothing can travel faster than light. HKUST's team is the first to experimentally show that optical precursors exist at the single-photon level, and that they are the fastest part of the single-photon even in a so called 'superluminal' medium.

"The results add to our understanding of how a single photon moves. They also confirm the upper bound on how fast information travels with light," said Prof Du. "By showing that single photons cannot travel faster than the speed of light, our results bring a closure to the debate on the true speed of information carried by a single photon. Our findings will also likely have potential applications by giving scientists a better picture on the transmission of quantum information."

HKUST President Tony F Chan said, "We are most delighted that Prof Shengwang Du and his research group have confirmed a key feature of a fundamental law of physics, which also has important implications for communication technology. It epitomizes the mission of our university - to both produce fundamental knowledge and technological impact."

Discovery of superluminal propagation of optical pulses in some specific medium 10 years ago has evoked the world's dream of time travel, but later scientists realized that it is only a visual effect where the superluminal 'group' velocity of many photons could not be used for transmitting any real information. Then people set their hope on single photons because in the strange quantum world nothing seems impossible -- a single photon may be possible to travel faster than the speed limit in the classical world. Because of lack of experimental evidence of single photon velocity, this is also an open debate among physicists. To tackle the problem, Prof Du's team measured the ultimate speed of a single photon with controllable waveforms. The study, which showed that single photons also obey the speed limit c, confirms Einstein's causality; that is, an effect cannot occur before its cause.

HKUST's team used a demonstration which required not only producing single photons, but separating the optical precursor, which is the wave-like propagation at the front of an optical pulse, from the rest of the photon wave packet. To do so, Prof Du's team generated a pair of photons, and then passed one of them through a group of laser-cooled rubidium atoms with an effect called electromagnetically induced transparency (EIT). For the first time, they successfully observed optical precursors of a single photon.

The team found that, as the fastest part of a single photon, the precursor wave front always travels at the speed of light in vacuum. The main wave packet of the single photon travels no faster than the speed of light in vacuum in any dispersive medium, and can be delayed up to 500 nanoseconds in a slow medium. Even in a superluminal medium where the group velocity (of an optical pulse peak) is faster than the in vacuum, the main part of the single photon has no possibility to travel faster than its precursor.

Explore further: New microscope collects dynamic images of the molecules that animate life

More information: Optical Precursor of a Single Photon, Phys. Rev. Lett. 106, 243602 (2011) DOI:10.1103/PhysRevLett.106.243602

Abstract
We report the direct observation of optical precursors of heralded single photons with step- and square-modulated wave packets passing through cold atoms. Using electromagnetically induced transparency and the slow-light effect, we separate the single-photon precursor, which always travels at the speed of light in vacuum, from its delayed main wave packet. In the two-level superluminal medium, our result suggests that the causality holds for a single photon.

See also Physics Synopsis

Provided by Hong Kong University of Science and Technology

5 /5 (15 votes)

Related Stories

Observing a Photon no Longer a Seek-and-Destroy Mission

Jun 02, 2004

A team of University of Queensland, Australia physicists has devised a sophisticated measurement system for single particles of light, or photons, enabling them to investigate fascinating behaviour in the quantum world. ...

Recommended for you

Cooling with molecules

Oct 22, 2014

An international team of scientists have become the first ever researchers to successfully reach temperatures below minus 272.15 degrees Celsius – only just above absolute zero – using magnetic molecules. ...

User comments : 56

Adjust slider to filter visible comments by rank

Display comments: newest first

hush1
5 / 5 (2) Jul 24, 2011
lol
Besides my reply here, what else is instantaneous?
Noumenon
4.5 / 5 (55) Jul 24, 2011
Entanglement. What's funny about the above?
TheCyndicate
1 / 5 (5) Jul 24, 2011
I didn't need this, to know that time travel was not possible.
Time travel was never going to be possible, because Time itself does not exist as a physical reality.

It is just a human invented measurement.

you can't "Inch" travel, so you sure as hell can't "Time" travel either.
bluehigh
1 / 5 (4) Jul 24, 2011
and can be delayed up to 500 nanoseconds in a slow light medium.


Cool! Variable length photons. Near zero length in a vacuum and 150 meters long in some cases. (between the wave front and trailing edge). No?
DGBEACH
1.3 / 5 (12) Jul 24, 2011
If I walk at 3 miles per hour while shining my flashlight in front of me, isn't the light traveling at c 3 miles per hour? If the entire universe is expanding, and we are therefore moving with it, then so are ALL of our light sources, including the one used in this article...thus it is traveling at c the speed of the universe's expansion (or - that speed, depending on the light's direction of course).
What we really need is a truly "Stationary position" in the universe.
DGBEACH
1 / 5 (9) Jul 24, 2011
That's c plus 3 mph...too late to edit
Noumenon
4.5 / 5 (58) Jul 24, 2011
There is no "truly "Stationary position" in the universe", i.e. no absolute rest frame. If you were in a spac ship traveling at half of c, and you tuned on a light in the direction of travel, ... to you and an observer traveling at any speed in the opposite direction (!), the light is still going at c,..'so no not c plus 3 mph, just c.
bluehigh
1 / 5 (3) Jul 24, 2011
The article clearly establishes that a photon can have variable length between the leading edge precursor and the trailing edge of the photon wave packet that is modulated by the medium in which the photon is travelling. Not much of a stretch (!) then to suggest that packet length modulation may occur with source velocity or other factors variability.

bluehigh
1 / 5 (3) Jul 24, 2011
.. and demonstrates that although time travel that violates cause and effect is unreal, delayed effect following cause is possible. So unless you know the factors that have modulated the photons (light) then the time interval between cause and effect is variable within bounds.
YouAreRight
4.3 / 5 (6) Jul 24, 2011
@DGBEACH
That's c plus 3 mph...too late to edit


The reason your comment has been demoted to one star is that c is a cosmic speed limit, c plus 3 mph is not possible.

You need to read up on Special Relativity to understand this.

Here is breif intro that might help.
http://www.squido...lanation

Happy Physics-ing
antialias_physorg
3.3 / 5 (7) Jul 24, 2011
isn't the light traveling at c (plus) 3 miles per hour?

No. The light will only be slightly blueshifted. Speed of light is constant.
bluehigh
1.3 / 5 (14) Jul 24, 2011
Speed of light is NOT constant. It has a maximum velocity (c), the known maximum velocity of any mass.

bluehigh
1.9 / 5 (14) Jul 24, 2011
The velocity at which photons travel is dependent on the properties of the medium. In air or glass for example photons are slowed and give rise to refraction.
hush1
1 / 5 (3) Jul 24, 2011
"Entanglement. What's funny about the above?"
Because we have been here before:

http://www.physor...ons.html

I sided with commentator toejam on that thread:

"Totally wrong. Measurements are temporal. The study ignores the wave function. Try again. Time is not a law of nature." - toejam

Entanglement. Fine. Nothing temporal about that.
Nothing temporal about information as well:
http://www.physor...lly.html

"Quantum no-hiding theorem experimentally confirmed for first time"

The speed of light is losing significance. Just like the speed of sound lost significance. The demand "no prior geometry" from 19th century physicists is met by 20th century physicists.
Einstein was a 'geometrist'. No wonder things had a spooky action AT A DISTANCE.

In that light, it is the highest form of irony that QM has a "measurement problem".
Noumenon
4.5 / 5 (52) Jul 24, 2011
@Bluehigh

True, but only because photons are absorbed and re-emitted by electrons that make up the medium, so such effects are merely epiphenomenon, or circumstancial. A photon can only travel at c. Is it the same photon that entered the glass as refracted from it?
bluehigh
1 / 5 (5) Jul 24, 2011
As Edward Teller once said of interactions 'at the interface' therein lies the unknown. This article provides an insight into other factors (in particular the time length of the photon wave packet) that may lead to a deeper understanding of the Physics involved when Photons transit between media. To propagate half baked notions that 'light speed' is a constant only confuses when known to be factually untrue. The constant is 'c', of which happens to be the velocity of light in 'empty space'. What this article is showing is that the length of a photon wave packet (and a photons velocity) can and does change but never exceeds 'c'. Perhaps instead of one Planck length at a time then stretched photons could jump multiple Planck lengths in each quantum leap, a kind of tunneling just to keep the time keeping in order. Who knows just yet. In any case suggesting that 'light speed' is constant is misinformation. 'c' is the maximum velocity constant and that is not always the 'speed of light'
Noumenon
4.5 / 5 (52) Jul 24, 2011
I would agree that time of itself is not a discoverable physical entity.
Corban
5 / 5 (6) Jul 24, 2011
Reflecting on the last time you made your bed and fluffed the blanket, ever notice how the speed of the wave never changes with how hard you fluff it, only its amplitude?

Only by changing the fabric, from cotton to latex, can you change the propagation speed. Same goes for light. Space is the blanket, and c is the speed of the fluff. Light speed is a function of space's properties. Unless you changed space itself, light isn't going any faster.
Forestgnome
1 / 5 (1) Jul 24, 2011
Since the subject of time travel came up, anyone have a simple explanation why the propagation speed of a wave has anything to do with the speed of an object? I don't see how they relate. I know how waves propagate at a given speed. Look at water. A wave in water propagates at a fairly constant speed. That doesn't have anything to do with how fast you can accelerate a drop of water.
Noumenon
4.5 / 5 (51) Jul 24, 2011
True, but in the case of light, since it is massless and a self-propagating wave, it's velocity is as fast as is possible given certain measurable constants relating to the components of light; electric and magnetic (See Maxwell). Massive objects have inertia and so require some energy to accelerate them,... as the object approaches c it gains mass (see special relativity) and so requires ever so much more energy to continue gaining velocity,.. eventually a intrinsic upper limit is reached at c requiring infinite energy to accelerate further for an object with mass.

Corban seems to imply above that space contains some medium (ether?), which would be like water waves, but this was shown to be an unnecessary assumption by Einstein,.. and so the idea was dropped, even by Lorentz.
Nik_2213
not rated yet Jul 24, 2011
"That's c plus 3 mph."
Wrong. As the other poster said, the light is blue-shifted. Although sound is a poor analogy, remember the Doppler effect as a sound source passes near you-- Whee\oooh...
aroc91
5 / 5 (2) Jul 24, 2011
Time travel was never going to be possible, because Time itself does not exist as a physical reality.

It is just a human invented measurement.


The MEASUREMENT of time is a human construct, not time itself. Explain gravity and relative velocity time dilation if time doesn't exist.
Noumenon
4.5 / 5 (53) Jul 24, 2011
The MEASUREMENT of time is a human construct, not time itself. Explain gravity and relative velocity time dilation if time doesn't exist.

How is time discoverable independently of it's use?

Time is a relation between things. To explain anything in the phenomenal world requires supplying the conceptual structure, and this is dependent on requirements of mind, so therefore a component of the phenomenal world is subjective (mind). Time is an artifact of experience, and has no meaning outside phenomenal reality.
KBK
1 / 5 (2) Jul 24, 2011
Reflecting on the last time you made your bed and fluffed the blanket, ever notice how the speed of the wave never changes with how hard you fluff it, only its amplitude?

Changing the fabric, from cotton to latex, you change the propagation speed. Same goes for light. Space is the blanket, and c is the speed of the fluff. Light speed is a function of space's properties. Unless you changed space itself, light isn't going any faster.


Issue: set field, set framework, set parameters. According to the set field of framework and parameters, light does not exceed a given 'speed'.

Try a different framework, then the results will be different.

Problem: Understanding the limits of the current framework of analysis.

Reason: Multiple fields outside of 'science' are indicating, repeatedly, in different ways, that c is not as limited as the current frameworks indicate.

Conclusion: Analysis frameworks is limited or faulty.

Extrapolation: Find external framework of allowance.
hush1
2 / 5 (4) Jul 24, 2011
"Explain gravity..." - aroc91
I'll pass. The best I can do is describe gravity's observable effects.

The outcomes of anti-hydrogen (antimatter) experiments will add to the descriptions of the effects of gravity. Just as the double slit experiments added descriptions to the effects of the wave nature of matter.

Or as KBK eloquently commented: Framesworks of analysis.
Ducklet
2 / 5 (5) Jul 24, 2011
There seems to be a lot of mixup between light and photons. Also, 'empty space' doesn't exist; there is always at least gravity and other forces as long as there is a single speck of energy in the universe. By this definition (mostly-empty) a photon ALWAYS travels through empty space, even though various forces may deflect it. When it hits something, most likely an electron, it's no longer in empty space. It then ceases to exist, its energy absorbed by the electron. Which then promptly reemits (refracts) it in the form of a DIFFERENT photon. This slows down light propagation, but it doesn't slow down the photon. A secondary effect, of photons traveling in strong force fields, may have a slowing effect as well but that's basically 'empty space'.
bluehigh
1 / 5 (3) Jul 24, 2011
.. only because photons are absorbed and re-emitted by electrons that make up the medium ..
- Noumenon

The simplistic quantum description (when a photon is absorbed and then re-emitted by any given atom) is misleading. The photon scatters off all atoms simultaneously, with appropriate phase shifts, and it is the interference of all such processes together with the original wave that looks like slowing down. Its worth emphasizing that the slowing effect is due to the distributed, wavelike part of the photon behavior (or the fact that the photon is an excitation of an extended mode of the EM field, if you want to put it in that language), and the collective effect of all the possible scattering paths.
BlankVellum
5 / 5 (4) Jul 24, 2011
@TheCyndicate


Time travel was never going to be possible, because Time itself does not exist as a physical reality.

It is just a human invented measurement.


This is false. Time is a property of entropy, which can be quantified using phase space. Our perception of time is a construct, but this is simply stating a truism.

Twin
1 / 5 (2) Jul 24, 2011
I'd like to "suppose" something (on topic, I hope).

I've heard that high velocity travelers will age less than their peers. If this is true, would return voyagers only perceive themselves younger or would they actually be younger than their peers?

Please indulge one more.
If traveling to a star four light years distant at 99.9% light speed, how long would the voyage be perceived to take by the traveler?

(not a physicist - real questions to me).
Pete1983
not rated yet Jul 24, 2011
BlankVellum:

This is false. Time is a property of entropy, which can be quantified using phase space. Our perception of time is a construct, but this is simply stating a truism.


While our perception of time is seemingly just a construct, I'd still love to know if time exists outside of time. I.e, if reality is probabilistic (which it seems to be), then is the only "time" that exists, this moment, i.e "now"? It's quite easy to dismiss the future given the probabilistic nature of reality, but what of the past? I can't help but wonder whether the past "exists", due to it's already happening, and the information of the past is stored up in the now (i.e the history of particles etc). So in this picture entropy would be the ever increasing hard disk usage of reality from all the "time" that has gone by.

Sorry I doubt the above makes much sense... early morning wandering mind syndrome...
Pete1983
not rated yet Jul 24, 2011
I'd like to "suppose" something (on topic, I hope).

I've heard that high velocity travelers will age less than their peers. If this is true, would return voyagers only perceive themselves younger or would they actually be younger than their peers?

Please indulge one more.
If traveling to a star four light years distant at 99.9% light speed, how long would the voyage be perceived to take by the traveler?

(not a physicist - real questions to me).


Hey Twin,

suppose away! That's where all the fun is!

1. They are actually younger. By travelling at near light speed (or by sitting near a black hole), these people would experience less "time" than those back on earth. Although we on earth experience a very small amount of time-dilation ourselves! So say for the earth group 10 years had gone by, the others might have only experienced 5 years of time.

2. I can't do the math right now, but a 4.3 light year journey to Alpha Centauri C at 99% lightspeed would take 7.4 months.
vacuum-mechanics
1 / 5 (2) Jul 24, 2011
There seems to be a lot of mixup between light and photons. Also, 'empty space' doesn't exist; there is always at least gravity and other forces as long as there is a single speck of energy in the universe. By this definition (mostly-empty) a photon ALWAYS travels through empty space, even though various forces may deflect it. When it hits something, most likely an electron, it's no longer in empty space. It then ceases to exist, its energy absorbed by the electron. Which then promptly reemits (refracts) it in the form of a DIFFERENT photon. This slows down light propagation, but it doesn't slow down the photon. A secondary effect, of photons traveling in strong force fields, may have a slowing effect as well but that's basically 'empty space'.

What is the different between light and photon?
Deesky
5 / 5 (2) Jul 24, 2011
I've heard that high velocity travelers will age less than their peers. If this is true, would return voyagers only perceive themselves younger or would they actually be younger than their peers?

It's both a matter of perception and fact. The voyagers would not perceive themselves to be any younger (or older) while traveling. Time for them would tick away as normal, ie 1 sec/sec. However, relative to Earth's frame of reference, their clocks will be ticking slower so that when they finally make it back to earth and compare ages, they will actually be younger then their peers.
Sanescience
2.3 / 5 (3) Jul 25, 2011
First: regarding the question of subjective experience of time. Two objects traveling toward each other can combine speeds to give rise to some statistics that read greater than the speed of light (as in modern atom smashers with counter rotating beams.)

Then there is this: "Scientists break speed of light"
http://www.cbc.ca...720.html
Sanescience
2.3 / 5 (3) Jul 25, 2011
Oh, and should add this to.

"there is just no time for these two photons to communicate."

"The experiment shows that in quantum mechanics at least, some things transcend space-time."

http://www.nature...038.html
Deesky
4.1 / 5 (9) Jul 25, 2011
Then there is this: "Scientists break speed of light"
http://www.cbc.ca...720.html

Phase velocity speed (as opposed to group velocity) is not an example of FTL and neither is entanglement. In both cases, no signal or information is transmitted at superluminal speed. Greater than c information transfer is still sci-fi.
GoodElf
not rated yet Jul 25, 2011
It is an impossibility for any particle to "accelerate through the speed of light" to a speed faster than light. That includes light itself since it just a massless particle subjected to a finite force (an impulse when it was created. So it undergoes infinite acceleration and instantly reached the "top speed" of C. This is the time frame for light and experiences entirely stationary time... infinite time dilation. External observers cannot observe any events occurring to a photon packet unless it becomes "scattered" by "events". All further attempts to "accelerate" light will fail since acceleration is a dynamic process... a dynamically changing velocity in time... however time is "frozen" in the packet frame so no "acceleration" can occur.

Light cannot travel faster than C so to produce an effective FTL velocity it must travel a shorter path in space but be calculated using the longer "effective travel path". A longer photon path must become a "frustrated" sum over shorter paths.
Twin
3 / 5 (2) Jul 25, 2011
Thank you for the responses.
I am a little confused by the idea that high speed travelers would actually be younger vs the idea that time is just an artificial construct.
Sanescience
1.8 / 5 (4) Jul 25, 2011
Twin: high speed travelers experience time at a "slower" frame rate than relatively slower travelers. So they would only be "younger" after a period of time passes for the two of them. Say, the high speed traveler will measure one minute passing by, and the slow traveler would measure two minutes going by. If the two could then meet again and compare clocks, it would seem that the high speed traveler is "younger" only because less time for him had passed.

Here is a question I haven't found a good answer for. If the mass-energy of particles increases with speed, can their gravitational attraction be used to judge their speed in absence of a relative frame?
vacuum-mechanics
1 / 5 (3) Jul 25, 2011
There seems to be a lot of mixup between light and photons. Also, 'empty space' doesn't exist; there is always at least gravity and other forces as long as there is a single speck of energy in the universe. By this definition (mostly-empty) a photon ALWAYS travels through empty space, even though various forces may deflect it. When it hits something, most likely an electron, it's no longer in empty space. It then ceases to exist, its energy absorbed by the electron. Which then promptly reemits (refracts) it in the form of a DIFFERENT photon. This slows down light propagation, but it doesn't slow down the photon. A secondary effect, of photons traveling in strong force fields, may have a slowing effect as well but that's basically 'empty space'.

This slows down light propagation, but it doesn't slow down the photon. Why, isnt photon moving by propagation?
alarson
1 / 5 (3) Jul 25, 2011
There are some excellent answers here. Perhaps I may add my 2 bits worth.
1. Time does not exist. There are only intervals between events. Time as we use it is only artifical measurement of these intervals. In the 'twins paradox' the traveling twin is said to age less than the stay-at home twin on earth (or on a stationary spaceship). One can also say that it is the twin that stays on earth or on the at-rest spaceship who is doing the traveling and therefore not ageing i.e. the traveling twin is now the one at rest and he sees the original stationary twin as traveling away. Answer to the paradox, time dilation does not occure for either twin.
2. Light's propagation through space. Light travels space as a projectile like a rifle bullet and uses no energy while doing so. Light propagates through different mediums at different speeds, therefore a light photon when it enters space is going at the speed that it is going when it exits the last atomic field. c varies.
Art
Deesky
4.5 / 5 (10) Jul 25, 2011
high speed travelers experience time at a "slower" frame rate than relatively slower travelers.

No, they experience time just like any other person in the universe. They're not moving in slow motion or fast motion - time passes for them as normal.

So they would only be "younger" after a period of time passes for the two of them.

But only if they meet up again.

the high speed traveler will measure one minute passing by, and the slow traveler would measure two minutes going by

No, they will both measure 1 s/s in their own frames of reference.

If the two could then meet again and compare clocks, it would seem that the high speed traveler is "younger"

Not only seem, but he will be younger. The effect is only apparent when clocks (people) are compared in the original (earth) frame of reference. You can only say time is going slower or faster relative to different frames of reference, not your own.
Deesky
4.6 / 5 (10) Jul 25, 2011
Time does not exist. There are only intervals between events. Time as we use it is only artifical measurement of these intervals.

The question of time is a vexing one, but in the above you have merely substituted the word 'time' with the word 'interval'. They're effectively the same thing, so you haven't done away with it.

One can also say that it is the twin that stays on earth or on the at-rest spaceship who is doing the traveling and therefore not ageing

No, you can't. There is a fundamental difference between a body 'at rest' (earth) and one undergoing non-inertial acceleration (ship). As the rest of your argument is based on this fundamental misunderstanding, it is moot.

Light's propagation through space. Light travels space as a projectile like a rifle bullet

Er, no, it doesn't. Why don't you consult the Physics FAQ pages for some insight.
antialias_physorg
4.2 / 5 (5) Jul 26, 2011
Speed of light is NOT constant. It has a maximum velocity (c), the known maximum velocity of any mass.

But since the minimum velocity is equal to the maximum velocity (in a vacuum), so the speed is constant.

Speed of light in a medium is another matter. There you can even have speeds that vary with the frequency.
Sanescience
1 / 5 (1) Jul 26, 2011
Deesky: you are being unnecessarily picky about laymen explanations...

high speed travelers experience time at a "slower" frame rate than relatively slower travelers.

No, they experience time just like any other person in the universe. They're not moving in slow motion or fast motion - time passes for them as normal.

Your comment is correct but does not apply.
As the topic is how a reference frame external to the two individuals would view them. Much like how GPS satellites really do seem to be experiencing (not their frame of reference!) time going by faster that we do on earth. Also you ignored my quotes around slower.

So they would only be "younger" after a period of time passes for the two of them.

But only if they meet up again.

Um, that is the point of this discussion.
Sanescience
1 / 5 (1) Jul 26, 2011
the high speed traveler will measure one minute passing by, and the slow traveler would measure two minutes going by

No, they will both measure 1 s/s in their own frames of reference.

Inaccurate. This is in contradiction with your earlier assertion that one person really would have aged less than another. Or your later statement that clocks when compared would be different.

If the two could then meet again and compare clocks, it would seem that the high speed traveler is "younger"

Not only seem, but he will be younger. The effect is only apparent when clocks (people) are compared in the original (earth) frame of reference. You can only say time is going slower or faster relative to different frames of reference, not your own.

If your going to be that picky, you shouldn't then make ambiguous statements that may imply the person became younger as if going backward in time. Other than that this is just rewording my statement.
Deesky
5 / 5 (3) Jul 26, 2011
Deesky: you are being unnecessarily picky about laymen explanations...

I'm not being picky, I'm trying to be accurate. The problem with 'layman' explanations of topics which are quite unintuitive is that the layperson is left with a misleading impression as to what is going on.

For example, it's easy for the layperson to think that people on a fast moving ship will be moving in slow motion because their clocks tick slower. This is not the case and is something I wanted to make clear.

Your comment is correct but does not apply.

If it's correct, how is it not applicable? Your reframing of what the 'topic' should be, notwithstanding.

Um, that is the point of this discussion.

And this is a crucial point which needed to be emphasized.
Deesky
5 / 5 (4) Jul 26, 2011
No, they will both measure 1 s/s in their own frames of reference.

Inaccurate. This is in contradiction with your earlier assertion that one person really would have aged less than another. Or your later statement that clocks when compared would be different.

How is it a contradiction? By making this objection it is clear to me that you do not really understand what is going on and why layman explanations often confuse already confusing concepts.

If your going to be that picky, you shouldn't then make ambiguous statements that may imply the person became younger as if going backward in time.

Where did I do that?

Other than that this is just rewording my statement.

It's a more correct restatement of what you tried to say, IMO, of course.
DarkHorse66
1 / 5 (2) Jul 27, 2011
@sanescience
re: increased gravitational mass vs mass-energy for a moving object. That is a very intriguing question! Even in my modern physics subject (at 200 level) that point was never talked about. I would be very interested to hear about that one too. Is anyone willing to try and formulate a possible answer?
Osiris1
1 / 5 (2) Jul 30, 2011
Propaganda! and Pandering! Think common sense Newtonian physics is better and makes more sense! Besides, no vacuum is truly a vacuum anyway, so no real measurement of so called 'c' in a bottle is really possible. We do not know as much as we think we do; after all, how does one spell 'ass - u -(&) me'! for 'assume'?
aroc91
not rated yet Jul 31, 2011
"Explain gravity..." - aroc91
I'll pass. The best I can do is describe gravity's observable effects.

The outcomes of anti-hydrogen (antimatter) experiments will add to the descriptions of the effects of gravity. Just as the double slit experiments added descriptions to the effects of the wave nature of matter.

Or as KBK eloquently commented: Framesworks of analysis.


I meant gravity-influenced time dilation, not just gravity.

ubavontuba
1 / 5 (4) Jul 31, 2011
That light always travels at c in a vacuum is an artifact of spacetime. That is, time dilation (local clock variance) creates the perception that light speed is constant.

Lately, I've been wondering if it's possible to create a time dilation paradox. That is, could the perception of the speed of light in an experiment set a clock rate that is paradoxically opposed to a clock rate needed for a simultaneous observation of another experiment?

Generally, all I get is a singularity where everything must occur at once, or not at all.

I have wondered though, if entanglement might offer a solution. Essentially, could particles be entangled outside of time? That is, could particle x be entangled with particle y in a different temporal frame of reference? If so, this would imply that observer z could predict the future, or affect the past, simply by making an observation of one, or the other particle.

Any thoughts?

Y8Q412VBZP21010
3 / 5 (2) Jul 31, 2011
Any thoughts?


[Raises hand:] "Yes. Are you joking, or are you on recreational drugs?"

If the latter, could you tell me where I can get some of that stuff, too?
ubavontuba
1 / 5 (4) Jul 31, 2011
Are you joking,
No.

No.

http://en.wikiped...nglement

Obviously, as each point in space is its own frame of reference, it's possible to separate entangled pairs in spacetime in terms of distance (space). Why not in the other spacetime component (time)?

If two entangled particles are separated vertically in a gravity well (say, like the earth's) they will, as per the theory of general relativity, have different clock rates.

Observing the effects of this clock differential on entangled particles might be interesting.

Would the "slow" particle not be affected by an observation of the "fast" particle until it catches up with its observed partner in time? Will the "fast" particle fall out of entanglement before the slow one is observed?

Is it even possible to retain entanglement, in regards to time dilation?

angelhkrillin
1 / 5 (2) Jul 31, 2011
For the people posting stuff like, "oh well if I'm driving 60mph in my car and have lights on aren't they going C plus 60mph?" NO, the light doesn't, go and please read Relativity - The Special and General Theory by Albert Einstein.
Kontrast
not rated yet Aug 02, 2011
Here is a question I haven't found a good answer for. If the mass-energy of particles increases with speed, can their gravitational attraction be used to judge their speed in absence of a relative frame?


Sadly, no. If you tried to measure their "absolute speed" by measuring gravitational attraction, you're still comparing two objects, so you don't get around having to use a relative frame.

Or in other words: The "scale" you would be using to measure how much (relative) mass an object has gained by accelerating would be the relative frame you're trying to avoid. If the scale was inside the object (e.g. a spaceship), it would show no relative mass, seeing that it's in the same reference frame. Outside of the object, it would show the appropriate relative mass for the speed the object is travelling relative to the scale.

There is as far as I know no such thing as an absolute speed without a "relative frame".