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What is "time" ?


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1 hour ago, Bleednipple said:

Yeah but what would happen if you did the cat-and-buttered-toast thing in a largish box without looking inside?

You explain, I.m not going there !  

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What I suggest is, over the next ten days we talk over each section of BG;s tak in turn up as far as "Motion  effects on space" ending at ca 1;50

By thne we should have a good idea of Special Relativty and space-time. Ponder  this; if i drive my TR past you, a stationary onlooker, at a deceent fraction of the speed of light (300,000 km/sec) you will see me and the TR shortened front to back, it does not get less wide or less tall. You will also see the clock on the  dashboard going very slow. These are length cpntraction and time dilation of Special Relativity. What do I see? the car loooks entirely normal, and my clcok works as normal. If I measured the car lenth with a tape it would be as per Brown Bible. but if you measured its length youd be certain it had been squashed. Optical illusions are not responsible. SR means that at high relative speed to the observer time dilates and space contracts in the direction of travel.

By the end of  next weekend I hope we will see how Einstein arrived  at that spectacular conclusion.

 

 

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OK, that's the guts of Special Relativty mastered. We have learned that  time slows for a clock in motion relative to a 'stationary' observer. And space shrinks in the direction of motion.

As an aside, to drive SR home, let's ask ourselves how much time passes for a photon, a particle of light  travelling at light speed ( 300,000 mph in vacuum). Put it this way: how old were the photons when recorded in the Hubble telelscope Ultra deep field https://en.wikipedia.org/wiki/Hubble_Ultra-Deep_Field

and form their frame of reference how far had they travelled ?

Peter

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14 minutes ago, john.r.davies said:

There once was a fencer called Fisk

Whose movements were so quick and brisk

By the speed of his action,

FitzGerald Contraction

Shortened his foil to a disc!

Poor old Lorentz, one syllable short of limeric fame.

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25 minutes ago, barkerwilliams said:

Ah Photons.

Two torches shining at each other, each stream of photons is traveling at the speed of light. From one photons perspective at what speed is the approaching photon travelling?

Alan

There are two answers.

One is c, the speed of light. It is constant from any exxternal reference frame

The second is "you are not allowed to ask that"  A measurring device has mass that at the speed of light beomes infinite, so no measurment cna ever be made. A photon is not able to measure anything.

Peter

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1 hour ago, Peter Cobbold said:

OK, that's the guts of Special Relativty mastered. We have learned that  time slows for a clock in motion relative to a 'stationary' observer. And space shrinks in the direction of motion.

As an aside, to drive SR home, let's ask ourselves how much time passes for a photon, a particle of light  travelling at light speed ( 300,000 mph in vacuum). Put it this way: how old were the photons when recorded in the Hubble telelscope Ultra deep field https://en.wikipedia.org/wiki/Hubble_Ultra-Deep_Field

and form their frame of reference how far had they travelled ?

Peter

From our persepctive ( reference frame in the jargon) the deep field galaxies are 13 billion years in our past. But in the photons' refrence froma tiem does not pass, they have not aged at all in that 13 billion year journey.  Nor have they travelled any distnace from thir viewpoint

https://phys.org/news/2014-05-does-light-experience-time.html

Time and space are relative !

Peter

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The most disturbing thing about Relativity is the inevitable conclusion that we love in a Block Universe (that loaf of bread) and live in Blcok Time. Blcok Time results from the linking of space and time together, and means that all the past, the present and alll the future are real, they all exist. Our futures exist, how and when we die etc ect. This determinstic nature of time denise us free will. It would be all too easy to dismiss SR as wrong, but it is not, Over the past 100 years it has wwithstood every test thrown at it, and is of great importnca practicall eg GPS, nuclear enrgy, Large hadron collider etcetc.

But we all sense time flowing. Time locked into Einstein-Mikovski space-time continuum is nto whhat we perceive. We rememeber the past but not the future. Most physicsita claim that  our sensation of time flowing is an illusion, a minority claim time really does flow. Two examples

https://www.scientificamerican.com/article/that-mysterious-flow-2006-02/

http://www.pitt.edu/~jdnorton/papers/Time_passes_final.pdf

 

I do not buy time is illusory. We have evolved memories for the past that enble us to make an informed guess about the future. We remember a leopard killing a friend here and guess it could attack again. But if block time is real we would surely have evolve a memort for the future. We would know the attack would com, be fore-warned. Promoters of "time is illusory" assume memoery as a given and ignore the fundamental driver of life: evolution. The past may be real only in our memories, the future not yet real - but SR and Blcok time do not fit that scenario. In following posts we will explore how SR, without being wrong in any way, might be "incomplete". Current ideas hint that time really does flow, and might not be as fundamental as we think.

Peter

 

 

 

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21 minutes ago, RogerH said:

So far so good.

I have watched 2 hours of Brian Greens's presentation and I am completely with him up to the point where he said 'Hello' at the beginning.

Then it got a bit fuzzy.

 

Roger

Roger,  You are not alone, even physics undergrads these days  are allowed to dodge "Relativty" options and do  something easier.

All you need to get from the talks is:

As an object move at a decent fraction of the speed of light, time slows (time dilation) and space shrinks in the direction of travel ( lenght contraction). This is only percieved by an observer who is stationary. An observer in the object sees no change of time or shrinking. Optical illusions are not involved at all.    This is Special Relativty, but we do not see it in everyday life as nothing we can seee travels relative to us at anywhere near the speed of light, 300,000km/sec. 

The upshot of SR is thta teh 3dimensions of space and one of time msut be linked together. Time is like another diemnsion fo space: the space-time continuum. Time is locked to space and since we do not think space flows, neither does time. Result: that loaf of bread, the Blcok Univers with Blccok Time that extends bakc ot all the past anf forward to all the future. In SR time does not flow as we experience it.  Thats the cocnucdrum I hopw tis threa willl help unravel.

Peter

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53 minutes ago, Lebro said:

:blink:

Indeed the implications of SR are counter intuitive. But later in the thread there should be clues to something more fundamental going on, adeepr layer underpinning SR. SR may be "incomplete"

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1 minute ago, RogerH said:

I can feel it all raveling in 'front' of me.  However that could be 'behind' an observer a long way away.

 

Roger

Roger,  you got it  ! your antennae helping? Peter

that bit about the alien on his bike is really difficult but we can skip over it withoutloosing thhe gist of time locked with space

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Peter has said that, for a photon, no time passes.   As velocity is distance over time, to photon velocity has no meaning. If time is zero, then distance over time gives infinity.   That's a problem with such thinking - you often end with infinity!

 

Edited by john.r.davies
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2 hours ago, john.r.davies said:

Peter has said that, for a photon, no time passes.   As velocity is distance over time, to photon velocity has no meaning. If time is zero, then distance over time gives infinity.   That's a problem with such thinking - you often end with infinity!

 

But the speed of light is nowhere near infinity, its 300,000 km/sec (in vacuo) It has been measured to great precision. It is Einsteins fundamentla postulate and is non-negotiable !  Given that, a deep field photon would take 13 billion years to reach Hubble. But that valid measurement is from our frame of reference. But on the photons own refernece frame no time passes and it travels no distance.** That is the guts of SR. Time and space are relative to the observer and dilate and contract as the relative velocities between reference frames increase. When the relative veocities of reference frames is at c (shorthand symbol for speed of light) then tiem and space are infintely dialted and  contracted in the direction of the motion, to zero.

** If a deep field photon could see earth, we would be frozen in time and contracted  to a disc. SR works both ways, no reference frame is preferred.

There is a phenomenon with solid, accepted exptl validation that two events can be co-ordiinated instantaneously ( infininately fast). Entanglement. Otheriwse known as "spooky action at a distance "  Since no information can travel faster than c entanglement and SR are at odds. 300,000 km/sec iis  fast but nowhere near  as fast as spooky action. We will return to this later in the thread.

Peter

Peter

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Looking at the Block Universe we see time-slices extending back anf ahead of a now-slice.  But our now-slice always advnces inot the future never the past. Why not into the past if is as real as the future.

What determines the arrow of time ?

Peter

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12 hours ago, Peter Cobbold said:

But the speed of light is nowhere near infinity, its 300,000 km/sec (in vacuo) It has been measured to great precision. It is Einsteins fundamentla postulate and is non-negotiable !  Given that, a deep field photon would take 13 billion years to reach Hubble. But that valid measurement is from our frame of reference. But on the photons own refernece frame no time passes and it travels no distance.** That is the guts of SR. Time and space are relative to the observer and dilate and contract as the relative velocities between reference frames increase. When the relative veocities of reference frames is at c (shorthand symbol for speed of light) then tiem and space are infintely dialted and  contracted in the direction of the motion, to zero.

** If a deep field photon could see earth, we would be frozen in time and contracted  to a disc. SR works both ways, no reference frame is preferred.

There is a phenomenon with solid, accepted exptl validation that two events can be co-ordiinated instantaneously ( infininately fast). Entanglement. Otheriwse known as "spooky action at a distance "  Since no information can travel faster than c entanglement and SR are at odds. 300,000 km/sec iis  fast but nowhere near  as fast as spooky action. We will return to this later in the thread.

Peter

Peter

Why the "But" in your response, Peter?   Surely we agree - how could we do else, faced with Einstein?   I said, "to photon velocity has no meaning" and you, "on the photons own refernece frame no time passes and it travels no distance."     Comes to the same thing!

 

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So what determines the arrow of time?  Hawking postulated three arrows, thermodynamic, psychological and cosmological.   

Reality trends to a more disordered state, so that its direction is away from a highly organised condition, pushing the thermodynamic arrow that way.    In the absence of real knowledge of our psychology. Hawking used computer memory as an analogy, and suggested that energy is used to put items into memory,  so that we perceive time  in the same direction as the thermodynamic arrow.   He first used  Big Bang and the "inflation" that almost instantly followed, causing a Universe that was smooth and ordered, but since has become disordered and lumpy with stars and humans,  to suggest that when expansion ceased and the Universe begins to contract then the arrow of time would reverse.     But he says he was mistaken!    There is no boundary to the Universe, no edge, so the trend to disorder would continue as contraction occurred.    He therefore resorts to the "Weak anthropic principle" (the Laws and Quantities of the Universe we see are so, because if they were different, we would not exist) and suggests that a no boundary Universe will not contract until after a very long interval, so long that the Universe will be almost completely disordered, and life could not exist, and while the cosmological arrow could reverse, we could not be there to see it.

Thus, says Hawking, time flows in one direction, driven by the three arrows of the Universe, as we see it in this moment.

I fear that my memory of Hawking's views, assisted by resort to his "A Brief History of Time" may not do him justice.

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4 hours ago, peejay4A said:

Who was it who said “time is what stops everything happening at once”?  

at a guess Richard Feynman or John Archibald Wheeler.

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4 hours ago, john.r.davies said:

So what determines the arrow of time?  Hawking postulated three arrows, thermodynamic, psychological and cosmological.   

Reality trends to a more disordered state, so that its direction is away from a highly organised condition, pushing the thermodynamic arrow that way.    In the absence of real knowledge of our psychology. Hawking used computer memory as an analogy, and suggested that energy is used to put items into memory,  so that we perceive time  in the same direction as the thermodynamic arrow.   He first used  Big Bang and the "inflation" that almost instantly followed, causing a Universe that was smooth and ordered, but since has become disordered and lumpy with stars and humans,  to suggest that when expansion ceased and the Universe begins to contract then the arrow of time would reverse.     But he says he was mistaken!    There is no boundary to the Universe, no edge, so the trend to disorder would continue as contraction occurred.    He therefore resorts to the "Weak anthropic principle" (the Laws and Quantities of the Universe we see are so, because if they were different, we would not exist) and suggests that a no boundary Universe will not contract until after a very long interval, so long that the Universe will be almost completely disordered, and life could not exist, and while the cosmological arrow could reverse, we could not be there to see it.

Thus, says Hawking, time flows in one direction, driven by the three arrows of the Universe, as we see it in this moment.

I fear that my memory of Hawking's views, assisted by resort to his "A Brief History of Time" may not do him justice.

Agreed. The thermodynamic model dates from Eddington - the arrow  reflects entropy increasing ( eggs break but do not reassemsble(. But that has the unfortunate consequence of requiring a very low entropt state of "perfection" at the big bang, and that is  regarded as improbable, The anthropic approach - we have memories and that creates an impression of time flow- is not attractive to biologisrs becas=use evolution cannnot respond to absnet selection forces. if tiem does not flwo there is no evolutionary pressure to eveolve a memory. I never grasped his cosmological argument.

There is another arrow we have not mentioned? it is almost a truism, so commonplace that it is hiding in plain sight, and you never need have read any physics to see it. Yet is may  well be fundamental to understanding time.   Anyone ?

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