The Naked Scientists

The Naked Scientists Forum

Author Topic: At the speed of light, does time stop?  (Read 17608 times)

Offline cheryl j

  • Neilep Level Member
  • ******
  • Posts: 1460
  • Thanked: 1 times
    • View Profile
At the speed of light, does time stop?
« on: 27/02/2013 15:53:56 »
I was listening to a lecture by astrophysicist Neil deGrasse Tyson and he made the remark that at the speed of light, time stops. He said from "the photons point of view," there is no period of time between when the photon is emitted from a star, and when it arrives millions of years of later, here on earth or somewhere else. No matter how hard I try to wrap my brain around that idea, it doesn't seem possible, because light does have a velocity, and velocity is distance/time. Hopefully I understood him correctly and am not misstating something.
« Last Edit: 27/02/2013 23:27:14 by chris »


 

Offline Bill S

  • Neilep Level Member
  • ******
  • Posts: 1818
  • Thanked: 11 times
    • View Profile
Re: photons and time
« Reply #1 on: 27/02/2013 23:17:23 »
It seems that within the scientific community one can find experts who say quite definitely that photons do not "experience" time.  However, the most common response to this question seems to be that because one cannot assign a relativistic frame of reference to a photon, the question makes no sense, and therefore cannot have an answer.  It is argued that concepts mean something only if you can define how to measure them practically.

While one has to accept that all this is undoubtedly true, that photons cannot be considered as observers, and that no physical observer can attain the speed of light; the feeling that there is a cop-out lurking there somewhere is difficult to shake off.

I have recently tried using the CMBR as a way of clarifying the issue, but with little success.  I learned quite a bit about the CMB, but made no progress with the photon/time question.

I wish you luck sorting this one out.  If you succeed, PLEASE post your findings.
 

Offline Bill S

  • Neilep Level Member
  • ******
  • Posts: 1818
  • Thanked: 11 times
    • View Profile
Re: At the speed of light, does time stop?
« Reply #2 on: 27/02/2013 23:52:39 »
Sascher Vongehr says:  http://www.science20.com/alpha_meme/fundamental_nature_light-75861

   Light has no time to see
       nor any space to be,
       nor even any energy.




 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Re: At the speed of light, does time stop?
« Reply #3 on: 28/02/2013 00:29:15 »
I hate to contradict Neil deGrasse Tyson, but if he claimed a photon has a "point of view," he's wrong.  This comes up a lot on the forum, so I'll just summarize the answer and link you to a previous thread.

The basic answer is that the idea of getting inside an object's "point of view" means we define a reference frame for that object.  Our theories tell us how to define reference frames for anything with rest mass (such as ourselves and clocks), but not for massless photons!  So we don't have a theory that tells us what a photon sees, and we don't know if it even makes sense to talk about time passing for a photon, since none of our measurement tools can ever move at the speed of light.

Here's a previous thread where it gets discussed in more detail: http://www.thenakedscientists.com/forum/index.php?topic=42037.0
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11993
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: At the speed of light, does time stop?
« Reply #4 on: 28/02/2013 19:34:00 »
Using your own position in time and space Cheryl you would define a photon as having a speed, wouldn't you? And we have a speed defined for it called 'c'. So from our perspective it must take 'time'. But we also find it to be time less, as the only thing annihilating that light will be its interaction, although astronomical and other redshifts possibly can do the same? I'm not sure there, as we also have the definition of a light quanta of a unchanging energy intrinsically. Redshifts are you relating to a wave, but thought of as a photon its intrinsic energy shouldn't change ever.
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Re: At the speed of light, does time stop?
« Reply #5 on: 01/03/2013 02:37:16 »
I agree with JP. It's not quite right to be speaking from a photons point of view. It's merely an extrapolation.
 

Offline cheryl j

  • Neilep Level Member
  • ******
  • Posts: 1460
  • Thanked: 1 times
    • View Profile
Re: At the speed of light, does time stop?
« Reply #6 on: 01/03/2013 04:43:24 »
Well.That is why I stick to biology I guess. Except for the very difficult mathematics, does studying physics feel like smoking marijuana half the time? It's so conceptionally bizarre. I assume the mathematics makes it less so.
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Re: At the speed of light, does time stop?
« Reply #7 on: 01/03/2013 05:00:18 »
Well.That is why I stick to biology I guess. Except for the very difficult mathematics, does studying physics feel like smoking marijuana half the time? It's so conceptionally bizarre. I assume the mathematics makes it less so.
He he! I guess it may feel like that to some people. I love it though. It gives me a better sense of the world in which I live. To me studying physics is a bit like looking up Mother Natures skirt. :)
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Re: At the speed of light, does time stop?
« Reply #8 on: 01/03/2013 14:31:33 »
Well.That is why I stick to biology I guess. Except for the very difficult mathematics, does studying physics feel like smoking marijuana half the time? It's so conceptionally bizarre. I assume the mathematics makes it less so.
He he! I guess it may feel like that to some people. I love it though. It gives me a better sense of the world in which I live. To me studying physics is a bit like looking up Mother Natures skirt. :)

I gave up on biology in high school!  All the memorization made my head hurt.  I find physics easier because much of it can be derived mathematically from simple rules.  I guess it's just a matter of how you learn best!
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11993
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: At the speed of light, does time stop?
« Reply #9 on: 02/03/2013 22:24:09 »
Hmm, first marijuana and now, memory loss?
Ah well, all disciplines have their ups and downs I guess :)
Even biology .
 

Offline Bill S

  • Neilep Level Member
  • ******
  • Posts: 1818
  • Thanked: 11 times
    • View Profile
Re: At the speed of light, does time stop?
« Reply #10 on: 02/03/2013 23:35:18 »
If anyone has the time and patience to follow the link to Sascher Vongehr's articles, I would be fascinated to hear some comments from those who have more scientific know-how than I.
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Re: At the speed of light, does time stop?
« Reply #11 on: 03/03/2013 00:13:01 »
He's making the same mistake mentioned above: that we can legitimately extrapolate to the "point of view of light" by just thinking about our own point of view if we could somehow reach the speed of light.  We can't, of course, and the theory that covers our sub-luminal speeds does not extrapolate to the reference frame of light.  This is a really common mistake, and it naturally comes from trying to extend the theory beyond it's limits--I used to post the same viewpoint on this site until I read up a bit more and realized why it was wrong. 

The problem is that in science we can only make theories about things that are testable, and "what does light experience"? is not testable by any means we currently have.  From a scientific viewpoint, it's unknowable for now.
 

Offline Ethos_

  • Neilep Level Member
  • ******
  • Posts: 1277
  • Thanked: 14 times
    • View Profile
Re: At the speed of light, does time stop?
« Reply #12 on: 03/03/2013 02:12:44 »
He's making the same mistake mentioned above: that we can legitimately extrapolate to the "point of view of light" by just thinking about our own point of view if we could somehow reach the speed of light.

Precisely my friend................Just because experiment proves that as we accelerate closer to light speed, and the passage of time slows down for the traveler, it does not extrapolate into zero time at light speed. And this is because it can't be shown experimentally, and to assume this result is not scientific. Likewise, just because the universe is expanding and we assume that it all started from a singularity at the Big Bang, is also not achieved by the scientific method.

Sadly, many individuals that call themselves scientists only classify a particular view as scientific truth when those views fit their personal likes. And then turn right around and dismiss another view with similar evidence.

There is just as much evidence for zero time at light speed as there is for the Big Bang! What should that tell us about scientific bias?
« Last Edit: 03/03/2013 02:20:33 by Ethos_ »
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Re: At the speed of light, does time stop?
« Reply #13 on: 03/03/2013 12:59:02 »
They're not analogous.  You can't extrapolate to light's point of view because our theories don't extrapolate to cover the case of moving at the speed of light and we can't possibly experiment there to develop a new theory that does cover that case.

In the case of the big bang, our theories do cover the period immediately after the big bang and we can access a lot of that period experimentally (through particle accelerators).  What we can't really talk about is what happened at the instant of the big bang, since that's where our theories break down and we can't do experiments. 

If you'd like to discuss the evidence for the big bang, let's start a new thread, since it's likely to derail this one.
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11993
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: At the speed of light, does time stop?
« Reply #14 on: 04/03/2013 12:32:47 »
From a point of measuring you use a two way experiment for deciding a 'photon speed'. You and your detector together, and then a mirror placed some way away. The particle 'bounce' if you like in a so called 'elastic collision' with the mirror, or just think of it as a wave 'bouncing' from the mirror back to our detector. Don't really know any other way to do it myself although I've seen some very clever ideas elsewhere. Anyway, what you do when you measure that speed is that you use your local clock and your local ruler. The ruler defines your distance to the mirror, the clock defines 'segments' of time, ticking. Combining those you get a arbitrarily defined 'speed', arbitrarily as you can use some other 'segmentation' of time (or ruler), reaching another definition of 'speed'. But what seems to be true is that no matter what segmentation you use the light/photon still becomes a constant, no matter where you do that experiment, at a event horizon or on Earth.

And that is 'locality'. Giving you a same speed, no matter where you are. That 'speed' can then be seen as a real constant, defining the space and time you see. Why :) well, how do you measure, and see?
« Last Edit: 04/03/2013 12:40:17 by yor_on »
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11993
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: At the speed of light, does time stop?
« Reply #15 on: 04/03/2013 12:54:07 »
There is one point more to it.
Gravity..

At the event horizon, assuming you being there measuring outwards into space, gravity should bend the 'path taken' as observed by you, making it take more 'time' for your measurement, all locally perceived. That situation is analogous to one in where you are inside a uniformly (constantly) accelerating spaceship (ignoring tidal forces) also finding the light to take more 'time'.

Now, why are those two analogous in my mind?

Gravity 'bends' SpaceTime, and inside that spaceship you must find a local 'gravity', making you 'weight something'. So strictly defined 'c' is 'c' as long as we only consider a 'flat SpaceTime', all as I get it. But using 'gravity' to define a 'shortest path' for that light, the two situations described becomes equivalent, and light will still be a local 'constant' relative the 'bending' of Space, measured locally. Well, as I get it.

It's not that complicated really, but the implications surely are.
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11993
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: At the speed of light, does time stop?
« Reply #16 on: 04/03/2013 13:28:52 »
And a third.

Now consider a light clock  (<- - ->) traveling in space.

It uniformly moves away from you very fast, near the speed of light as you measure, and you see it as depicted above.
The light 'bounce' between those two mirrors ' ( <-and ->) becoming a 'clock', ticking.

From your perspective it must 'slow down'  as that light clock is moving away from you very fast, making the 'light bounces' traverse a lot of more space each time, than they would need to if the clock was perfectly still relative yourself (as being in the same room).

And it goes all out from light being a 'constant', same anywhere.

The question then becomes if this is true, does that light clock really slow down its 'time', and relative what If so?
Only your measurments, or inside that ship too?

That's what the 'twin experiment' discuss. And one twin are indeed expected to have aged less, the 'traveling one', as measured/compared when returning to Earth.

But it is also true that using the two way experiment to define a local 'time' inside the Spaceship, splitting that light into even 'chunks' relative your ruler, that speed never differs. We could instead have defined it as the light clock uniformly accelerating, and then found a 'bent space' between the mirror and detector inside that ship, but to point it out real good we can use a uniform motion instead, as all uniform motions becomes equivalent in matter of locally measuring a 'speed'. The mass may impose a 'bending', or if we talk accelerations, the 'relative mass' locally perceived to change your 'weight'. But in all uniform motions that relative mass is experimentally absent, locally measured, and, as I get it of course :)
=

What you can use is 'potential energy' to describe uniform motions. But all such 'energy' is relations between 'two frames of reference', as comparisons between points in space and time. And using suns to describe it they all move in different uniform motions relative you (Earth), also having different masses.

So depending on what Sun you compare your 'potential energy' to, in some thought up collision, you get different results. So what is your 'potential energy' alone? It got to be your mass, and it should be your motion. But all uniform motions are equivalent, locally measured. You need that other 'frame of reference' to find your 'potential energy'.


« Last Edit: 04/03/2013 13:48:14 by yor_on »
 

Offline Bill S

  • Neilep Level Member
  • ******
  • Posts: 1818
  • Thanked: 11 times
    • View Profile
Re: At the speed of light, does time stop?
« Reply #17 on: 05/03/2013 19:13:46 »
Quote from: JP
The problem is that in science we can only make theories about things that are testable, and "what does light experience"? is not testable by any means we currently have.  From a scientific viewpoint, it's unknowable for now.

Is it me, or is there a dichotomy here?

Does a photon have a frame of reference?  We cannot say it does because relativity does not permit the designation of a frame of reference for a massless object.  Therefore the subject is anathema; it is simply speculation.  On the other hand a singularity, for which relativity provides no explanation, is quite acceptable on the basis that relativity breaks down at this point, so something else must provide the explanation, and undoubtedly will do so when we can find it, so it's OK to go on speculating about singularities.     



 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Re: At the speed of light, does time stop?
« Reply #18 on: 05/03/2013 19:45:20 »
Quote from: JP
The problem is that in science we can only make theories about things that are testable, and "what does light experience"? is not testable by any means we currently have.  From a scientific viewpoint, it's unknowable for now.

Is it me, or is there a dichotomy here?

Does a photon have a frame of reference?  We cannot say it does because relativity does not permit the designation of a frame of reference for a massless object.  Therefore the subject is anathema; it is simply speculation.  On the other hand a singularity, for which relativity provides no explanation, is quite acceptable on the basis that relativity breaks down at this point, so something else must provide the explanation, and undoubtedly will do so when we can find it, so it's OK to go on speculating about singularities. 
But you don't put frames of reference in a singularity...
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Re: At the speed of light, does time stop?
« Reply #19 on: 05/03/2013 20:25:01 »
Quote from: JP
The problem is that in science we can only make theories about things that are testable, and "what does light experience"? is not testable by any means we currently have.  From a scientific viewpoint, it's unknowable for now.

Is it me, or is there a dichotomy here?


As lightarrow pointed out, a reference frame and a singularity are not the same thing!   The issue is that one of the postulates of SR is that the speed of light is always measured as a constant in any inertial reference frame.  It is impossible for light to be in an inertial reference frame, since it would measure it's own speed at 0, rather than the speed of light, meaning that whatever it's "frame" means, it's not an inertial reference frame.

A singularity is where equations break down in a particular way, although it commonly gets used in physics to mean the region of space-time at the center of black holes where the equations of general relativity break down.  This is different than the light reference frame issue, since we don't formulate GR by excluding this break down region.  Rather, it's formulated from different considerations, and these breakdown points fall out of the equations, signifying the theory doesn't apply there and something more must be used. 

It's a subtle point, but while in GR, singularities exist over real regions of space-time and are approachable by experiment and current theory, reference frames for photons aren't even a valid thing in special relativity, so they can't be approached by theory or experiment--at least not in terms of current theories.
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Re: At the speed of light, does time stop?
« Reply #20 on: 05/03/2013 22:09:25 »
Quote from: JP
I used to post the same viewpoint on this site until I read up a bit more and realized why it was wrong. 
Where did you read about it? I'm always looking for better ways to explain/describe things.
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Re: At the speed of light, does time stop?
« Reply #21 on: 05/03/2013 22:17:00 »
Quote from: Ethos
\He's making the same mistake mentioned above: that we can legitimately extrapolate to the "point of view of light" by just thinking about our own point of view if we could somehow reach the speed of light.
It can be viewed as a limit though. We do this when we say that the proper time between two events seperated by a lightlike spacetime interval. This is the reason one cannot define a 3-momentum for a photon, i.e. because you'd be dividing by zero.

Regarding the so-called Big Bang singularity. In the Big Bang theory there is no event which can be called the big bang. As Peebles explains "If there were an instant, at a 'big bang,' when our universe started expanding it is not in the cosmology as now accepted, because no one has thought of a way to adduce objective physical evidence that such an event really happened."
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Re: At the speed of light, does time stop?
« Reply #22 on: 06/03/2013 01:18:27 »
Quote from: JP
I used to post the same viewpoint on this site until I read up a bit more and realized why it was wrong. 
Where did you read about it? I'm always looking for better ways to explain/describe things.

One that I recall was this: http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/headlights.html
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Re: At the speed of light, does time stop?
« Reply #23 on: 06/03/2013 02:06:58 »
Quote from: JP
I used to post the same viewpoint on this site until I read up a bit more and realized why it was wrong. 
Where did you read about it? I'm always looking for better ways to explain/describe things.

One that I recall was this: http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/headlights.html
Nice!!!
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Re: At the speed of light, does time stop?
« Reply #24 on: 06/03/2013 17:10:22 »
The other neat thing about this question is that it shows that physics is about tying equations into nature, not just blindly applying math.  The reason I used to think that photons didn't "experience" time was that if you blindly apply the equations of SR to photons, you can get to that conclusion (technically, you'd have to take a limiting case). 

But if you look at nature, which says the speed of light is constant for all inertial reference frames, then you realized immediately that using the equations in that case is nonsense--since you're trying to define a frame where the speed of light has changed to 0 m/s.  And in fact, you can verify that the derivation of the equations of SR precludes them from covering the reference frame of photons.
 

The Naked Scientists Forum

Re: At the speed of light, does time stop?
« Reply #24 on: 06/03/2013 17:10:22 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums