Distances for classical objects vs. photons

In summary: Essentially, it states that the laws of physics (like the equation d = vt) remain the same when an observer is in a moving frame of reference.However, this only holds for objects with a constant speed; if an object is accelerating, then the laws of physics would be different in its moving frame of reference.In summary, it seems that classical objects - like us - experience vast spatial extents, while photons do not. This is due to the fact that photons have a "rest frame" in which the concepts of "distance" and "elapsed time" make sense, while classical objects do not. However, this effect might never apply to non-photon objects if acceleration to near speed of light was a technological possibility
  • #1
TRB8985
74
15
Good afternoon all,

A few days ago, I had been reading a book on general relativity and cosmology by Dr. Brian Greene, in which something was written that I found to be very profound. (At least, from the standpoint of my own ignorance on the subject.) I was wondering if any professionals could point me in the right direction to learning more about this quandary.

In the book, Dr. Greene went on to describe how classical objects, like us, experience the vast enormity of spatial extent in the heavens, yet photons do not. More specifically, it was mentioned that the distance between objects from a photons perspective is zero.

This utterly blows my mind. How is this even possible? Is there some kind of underlying prevailing wisdom I'm unaware of wherein distances are just an artifact of spacetime experienced by classical objects?

Forgive my speculation, but I'm completely unaware of how to reconcile this statement.
 
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  • #2
TRB8985 said:
In the book, Dr. Greene went on to describe how classical objects, like us, experience the vast enormity of spatial extent in the heavens, yet photons do not.

While Brian Greene is a well-known physicist, his pop science books are not good sources if you actually want to learn about physics. In this particular case, his description depends on an implicit assumption that a photon has a "rest frame" in which the concepts of "distance" and "elapsed time" make sense. That assumption is false: see the PF FAQ entry on this topic, and also the Usenet Physics FAQ entry on a similar topic.
 
  • #3
Indeed; thanks Peter! Appreciate your help.
 
  • #4
Yes, under relativity the passage of time is relative to the observer's inertial reference frame. Since a photon does not have an inertial reference frame, it does not perceive the passage of time - which renders the concept of distance meaningless under the familiar equation d = vt.
 
  • #5
Chronos said:
it does not perceive the passage of time

It's probably better to say that the concept of "perceived passage of time" does not even apply to a photon, to avoid any possible misunderstanding.
 
  • #6
Incredible. I'm reminded daily that I picked the right career to get into.

Let me push this a little further, and please reign me back into the correct ideas if I'm getting too far into left field.

Could this effect *ever* apply to non-photon objects if acceleration to near speed of light was a technological possibility? Or do these explanations apply explicitly only to photons?

Again, thank you both.
 
  • #8
Excuse me, sorry; the effect analogous to a photon experiencing timelessness and a distance of zero.

It seems strange that this is something only experienced by a photon in its own perspective. The whole scenario makes me speculate whether this is some underlying fundamental characteristic of the universe itself, yet we experience things completely differently and are completely unaware of it in our own inertial frames of reference. (i.e. - is the universe really of "zero" size?)

However, I feel like I'm getting far too into the lands of philosophy and armchair rationalization.
 
  • #9
TRB8985 said:
the effect analogous to a photon experiencing timelessness and a distance of zero.

Ok, but the whole point is that the photon does not "experience" these things; the concepts of "experienced time" and "distance" are meaningless for a photon.

TRB8985 said:
It seems strange that this is something only experienced by a photon in its own perspective.

A photon does not have a "perspective" in this sense; as the links I gave say, there is no such thing as an inertial reference frame in which a photon is at rest. This is a fundamental difference between lightlike objects (like photons), which travel on null worldlines, and other objects (like us) which travel on timelike worldlines. The concepts of "experienced time" and "distance" only make sense for objects traveling on timelike worldlines.

TRB8985 said:
The whole scenario makes me speculate whether this is some underlying fundamental characteristic of the universe itself

In terms of SR and GR, the difference between null and timelike worldlines (and objects that travel on them) is a geometric property of spacetime. So in that sense it is an "underlying fundamental characteristic".
 
  • #10
Okay. I think it finally clicked now.

I apologize for making you repeat yourself, Peter - my understanding in physics is purely Newtonian at this point (classical undergraduate), so shaking the incorrect, preconceived notions I've carried all my life for the prevailing wisdom in SR and GR is still very alien and counterintuitive.
 
  • #11

Related to Distances for classical objects vs. photons

1. How do we measure distances for classical objects?

Distances for classical objects can be measured using various techniques such as parallax, radar ranging, and triangulation. Parallax involves measuring the change in position of an object relative to a background as the observer changes locations. Radar ranging uses the time it takes for a radio signal to bounce off an object and return to the sender to determine its distance. Triangulation involves measuring the angles to an object from two different locations and using trigonometry to calculate the distance.

2. How are distances for photons measured?

Distances for photons are measured using the speed of light and the time it takes for a photon to reach an observer. This is known as the redshift method, where the wavelength of light emitted by an object is stretched as it travels through the expanding universe. By measuring this wavelength, scientists can determine the distance of the object.

3. Why do we use different methods to measure distances for classical objects and photons?

The methods used to measure distances for classical objects and photons are different because they have different properties. Classical objects have a physical size and can be measured using techniques such as parallax and triangulation. On the other hand, photons do not have a physical size and travel at the speed of light, making it impossible to measure their distance using traditional methods. Therefore, the speed of light and the redshift method are used to measure distances for photons.

4. Can we use the same method to measure distances for all objects in the universe?

No, we cannot use the same method to measure distances for all objects in the universe. Different objects have different properties and therefore require different methods to measure their distances. For example, the redshift method is only applicable to objects that emit light, while parallax can only be used for objects that are relatively close to Earth. Each method has its own limitations and is used based on the properties of the object being measured.

5. How accurate are the distance measurements for classical objects and photons?

The accuracy of distance measurements for classical objects and photons depends on the method used and the precision of the instruments. For nearby objects, such as planets in our solar system, measurements can be very accurate. However, for objects that are extremely far away, such as galaxies, the margin of error can be larger. Additionally, the expansion of the universe and other factors can also affect the accuracy of distance measurements.

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