# Length

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With length contraction is there a change in density? If there is would that in part/maybe in whole account for the increase in mass.  Then it leads to other questions like if we slow something down that is moving really fast like a particle does it get bigger?, or maybe change identities completely, because it changes energy levels,and can we expect a particle to be contracted and more dense on the side of the direction it is moving.

Do photons increase in volume when passing through water? If it doesn't, any thoughts on why?

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As measured from the frame that an object is moving with respect to, there would be an apparent density increase, merely because it would measure more mass in less volume.  But this is not in any way connected to the apparent mass increase.  That is simply due to the fact that The moving object has a kinetic energy as measured with respect to the measuring frame.  That KE has a mass equivalence.    One way to think of it is that energy imbues an object with properties ( such as inertia) that used to be just considered associated with "mass". In modern parlance, the "mass" of the object does not change as mass is generally  restricted to mean "rest" or "invariant" mass.

You have to be careful when dealing with changes in velocity of extended objects.   If you have an object that measures itself so that it is x units long, and that object is moving at 0.866c relative to you, then you will measure it to be x/2 units long.     If you now bring it to a rest with respect to yourself, you have to consider how this is done.   Let's say that it's done so that the object never measures a change in its size during the process. This mean that you will measure its length to increase as it slows.  This is due to the relativity of simultaneity . In the object's frame, all its parts "slow down at the same rate at the same time.  But events that are simultaneous for the object will not be simultaneous according to you.   You will measure the trailing end of the object start to slow down before the leading end does and as a result by the time the whole thing comes to rest, it will be x units long.

I don't get where you would get the idea that an object would be more contracted at the leading edge.

Photon don't have a "volume".   While they have particle-like properties, you can't think of them as little spheres of something.

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12 hours ago, Janus said:

I don't﻿﻿﻿ get where you would get the idea that an object would be more contracted at the leading edge.

Photon ﻿don't have a "volume".   While they have particle-like properties, you can't think of them as little spheres of something﻿.﻿

I will try to remember. I've been thinking it might be faulty memory. In part it probably is. In part it might be my inabiltyy to distinguish fake science from real science. I'm beginning to realize that just about everything is for sell, and that I should probably get off the band wagon until every little thing stops being trendy and ending up in my news feeds.

About the photons, why not?

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• 4 weeks later...
On 4/6/2019 at 7:54 AM, jajrussel said:

I will try to remember. I've been thinking it might be faulty memory. In part it probably is. In part it might be my inabiltyy to distinguish fake science from real science. I'm beginning to realize that just about everything is for sell, and that I should probably get off the band wagon until every little thing stops being trendy and ending up in my news feeds.

About the photons, why not?

Sorry, I assumed any object of energy would have volume, I also assumed that kinetic energy would present as Mass. Yes at rest a photon has no Mass, but then I am not sure that  at  rest a photon exists. Actually I'm fairly certain that if an object of zero Mass has to move at c then it can only exists at c, and at c it must have Mass in the form of kinetic energy.

I think I remember reading that contraction occurs from the direction the object is moving, and I assumed that that side would be more dense. I also assumed that space would be more dense where it meets the object..

With an increase in velocity the object becomes smaller. Does this not occur from a direction? If you were to measure the objects kinetic energy would that energy be the same throughout the object, or greater at the leading edge.

With a photon you would only get one reading? And all of it would be kinetic? But with a solid object?

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4 minutes ago, jajrussel said:

With an increase in velocity the object becomes smaller. Does this not occur from a direction?

No, not as such.

4 minutes ago, jajrussel said:

If you were to measure the objects kinetic energy would that energy be the same throughout the object, or greater at the leading edge.

There is no way to associate KE with individual parts of a rigid object. It's a property of the whole thing.

The only circumstance I can think of where this would not apply is on time scales short enough where shock waves (or related phenomena) propagating through the object matter to the interaction. But then, you aren't treating it as a rigid object.

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On 4/6/2019 at 1:09 AM, Janus said:

Photon don't have a "volume".   While they have particle-like properties, you can't think of them as little spheres of something.

Could we think about them as a "3D wave-shaped" energy distribution? In the wave, they take up some space, which can be considered as "volume". They take up some space when they are in the particle "form".

Edited by FreeWill
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2 hours ago, FreeWill said:

Could we think about them as a "3D wave-shaped" energy distribution? In the wave, they take up some space, which can be considered as "volume". They take up some space when they are in the particle "form".

If you lack rigor in formulating the question, you aren't going to get a meaningful answer.

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