Last Updated：2025/12/08
(physics) A physical paradox that arises when considering the buoyant force exerted on a relativistic bullet (or in a submarine) immersed in a fluid subject to an ambient gravitational field. If a bullet has neutral buoyancy at rest in a perfect fluid and is then launched at relativistic speed, observers at rest within the fluid would conclude that the bullet should sink, since its density will increase due to the length contraction effect. On the other hand, in the bullet's proper frame it is the moving fluid that becomes denser and hence the bullet would float.
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Supplee's paradox
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Source Word
Supplee's paradox
Proper noun
(physics)
A
physical
paradox
that
arises
when
considering
the
buoyant
force
exerted
on
a
relativistic
bullet
(or
in
a
submarine)
immersed
in
a
fluid
subject
to
an
ambient
gravitational
field.
If
a
bullet
has
neutral
buoyancy
at
rest
in
a
perfect
fluid
and
is
then
launched
at
relativistic
speed,
observers
at
rest
within
the
fluid
would
conclude
that
the
bullet
should
sink,
since
its
density
will
increase
due
to
the
length
contraction
effect.
On
the
other
hand,
in
the
bullet's
proper
frame
it
is
the
moving
fluid
that
becomes
denser
and
hence
the
bullet
would
float.
Japanese Meaning
(物理学) 相対論的状況下での浮力に関する逆説。具体的には、完全な流体中で中性浮力を保つ弾丸（または潜水艦）が、相対論的速度で運動させられた場合、静止している流体の観測者からは弾丸の密度が長さ収縮により増加して沈むと判断される一方、弾丸の固有系では移動する流体の密度が増すため、弾丸が浮くと見なされるという矛盾が生じる現象
Sense（1）
(physics)
A
physical
paradox
that
arises
when
considering
the
buoyant
force
exerted
on
a
relativistic
bullet
(or
in
a
submarine)
immersed
in
a
fluid
subject
to
an
ambient
gravitational
field.
If
a
bullet
has
neutral
buoyancy
at
rest
in
a
perfect
fluid
and
is
then
launched
at
relativistic
speed,
observers
at
rest
within
the
fluid
would
conclude
that
the
bullet
should
sink,
since
its
density
will
increase
due
to
the
length
contraction
effect.
On
the
other
hand,
in
the
bullet's
proper
frame
it
is
the
moving
fluid
that
becomes
denser
and
hence
the
bullet
would
float.
